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Physicists Still Don_t Know What Happens at the Centre of a Black Hole · Night Science Stories

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  Speaker 1: Good evening and welcome to The Sleepy Scientist. It's lovely
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  Speaker 1: long way, which is more than can be said for
• 0:50
  Speaker 1: anything in tonight's episode. If you're already with us, thank you.
• 0:55
  Speaker 1: Your company on these evenings means a great deal. Now
• 0:58
  Speaker 1: we know a remarkable amount about black holes, how they
• 1:01
  Speaker 1: bend light, how they warp time, how matter behaves as
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  Speaker 1: it spirals inwards, growing hotter and brighter in its final moments,
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  Speaker 1: and then it crosses the edge, and our understanding goes quiet.
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  Speaker 1: I'll do my best not to make that so interesting
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  Speaker 1: that it keeps you up. But if it does, at
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  Speaker 1: least you'll drift off knowing why physicists can describe the
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  Speaker 1: outside of a black hole with extraordinary precision and and
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  Speaker 1: yet still cannot tell you, with any confidence at all,
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  Speaker 1: what's waiting right at the middle of one. Not the
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  Speaker 1: worst reason to lose a little sleep. So shall we begin?
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  Speaker 1: What actually happens at the centre of a black hole?
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  Speaker 1: And why does our very best theory of gravity seem
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  Speaker 1: to stop giving trustworthy answers at exactly the moment things
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  Speaker 1: get interesting? If a black hole has a centre, then
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  Speaker 1: what is actually there? It sounds like the sort of
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  Speaker 1: question that ought to have a tidy answer. After all,
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  Speaker 1: physicists can calculate how black holes bend light, how fast
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  Speaker 1: matter can orbit around them, how two black holes spiral together,
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  Speaker 1: and even how the fabric of space-time itself vibrates when
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  Speaker 1: they collide. We can detect black holes thousands or millions
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  Speaker 1: of light-years away. We can estimate their masses, measure their spins,
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  Speaker 1: and in some cases even image the glowing material surrounding
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  Speaker 1: the darkness they create. And yet, when we ask what
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  Speaker 1: ultimately exists at the very centre, The answer becomes unexpectedly uncertain.
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  Speaker 1: That is because a black hole is not really an
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  Speaker 1: object in the ordinary sense. The Earth has a centre
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  Speaker 1: because it is made from layers of matter. There is
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  Speaker 1: a crust, a mantle, a liquid outer core and a
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  Speaker 1: solid inner core. The Sun has a centre because it
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  Speaker 1: too contains matter arranged in layers, with temperature, pressure and
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  Speaker 1: density increasing as you move inward toward the core where
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  Speaker 1: nuclear fusion occurs. A black hole is different. There is
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  Speaker 1: no known solid surface to land on, no ordinary crust
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  Speaker 1: to pass through, and no conventional central core surrounded by
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  Speaker 1: progressively deeper layers of material. A black hole is better
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  Speaker 1: understood as a region of spacetime whose geometry has become
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  Speaker 1: so extreme that, beyond a certain boundary, nothing moving at
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  Speaker 1: or below the speed of light can return to the
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  Speaker 1: outside universe. That boundary is called the event horizon. The
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  Speaker 1: event horizon is perhaps the most famous feature of a
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  Speaker 1: black hole, but it is also one of the easiest
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  Speaker 1: to misunderstand. It is not a shell floating in space.
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  Speaker 1: It is not a physical membrane. It is not a
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  Speaker 1: dark surface made from mysterious material. If you could somehow
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  Speaker 1: fall through the event horizon of a sufficiently large black hole,
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  Speaker 1: there would not necessarily be a sudden collision or a
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  Speaker 1: bright flash announcing that you had crossed it. Instead, the
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  Speaker 1: event horizon is a boundary defined by causality. Outside it,
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  Speaker 1: there are possible paths through space-time that can still lead
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  Speaker 1: away from the black hole. Light can escape. A spacecraft,
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  Speaker 1: at least in principle, can move outward. Signals can still
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  Speaker 1: travel back into the wider universe. Inside it, those outward
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  Speaker 1: paths no longer exist. Once something crosses the event horizon,
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  Speaker 1: no light signal it sends can return to a distant observer.
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  Speaker 1: No radio transmission can escape. No beam of light can
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  Speaker 1: climb back out. The event horizon therefore creates a very
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  Speaker 1: unusual scientific problem. It separates regions of the universe not
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  Speaker 1: merely by distance, but by whether information can ever travel
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  Speaker 1: from one region to another, and this is where several
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  Speaker 1: different black hole ideas are often mixed together. The event
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  Speaker 1: horizon is one thing, the black hole shadow is another,
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  Speaker 1: and the singularity predicted by classical general relativity is something
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  Speaker 1: else again. When astronomers produce an image showing a dark
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  Speaker 1: central region surrounded by glowing material, they are not photographing
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  Speaker 1: the singularity. They are not even directly photographing the event horizon,
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  Speaker 1: in the ordinary sense. The dark region is known as
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  Speaker 1: the black hole shadow. It appears because light travelling near
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  Speaker 1: the black hole follows strongly curved paths through spacetime. Some
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  Speaker 1: of that light escapes toward us, while some is captured.
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  Speaker 1: The result is a dark area whose apparent size is
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  Speaker 1: larger than the event horizon itself, surrounded by radiation emitted
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  Speaker 1: by hot plasma nearby. So even one of our most
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  Speaker 1: direct-looking images of a black hole is really an image
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  Speaker 1: of light behaving under extreme gravity. The true interior remains hidden.
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  Speaker 1: Astronomers can still learn an extraordinary amount from what happens
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  Speaker 1: just outside the event horizon. Gas falling toward a black
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  Speaker 1: hole can form an accretion disk and heat to enormous temperatures.
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  Speaker 1: Magnetic fields can become twisted through the surrounding plasma. In
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  Speaker 1: some systems, narrow jets of energetic particles can be launched
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  Speaker 1: across distances far larger than the black hole itself. The
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  Speaker 1: motion of stars around a black hole can reveal its mass.
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  Speaker 1: The motion of hot gas can reveal how spacetime is curved.
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  Speaker 1: Gravitational waves from black hole collisions allow physicists to test
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  Speaker 1: gravity in conditions far beyond anything achievable in a laboratory.
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  Speaker 1: But all of these measurements have one limitation in common.
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  Speaker 1: They come from regions that can still communicate with us.
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  Speaker 1: Anything that happens after matter has crossed the event horizon
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  Speaker 1: cannot send an ordinary message back out. This restriction would
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  Speaker 1: already make the centre of a black hole difficult to understand.
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  Speaker 1: But the deeper problem comes from the theory that describes
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  Speaker 1: black holes in the first place. That theory is Albert
• 6:40
  Speaker 1: Einstein's General Theory of Relativity. Completed in 1915, General Relativity
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  Speaker 1: describes gravity not as a conventional pulling force acting across
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  Speaker 1: empty space, but as a consequence of curved spacetime. Matter
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  Speaker 1: and energy affect the geometry of spacetime, while that geometry
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  Speaker 1: influences how matter and light move. The theory has been
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  Speaker 1: tested with remarkable success. It explains the orbit of Mercury
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  Speaker 1: more accurately than Newtonian gravity alone. It predicts gravitational lensing,
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  Speaker 1: where massive objects bend passing light. It predicts gravitational time dilation.
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  Speaker 1: It predicted gravitational waves, which were directly detected a century later.
• 7:22
  Speaker 1: And it predicts black holes. Yet if general relativity is
• 7:26
  Speaker 1: followed deeper and deeper into an ideal black hole, something
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  Speaker 1: troubling happens. The curvature of spacetime becomes more extreme. In
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  Speaker 1: the simplest classical solutions, physical quantities eventually diverge. The mathematics
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  Speaker 1: leads toward what is called a singularity. The word can
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  Speaker 1: make it sound as though physicists know exactly what sits there.
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  Speaker 1: A tiny dot, infinite density, infinite gravity, perhaps a little
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  Speaker 1: cosmic bead at the absolute centre of everything that has
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  Speaker 1: fallen in. But that picture is much less certain than
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  Speaker 1: it first appears. A singularity is not comfortably understood as
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  Speaker 1: an ordinary physical object. Instead, it is a sign that
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  Speaker 1: the equations of classical general relativity have been pushed into
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  Speaker 1: a regime where they stop providing a complete physical description.
• 8:18
  Speaker 1: This is a subtle distinction, but it matters. Physicists are
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  Speaker 1: not staring at the centre of a black hole with
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  Speaker 1: absolutely no idea what should happen. General relativity gives them
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  Speaker 1: a very strong prediction that collapse continues toward a singular regime.
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  Speaker 1: The uncertainty is about what that regime actually means in nature.
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  Speaker 1: Does matter truly reach infinite density? Does space-time itself break down?
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  Speaker 1: Does quantum physics prevent the singularity from forming? Does something
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  Speaker 1: entirely different happen before infinity is ever reached? At the moment,
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  Speaker 1: there is no experimentally confirmed answer. This is why the
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  Speaker 1: centre of a black hole sits in such an interesting
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  Speaker 1: position within modern physics. The mystery is not simply that
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  Speaker 1: something is hidden from view. The real difficulty is that
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  Speaker 1: our best classical theory of gravity predicts a destination that
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  Speaker 1: seems to demand physics beyond that theory itself. We know
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  Speaker 1: a great deal about the road leading inward. We know
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  Speaker 1: the event horizon is not the centre. We know the
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  Speaker 1: darkness seen in astronomical images is not the singularity. We
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  Speaker 1: know general relativity predicts that the interior cannot remain ordinary.
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  Speaker 1: and we know that somewhere deep inside, the confidence provided
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  Speaker 1: by familiar physics begins to run out. So the problem
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  Speaker 1: is not that physics gives us no answer at all.
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  Speaker 1: It is that physics gives us an answer powerful enough
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  Speaker 1: to tell us that something profound must happen, while still
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  Speaker 1: leaving open the question of what that something really is.
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  Speaker 1: To understand why the centre of a black hole causes
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  Speaker 1: such trouble, it helps to begin with a question that
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  Speaker 1: sounds almost simpler than it really is. What exactly is gravity?
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  Speaker 1: For Isaac Newton, gravity was a force acting between masses.
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  Speaker 1: The Earth pulls on the Moon, the Sun pulls on
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  Speaker 1: the planets, drop an apple and the Earth pulls it downward.
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  Speaker 1: Newton's description was extraordinarily successful, and for many everyday situations
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  Speaker 1: it remains more than accurate enough. But Albert Einstein realised
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  Speaker 1: that gravity could be understood in a very different way.
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  Speaker 1: In the general theory of relativity, gravity is not primarily
• 10:28
  Speaker 1: treated as an invisible force, reaching across space. Instead, matter
• 10:34
  Speaker 1: and energy affect the geometry of spacetime itself. Spacetime is
• 10:38
  Speaker 1: the combined framework of space and time in which physical
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  Speaker 1: events occur. When mass and energy are present, that framework
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  Speaker 1: becomes curved. Objects then follow the natural paths available through
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  Speaker 1: that curved geometry. The Earth does not orbit the Sun
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  Speaker 1: because a mysterious rope is pulling it sideways through empty space.
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  Speaker 1: It follows a path through spacetime shaped by the Sun's mass.
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  Speaker 1: Light behaves in the same way. Even though light has
• 11:06
  Speaker 1: no rest mass, its path bends near massive objects because
• 11:09
  Speaker 1: the spacetime through which it travels is curved. This is
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  Speaker 1: one of the central ideas of general relativity. and it
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  Speaker 1: becomes especially important when gravity is extremely strong. Black holes
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  Speaker 1: are not simply places where Newtonian gravity has become very large,
• 11:25
  Speaker 1: they are places where the geometry of spacetime has become extreme.
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  Speaker 1: Soon after Einstein published the field equations of general relativity
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  Speaker 1: in 1915, the German physicist Karl Schwarzschild found an exact
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  Speaker 1: solution to them. Schwarzschild was working during the First World War,
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  Speaker 1: and remarkably he produced the solution while serving in the
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  Speaker 1: German army. His calculation described the spacetime outside an idealised
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  Speaker 1: spherical mass that does not rotate and carries no electric charge. Today,
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  Speaker 1: this is known as the Schwarzschild solution. It is the
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  Speaker 1: simplest mathematical description of a black hole. Real astrophysical black
• 12:03
  Speaker 1: holes are expected to rotate, sometimes very rapidly, so the
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  Speaker 1: Schwarzschild solution is not a perfect description of most black
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  Speaker 1: holes in the universe. but it remains extraordinarily useful because
• 12:16
  Speaker 1: it strips the problem down to its essentials. No spin,
• 12:19
  Speaker 1: no electrical charge, perfect spherical symmetry, just mass and curved spacetime.
• 12:26
  Speaker 1: The Schwarzschild solution contains a particular distance that has become
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  Speaker 1: one of the most familiar quantities in black hole physics.
• 12:34
  Speaker 1: It is called the Schwarzschild radius. For a non-rotating black hole,
• 12:39
  Speaker 1: the Schwarzschild radius marks the location of the event horizon.
• 12:43
  Speaker 1: Its size depends directly on the mass of the black hole.
• 12:46
  Speaker 1: If the Sun could somehow be compressed until it formed
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  Speaker 1: a black hole without changing its mass, its event horizon
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  Speaker 1: would have a radius of roughly three kilometres. That means
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  Speaker 1: almost the entire mass of the Sun, currently spread across
• 13:00
  Speaker 1: a sphere roughly 1.4 million kilometres wide, would need to
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  Speaker 1: be contained within a region only a few kilometres across.
• 13:08
  Speaker 1: The Earth would need to be compressed much further still.
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  Speaker 1: its Schwarzschild radius would be less than a centimetre. These
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  Speaker 1: comparisons give some sense of how dramatically matter must be
• 13:20
  Speaker 1: compacted before an event horizon forms. But the mathematics of
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  Speaker 1: the Schwarzschild solution originally seemed to contain a disturbing feature.
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  Speaker 1: At the Schwarzschild radius, certain parts of the equations appear
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  Speaker 1: to become infinite. That sounds alarming. If a mathematical quantity
• 13:41
  Speaker 1: becomes infinite at exactly the location of the event horizon,
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  Speaker 1: it is tempting to conclude that something physically catastrophic must
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  Speaker 1: happen there. For a time, this contributed to confusion about
• 13:53
  Speaker 1: what the Schwarzschild radius actually represented, but the infinity at
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  Speaker 1: the horizon turned out not to be a genuine physical singularity.
• 14:03
  Speaker 1: It was a coordinate singularity. That phrase may sound rather technical,
• 14:08
  Speaker 1: but the basic idea is surprisingly gentle. Coordinates are simply
• 14:12
  Speaker 1: a system for labelling positions and events. On Earth we
• 14:16
  Speaker 1: might use latitude and longitude. Near the North Pole, longitude
• 14:21
  Speaker 1: behaves strangely because all lines of longitude meet. That does
• 14:25
  Speaker 1: not mean the North Pole itself is physically broken. The
• 14:29
  Speaker 1: problem lies in the coordinate system. Something similar happens in
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  Speaker 1: the original Schwarzschild coordinates at the event horizon. The mathematical
• 14:38
  Speaker 1: description becomes badly behaved there, but spacetime itself does not
• 14:42
  Speaker 1: necessarily possess infinite curvature at that boundary. Other coordinate systems
• 14:48
  Speaker 1: reveal this much more clearly. One important contribution came from
• 14:53
  Speaker 1: the physicist David Finkelstein in the 20th century. By using
• 14:57
  Speaker 1: a different way of describing spacetime, Finkelstein helped make it
• 15:02
  Speaker 1: clear that the event horizon is not an infinitely curved wall.
• 15:06
  Speaker 1: An observer falling freely toward a sufficiently large black hole could,
• 15:11
  Speaker 1: in principle, cross the horizon without encountering any locally infinite
• 15:15
  Speaker 1: physical barrier at that exact location. The horizon is still
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  Speaker 1: profoundly important. Once crossed, the causal structure of spacetime changes
• 15:26
  Speaker 1: in such a way that returning to the outside universe
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  Speaker 1: becomes impossible. but the horizon itself is not where the
• 15:33
  Speaker 1: mathematics of general relativity truly breaks down. The more serious
• 15:38
  Speaker 1: problem lies deeper inside. In the Schwarzschild solution, the troublesome
• 15:44
  Speaker 1: region appears at the central value corresponding to zero radius.
• 15:49
  Speaker 1: As that central region is approached, measures of spacetime curvature
• 15:53
  Speaker 1: grow without bound. Unlike the apparent problem at the horizon,
• 15:58
  Speaker 1: This behaviour cannot simply be removed by choosing a more
• 16:01
  Speaker 1: convenient coordinate system. It reflects something much more fundamental. The
• 16:07
  Speaker 1: geometry itself is becoming singular. This distinction is crucial. At
• 16:12
  Speaker 1: the event horizon, the original mathematics looked badly behaved, even
• 16:17
  Speaker 1: though the physical spacetime could remain locally regular. At the centre,
• 16:22
  Speaker 1: the geometry predicted by the theory itself becomes pathological. Curvature
• 16:27
  Speaker 1: grows without limit. The classical description stops extending smoothly. And suddenly,
• 16:34
  Speaker 1: an extraordinarily successful theory seems to be warning us that
• 16:38
  Speaker 1: it is being pushed too far. Physics has encountered this
• 16:42
  Speaker 1: kind of warning before. When a model produces an infinite
• 16:46
  Speaker 1: value for a quantity that should represent something physical, scientists
• 16:51
  Speaker 1: do not usually celebrate the discovery of an actual infinity
• 16:54
  Speaker 1: and go home early. They become suspicious, and infinity often
• 16:59
  Speaker 1: suggests that the assumptions behind the model are no longer
• 17:02
  Speaker 1: adequate in that regime. That does not mean the theory
• 17:06
  Speaker 1: was wrong everywhere else. Quite the opposite. General relativity has
• 17:10
  Speaker 1: passed an extraordinary range of experimental and observational tests. It
• 17:16
  Speaker 1: explains gravitational time dilation. It predicts the bending of light
• 17:20
  Speaker 1: around massive objects. It describes the motion of planets with
• 17:24
  Speaker 1: remarkable precision. It predicted gravitational waves long before instruments were
• 17:30
  Speaker 1: sensitive enough to detect them. And it provides the basic
• 17:33
  Speaker 1: framework for understanding black holes across the universe. The problem
• 17:38
  Speaker 1: is not that general relativity suddenly becomes useless. The problem
• 17:42
  Speaker 1: is that if we follow it relentlessly toward the deepest
• 17:45
  Speaker 1: part of an ideal black hole, it eventually produces a
• 17:48
  Speaker 1: result that appears physically incomplete. This is one of the
• 17:52
  Speaker 1: most important things to understand about black hole physics. The
• 17:56
  Speaker 1: same theory that describes the exterior so beautifully is also
• 18:00
  Speaker 1: the theory that leads us toward a place where its
• 18:03
  Speaker 1: own description begins to fail. Outside the horizon, general relativity
• 18:09
  Speaker 1: is astonishingly powerful. At the horizon, it remains well behaved
• 18:14
  Speaker 1: when the correct coordinates are used. Deep inside, however, the
• 18:18
  Speaker 1: prediction becomes more extreme. And at the centre of the
• 18:22
  Speaker 1: ideal Schwarzschild black hole, the mathematics no longer gives us
• 18:26
  Speaker 1: a comfortable physical picture of what nature is actually doing.
• 18:30
  Speaker 1: So the event horizon is not the place where physics breaks.
• 18:34
  Speaker 1: It is the boundary beyond which our access to information
• 18:36
  Speaker 1: is lost. The true theoretical crisis lies farther inward, where
• 18:41
  Speaker 1: spacetime itself is driven toward a singular state. That is
• 18:46
  Speaker 1: where the simple question of what lies at the centre
• 18:49
  Speaker 1: begins to become much harder than it first appeared. Imagine
• 18:53
  Speaker 1: falling toward a black hole. At first, depending on how
• 18:57
  Speaker 1: far away you begin, the experience might not seem especially unusual.
• 19:02
  Speaker 1: You would be in free fall, following the natural curvature
• 19:05
  Speaker 1: of spacetime created by the black hole's mass. There would
• 19:09
  Speaker 1: be no invisible hand tugging on your clothes, and no
• 19:13
  Speaker 1: sudden sensation of speed in the ordinary sense. Just as
• 19:17
  Speaker 1: an astronaut orbiting Earth feels weightless while continually falling around
• 19:21
  Speaker 1: the planet, you too would be following a freely falling path.
• 19:26
  Speaker 1: The stranger effects appear when we compare your experience with
• 19:30
  Speaker 1: that of someone watching from far away. Suppose you carry
• 19:34
  Speaker 1: a small lamp and send a flash of light outward
• 19:37
  Speaker 1: once every second according to your own clock. To you,
• 19:42
  Speaker 1: Nothing particularly odd happens to the lamp. Your clock ticks normally.
• 19:47
  Speaker 1: Your heart, if we are allowing ourselves the luxury. of
• 19:50
  Speaker 1: imagining that you are still calm about all of this,
• 19:53
  Speaker 1: beats normally. Each flash leaves the lamp in the usual way,
• 19:58
  Speaker 1: but the distant observer receives those flashes very differently. As
• 20:03
  Speaker 1: you move deeper into the black hole's gravitational field, Light
• 20:06
  Speaker 1: climbing away from you loses energy from the distant observer's
• 20:10
  Speaker 1: point of view. Its wavelength becomes longer. Visible light can
• 20:15
  Speaker 1: shift toward the red, then into infrared wavelengths, then radio waves,
• 20:20
  Speaker 1: and eventually to wavelengths so stretched and faint that detection
• 20:25
  Speaker 1: becomes effectively impossible. This is gravitational redshift. At the same time,
• 20:31
  Speaker 1: the intervals between the flashes appear increasingly stretched. Your distant
• 20:36
  Speaker 1: observer sees your clock apparently slowing down as you approach
• 20:39
  Speaker 1: the event horizon. Each new signal takes longer to arrive
• 20:44
  Speaker 1: and arrives more redshifted than the last. In the ideal
• 20:48
  Speaker 1: mathematical description used for a non-rotating black hole, the distant
• 20:53
  Speaker 1: observer never receives a signal showing you actually crossing the horizon. Instead,
• 20:59
  Speaker 1: your image appears to slow, fade and become increasingly red.
• 21:05
  Speaker 1: This can create a very misleading impression. It can sound
• 21:09
  Speaker 1: as though you yourself never cross the event horizon. But
• 21:12
  Speaker 1: you do. According to your own clock, known in relativity
• 21:17
  Speaker 1: as proper time, you reach and cross the horizon after
• 21:21
  Speaker 1: a finite duration. There is no infinite wait from your
• 21:25
  Speaker 1: point of view. The apparent freezing belongs to the way
• 21:28
  Speaker 1: signals from you reach a distant observer. It is not
• 21:32
  Speaker 1: an experience you personally have while falling. This distinction between
• 21:36
  Speaker 1: distant time and proper time is one of the places
• 21:39
  Speaker 1: where general relativity quietly removes some of our everyday expectations
• 21:44
  Speaker 1: about how time ought to behave, and the event horizon
• 21:48
  Speaker 1: itself is stranger still. For a sufficiently large black hole,
• 21:53
  Speaker 1: you might cross it without noticing anything special at that
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  Speaker 1: exact moment. There is no physical wall, there is no
• 22:00
  Speaker 1: shell of matter marking the boundary. No alarm sounds because
• 22:04
  Speaker 1: space-time has noticed that you have entered the wrong neighbourhood.
• 22:08
  Speaker 1: The event horizon is defined by the paths that light
• 22:11
  Speaker 1: and matter can take through space-time. If the black hole
• 22:15
  Speaker 1: is large enough, local gravitational conditions at the horizon can
• 22:19
  Speaker 1: actually be quite gentle. What makes the horizon important is
• 22:23
  Speaker 1: not necessarily what you feel there, it is what becomes
• 22:27
  Speaker 1: possible after you cross it. A useful way to picture
• 22:30
  Speaker 1: this involves something called a light cone. At any event
• 22:35
  Speaker 1: in spacetime, there are directions that light can travel and
• 22:38
  Speaker 1: directions that massive objects can travel while still moving more
• 22:42
  Speaker 1: slowly than light. If these possible future paths are drawn
• 22:46
  Speaker 1: on a spacetime diagram, they form a shape rather like
• 22:50
  Speaker 1: a cone. The cone represents your possible causal future. Anything
• 22:55
  Speaker 1: inside that future light cone is somewhere you could at
• 22:58
  Speaker 1: least in principle, influence later. Far from a black hole,
• 23:02
  Speaker 1: part of your future light cone points outward and part
• 23:05
  Speaker 1: points inward. You can choose to move closer to the
• 23:09
  Speaker 1: black hole or farther away. As you fall deeper into
• 23:12
  Speaker 1: its gravitational field, the structure of those possible paths changes.
• 23:17
  Speaker 1: The light cones become increasingly tilted toward the black hole.
• 23:21
  Speaker 1: At the event horizon, even a light ray aimed directly
• 23:24
  Speaker 1: outward is balanced on the causal boundary. Inside the horizon,
• 23:29
  Speaker 1: something decisive has changed. Every future-directed light path points towards
• 23:34
  Speaker 1: smaller radius, and because nothing carrying matter or information can
• 23:39
  Speaker 1: outrun light, every future-directed path available to you also leads
• 23:43
  Speaker 1: farther inward. This is a deeper explanation of why escape
• 23:48
  Speaker 1: becomes impossible. Black holes are often introduced using escape velocity.
• 23:53
  Speaker 1: We might say that the escape velocity at the horizon
• 23:56
  Speaker 1: has reached the speed of light. And because nothing can
• 23:58
  Speaker 1: travel faster than light, nothing can escape. That description can
• 24:03
  Speaker 1: be useful as a first approximation. But general relativity gives
• 24:07
  Speaker 1: us a more complete picture. Inside the event horizon, the
• 24:11
  Speaker 1: problem is not simply that your rocket is too weak.
• 24:14
  Speaker 1: A stronger rocket does not solve it. Even an imaginary
• 24:18
  Speaker 1: engine capable of producing an absurd amount of thrust cannot
• 24:22
  Speaker 1: create an outward future path if that path is no
• 24:25
  Speaker 1: longer part of the causal structure of spacetime. The route
• 24:28
  Speaker 1: back out is not merely difficult. It is absent. That
• 24:33
  Speaker 1: is a much stranger idea than being trapped at the
• 24:35
  Speaker 1: bottom of a deep gravitational well. A sufficiently powerful spacecraft
• 24:40
  Speaker 1: can escape from a planet because outward trajectories still exist.
• 24:45
  Speaker 1: Inside a black hole, all allowed future trajectories lead inward.
• 24:49
  Speaker 1: Light itself cannot choose otherwise. How dramatic the horizon feels
• 24:54
  Speaker 1: depends strongly on the black hole's mass. For a stellar-mass
• 24:57
  Speaker 1: black hole, perhaps several times the mass of the Sun,
• 25:01
  Speaker 1: the gravitational field changes rapidly over short distances near the horizon.
• 25:06
  Speaker 1: That produces strong tidal forces. Your feet, if they are
• 25:10
  Speaker 1: closer to the black hole than your head, can experience
• 25:13
  Speaker 1: noticeably stronger gravity. The difference stretches objects along the direction
• 25:19
  Speaker 1: of fall while squeezing them sideways. In extreme cases, this
• 25:23
  Speaker 1: process becomes the rather memorable phenomenon known as spaghettification. For
• 25:28
  Speaker 1: a supermassive black hole, however, the situation near the event
• 25:31
  Speaker 1: horizon can be surprisingly different. A black hole containing millions
• 25:36
  Speaker 1: or billions of times the mass of the Sun has
• 25:38
  Speaker 1: a much larger horizon. Because its characteristic size grows with mass,
• 25:44
  Speaker 1: the variation in gravity across a human-sized object at the
• 25:47
  Speaker 1: horizon can be much smaller. You could cross the event
• 25:51
  Speaker 1: horizon of a sufficiently massive black hole without being immediately
• 25:55
  Speaker 1: torn apart by tidal forces. This does not make the
• 25:58
  Speaker 1: black hole safer in any long-term sense. It simply means
• 26:03
  Speaker 1: that the event horizon itself need not be the violent part.
• 26:07
  Speaker 1: The truly unavoidable behaviour lies deeper inside. Once the horizon
• 26:11
  Speaker 1: has been crossed, moving towards smaller radius becomes increasingly unlike
• 26:15
  Speaker 1: choosing a direction through ordinary space. It becomes tied to
• 26:19
  Speaker 1: the structure of the future itself. Outside the black hole,
• 26:23
  Speaker 1: travelling inward is optional. Inside, it is not. This is
• 26:28
  Speaker 1: one reason why saying that everything is pulled toward the
• 26:31
  Speaker 1: centre does not quite capture what general relativity is describing.
• 26:36
  Speaker 1: You're not simply falling toward a destination because a force
• 26:39
  Speaker 1: is dragging you there. You're following the future-directed path that
• 26:43
  Speaker 1: spacetime allows. In that sense, Moving inward becomes rather like
• 26:48
  Speaker 1: moving toward tomorrow. You can change what you do along
• 26:52
  Speaker 1: the way. You can turn around. You can fire engines.
• 26:55
  Speaker 1: You can shine a torch behind you or in front
• 26:57
  Speaker 1: of you. But none of those actions creates a route
• 27:00
  Speaker 1: back into the exterior universe once the horizon has been crossed.
• 27:05
  Speaker 1: The event horizon therefore marks a profound change, but not
• 27:09
  Speaker 1: because it is the centre of the black hole. It
• 27:11
  Speaker 1: is the point beyond which the centre becomes part of
• 27:14
  Speaker 1: your unavoidable future. and that is what makes the interior
• 27:18
  Speaker 1: so different from the space outside. Crossing the horizon does
• 27:22
  Speaker 1: not mean you have reached the place where general relativity
• 27:25
  Speaker 1: breaks down. For a large enough black hole, the crossing
• 27:28
  Speaker 1: itself may be locally uneventful. The deeper problem lies ahead.
• 27:33
  Speaker 1: From that moment onward, every possible future direction through space-time
• 27:37
  Speaker 1: carries you farther inward, toward regions of increasing curvature, and
• 27:42
  Speaker 1: eventually toward the place where the classical description itself begins
• 27:45
  Speaker 1: to fail. Once the event horizon has been crossed, the
• 27:50
  Speaker 1: black hole becomes stranger in a way that is difficult
• 27:53
  Speaker 1: to capture with the usual language of gravity. It is
• 27:56
  Speaker 1: tempting to picture the interior as an extremely deep pit.
• 28:00
  Speaker 1: Somewhere at the bottom sits the centre, and gravity simply
• 28:04
  Speaker 1: pulls everything toward it with overwhelming strength. That picture is
• 28:08
  Speaker 1: useful only up to a point. Inside an ideal non-rotating
• 28:13
  Speaker 1: black hole, The deeper explanation is not merely that the
• 28:16
  Speaker 1: gravitational pull becomes too strong to resist. The geometry of
• 28:21
  Speaker 1: spacetime itself is arranged so that moving towards smaller radius
• 28:25
  Speaker 1: becomes part of moving into the future. This is one
• 28:28
  Speaker 1: of the most unusual consequences of the Schwarzschild solution. Outside
• 28:34
  Speaker 1: the event horizon, radius behaves more or less as we
• 28:37
  Speaker 1: expect a spatial direction to behave. You can move toward
• 28:41
  Speaker 1: the black hole or away from it. With enough acceleration,
• 28:45
  Speaker 1: you can change your radial motion. You can stop approaching,
• 28:48
  Speaker 1: turn around, and head elsewhere. Time behaves differently. Whatever you do,
• 28:54
  Speaker 1: you continue toward your future. You can travel north or south,
• 28:58
  Speaker 1: remain in one place, walk in circles, or spend an
• 29:02
  Speaker 1: unreasonable amount of time deciding which biscuit to have with tea,
• 29:06
  Speaker 1: but none of those decisions allows you to travel away
• 29:09
  Speaker 1: from tomorrow. Inside the Schwarzschild horizon, The mathematical roles associated
• 29:15
  Speaker 1: with the radial and temporal coordinates change in an important sense.
• 29:20
  Speaker 1: This is sometimes described by saying that space and time
• 29:23
  Speaker 1: swap places. That phrase is memorable, but it can also
• 29:27
  Speaker 1: be misleading if taken too literally. Your wristwatch does not
• 29:31
  Speaker 1: suddenly begin measuring kilometres. A ruler does not become a clock.
• 29:35
  Speaker 1: You still experience one local direction of time and nearby
• 29:39
  Speaker 1: physics does not abruptly rearrange itself into complete nonsense. What
• 29:44
  Speaker 1: changes is the causal structure of spacetime. The direction of
• 29:48
  Speaker 1: decreasing radius becomes unavoidable for anything following a future-directed path.
• 29:53
  Speaker 1: In that limited but important sense, the centre is no
• 29:57
  Speaker 1: longer simply somewhere in space. It lies in your future.
• 30:01
  Speaker 1: Imagine standing outside the event horizon with an extremely powerful rocket.
• 30:06
  Speaker 1: You can point the rocket away from the black hole,
• 30:09
  Speaker 1: fire the engines and perhaps escape. Whether you succeed depends
• 30:13
  Speaker 1: on where you are, how fast you're already moving and
• 30:16
  Speaker 1: what acceleration you can produce. Once inside the horizon, however,
• 30:20
  Speaker 1: increasing the engine power does not restore the missing route.
• 30:24
  Speaker 1: You can still fire the rocket outward. you can still
• 30:27
  Speaker 1: accelerate relative to a freely falling observer nearby. You can
• 30:31
  Speaker 1: alter the details of your journey, but you cannot produce
• 30:35
  Speaker 1: a future-directed path that carries you back across the horizon
• 30:39
  Speaker 1: into the exterior universe. The reason becomes clearer if we
• 30:43
  Speaker 1: return to light. Light always follows what are called null
• 30:47
  Speaker 1: paths through spacetime, representing the fastest possible routes along which
• 30:52
  Speaker 1: causal influence can travel. Outside a black hole, One beam
• 30:57
  Speaker 1: of light can be aimed inward, while another can be
• 30:59
  Speaker 1: aimed outward. Inside the event horizon, even the beam aimed
• 31:04
  Speaker 1: as outward as physically possible still moves towards smaller values
• 31:08
  Speaker 1: of the radial coordinate as its future unfolds. If light
• 31:11
  Speaker 1: cannot create an outward escape path, neither can a rocket.
• 31:15
  Speaker 1: This is why describing a black hole entirely in terms
• 31:18
  Speaker 1: of enormous gravitational force misses something fundamental. The issue is
• 31:23
  Speaker 1: not simply that gravity wins a contest against your engines.
• 31:26
  Speaker 1: there is no outward future route available for the engines
• 31:29
  Speaker 1: to follow. A helpful way of seeing this comes from
• 31:32
  Speaker 1: space-time diagrams. A space-time diagram is a drawing that represents
• 31:37
  Speaker 1: space and time together. Instead of showing only where objects
• 31:41
  Speaker 1: are located, it shows how their positions change as time passes.
• 31:46
  Speaker 1: Light rays provide a natural structure for these diagrams because
• 31:50
  Speaker 1: the speed of light defines the boundary between events that
• 31:54
  Speaker 1: can influence one another and events that cannot. The possible
• 31:59
  Speaker 1: future paths from a particular event can be represented using
• 32:03
  Speaker 1: light cones. Far from a black hole, the future light
• 32:06
  Speaker 1: cone contains directions that lead both toward and away from
• 32:10
  Speaker 1: the black hole. Closer to the horizon, the geometry increasingly
• 32:14
  Speaker 1: tips those future directions inward. At the event horizon, the
• 32:19
  Speaker 1: outward edge of the future light cone lies along the
• 32:21
  Speaker 1: horizon itself. Inside, the entire future-like cone points towards smaller radius.
• 32:27
  Speaker 1: The diagrams can become complicated when drawn using ordinary coordinates,
• 32:32
  Speaker 1: particularly because an entire infinite universe has to compete for
• 32:35
  Speaker 1: space on a finite piece of paper. Roger Penrose developed
• 32:40
  Speaker 1: an elegant way of dealing with this. Penrose diagrams, also
• 32:44
  Speaker 1: called conformal diagrams, compressed enormous or even infinite regions of
• 32:49
  Speaker 1: spacetime into finite drawings while preserving the causal relationships between events.
• 32:55
  Speaker 1: Distances are distorted, so they are not ordinary maps. But
• 32:59
  Speaker 1: light still travels along characteristic diagonal lines, making it possible
• 33:04
  Speaker 1: to see which regions can communicate with which others. In
• 33:08
  Speaker 1: a Penrose diagram of an ideal Schwarzschild black hole, the
• 33:12
  Speaker 1: event horizon appears as a causal boundary. Beyond it lies
• 33:16
  Speaker 1: the black hole interior. and the singularity does not appear
• 33:20
  Speaker 1: as a tiny dot waiting at some ordinary spatial location
• 33:23
  Speaker 1: in the middle. For the ideal Schwarzschild geometry, the future
• 33:27
  Speaker 1: singularity is represented as a space-like boundary. That distinction matters enormously.
• 33:34
  Speaker 1: It means the singularity is not like a rock sitting
• 33:36
  Speaker 1: somewhere ahead that an agile traveller might steer around. Once
• 33:41
  Speaker 1: you are sufficiently inside the black hole, reaching that singular
• 33:45
  Speaker 1: boundary is built into the future-directed structure of the spacetime.
• 33:49
  Speaker 1: Avoiding it would be rather like trying to avoid the
• 33:51
  Speaker 1: passage of time itself. You can influence how your journey proceeds,
• 33:56
  Speaker 1: but you cannot simply turn around and arrange for tomorrow
• 33:59
  Speaker 1: never to arrive. Again, this is an analogy. The black
• 34:03
  Speaker 1: hole interior does not literally turn the centre into an
• 34:06
  Speaker 1: ordinary moment on a calendar, but the comparison captures why
• 34:10
  Speaker 1: the usual picture of falling toward a central object is incomplete.
• 34:14
  Speaker 1: A black hole is not best imagined. as a tunnel
• 34:18
  Speaker 1: drilled into familiar three-dimensional space. Nor is it merely an
• 34:22
  Speaker 1: invisible sphere containing an especially massive object in the middle.
• 34:26
  Speaker 1: It is a region of spacetime defined by its causal geometry.
• 34:31
  Speaker 1: That geometry determines which events can affect which others, and
• 34:35
  Speaker 1: which directions remain available to matter and light. Once the
• 34:40
  Speaker 1: horizon is crossed, every physically allowed future for a freely
• 34:44
  Speaker 1: falling observer leads farther inward. This gives the predicted central
• 34:49
  Speaker 1: singularity a much more serious role than simply being a
• 34:53
  Speaker 1: location where crushed matter accumulates. Classical general relativity is not
• 34:58
  Speaker 1: merely saying that a great deal of material gets squeezed
• 35:01
  Speaker 1: into a very small volume. It is saying that future-directed
• 35:05
  Speaker 1: paths through the black hole eventually encounter a boundary where
• 35:09
  Speaker 1: the classical spacetime description can no longer be continued in
• 35:13
  Speaker 1: the usual way. That is a far deeper problem. If
• 35:18
  Speaker 1: the centre were merely an incredibly dense object, physicists could
• 35:22
  Speaker 1: at least ask what material it was made from, calculate
• 35:25
  Speaker 1: its pressure, study its internal forces and try to describe
• 35:29
  Speaker 1: the state of matter there. But the Schwarzschild singularity represents
• 35:34
  Speaker 1: something more radical. The geometry that normally provides the arena
• 35:38
  Speaker 1: in which matter exists is itself becoming incomplete. Space-time is
• 35:43
  Speaker 1: not simply containing an extreme event. Spacetime is part of
• 35:47
  Speaker 1: the problem, and this is why the journey toward the
• 35:50
  Speaker 1: centre cannot be understood purely as a journey through increasingly
• 35:53
  Speaker 1: strong gravity. Once inside the horizon, the shape of the
• 35:58
  Speaker 1: future has changed. The centre is no longer somewhere you
• 36:01
  Speaker 1: might visit if you continue in the wrong direction. Within
• 36:05
  Speaker 1: the classical Schwarzschild description, it is where every allowed future
• 36:09
  Speaker 1: direction eventually leads. The equations are not merely compressing matter
• 36:14
  Speaker 1: more and more tightly. They're carrying every causal path toward
• 36:18
  Speaker 1: the edge of the spacetime that those equations know how
• 36:21
  Speaker 1: to describe, and it is at that edge that the
• 36:24
  Speaker 1: question of what actually happens becomes unavoidable. The word singularity
• 36:29
  Speaker 1: is one of those scientific terms that has escaped into
• 36:32
  Speaker 1: popular culture with a slightly more dramatic reputation than it deserves.
• 36:37
  Speaker 1: It is often pictured as an infinitely dense point at
• 36:40
  Speaker 1: the exact centre of a black hole. Everything falls inward.
• 36:44
  Speaker 1: Matter is crushed smaller and smaller, and eventually all of
• 36:47
  Speaker 1: it is squeezed into something with zero size and infinite density.
• 36:52
  Speaker 1: That image is useful as a rough starting point, but
• 36:55
  Speaker 1: it is not the most careful way physicists describe what
• 36:59
  Speaker 1: general relativity is actually predicting. A gravitational singularity is better
• 37:04
  Speaker 1: understood as a place where the classical description of space-time
• 37:07
  Speaker 1: becomes incomplete. In the simplest black hole model, the Schwarzschild solution,
• 37:12
  Speaker 1: The problem appears as the radial coordinate approaches zero. As
• 37:17
  Speaker 1: that happens, the curvature of spacetime grows without bound. This
• 37:21
  Speaker 1: is not merely a strange feature of one badly chosen
• 37:24
  Speaker 1: coordinate system. There are mathematical quantities that describe spacetime curvature
• 37:29
  Speaker 1: independently of the coordinates being used, and one of the
• 37:32
  Speaker 1: best known is called the Kretschmann Scala. The Kretschmann Scala
• 37:37
  Speaker 1: is built from the curvature of spacetime in such a
• 37:39
  Speaker 1: way that it gives physicists a useful measure of how
• 37:43
  Speaker 1: strongly the geometry is being distorted. At the Schwarzschild event horizon,
• 37:48
  Speaker 1: the Kretschmann scalar remains finite. That is one reason physicists
• 37:53
  Speaker 1: know the apparent mathematical problem at the horizon can be
• 37:56
  Speaker 1: removed by changing coordinates. At the central value corresponding to
• 38:01
  Speaker 1: zero radius, however, the Kretschmann scalar diverges. It grows without limit.
• 38:08
  Speaker 1: This tells us that the central problem is not simply
• 38:11
  Speaker 1: caused by an awkward way of labelling spacetime. The curvature
• 38:15
  Speaker 1: itself is becoming singular. This is where phrases such as
• 38:19
  Speaker 1: infinite curvature and infinite density come from. If an ordinary
• 38:24
  Speaker 1: object is compressed into a smaller and smaller volume, while
• 38:28
  Speaker 1: keeping the same total mass, its average density rises. So
• 38:33
  Speaker 1: if a mathematical model continues compressing matter towards zero volume,
• 38:37
  Speaker 1: The corresponding density would appear to grow without bound. But
• 38:42
  Speaker 1: there is an important difference between a mathematical expression approaching
• 38:45
  Speaker 1: infinity and physicists having experimental proof that nature literally contains
• 38:51
  Speaker 1: an infinite physical quantity. Infinity is often a warning sign.
• 38:56
  Speaker 1: It can mean that a theory is being asked to
• 38:58
  Speaker 1: describe conditions beyond the range in which it should be trusted.
• 39:02
  Speaker 1: That does not automatically make the theory wrong. The theory
• 39:06
  Speaker 1: can be extraordinarily accurate across an enormous range of conditions
• 39:10
  Speaker 1: and still fail at an extreme boundary. General relativity is
• 39:14
  Speaker 1: a perfect example. It successfully describes the motion of planets,
• 39:19
  Speaker 1: gravitational lensing, the expansion of the universe, gravitational time dilation
• 39:24
  Speaker 1: and gravitational waves. It also describes the exterior geometry of
• 39:29
  Speaker 1: black holes with astonishing precision. But none of that guarantees
• 39:33
  Speaker 1: that the same classical equations must remain complete when spacetime
• 39:38
  Speaker 1: curvature is driven toward arbitrarily extreme values. To understand the
• 39:43
  Speaker 1: singularity more precisely, physicists often talk about geodesics. A geodesic
• 39:50
  Speaker 1: is the natural path followed by an object moving freely
• 39:53
  Speaker 1: through curved spacetime. In flat space, the closest comparison would
• 39:58
  Speaker 1: be travelling in a straight line without being pushed or pulled.
• 40:02
  Speaker 1: In curved spacetime, the geometry itself determines what the natural
• 40:06
  Speaker 1: path looks like. Planets orbit stars along geodesic paths. Light
• 40:11
  Speaker 1: follows its own type of geodesic. A freely falling observer
• 40:16
  Speaker 1: moving toward a black hole follows one as well. Now
• 40:19
  Speaker 1: imagine following one of these geodesics into the interior of
• 40:22
  Speaker 1: an ideal Schwarzschild black hole. General relativity says that the
• 40:27
  Speaker 1: path reaches the central singular region after a finite amount
• 40:31
  Speaker 1: of proper time, and then something unusual happens. The geodesic
• 40:36
  Speaker 1: cannot simply be continued beyond that point within the classical spacetime.
• 40:41
  Speaker 1: It ends. This idea is called geodesic incompleteness. And in
• 40:45
  Speaker 1: modern relativity, geodesic incompleteness is a more precise way of
• 40:50
  Speaker 1: thinking about singular behaviour than simply saying that there is
• 40:53
  Speaker 1: an infinitely dense dot sitting in space. The distinction matters
• 40:58
  Speaker 1: because a singularity is not necessarily an object. It is
• 41:02
  Speaker 1: not a tiny ball with a surface. It is not
• 41:05
  Speaker 1: a lump of exotic material. It is not something that
• 41:08
  Speaker 1: sits comfortably inside spacetime in the same way that a
• 41:11
  Speaker 1: planet sits inside the solar system. Instead, The problem is
• 41:16
  Speaker 1: that space-time itself has reached a boundary where the classical
• 41:20
  Speaker 1: theory no longer provides a complete continuation. That is a
• 41:24
  Speaker 1: much stranger idea than ordinary crushing. If a star became
• 41:28
  Speaker 1: merely very dense, physicists could still ask familiar questions. What
• 41:33
  Speaker 1: particles are present? What pressure supports the material? How hot
• 41:38
  Speaker 1: is it? What forces operate inside it? but once the
• 41:42
  Speaker 1: classical spacetime description itself becomes incomplete, even the stage on
• 41:47
  Speaker 1: which those questions are usually asked has become uncertain. The
• 41:52
  Speaker 1: coordinates are not merely describing extreme matter. The geometry that
• 41:56
  Speaker 1: gives meaning to distance, duration and causal structure is part
• 42:01
  Speaker 1: of what has broken down. This is why physicists are
• 42:04
  Speaker 1: cautious about saying that the singularity is definitely a literal
• 42:08
  Speaker 1: object of infinite density. General relativity contains no quantum mechanics.
• 42:14
  Speaker 1: It is a classical theory. That is not a criticism.
• 42:18
  Speaker 1: It simply tells us what sort of theory it is.
• 42:21
  Speaker 1: Quantum mechanics becomes essential when nature is examined at sufficiently
• 42:26
  Speaker 1: small scales. Particles, fields and interactions behave in ways that
• 42:31
  Speaker 1: classical physics cannot reproduce. So when general relativity predicts that
• 42:36
  Speaker 1: matter and space-time are driven toward increasingly extreme conditions, there
• 42:42
  Speaker 1: is no reason to assume that quantum effects will politely
• 42:45
  Speaker 1: remain irrelevant forever. At some point, a complete description is
• 42:50
  Speaker 1: expected to require gravity itself to be understood quantum mechanically.
• 42:55
  Speaker 1: And that is exactly the theory physicists do not yet
• 42:59
  Speaker 1: possess in experimentally confirmed form. There are useful historical reminders
• 43:05
  Speaker 1: that infinities do not always represent literal features of nature.
• 43:09
  Speaker 1: Classical electromagnetic theory once produced severe mathematical problems when charged
• 43:15
  Speaker 1: particles were treated as perfect points, with their own electric
• 43:18
  Speaker 1: fields extending all the way down to zero distance. Early
• 43:23
  Speaker 1: calculations could produce divergent self-energies. Quantum field theory later changed
• 43:28
  Speaker 1: the description dramatically, though dealing with infinities remained a subtle
• 43:32
  Speaker 1: part of the mathematics. Classical physics also predicted what became
• 43:37
  Speaker 1: known as the ultraviolet catastrophe, when older ideas were applied
• 43:41
  Speaker 1: to thermal radiation at very high frequencies. The resulting divergence
• 43:45
  Speaker 1: was not a real physical catastrophe. It was a sign
• 43:49
  Speaker 1: that the classical theory was missing something fundamental. Max Planck's
• 43:54
  Speaker 1: introduction of energy quantization helped open the door to quantum
• 43:57
  Speaker 1: theory and remove that particular failure. These examples do not
• 44:02
  Speaker 1: prove that black hole singularities will disappear in exactly the
• 44:05
  Speaker 1: same way. That would be far too easy. But they
• 44:10
  Speaker 1: show why physicists are cautious whenever infinity appears in a
• 44:14
  Speaker 1: physical prediction. Sometimes the infinity is telling you less about
• 44:18
  Speaker 1: nature and more about the limits of the model being used.
• 44:22
  Speaker 1: The black hole singularity may be doing something similar. Classical
• 44:26
  Speaker 1: general relativity predicts the approach towards singular behaviour with extraordinary strength.
• 44:32
  Speaker 1: The result is not a fragile curiosity that disappears as
• 44:35
  Speaker 1: soon as the equations are treated carefully. The deeper problem survives.
• 44:40
  Speaker 1: Curvature grows without bound in the ideal solution. Future-directed geodesics terminate.
• 44:47
  Speaker 1: The spacetime becomes incomplete, so physicists cannot simply ignore the
• 44:52
  Speaker 1: singularity because it looks inconvenient. At the same time, They
• 44:56
  Speaker 1: cannot confidently claim that the classical infinity is the final
• 45:00
  Speaker 1: physical truth. The theory itself gives no description of quantum spacetime.
• 45:05
  Speaker 1: It cannot tell us whether the singularity is replaced by
• 45:08
  Speaker 1: some finite structure. It cannot tell us whether matter undergoes
• 45:12
  Speaker 1: a new phase. It cannot tell us whether spacetime continues
• 45:15
  Speaker 1: in some unfamiliar form. And it cannot tell us whether
• 45:18
  Speaker 1: the very concepts of distance, time and location remain meaningful
• 45:23
  Speaker 1: in the way we normally use them. That is what
• 45:25
  Speaker 1: makes the singularity such an important clue. It is not
• 45:29
  Speaker 1: merely the frightening object at the bottom of a black hole.
• 45:33
  Speaker 1: It is a place where an extraordinarily successful theory points
• 45:37
  Speaker 1: toward its own boundary. General relativity carries the calculation inward
• 45:42
  Speaker 1: with remarkable confidence. It tells us that classical collapse does
• 45:47
  Speaker 1: not simply stop because the numbers become uncomfortable. It drives
• 45:51
  Speaker 1: the geometry towards singular behaviour. But when we ask what
• 45:55
  Speaker 1: physically exists at the end of that process, the theory
• 45:59
  Speaker 1: no longer gives us an answer we can safely interpret
• 46:01
  Speaker 1: as complete. So the mystery is more precise than the
• 46:05
  Speaker 1: familiar phrase about infinite density suggests. Classical relativity predicts that
• 46:11
  Speaker 1: something unavoidable happens. It predicts that spacetime itself reaches a
• 46:15
  Speaker 1: singular limit. What it does not tell us is what
• 46:19
  Speaker 1: nature places there once the classical description has run out.
• 46:24
  Speaker 1: For a while, there was a tempting way to dismiss
• 46:26
  Speaker 1: the singularity. Perhaps it was simply too perfect. The Schwarzschild
• 46:31
  Speaker 1: solution describes an ideal black hole that is completely spherical,
• 46:36
  Speaker 1: carries no electric charge, does not rotate, and sits in
• 46:41
  Speaker 1: an otherwise beautifully organised mathematical universe. Real stars are nothing
• 46:46
  Speaker 1: like that. They spin. They wobble. Their densities vary from
• 46:51
  Speaker 1: place to place. Magnetic fields thread through them. Matter moves turbulently.
• 46:57
  Speaker 1: Their surroundings are filled with radiation, gas and gravitational disturbances.
• 47:02
  Speaker 1: So it was reasonable to wonder whether the singularity at
• 47:05
  Speaker 1: the centre of the Schwarzschild solution was merely a consequence
• 47:08
  Speaker 1: of making the problem unrealistically neat. Perhaps, once all the
• 47:13
  Speaker 1: messiness of a genuine collapsing star was included, the singularity
• 47:17
  Speaker 1: would disappear. This was an important question because exact solutions
• 47:21
  Speaker 1: in physics, can sometimes contain features that depend heavily on
• 47:25
  Speaker 1: the assumptions used to construct them. Perfect symmetry is extremely
• 47:30
  Speaker 1: convenient mathematically, but nature has never shown much enthusiasm for
• 47:34
  Speaker 1: keeping things perfectly tidy. The mathematician and physicist Roger Penrose
• 47:40
  Speaker 1: changed the situation dramatically in 1965. Instead of solving Einstein's
• 47:46
  Speaker 1: equations for one particular collapsing star, Penrose approached the problem
• 47:50
  Speaker 1: in a more general way. He asked what general relativity
• 47:54
  Speaker 1: itself required once gravitational collapse had become sufficiently severe. The
• 47:59
  Speaker 1: result was the first modern singularity theorem. Its importance was enormous.
• 48:05
  Speaker 1: Penrose showed that singular behaviour was not merely a strange
• 48:08
  Speaker 1: accident produced by the perfect spherical symmetry of the Schwarzschild solution.
• 48:14
  Speaker 1: Under certain physically reasonable conditions, gravitational collapse could lead inevitably
• 48:20
  Speaker 1: to a spacetime that was geodesically incomplete. In other words,
• 48:25
  Speaker 1: the problem survived even when perfect symmetry was removed. One
• 48:29
  Speaker 1: of the central ideas behind Penrose's reasoning is something called
• 48:33
  Speaker 1: a trapped surface. To understand a trapped surface, imagine a
• 48:37
  Speaker 1: spherical surface surrounding a collapsing region. At every point on
• 48:42
  Speaker 1: that surface, light can be sent in two broadly different directions.
• 48:46
  Speaker 1: Some light rays are aimed inward, others are aimed outward.
• 48:51
  Speaker 1: Under ordinary circumstances, we would expect the inward-moving light rays
• 48:55
  Speaker 1: to converge while the outward-moving light rays spread apart. That
• 48:59
  Speaker 1: is what happens around an ordinary star. Shine light outward,
• 49:03
  Speaker 1: and the beam expands into larger and larger spheres as
• 49:06
  Speaker 1: it travels away. But gravity can become so strong that
• 49:10
  Speaker 1: this familiar behaviour changes. A trapped surface is a closed surface.
• 49:16
  Speaker 1: from which both sets of future-directed light rays converge. Even
• 49:20
  Speaker 1: the light initially aimed outward finds that the geometry of
• 49:24
  Speaker 1: spacetime is forcing its bundle of rays towards smaller, effective areas.
• 49:30
  Speaker 1: The outward direction has stopped behaving in the way our
• 49:32
  Speaker 1: intuition expects. This is a geometric sign that gravity has
• 49:36
  Speaker 1: passed a profound threshold. The region has become trapped. Once
• 49:41
  Speaker 1: a trapped surface forms and provided certain assumptions about the
• 49:45
  Speaker 1: matter and global structure of spacetime are satisfied, Penrose showed
• 49:50
  Speaker 1: that the future development of the spacetime cannot remain completely regular.
• 49:54
  Speaker 1: Some geodesics must come to an end after only a
• 49:57
  Speaker 1: finite amount of their natural parameter. That is geodesic incompleteness.
• 50:02
  Speaker 1: This is where the meaning of the theorem requires some care.
• 50:06
  Speaker 1: Penrose did not produce a microscopic description of a tiny
• 50:09
  Speaker 1: object at the centre of a black hole. The theorem
• 50:13
  Speaker 1: does not say that physicists have photographed, measured or otherwise
• 50:17
  Speaker 1: established a literal point of infinite density. It does something
• 50:21
  Speaker 1: subtler and in many ways more powerful. It shows that,
• 50:24
  Speaker 1: within classical general relativity and under the relevant assumptions, the
• 50:29
  Speaker 1: spacetime description cannot remain smoothly complete. Some future-directed paths simply
• 50:36
  Speaker 1: cannot be extended indefinitely. The theory reaches a boundary. That
• 50:40
  Speaker 1: result matters because it avoids depending on the special symmetry
• 50:44
  Speaker 1: of the Schwarzschild black hole. A collapsing star can be distorted.
• 50:49
  Speaker 1: It can contain irregularities. Its geometry need not be arranged
• 50:53
  Speaker 1: with perfect mathematical elegance. Once the conditions of the theorem
• 50:58
  Speaker 1: are met, the classical problem does not simply vanish because
• 51:01
  Speaker 1: the star is a bit lumpy. This transformed the scientific
• 51:05
  Speaker 1: status of black hole singularities. Before this work, it was
• 51:09
  Speaker 1: easier to imagine that the singularity might be a mathematical
• 51:12
  Speaker 1: curiosity belonging to a special exact solution. After Penrose, singular
• 51:18
  Speaker 1: behaviour became much harder to dismiss. The theory itself appeared
• 51:22
  Speaker 1: to be telling physicists that gravitational collapse naturally pushes classical
• 51:26
  Speaker 1: spacetime toward incompleteness. Roger Penrose's approach also helped establish the
• 51:32
  Speaker 1: modern global study of spacetime. Rather than concentrating only on
• 51:36
  Speaker 1: what happens at one location, the mathematics considers the overall
• 51:40
  Speaker 1: causal structure of the geometry. Which events can influence which others?
• 51:45
  Speaker 1: Where can light travel? Can geodesics continue forever? Do trapped
• 51:50
  Speaker 1: regions form? These questions became extraordinary powerful tools for understanding
• 51:57
  Speaker 1: black holes. In the following years, Stephen Hawking and Roger
• 52:01
  Speaker 1: Penrose developed further singularity theorems. Their work extended the basic
• 52:05
  Speaker 1: ideas into broader settings, including cosmology. The exact assumptions vary
• 52:11
  Speaker 1: between different theorems, but the recurring message is striking. Under
• 52:16
  Speaker 1: suitable conditions, general relativity naturally produces situations in which spacetime
• 52:21
  Speaker 1: cannot be continued as a complete classical geometry. The implications
• 52:26
  Speaker 1: reach beyond black holes. When general relativity is extrapolated backward
• 52:31
  Speaker 1: into the early universe, similar questions about singular behaviour arise
• 52:35
  Speaker 1: around the Big Bang. Once again, this does not necessarily
• 52:39
  Speaker 1: prove that nature literally began with a physical point of
• 52:43
  Speaker 1: infinite density. It tells us that the classical spacetime description
• 52:47
  Speaker 1: cannot simply be extended forever without encountering a fundamental problem.
• 52:53
  Speaker 1: The black hole case remains especially powerful because gravitational collapse
• 52:56
  Speaker 1: can be understood as a physical process occurring within an
• 52:59
  Speaker 1: already existing universe. A massive star exhausts its nuclear fuel.
• 53:05
  Speaker 1: Its core loses the pressure support required to resist gravity.
• 53:09
  Speaker 1: Collapse continues. If enough mass is compressed within a sufficiently
• 53:13
  Speaker 1: small region, trapped surfaces can form. And the singularity theorem
• 53:17
  Speaker 1: then tells us that classical spacetime cannot remain complete under
• 53:21
  Speaker 1: the stated conditions. Roger Penrose was awarded half of the
• 53:25
  Speaker 1: Nobel Prize in Physics in 2020 for the discovery that
• 53:29
  Speaker 1: black hole formation is a robust prediction of the general
• 53:32
  Speaker 1: theory of relativity. The other half was shared by Reinhard
• 53:36
  Speaker 1: Genzel and Andrea Ghez for the discovery of a supermassive
• 53:40
  Speaker 1: compact object at the centre of the Milky Way. Penrose's
• 53:43
  Speaker 1: prize recognised something deeper than the study of one ideal equation.
• 53:48
  Speaker 1: His work helped establish that black holes were not merely
• 53:51
  Speaker 1: mathematical oddities. They emerged naturally from Einstein's theory, and the
• 53:56
  Speaker 1: singular behaviour associated with gravitational collapse was not easily removed
• 54:01
  Speaker 1: by making the model more realistic. But the theorem also
• 54:05
  Speaker 1: leaves a crucial question unanswered. It proves incompleteness. It does
• 54:10
  Speaker 1: not tell us what replaces that incompleteness. That difference is
• 54:14
  Speaker 1: easy to miss. Suppose a road map shows every available
• 54:18
  Speaker 1: route ending at the edge of the paper. That tells
• 54:20
  Speaker 1: you something important about the map. It does not automatically
• 54:24
  Speaker 1: tell you what exists beyond the page. General relativity leads
• 54:28
  Speaker 1: future-directed paths toward a region where the classical description becomes incomplete.
• 54:35
  Speaker 1: The theorem tells us that this problem is real within
• 54:38
  Speaker 1: the theory and cannot simply be blamed on perfect symmetry.
• 54:42
  Speaker 1: But it does not supply the missing physics. It does
• 54:45
  Speaker 1: not tell us whether matter reaches an actual infinity. It
• 54:49
  Speaker 1: does not describe a quantum state of spacetime. It does
• 54:52
  Speaker 1: not reveal whether the singular regime is replaced by some
• 54:56
  Speaker 1: finite structure, a new phase of geometry, or something that
• 55:00
  Speaker 1: current language does not yet describe well. This is why
• 55:03
  Speaker 1: the singularity theorems make the mystery sharper rather than removing it.
• 55:08
  Speaker 1: They close one easy escape route. Physicists cannot simply say
• 55:12
  Speaker 1: that the centre of the Schwarzschild black hole looked troublesome
• 55:16
  Speaker 1: because the model was too tidy. The trouble survives much
• 55:19
  Speaker 1: more general conditions. classical general relativity genuinely points toward the
• 55:24
  Speaker 1: incompleteness of spacetime during sufficiently extreme collapse. Yet precisely what
• 55:30
  Speaker 1: nature does when that classical description reaches its limit remains unknown,
• 55:35
  Speaker 1: so the theorem gives us one of the strongest statements
• 55:37
  Speaker 1: available about the centre of a black hole. Something profound
• 55:41
  Speaker 1: cannot simply be avoided. But knowing that the classical road
• 55:45
  Speaker 1: must end is still not the same as knowing what
• 55:47
  Speaker 1: waits where the road disappears. The simplest black hole is
• 55:51
  Speaker 1: wonderfully tidy. It does not rotate. It has no electric charge.
• 55:56
  Speaker 1: It is perfectly spherical. Its entire behaviour can be described
• 56:00
  Speaker 1: using only its mass. Nature, unfortunately for anyone who enjoys
• 56:05
  Speaker 1: tidy equations, is rarely quite so cooperative. Real black holes
• 56:10
  Speaker 1: are expected to spin. That is because almost everything involved
• 56:15
  Speaker 1: in making them is already moving. Stars rotate. gas clouds swirl,
• 56:20
  Speaker 1: binary systems orbit, galaxies turn. When a massive star collapses,
• 56:27
  Speaker 1: it does not usually lose all of its angular momentum.
• 56:30
  Speaker 1: When two black holes merge, the orbital motion of the
• 56:33
  Speaker 1: pair contributes angular momentum to the final object. Matter falling
• 56:38
  Speaker 1: in from an accretion disk can spin a black hole
• 56:40
  Speaker 1: up even further. So although the non-rotating Schwarzschild black hole
• 56:45
  Speaker 1: is an essential starting point, The more realistic description for
• 56:49
  Speaker 1: an astrophysical black hole is usually a rotating one, and
• 56:52
  Speaker 1: rotation changes the geometry dramatically. In 1963, the New Zealand
• 56:58
  Speaker 1: mathematician Roy Kerr discovered an exact solution to Albert Einstein's
• 57:03
  Speaker 1: field equations describing the spacetime around an ideal rotating black
• 57:07
  Speaker 1: hole with no electric charge. Today, this is known as
• 57:11
  Speaker 1: the Kerr solution. It was a remarkable discovery because rotation
• 57:15
  Speaker 1: had made Einstein's equations far more difficult to solve than
• 57:19
  Speaker 1: in the perfectly spherical Schwarzschild case. The Kerr black hole
• 57:23
  Speaker 1: is still highly idealised. It is assumed to be isolated,
• 57:27
  Speaker 1: stationary and described by a clean mathematical geometry. But it
• 57:32
  Speaker 1: captures something the Schwarzschild solution cannot. It shows what rotation
• 57:36
  Speaker 1: does to spacetime itself. A rotating black hole does not
• 57:40
  Speaker 1: simply spin like a solid ball. There is no ordinary
• 57:44
  Speaker 1: material surface carrying painted dots around in circles. Instead, the
• 57:49
  Speaker 1: rotation affects the geometry surrounding the black hole. Spacetime is
• 57:53
  Speaker 1: dragged around with it. This effect is called frame dragging.
• 57:58
  Speaker 1: Frame dragging is not unique to black holes. General relativity
• 58:02
  Speaker 1: predicts that any rotating mass should slightly twist the spacetime
• 58:06
  Speaker 1: around it. For ordinary objects, the effect is tiny. Earth
• 58:10
  Speaker 1: produces frame dragging, and it has been measured experimentally using
• 58:15
  Speaker 1: satellites and precision instruments. Around a rapidly rotating black hole, however,
• 58:21
  Speaker 1: the effect becomes extreme. Close enough to the black hole,
• 58:25
  Speaker 1: spacetime is being dragged around so strongly that remaining stationary
• 58:29
  Speaker 1: relative to the distant universe becomes impossible. This leads to
• 58:34
  Speaker 1: another important region known as the ergosphere. The ergosphere lies
• 58:38
  Speaker 1: outside the event horizon of a rotating black hole. Its
• 58:42
  Speaker 1: outer boundary is called the static limit. Inside that limit,
• 58:47
  Speaker 1: an object cannot simply hover in one fixed position relative
• 58:50
  Speaker 1: to distant stars. It must move around the black hole
• 58:54
  Speaker 1: in the same general direction as the black hole's rotation.
• 58:58
  Speaker 1: This does not mean the object has already crossed the
• 59:00
  Speaker 1: event horizon. Escape from the ergosphere is still possible, but
• 59:04
  Speaker 1: standing still is not. The spacetime itself has become too
• 59:08
  Speaker 1: strongly twisted. This difference between the ergosphere and the event
• 59:12
  Speaker 1: horizon is important because rotating black holes no longer possess
• 59:17
  Speaker 1: the simple spherical causal structure of the Schwarzschild case. Their
• 59:21
  Speaker 1: geometry contains more layers. There is still an outer event horizon.
• 59:27
  Speaker 1: Once crossed, This remains the boundary beyond which ordinary signals
• 59:31
  Speaker 1: cannot return to the distant universe. But the ideal Kerr
• 59:35
  Speaker 1: solution also contains a second horizon deeper inside. This is
• 59:41
  Speaker 1: known as the inner horizon, or Cauchy horizon. Already the
• 59:45
  Speaker 1: picture of one horizon surrounding one central point has become
• 59:49
  Speaker 1: much less accurate. And then we reach the predicted singularity.
• 59:53
  Speaker 1: For a non-rotating Schwarzschild black hole, The classical singularity is
• 59:58
  Speaker 1: associated with the centre at zero radius. In the ideal
• 1:00:02
  Speaker 1: Kerr geometry, the mathematics produces something different. The singularity takes
• 1:00:07
  Speaker 1: the form of a ring, not a tiny spherical point,
• 1:00:11
  Speaker 1: a ring. This ring singularity is one of the reasons
• 1:00:15
  Speaker 1: the phrase the point at the centre of a black
• 1:00:17
  Speaker 1: hole should be treated carefully. For realistic rotating black holes,
• 1:00:22
  Speaker 1: Even the classical equations do not predict the same simple
• 1:00:26
  Speaker 1: central structure as the non-rotating model. Rotation fundamentally changes the geometry.
• 1:00:33
  Speaker 1: The complete mathematical extension of the Kerr solution becomes stranger still.
• 1:00:38
  Speaker 1: If the ideal equations are followed without interruption, the geometry
• 1:00:43
  Speaker 1: contains regions that can be extended beyond the inner horizon.
• 1:00:47
  Speaker 1: Some mathematical paths appear to pass through the ring. Others
• 1:00:51
  Speaker 1: lead into additional regions of space-time. Certain extensions have sometimes
• 1:00:56
  Speaker 1: been described using language involving other universes, wormhole-like passages or
• 1:01:01
  Speaker 1: paths with very unusual causal behaviour. These possibilities sound irresistible.
• 1:01:07
  Speaker 1: They are also exactly where caution becomes essential. The fully
• 1:01:11
  Speaker 1: extended Kerr solution describes an eternal, perfectly ideal rotating black hole.
• 1:01:19
  Speaker 1: A real black hole formed by the collapse of a
• 1:01:21
  Speaker 1: star is not eternal. It has a history. Matter falls in.
• 1:01:26
  Speaker 1: Radiation crosses the horizon. Gravitational disturbances are present. Other objects
• 1:01:32
  Speaker 1: interact with it. The interior is not guaranteed to preserve
• 1:01:35
  Speaker 1: the pristine mathematical structure of the ideal Kerr geometry. In particular,
• 1:01:41
  Speaker 1: the inner horizon turns out to be extremely sensitive to disturbances.
• 1:01:45
  Speaker 1: even tiny amounts of infalling radiation can become enormously blue-shifted there.
• 1:01:51
  Speaker 1: This suggests that the elegant inner structure of the exact
• 1:01:54
  Speaker 1: Kerr solution may be violently unstable in a realistic black hole.
• 1:01:59
  Speaker 1: So although the equations allow exotic extensions on paper, physicists
• 1:02:04
  Speaker 1: do not generally treat them as established roots through actual
• 1:02:07
  Speaker 1: astrophysical black holes. A diagram showing another universe on the
• 1:02:12
  Speaker 1: far side of a Kerr geometry is not a travel brochure.
• 1:02:17
  Speaker 1: It is a feature of an ideal mathematical solution whose
• 1:02:21
  Speaker 1: physical relevance depends on whether that interior structure can survive
• 1:02:25
  Speaker 1: under realistic conditions. What astronomers can test much more confidently
• 1:02:29
  Speaker 1: is the exterior. There, the Kerr description has performed remarkably well.
• 1:02:35
  Speaker 1: Matter orbiting close to black holes behaves broadly as expected
• 1:02:39
  Speaker 1: around strongly curved rotating spacetimes. The inner edge of accretion
• 1:02:43
  Speaker 1: disks depends on black hole spin, allowing astronomers to estimate
• 1:02:48
  Speaker 1: angular momentum in some systems. Gravitational waves from merging black
• 1:02:52
  Speaker 1: holes contain information about the masses and spins of the
• 1:02:55
  Speaker 1: objects before the collision and about the final black hole
• 1:02:59
  Speaker 1: left behind. After a merger, the newly formed black hole
• 1:03:03
  Speaker 1: settles down by emitting gravitational waves in a process known
• 1:03:06
  Speaker 1: as ring down. The observed behaviour is consistent with the
• 1:03:10
  Speaker 1: kind of rotating compact object predicted by general relativity. Horizon-scale
• 1:03:16
  Speaker 1: observations also support the broad picture of black holes possessing
• 1:03:20
  Speaker 1: the compact geometries expected from Einstein's theory. None of this
• 1:03:25
  Speaker 1: means that astronomers have looked inside and confirmed every feature
• 1:03:28
  Speaker 1: of the ideal Kerr solution. That distinction matters. The exterior
• 1:03:33
  Speaker 1: can agree beautifully with general relativity while the deepest interior
• 1:03:37
  Speaker 1: remains uncertain. A black hole may behave almost exactly like
• 1:03:41
  Speaker 1: a Kerr black hole from the outside, even if quantum
• 1:03:45
  Speaker 1: gravity eventually changes what happens near the would-be singular region.
• 1:03:50
  Speaker 1: This is one of the reasons black holes are so
• 1:03:52
  Speaker 1: scientifically frustrating, and so useful at the same time. The
• 1:03:57
  Speaker 1: part we can observe may follow the theory with extraordinary precision.
• 1:04:01
  Speaker 1: The part we most want to understand may remain permanently
• 1:04:04
  Speaker 1: hidden behind the horizons. Rotations therefore changes the central mystery
• 1:04:09
  Speaker 1: in an important way. Instead of asking only what happens
• 1:04:13
  Speaker 1: at one tiny point in the middle, we have to
• 1:04:16
  Speaker 1: ask what the entire internal geometry of a realistic spinning
• 1:04:19
  Speaker 1: black hole ultimately becomes. Does the inner horizon survive? Does
• 1:04:25
  Speaker 1: instability destroy the simple Kerr interior? Does the ring singularity
• 1:04:30
  Speaker 1: remain meaningful once real disturbances are included? And even if
• 1:04:34
  Speaker 1: classical relativity carries the geometry towards singular behaviour, what happens
• 1:04:39
  Speaker 1: when quantum effects can no longer be ignored? A realistic
• 1:04:43
  Speaker 1: black hole is not merely a darker version of a
• 1:04:45
  Speaker 1: spherical star. Its rotation twists spacetime, it creates an ergosphere,
• 1:04:51
  Speaker 1: it changes the shape of the horizons, and in the
• 1:04:54
  Speaker 1: ideal classical solution, it changes the singularity from a point-like
• 1:04:59
  Speaker 1: structure into a ring. So the closer we move toward
• 1:05:03
  Speaker 1: what a real black hole is likely to be, the
• 1:05:05
  Speaker 1: less useful the simple picture of a single point at
• 1:05:07
  Speaker 1: the centre becomes. The mystery is no longer just what
• 1:05:12
  Speaker 1: sits in the middle. It is what the interior of
• 1:05:14
  Speaker 1: rotating spacetime ultimately becomes. The ideal Kerr black hole is elegant,
• 1:05:20
  Speaker 1: perhaps a little too elegant. Its mathematics describes a perfectly
• 1:05:24
  Speaker 1: stationary rotating black hole, isolated from disturbance and allowed to
• 1:05:28
  Speaker 1: exist forever. Under those conditions, the equations contain an outer
• 1:05:33
  Speaker 1: event horizon, an inner horizon, and, deeper still, the famous
• 1:05:39
  Speaker 1: ring singularity. But a real black hole is not left alone.
• 1:05:43
  Speaker 1: Matter falls in. Light falls in. Gravitational waves pass through.
• 1:05:48
  Speaker 1: Radiation from the surrounding universe crosses the horizon. Even if
• 1:05:52
  Speaker 1: these disturbances are incredibly faint, the interior of a rotating
• 1:05:57
  Speaker 1: black hole may amplify them in a remarkable way. and
• 1:06:00
  Speaker 1: that amplification becomes especially important near the inner horizon. The
• 1:06:05
  Speaker 1: inner horizon is also called the Cauchy horizon. It appears
• 1:06:09
  Speaker 1: in the exact Kerr solution deeper inside the black hole
• 1:06:12
  Speaker 1: than the ordinary event horizon. Its name comes from mathematics.
• 1:06:16
  Speaker 1: In general relativity, a Cauchy surface is, roughly speaking, a
• 1:06:21
  Speaker 1: surface containing enough information to determine how the surrounding spacetime
• 1:06:25
  Speaker 1: evolves according to the equations. The Cauchy horizon marks a
• 1:06:30
  Speaker 1: boundary beyond which that straightforward predictability becomes problematic in the
• 1:06:34
  Speaker 1: ideal solution. Already, that should make the region sound slightly
• 1:06:38
  Speaker 1: less comfortable than the average holiday destination, but the deeper
• 1:06:42
  Speaker 1: problem is what happens to radiation approaching it. Imagine a
• 1:06:46
  Speaker 1: small amount of light falling into the black hole. Perhaps
• 1:06:49
  Speaker 1: it came from the original collapsing star. Perhaps it entered
• 1:06:53
  Speaker 1: much later from the surrounding accretion flow. Perhaps it is
• 1:06:57
  Speaker 1: nothing more than the faint background radiation that exists throughout
• 1:07:01
  Speaker 1: the universe. Near the inner horizon, the geometry can subject
• 1:07:05
  Speaker 1: this radiation to an enormous blueshift. Blueshift is the opposite
• 1:07:10
  Speaker 1: of redshift. When light is redshifted, its wavelength becomes longer
• 1:07:15
  Speaker 1: and its energy appears lower. When it is blueshifted, the
• 1:07:19
  Speaker 1: wavelength becomes shorter and the measured energy rises. Near the
• 1:07:23
  Speaker 1: Cauchy horizon, An infalling observer can encounter radiation that has
• 1:07:27
  Speaker 1: been amplified to astonishing energies by the geometry of the
• 1:07:31
  Speaker 1: black hole. The effect is not because the original radiation
• 1:07:34
  Speaker 1: had to be particularly powerful. The disturbing part is that
• 1:07:38
  Speaker 1: even very weak disturbances can become important. This is where
• 1:07:42
  Speaker 1: the work of Eric Poisson and Werner Israel enters the story.
• 1:07:47
  Speaker 1: Around 1990, Poisson and Israel studied the interior of charged
• 1:07:51
  Speaker 1: and rotating black holes. and showed that the inner horizon
• 1:07:54
  Speaker 1: could suffer an instability known as mass inflation. The name
• 1:07:58
  Speaker 1: can be slightly confusing. Mass inflation has nothing to do
• 1:08:02
  Speaker 1: with cosmological inflation, the rapid expansion thought to have occurred
• 1:08:06
  Speaker 1: in the extremely early universe. Nothing inside the black hole
• 1:08:10
  Speaker 1: is suddenly expanding into a miniature cosmos, merely because the
• 1:08:13
  Speaker 1: word inflation appears. Instead, mass inflation refers to the runaway
• 1:08:18
  Speaker 1: growth of an internal gravitational mass parameter, associated with the
• 1:08:22
  Speaker 1: black hole geometry. Near the inner horizon, ingoing and outgoing
• 1:08:27
  Speaker 1: streams of radiation can interact gravitationally. Because of the extreme
• 1:08:32
  Speaker 1: blue shift, even tiny fluxes can produce enormous effects. The
• 1:08:36
  Speaker 1: effective internal mass parameter can grow without bound in the
• 1:08:40
  Speaker 1: classical calculation. Space-time curvature can become correspondingly extreme. What looked
• 1:08:46
  Speaker 1: in the ideal Kerr solution like a smooth inner horizon
• 1:08:49
  Speaker 1: can therefore become violently unstable. This matters because the pristine
• 1:08:54
  Speaker 1: mathematical Kerr geometry contains some very exotic possibilities beyond the
• 1:08:59
  Speaker 1: inner horizon. If the exact eternal solution is extended formally,
• 1:09:04
  Speaker 1: the equations allow access to other regions of spacetime. Certain
• 1:09:08
  Speaker 1: paths seem capable of avoiding the simple, central fate of
• 1:09:12
  Speaker 1: the Schwarzschild black hole. The geometry becomes a complicated network
• 1:09:17
  Speaker 1: of horizons and additional regions. but mass inflation suggests that
• 1:09:21
  Speaker 1: nature may never provide such a perfectly maintained structure. A
• 1:09:26
  Speaker 1: realistic black hole always contains disturbances. There is essentially no
• 1:09:30
  Speaker 1: way to construct an astrophysical black hole that has never
• 1:09:33
  Speaker 1: interacted with anything. Even the quietest black hole in deep
• 1:09:37
  Speaker 1: space would still exist in a universe containing radiation and
• 1:09:41
  Speaker 1: gravitational perturbations. Those tiny imperfections may be enough to destabilise
• 1:09:46
  Speaker 1: the Cauchy horizon. So the inner horizon that looks perfectly
• 1:09:50
  Speaker 1: respectable on a clean mathematical page may behave very differently
• 1:09:55
  Speaker 1: inside an actual black hole. Later analytical studies and numerical
• 1:09:59
  Speaker 1: simulations have explored this problem in greater detail. The picture
• 1:10:03
  Speaker 1: that emerges is complicated. Depending on the assumptions in the
• 1:10:07
  Speaker 1: particular black hole model, the instability can generate singular structures
• 1:10:11
  Speaker 1: associated with the inner horizons. Some of these singularities can
• 1:10:15
  Speaker 1: initially be described as null singularities. The word null here
• 1:10:20
  Speaker 1: is connected with the paths followed by light. A null
• 1:10:23
  Speaker 1: singularity is associated with a light-like direction in spacetime, rather
• 1:10:28
  Speaker 1: than resembling the space-like singularity of the simplest Schwarzschild interior.
• 1:10:33
  Speaker 1: Deeper into the evolving interior, calculations can also produce space-like
• 1:10:38
  Speaker 1: singular behaviour. This means that instead of finding one neat
• 1:10:42
  Speaker 1: ring sitting quietly at the centre, a realistic rotating black
• 1:10:46
  Speaker 1: hole may contain an evolving network of extreme curvature shaped
• 1:10:50
  Speaker 1: by the history of everything that has fallen into it.
• 1:10:54
  Speaker 1: That is a much less tidy picture, and probably a
• 1:10:57
  Speaker 1: much more realistic one. The original Kerr ring remains an
• 1:11:01
  Speaker 1: important feature of the exact mathematical solution. But once perturbations
• 1:11:06
  Speaker 1: are included, the physical interior may not preserve the smooth
• 1:11:09
  Speaker 1: route leading toward that ring in the idealised way textbooks
• 1:11:13
  Speaker 1: sometimes suggest. This is a broader lesson in physics. Exact
• 1:11:18
  Speaker 1: solutions are enormously valuable because they reveal what equations are
• 1:11:22
  Speaker 1: capable of doing. But nature rarely supplies the exact conditions
• 1:11:26
  Speaker 1: required to preserve every feature of those solutions. A perfectly
• 1:11:31
  Speaker 1: balanced pencil can remain upright in an ideal calculation. In
• 1:11:35
  Speaker 1: a real room, the smallest disturbance eventually makes it fall.
• 1:11:40
  Speaker 1: The Cauchy horizon may be rather more dramatic than a pencil,
• 1:11:43
  Speaker 1: but the principle is similar. If a structure is unstable
• 1:11:47
  Speaker 1: to tiny disturbances, then its existence in a perfectly symmetric
• 1:11:51
  Speaker 1: mathematical model does not guarantee that the same structure survives
• 1:11:55
  Speaker 1: in the physical universe. There is another reason mass inflation
• 1:11:59
  Speaker 1: is so important. It may connect the relatively well-understood exterior
• 1:12:03
  Speaker 1: of a black hole to the region where classical relativity
• 1:12:06
  Speaker 1: becomes unreliable. Outside the black hole, spacetime can be described
• 1:12:10
  Speaker 1: with tremendous accuracy. Cross the event horizon, and classical general
• 1:12:16
  Speaker 1: relativity can still describe much of the journey. But near
• 1:12:19
  Speaker 1: the unstable inner horizon of a rotating black hole, curvature
• 1:12:23
  Speaker 1: may grow enormously. Eventually the conditions can become so extreme
• 1:12:28
  Speaker 1: that quantum gravitational effects should matter, and at that point
• 1:12:32
  Speaker 1: the calculation meets the same wall that appears elsewhere in
• 1:12:35
  Speaker 1: black hole physics. General relativity is classical. Mass inflation is
• 1:12:41
  Speaker 1: itself a prediction made within classical general relativity. It can
• 1:12:45
  Speaker 1: tell us that the geometry becomes violently distorted. It can
• 1:12:49
  Speaker 1: tell us that curvature may become extraordinarily large. It can
• 1:12:53
  Speaker 1: tell us that the ideal inner horizon is unstable. But
• 1:12:57
  Speaker 1: it cannot tell us what finally happens once the curvature
• 1:13:00
  Speaker 1: enters a regime where spacetime itself should behave quantum mechanically.
• 1:13:05
  Speaker 1: That requires a theory of quantum gravity. And no experimentally
• 1:13:09
  Speaker 1: confirmed theory currently provides the final answer. So mass inflation
• 1:13:14
  Speaker 1: does not solve the mystery of the black hole centre.
• 1:13:17
  Speaker 1: In some ways, it makes the mystery more interesting. The
• 1:13:21
  Speaker 1: simplest picture begins with a non-rotating black hole and a
• 1:13:25
  Speaker 1: central singularity. Then rotation changes the singularity into a ring
• 1:13:30
  Speaker 1: and introduces an inner horizon. For a moment the geometry
• 1:13:34
  Speaker 1: seems to become more elaborate but still mathematically organised. Then
• 1:13:38
  Speaker 1: realistic disturbances enter the story. The inner horizon destabilises. Radiation
• 1:13:44
  Speaker 1: is enormously blueshifted. The internal gravitational field can undergo runaway growth.
• 1:13:50
  Speaker 1: curvature can become extreme, and the beautifully extended Kerr spacetime
• 1:13:55
  Speaker 1: begins to look less like the interior of something nature
• 1:13:58
  Speaker 1: would actually build. This is why asking what lies at
• 1:14:02
  Speaker 1: the centre of a real black hole is more complicated
• 1:14:05
  Speaker 1: than pointing to the ring drawn in a diagram. Even
• 1:14:09
  Speaker 1: before quantum mechanics is included, Classical relativity suggests that the
• 1:14:14
  Speaker 1: interior of a realistic rotating black hole may be far
• 1:14:17
  Speaker 1: more violent and structurally complex than the exact Kerr solution
• 1:14:21
  Speaker 1: first appears. The central region may depend on instability, collapse history,
• 1:14:27
  Speaker 1: incoming radiation and evolving curvature. And then, just when the
• 1:14:31
  Speaker 1: mathematics becomes most extreme, the missing physics returns. Quantum gravity,
• 1:14:37
  Speaker 1: the very ingredient classical general relativity does not contain. So
• 1:14:42
  Speaker 1: the closer we move toward a realistic black hole, the
• 1:14:45
  Speaker 1: less the centre resembles a simple destination. It becomes an
• 1:14:48
  Speaker 1: evolving region where the geometry itself may be unstable, where
• 1:14:52
  Speaker 1: tiny disturbances can grow into enormous effects, and where the
• 1:14:57
  Speaker 1: clean mathematical picture begins to give way to physics that
• 1:15:00
  Speaker 1: is still not fully understood. The deeper we travel into
• 1:15:03
  Speaker 1: the mathematics of a black hole, the more extreme the
• 1:15:06
  Speaker 1: conditions become. Curvature increases, tidal effects strengthen, In a rotating
• 1:15:13
  Speaker 1: black hole, the inner geometry may become unstable, and eventually
• 1:15:17
  Speaker 1: the equations of classical general relativity lead toward conditions so
• 1:15:21
  Speaker 1: severe that another great framework of modern physics can no
• 1:15:25
  Speaker 1: longer be ignored, quantum physics. This is where the centre
• 1:15:29
  Speaker 1: of a black hole becomes more than a problem about gravity.
• 1:15:33
  Speaker 1: It becomes a meeting place between two extraordinarily successful descriptions
• 1:15:38
  Speaker 1: of nature that have never been completely joined together. General
• 1:15:42
  Speaker 1: relativity describes gravity and spacetime. Quantum mechanics describes the strange
• 1:15:47
  Speaker 1: but remarkably precise behaviour of nature at microscopic scales. Both
• 1:15:52
  Speaker 1: theories work exceptionally well within the domains where they have
• 1:15:55
  Speaker 1: been tested. The difficulty appears when nature demands that we
• 1:16:00
  Speaker 1: use both at the same time. General relativity is a
• 1:16:03
  Speaker 1: classical theory. In Albert Einstein's description, spacetime is treated as
• 1:16:08
  Speaker 1: a smooth geometric structure. mass and energy affect its curvature,
• 1:16:13
  Speaker 1: and that curved geometry tells matter and light how to move.
• 1:16:17
  Speaker 1: There is no fundamental quantum uncertainty built into spacetime itself.
• 1:16:23
  Speaker 1: Quantum mechanics works very differently. At microscopic scales, physical systems
• 1:16:29
  Speaker 1: are described using quantum states. Quantities that seem definite in
• 1:16:34
  Speaker 1: classical physics can instead be represented by probability distributions. Particles
• 1:16:39
  Speaker 1: display wave-like behaviour. Energy often comes in discrete amounts. Measurements
• 1:16:45
  Speaker 1: reveal outcomes governed by rules that have no ordinary classical equivalent.
• 1:16:51
  Speaker 1: Modern particle physics develops these ideas further through quantum field theory.
• 1:16:56
  Speaker 1: In quantum field theory, particles such as electrons and photons
• 1:17:00
  Speaker 1: are understood as quantum excitations of underlying fields. The electromagnetic
• 1:17:06
  Speaker 1: interaction The strong nuclear interaction and the weak nuclear interaction
• 1:17:11
  Speaker 1: are all described within quantum frameworks of extraordinary precision. Gravity
• 1:17:17
  Speaker 1: remains the awkward guest at the table. General relativity describes
• 1:17:21
  Speaker 1: it beautifully at large scales. Quantum theory describes the other
• 1:17:25
  Speaker 1: known interactions beautifully at microscopic scales. But the two frameworks
• 1:17:30
  Speaker 1: are built using very different assumptions. Usually this does not
• 1:17:35
  Speaker 1: cause much trouble. When studying an orbiting planet, quantum effects
• 1:17:39
  Speaker 1: in spacetime can be safely ignored. When studying an electron
• 1:17:43
  Speaker 1: in an atom, the curvature of spacetime produced by the
• 1:17:47
  Speaker 1: electron is absurdly small. Each theory can work within its
• 1:17:51
  Speaker 1: own comfortable territory. The centre of a black hole does
• 1:17:54
  Speaker 1: not offer that luxury. Classical general relativity predicts that curvature
• 1:17:59
  Speaker 1: becomes increasingly extreme Matter is driven into conditions far beyond
• 1:18:05
  Speaker 1: anything found in ordinary stars or laboratories. Eventually, physicists expect
• 1:18:10
  Speaker 1: that the quantum nature of gravity itself must become important.
• 1:18:15
  Speaker 1: A useful scale for discussing this boundary is the Planck scale.
• 1:18:19
  Speaker 1: The Planck length is roughly 1.6 times 10 to the
• 1:18:23
  Speaker 1: minus 35 metres. That is an almost incomprehensibly small distance.
• 1:18:28
  Speaker 1: A proton is already tiny. a characteristic size of roughly
• 1:18:32
  Speaker 1: 10 to the minus 15 metres, the Planck length is
• 1:18:36
  Speaker 1: about 20 orders of magnitude smaller still. If a proton
• 1:18:40
  Speaker 1: were enlarged until it was comparable in size to the
• 1:18:43
  Speaker 1: observable universe, the Planck length would remain astonishingly small by comparison.
• 1:18:49
  Speaker 1: The precise analogy should not be taken too literally, but
• 1:18:52
  Speaker 1: the difference in scale is the important point. The Planck
• 1:18:56
  Speaker 1: length lies far beyond anything that current microscopes or particle
• 1:19:00
  Speaker 1: accelerators can probe directly. There is also a characteristic Planck energy.
• 1:19:06
  Speaker 1: For an individual particle, the Planck energy is enormous compared
• 1:19:10
  Speaker 1: with energies reached in present-day particle physics. It corresponds to
• 1:19:15
  Speaker 1: roughly 10 to the 19 billion electron volts multiplied by
• 1:19:19
  Speaker 1: a billion, or about 10 to the 19 gigaelectron volts.
• 1:19:24
  Speaker 1: The important point is not the exact number. It is
• 1:19:27
  Speaker 1: that the Planck scale marks a regime in which The
• 1:19:31
  Speaker 1: usual distinction between matter behaving quantum mechanically and space-time behaving
• 1:19:35
  Speaker 1: classically is expected to become inadequate. This does not mean
• 1:19:39
  Speaker 1: physicists know that a tiny plank-sized ball exists at the
• 1:19:43
  Speaker 1: centre of every black hole. There is no established object
• 1:19:48
  Speaker 1: called a plank core waiting neatly in the middle. The
• 1:19:51
  Speaker 1: plank length should not be imagined as the smallest possible
• 1:19:54
  Speaker 1: ruler mark carved into the universe unless a particular theory
• 1:19:58
  Speaker 1: specifically gives it that interpretation. Instead, the Planck scale is
• 1:20:03
  Speaker 1: constructed from fundamental constants associated with gravity, quantum mechanics, and
• 1:20:10
  Speaker 1: the speed of light. Its significance is that these ingredients
• 1:20:14
  Speaker 1: become comparable there. That strongly suggests that a quantum theory
• 1:20:19
  Speaker 1: of gravity is needed. The difficulty is that physicists do
• 1:20:23
  Speaker 1: not yet know exactly what such a theory looks like,
• 1:20:26
  Speaker 1: It might be tempting simply to take ordinary quantum mechanics
• 1:20:30
  Speaker 1: and apply it to matter falling through an already curved
• 1:20:34
  Speaker 1: black hole spacetime. And physicists do something related to this
• 1:20:38
  Speaker 1: in a subject called quantum field theory in curved spacetime.
• 1:20:43
  Speaker 1: This framework treats quantum fields while the spacetime geometry itself
• 1:20:47
  Speaker 1: remains essentially classical. It is extremely useful. Stephen Hawking's prediction
• 1:20:54
  Speaker 1: that black holes emit thermal radiation comes from this kind
• 1:20:57
  Speaker 1: of approach, but there is a limit to what it
• 1:20:59
  Speaker 1: can do. If spacetime itself is becoming violently curved on
• 1:21:04
  Speaker 1: quantum scales, then treating the geometry as a fixed classical
• 1:21:08
  Speaker 1: background may no longer be enough. It would be rather
• 1:21:11
  Speaker 1: like trying to describe waves on an ocean, while insisting
• 1:21:15
  Speaker 1: that the water itself is not allowed to participate. At
• 1:21:18
  Speaker 1: some point, the background can no longer simply sit there
• 1:21:21
  Speaker 1: behaving classically while everything else becomes quantum. The geometry must
• 1:21:27
  Speaker 1: somehow enter the quantum description too. That is the central
• 1:21:32
  Speaker 1: ambition of quantum gravity. A theory of quantum gravity would
• 1:21:36
  Speaker 1: provide a framework in which quantum principles and gravitational spacetime
• 1:21:41
  Speaker 1: are described consistently together. It should reproduce general relativity when
• 1:21:47
  Speaker 1: gravity behaves classically, It should respect the well-tested principles of
• 1:21:51
  Speaker 1: quantum physics, and it should remain meaningful in regimes where
• 1:21:55
  Speaker 1: both become simultaneously important. The centre of a black hole
• 1:21:59
  Speaker 1: is one of the most obvious places where such a
• 1:22:02
  Speaker 1: theory may be required. The early universe is another. Both
• 1:22:06
  Speaker 1: involve extreme conditions in which classical spacetime appears to approach
• 1:22:11
  Speaker 1: a limit. Several major approaches to quantum gravity have been developed.
• 1:22:16
  Speaker 1: String theory attempts to place gravity and quantum physics within
• 1:22:19
  Speaker 1: a framework built from tiny extended objects rather than fundamental
• 1:22:23
  Speaker 1: point particles. Loop quantum gravity investigates the possibility that spacetime
• 1:22:29
  Speaker 1: geometry itself has a discrete quantum structure. Other approaches explore
• 1:22:34
  Speaker 1: causal sets, asymptotic safety, emergent spacetime and various ways in
• 1:22:40
  Speaker 1: which geometry might arise from more fundamental quantum relationships. These
• 1:22:44
  Speaker 1: are serious areas of theoretical research. None has yet supplied
• 1:22:49
  Speaker 1: an experimentally confirmed and universally accepted description of what actually
• 1:22:53
  Speaker 1: happens at the centre of an astrophysical black hole. That
• 1:22:58
  Speaker 1: distinction is essential. Physicists have ideas. They have sophisticated mathematical frameworks.
• 1:23:05
  Speaker 1: They have models in which singularities may be avoided. They
• 1:23:09
  Speaker 1: have calculations suggesting that classical spacetime could be replaced by
• 1:23:13
  Speaker 1: some quantum structure. What they do not yet have is
• 1:23:16
  Speaker 1: decisive observational evidence identifying one of those possibilities as nature's answer.
• 1:23:23
  Speaker 1: And obtaining such evidence is especially difficult because the relevant
• 1:23:27
  Speaker 1: region is hidden behind an event horizon. Even if astonishing
• 1:23:31
  Speaker 1: quantum behaviour occurs deep inside, it may leave only subtle
• 1:23:35
  Speaker 1: traces accessible to the outside universe. This places black hole
• 1:23:40
  Speaker 1: physics in a peculiar position. The problem is not that
• 1:23:44
  Speaker 1: the existing theories are unsuccessful. General relativity is one of
• 1:23:48
  Speaker 1: the most accurately tested theories of gravity ever constructed. Quantum
• 1:23:53
  Speaker 1: mechanics and quantum field theory underpin much of modern physics,
• 1:23:57
  Speaker 1: chemistry and technology. Their successes are precisely what make the
• 1:24:02
  Speaker 1: conflict so interesting. We are not trying to reconcile two
• 1:24:06
  Speaker 1: theories that obviously failed We are trying to reconcile two
• 1:24:10
  Speaker 1: theories that work extremely well. They simply speak very different
• 1:24:14
  Speaker 1: mathematical languages when asked about the deepest structure of reality.
• 1:24:18
  Speaker 1: General relativity says spacetime is dynamical geometry. Quantum physics says
• 1:24:24
  Speaker 1: fundamental systems possess quantum states, fluctuations and uncertainty. Near the
• 1:24:31
  Speaker 1: centre of a black hole, physicists expect that spacetime must
• 1:24:35
  Speaker 1: somehow be both geometric and quantum. exactly what that means
• 1:24:39
  Speaker 1: remains unknown. Perhaps the classical singularity is replaced before infinite
• 1:24:45
  Speaker 1: curvature is reached. Perhaps spacetime possesses a smallest meaningful structure.
• 1:24:51
  Speaker 1: Perhaps geometry emerges from something more fundamental. Perhaps the concepts
• 1:24:56
  Speaker 1: of space and time themselves cease to be useful in
• 1:25:00
  Speaker 1: their familiar forms. These possibilities remain questions rather than conclusions.
• 1:25:06
  Speaker 1: What can be said with greater confidence is that blindly
• 1:25:09
  Speaker 1: extending classical general relativity all the way into the singular
• 1:25:14
  Speaker 1: regime is unlikely to provide the final physical story. The
• 1:25:18
  Speaker 1: theory itself is signalling extreme conditions while containing no quantum
• 1:25:23
  Speaker 1: description of spacetime. That does not make general relativity a
• 1:25:27
  Speaker 1: failed theory. Newtonian gravity did not become useless when Einstein
• 1:25:32
  Speaker 1: improved our understanding of gravity. remains extraordinarily accurate for countless
• 1:25:38
  Speaker 1: ordinary situations. In much the same way, a future theory
• 1:25:42
  Speaker 1: of quantum gravity would be expected to contain general relativity
• 1:25:47
  Speaker 1: as an excellent approximation wherever quantum gravitational effects are negligible.
• 1:25:52
  Speaker 1: The boundary inside a black hole therefore represents something quite remarkable.
• 1:25:58
  Speaker 1: It is not simply a place where physicists ran out
• 1:26:00
  Speaker 1: of mathematical enthusiasm, It marks a point where two of
• 1:26:04
  Speaker 1: the deepest achievements in modern science are expected to meet,
• 1:26:08
  Speaker 1: while the framework capable of describing that meeting remains unfinished.
• 1:26:14
  Speaker 1: General relativity brings us astonishingly far inward. Quantum physics tells
• 1:26:19
  Speaker 1: us why we should become cautious when the scales grow
• 1:26:22
  Speaker 1: sufficiently extreme. And somewhere between them lies the missing descriptions.
• 1:26:29
  Speaker 1: The centre of a black hole is therefore not merely
• 1:26:32
  Speaker 1: hidden by an event horizon. It is hidden behind a
• 1:26:35
  Speaker 1: theoretical boundary as well. Beyond that boundary, gravity can no
• 1:26:39
  Speaker 1: longer be treated only as smooth classical geometry, while quantum
• 1:26:44
  Speaker 1: physics can no longer be applied as though spacetime were
• 1:26:46
  Speaker 1: simply an unchanging stage. Both must somehow become part of
• 1:26:51
  Speaker 1: the same story. And that story is one physicists are
• 1:26:54
  Speaker 1: still learning how to tell. If general relativity breaks down
• 1:26:58
  Speaker 1: near the centre of a black hole, then an obvious
• 1:27:01
  Speaker 1: possibility presents itself. Perhaps the singularity is not really there,
• 1:27:06
  Speaker 1: not in the literal sense suggested by the classical equations
• 1:27:09
  Speaker 1: at least. Perhaps the infinity appears because general relativity is
• 1:27:14
  Speaker 1: being pushed beyond the regime where it can be trusted,
• 1:27:17
  Speaker 1: and a quantum description of spacetime would replace that singular
• 1:27:21
  Speaker 1: behaviour with something finite. This idea is known more broadly
• 1:27:25
  Speaker 1: as singularity resolution. The phrase does not refer to one
• 1:27:29
  Speaker 1: specific theory or one agreed solution. It simply describes the
• 1:27:34
  Speaker 1: possibility that a more complete theory of gravity could remove
• 1:27:37
  Speaker 1: the infinities predicted by classical relativity and replace them with
• 1:27:42
  Speaker 1: a physically meaningful quantum state. One of the best known
• 1:27:46
  Speaker 1: approaches exploring this possibility is loop quantum gravity. Loop quantum
• 1:27:51
  Speaker 1: gravity is associated with researchers including Abhayashtakar, Carlo Rovelli and
• 1:27:56
  Speaker 1: Lee Smolin. Its starting point is very different from string theory.
• 1:28:01
  Speaker 1: Rather than treating gravity as emerging from tiny vibrating strings,
• 1:28:05
  Speaker 1: loop quantum gravity attempts to quantise spacetime geometry itself. The
• 1:28:10
  Speaker 1: basic idea is that quantities such as area and volume
• 1:28:13
  Speaker 1: may not remain perfectly continuous at the deepest level. Instead,
• 1:28:18
  Speaker 1: they may come in discrete quantum units. This does not
• 1:28:21
  Speaker 1: mean spacetime is necessarily made of tiny cubes arranged like
• 1:28:25
  Speaker 1: bathroom tiles– the mathematics is far subtler than that. But
• 1:28:29
  Speaker 1: it does suggest that the smooth geometry of general relativity
• 1:28:33
  Speaker 1: may be an approximation that emerges only at scales much
• 1:28:37
  Speaker 1: larger than the Planck scale. At ordinary distances spacetime would
• 1:28:41
  Speaker 1: appear smooth in much the same way that a sheet
• 1:28:44
  Speaker 1: of paper appears smooth from across a room. Look closely
• 1:28:48
  Speaker 1: enough and the underlying structure may be very different. If
• 1:28:52
  Speaker 1: spacetime geometry is genuinely quantum then the classical prediction of
• 1:28:57
  Speaker 1: infinite compression might change This possibility has been explored in
• 1:29:02
  Speaker 1: several simplified models inspired by loop quantum gravity. In some
• 1:29:06
  Speaker 1: of these calculations, the singularity does not survive. Instead of
• 1:29:11
  Speaker 1: curvature increasing without limit, quantum geometric effects become important before
• 1:29:16
  Speaker 1: the classical infinity is reached. The collapse may then pass
• 1:29:20
  Speaker 1: through a finite region, where the classical description fails, but
• 1:29:25
  Speaker 1: the quantum description remains meaningful. In certain models, this produces
• 1:29:30
  Speaker 1: something resembling a bounce. Matter collapses inward. Curvature grows. Quantum
• 1:29:36
  Speaker 1: gravitational effects become dominant. And rather than continuing toward an
• 1:29:40
  Speaker 1: infinite singularity, the geometry undergoes a transition. The word bounce
• 1:29:46
  Speaker 1: makes the process sound almost cheerful, but it should be
• 1:29:49
  Speaker 1: handled carefully. It does not necessarily mean that a star
• 1:29:52
  Speaker 1: falls inward and then immediately springs back out through the same...
• 1:29:57
  Speaker 1: a vent horizon like a rubber ball. The causal structure
• 1:30:01
  Speaker 1: can be far more complicated. Some models connect a collapsing
• 1:30:05
  Speaker 1: black hole interior to an expanding region. Others suggest transitions
• 1:30:09
  Speaker 1: that may resemble white hole-like behaviour. Still others produce finite
• 1:30:14
  Speaker 1: quantum cores or changes in the structure of spacetime without
• 1:30:17
  Speaker 1: a simple outward explosion. These are possibilities within theoretical models.
• 1:30:23
  Speaker 1: They are not observations. That distinction is especially important because
• 1:30:27
  Speaker 1: many of the most detailed calculations are performed under simplifying assumptions.
• 1:30:32
  Speaker 1: A model may assume perfect spherical symmetry. It may ignore rotation.
• 1:30:37
  Speaker 1: It may use an idealised form of matter. It may
• 1:30:40
  Speaker 1: reduce the full complexity of spacetime to a smaller set
• 1:30:44
  Speaker 1: of variables that can actually be handled mathematically. These simplified
• 1:30:49
  Speaker 1: systems are enormously useful. They allow physicists to investigate what
• 1:30:53
  Speaker 1: quantum geometry might do when classical gravity predicts a singularity.
• 1:30:58
  Speaker 1: But they are not the same as solving the complete
• 1:31:00
  Speaker 1: quantum dynamics of a realistic astrophysical black hole. A real
• 1:31:04
  Speaker 1: black hole probably rotates. It may interact with surrounding matter.
• 1:31:09
  Speaker 1: Radiation may be falling into it. Its interior may contain
• 1:31:13
  Speaker 1: the instabilities associated with an inner horizon. Its formation history
• 1:31:18
  Speaker 1: may matter. Describing all of this, while also treating spacetime
• 1:31:22
  Speaker 1: quantum mechanically, is vastly harder than analysing a perfectly symmetric model.
• 1:31:28
  Speaker 1: So when a paper shows that a singularity can be
• 1:31:30
  Speaker 1: removed in a particular loop quantum gravity-inspired calculation, the result
• 1:31:35
  Speaker 1: is interesting. But it does not mean the centre of
• 1:31:38
  Speaker 1: every real black hole has been solved. The broader lesson
• 1:31:42
  Speaker 1: is more modest and more useful. Quantum gravity may be
• 1:31:45
  Speaker 1: capable of changing the fate predicted by classical relativity. And
• 1:31:50
  Speaker 1: there are mathematically serious examples in which this actually happens.
• 1:31:55
  Speaker 1: The same possibility appears in other approaches too. Some theories
• 1:32:00
  Speaker 1: replace the singularity with a finite region of extremely high
• 1:32:03
  Speaker 1: but bounded curvature. Others propose that spacetime undergoes a phase transition.
• 1:32:10
  Speaker 1: Some suggest that the concepts of ordinary space and time
• 1:32:13
  Speaker 1: cease to be fundamental in the deepest regime and emerge
• 1:32:16
  Speaker 1: again only when conditions become less extreme. Different theories can
• 1:32:21
  Speaker 1: produce very different pictures. There is no requirement that every
• 1:32:24
  Speaker 1: successful theory of quantum gravity must contain a bounce. There
• 1:32:28
  Speaker 1: is no requirement that every theory must contain a tiny core.
• 1:32:32
  Speaker 1: And there is certainly no evidence that nature has chosen
• 1:32:35
  Speaker 1: whichever version happens to make the nicest illustration. Removing the
• 1:32:40
  Speaker 1: infinity is only the beginning. A successful theory has to
• 1:32:44
  Speaker 1: recover the parts of general relativity that already work. That
• 1:32:48
  Speaker 1: requirement is extremely demanding. Whatever replaces Einstein's classical description near
• 1:32:54
  Speaker 1: the singular regime must still reproduce familiar spacetime at larger scales.
• 1:33:00
  Speaker 1: It must explain why planets follow the orbits we observe.
• 1:33:03
  Speaker 1: It must agree with gravitational lensing. It must allow the
• 1:33:07
  Speaker 1: gravitational waves detected from merging black holes. It must produce
• 1:33:11
  Speaker 1: the successful exterior black hole geometry, wherever quantum effects are negligible.
• 1:33:17
  Speaker 1: A replacement theory cannot simply remove one unpleasant infinity while
• 1:33:22
  Speaker 1: breaking everything else. It must also behave consistently with quantum principles.
• 1:33:27
  Speaker 1: Probability must make sense. Quantum states must evolve according to
• 1:33:32
  Speaker 1: a coherent framework. The theory must explain what happens to
• 1:33:36
  Speaker 1: matter and information, and ideally it must produce consequences that
• 1:33:41
  Speaker 1: can eventually be tested. That final requirement may be the
• 1:33:45
  Speaker 1: most difficult. The most dramatic quantum gravity effects are expected
• 1:33:49
  Speaker 1: in regions that are extraordinarily small, extraordinarily energetic, or hidden
• 1:33:55
  Speaker 1: behind event horizons. Experiments cannot simply build a black hole
• 1:33:59
  Speaker 1: interior in a laboratory and place a detector beside the
• 1:34:03
  Speaker 1: interesting part. So physicists look for indirect signatures. They examine
• 1:34:08
  Speaker 1: gravitational waves. They study black hole thermodynamics. They investigate the
• 1:34:13
  Speaker 1: behaviour of the early universe. They search for tiny departures
• 1:34:17
  Speaker 1: from general relativity that might survive into observable conditions. So far,
• 1:34:22
  Speaker 1: no experiment has identified a particular singularity resolution model as
• 1:34:27
  Speaker 1: the correct description of nature. This leaves the centre of
• 1:34:31
  Speaker 1: the black hole in a fascinating state of uncertainty. Classical
• 1:34:35
  Speaker 1: general relativity predicts singular behaviour. Quantum gravity gives us reasons
• 1:34:40
  Speaker 1: to suspect that this may not be the end of
• 1:34:43
  Speaker 1: the story. Some calculations show that an infinity can be
• 1:34:47
  Speaker 1: replaced by finite quantum evolution, but no observation has yet
• 1:34:51
  Speaker 1: told us which, if any, of these models describes real
• 1:34:55
  Speaker 1: black holes. The singularity may therefore be less like an
• 1:34:59
  Speaker 1: object and more like a warning sign. It tells us
• 1:35:03
  Speaker 1: that the classical theory has reached a regime where another
• 1:35:05
  Speaker 1: description is required. What replaces it might be a finite
• 1:35:10
  Speaker 1: quantum region. It might be a transition into another phase
• 1:35:13
  Speaker 1: of spacetime. It might involve a bounce. It might involve
• 1:35:18
  Speaker 1: something that does not resemble any familiar geometric picture at all.
• 1:35:22
  Speaker 1: And there is another subtle possibility. Perhaps asking what is
• 1:35:27
  Speaker 1: physically sitting at the centre assumes too much. If ordinary
• 1:35:30
  Speaker 1: space-time emerges only approximately from a deeper quantum structure, then
• 1:35:35
  Speaker 1: concepts such as centre, distance, and even before and after
• 1:35:39
  Speaker 1: may lose their ordinary meaning at the most extreme scales.
• 1:35:43
  Speaker 1: The question may eventually have to be reformulated. For now,
• 1:35:48
  Speaker 1: the important conclusion is much simpler. General relativity's singularity is
• 1:35:53
  Speaker 1: a powerful classical prediction. It is not necessarily the final
• 1:35:57
  Speaker 1: physical object. Quantum gravity may remove it. Several serious theoretical
• 1:36:03
  Speaker 1: frameworks contain mechanisms capable of doing exactly that. But none
• 1:36:08
  Speaker 1: has yet been experimentally confirmed as the description of the
• 1:36:12
  Speaker 1: centre of an actual black hole. So the infinity may disappear.
• 1:36:16
  Speaker 1: The collapse may continue into a new form of quantum geometry.
• 1:36:20
  Speaker 1: The deepest interior may remain finite. All of these remain
• 1:36:24
  Speaker 1: genuine possibilities. What nature actually does is still unknown. Another
• 1:36:29
  Speaker 1: major attempt to unite gravity with quantum physics begins from
• 1:36:33
  Speaker 1: a very different idea. Instead of asking whether spacetime itself
• 1:36:38
  Speaker 1: is built from discrete quantum units, string theory changes the
• 1:36:42
  Speaker 1: way fundamental particles are described. In ordinary particle physics, an
• 1:36:47
  Speaker 1: electron is treated as point-like. So is a quark. So,
• 1:36:52
  Speaker 1: in the simplest mathematical description, are the other elementary particles.
• 1:36:56
  Speaker 1: String theory replaces that picture with extraordinarily small extended objects.
• 1:37:02
  Speaker 1: These objects are called strings. A string can vibrate in
• 1:37:06
  Speaker 1: different ways, and different patterns of vibration appear as different particles.
• 1:37:11
  Speaker 1: One mode may behave like a particle of matter, another
• 1:37:15
  Speaker 1: may behave like a particle associated with a force, and importantly,
• 1:37:19
  Speaker 1: the mathematics naturally contains a vibration with the properties expected
• 1:37:23
  Speaker 1: of a quantum particle of gravity. That is one reason
• 1:37:28
  Speaker 1: string theory became such an important candidate for quantum gravity.
• 1:37:32
  Speaker 1: Gravity is not simply bolted onto the theory afterwards. It
• 1:37:36
  Speaker 1: appears naturally within the framework. The strings themselves are thought
• 1:37:41
  Speaker 1: to be unimaginably small, with characteristic scales often associated with
• 1:37:46
  Speaker 1: a regime where quantum gravity becomes important. No experiment has
• 1:37:51
  Speaker 1: directly observed a fundamental string. String theory therefore remains a
• 1:37:56
  Speaker 1: theoretical framework rather than an experimentally confirmed description of nature.
• 1:38:01
  Speaker 1: But it has developed an enormous mathematical structure, and black
• 1:38:06
  Speaker 1: holes have become one of its most important testing grounds.
• 1:38:09
  Speaker 1: That makes sense. Black holes force several deep ideas together
• 1:38:14
  Speaker 1: in one place. They involve gravity, because they are regions
• 1:38:17
  Speaker 1: of extreme spacetime curvature. They involve quantum mechanics because quantum
• 1:38:22
  Speaker 1: fields behave non-trivially around their horizons. They involve thermodynamics because
• 1:38:28
  Speaker 1: black holes can be assigned temperamentally. and entropy. And they
• 1:38:31
  Speaker 1: involve information because quantum theory raises the difficult question of
• 1:38:36
  Speaker 1: what happens to information carried by matter that falls inside.
• 1:38:41
  Speaker 1: Very few systems place so many fundamental pieces of physics
• 1:38:45
  Speaker 1: on the same table at once. Black holes therefore became
• 1:38:48
  Speaker 1: a natural place to ask whether string theory could reproduce
• 1:38:52
  Speaker 1: features already discovered through other approaches. One particularly important feature
• 1:38:58
  Speaker 1: is black hole entropy. Jacob Bekenstein had argued that a
• 1:39:02
  Speaker 1: black hole should possess entropy related to the area of
• 1:39:05
  Speaker 1: its event horizon. Stephen Hawking later showed that black holes
• 1:39:09
  Speaker 1: should emit thermal radiation and possess a temperature. Together, these
• 1:39:13
  Speaker 1: results produced what is known as Bekenstein-Hawking entropy. The striking
• 1:39:19
  Speaker 1: feature is that the entropy is proportional to the area
• 1:39:22
  Speaker 1: of the event horizon, rather than the volume enclosed by it.
• 1:39:27
  Speaker 1: Entropy is usually associated with the number of microscopic arrangements
• 1:39:31
  Speaker 1: available to a physical system. A gas can have many
• 1:39:35
  Speaker 1: different microscopic configurations, even when its large-scale properties such as
• 1:39:40
  Speaker 1: temperature and pressure remain the same. So if a black
• 1:39:43
  Speaker 1: hole has entropy, an obvious question follows. What are its
• 1:39:47
  Speaker 1: microscopic states? In ordinary thermodynamics, entropy points toward hidden microscopic structure.
• 1:39:55
  Speaker 1: If black hole entropy behaves in a similar way, perhaps
• 1:39:58
  Speaker 1: a black hole also possesses an enormous number of underlying
• 1:40:02
  Speaker 1: quantum configurations. In 1996, Andrew Strominger and Kumran-Waffer produced a
• 1:40:09
  Speaker 1: celebrated result within string theory. They studied a special class
• 1:40:14
  Speaker 1: of theoretical black holes and counted the number of microscopic
• 1:40:17
  Speaker 1: quantum states available within the string theory description. The calculation
• 1:40:23
  Speaker 1: reproduced the same entropy expected from the Bekenstein-Hawking formula. This
• 1:40:28
  Speaker 1: was a major achievement. It showed, at least for certain
• 1:40:32
  Speaker 1: highly idealised black holes, that the mysterious thermodynamic entropy of
• 1:40:37
  Speaker 1: a black hole could emerge from identifiable underlying quantum microstates.
• 1:40:42
  Speaker 1: In other words, the entropy did not have to be
• 1:40:44
  Speaker 1: treated merely as a formal quantity attached to the horizon.
• 1:40:48
  Speaker 1: String theory could give a microscopic interpretation. That result did
• 1:40:53
  Speaker 1: not immediately reveal what happens inside an ordinary astrophysical black hole.
• 1:40:58
  Speaker 1: The black holes involved in the calculation possessed special properties
• 1:41:02
  Speaker 1: that made the mathematics manageable. But the calculation provided powerful
• 1:41:06
  Speaker 1: evidence that black holes may have a rich quantum structure
• 1:41:09
  Speaker 1: hidden beneath their classical appearance. This possibility becomes even more
• 1:41:14
  Speaker 1: dramatic in an idea known as the Fuzzball Proposal. The
• 1:41:18
  Speaker 1: proposal is associated especially with the physicist Samir Mathur. In
• 1:41:23
  Speaker 1: the simplest classical picture, a black hole has an event
• 1:41:27
  Speaker 1: horizon surrounding an interior that may remain relatively uneventful for
• 1:41:31
  Speaker 1: a falling observer until the deepest region is reached. Eventually,
• 1:41:36
  Speaker 1: the classical geometry leads toward a singularity. The fuzzball idea
• 1:41:41
  Speaker 1: suggests something radically different. In certain string theory constructions, the
• 1:41:46
  Speaker 1: quantum microstates corresponding to a black hole do not necessarily
• 1:41:50
  Speaker 1: look like one common smooth geometry, with an empty interior
• 1:41:54
  Speaker 1: hidden behind a featureless horizon. Instead, individual states can possess
• 1:41:59
  Speaker 1: complicated stringy and brain-like structure. The conventional interior may be
• 1:42:04
  Speaker 1: replaced or at least profoundly modified by quantum structure extending
• 1:42:09
  Speaker 1: across a region comparable to the scale of the would-be horizon.
• 1:42:13
  Speaker 1: The word fuzzball is meant to capture this idea of
• 1:42:16
  Speaker 1: a complicated extended object rather than a simple empty region
• 1:42:20
  Speaker 1: surrounding a central singularity. The details are highly mathematical. The
• 1:42:25
  Speaker 1: important physical point is much simpler. In this picture, the
• 1:42:30
  Speaker 1: classical black hole may be a coarse-grained description. From far away,
• 1:42:35
  Speaker 1: many different quantum microstates could look almost identical. They could
• 1:42:40
  Speaker 1: have the same mass, the same charge, the same angular momentum,
• 1:42:45
  Speaker 1: the same broad gravitational influence. But microscopically, their structures could
• 1:42:50
  Speaker 1: be different. This would be rather like looking at a
• 1:42:53
  Speaker 1: cloud from a great distance. At large scales, it appears smooth.
• 1:42:59
  Speaker 1: Up close, it contains extraordinary microscopic complexity. If something similar
• 1:43:05
  Speaker 1: happens with black holes, then the familiar smooth horizon and
• 1:43:09
  Speaker 1: interior geometry may emerge only as an approximate description. This
• 1:43:13
  Speaker 1: possibility has important consequences for the black hole information problem.
• 1:43:18
  Speaker 1: If the black hole possesses detailed quantum microstates, rather than
• 1:43:22
  Speaker 1: a completely featureless interior ending in a singularity, then information
• 1:43:27
  Speaker 1: might be stored and eventually returned in ways the simplest
• 1:43:31
  Speaker 1: semi-classical picture does not capture. But once again, caution is essential.
• 1:43:37
  Speaker 1: Fuzzballs are not an established description of every black hole
• 1:43:41
  Speaker 1: in the universe. String theory has produced explicit microstate geometries
• 1:43:46
  Speaker 1: and related constructions for certain special systems. These results are
• 1:43:52
  Speaker 1: mathematically important. They show that black hole-like states with rich
• 1:43:57
  Speaker 1: quantum structure can exist within the framework. but they do
• 1:44:01
  Speaker 1: not prove that the supermassive black hole at the centre
• 1:44:04
  Speaker 1: of the Milky Way is literally a fuzzball in the
• 1:44:06
  Speaker 1: straightforward sense. They do not establish that every black hole
• 1:44:10
  Speaker 1: formed from a collapsing star possesses the same kind of structure,
• 1:44:15
  Speaker 1: and they do not provide a universally accepted replacement for
• 1:44:18
  Speaker 1: the classical interior. This is one of the clearest examples
• 1:44:22
  Speaker 1: of why the central question remains unresolved. Different approaches to
• 1:44:27
  Speaker 1: quantum gravity can suggest very different pictures, In some loop-quantum
• 1:44:32
  Speaker 1: gravity-inspired models, classical spacetime remains a good approximation through much
• 1:44:37
  Speaker 1: of the black hole interior. Only when curvature becomes extremely
• 1:44:41
  Speaker 1: large does a compact quantum region replace the singularity. In
• 1:44:46
  Speaker 1: that sort of picture, the event horizon may look ordinary
• 1:44:49
  Speaker 1: to a falling observer, while the radical new physics remains
• 1:44:53
  Speaker 1: concentrated much deeper inside. The fuzzball proposal moves the change
• 1:44:58
  Speaker 1: much farther outward. Quantum structure may extend over a region
• 1:45:03
  Speaker 1: comparable to the horizon itself. So one family of ideas
• 1:45:07
  Speaker 1: says that the familiar interior mostly survives until extremely small
• 1:45:11
  Speaker 1: scales are reached. Another suggests that the classical interior may
• 1:45:15
  Speaker 1: never exist in quite the way Einstein's equations imply. Both
• 1:45:19
  Speaker 1: possibilities arise from serious attempts to combine gravity and quantum physics.
• 1:45:25
  Speaker 1: Neither has been experimentally confirmed as the correct description of
• 1:45:28
  Speaker 1: astrophysical black holes. That disagreement is not a weakness of
• 1:45:33
  Speaker 1: the subject. It is evidence of how difficult the problem
• 1:45:36
  Speaker 1: really is. Physicists are trying to infer the microscopic structure
• 1:45:41
  Speaker 1: of a region they cannot directly observe using theories that
• 1:45:45
  Speaker 1: operate far beyond the energies accessible to present experiments. Different
• 1:45:50
  Speaker 1: mathematical frameworks therefore explore different ways in which the classical
• 1:45:54
  Speaker 1: singularity might be avoided. String theory asks whether the apparent
• 1:45:59
  Speaker 1: simplicity of a black hole is hiding an enormous number
• 1:46:03
  Speaker 1: of quantum states. Loop-based approaches ask whether geometry itself becomes quantized.
• 1:46:10
  Speaker 1: Other ideas modify the central region in still different ways.
• 1:46:14
  Speaker 1: The fact that these pictures differ so dramatically is precisely
• 1:46:17
  Speaker 1: why no honest description of the centre can currently be
• 1:46:20
  Speaker 1: reduced to one confident sentence. The classical singularity forces the question.
• 1:46:27
  Speaker 1: Something in the ordinary description stops being adequate. String theory
• 1:46:32
  Speaker 1: offers one possible response by replacing point-like fundamental ingredients with
• 1:46:37
  Speaker 1: extended quantum objects and giving black holes microscopic states that
• 1:46:42
  Speaker 1: classical general relativity cannot see. Some versions go further, suggesting
• 1:46:48
  Speaker 1: that the familiar interior itself may be replaced by intricate
• 1:46:51
  Speaker 1: quantum structure. Whether nature actually works this way remains unknown.
• 1:46:58
  Speaker 1: But the possibility reveals something important. The centre of a
• 1:47:02
  Speaker 1: black hole may not simply contain an exotic object waiting
• 1:47:05
  Speaker 1: to be discovered. It may require us to rethink what
• 1:47:09
  Speaker 1: we mean by space-time, matter, horizon and interior in the
• 1:47:13
  Speaker 1: first place. Once the classical singularity is treated as a
• 1:47:18
  Speaker 1: sign that something deeper may be required, an interesting possibility
• 1:47:22
  Speaker 1: opens up. Perhaps there is not one single way for
• 1:47:26
  Speaker 1: nature to avoid infinity. Loop quantum gravity offers one family
• 1:47:30
  Speaker 1: of ideas, string theory offers another, but neither approach has
• 1:47:35
  Speaker 1: a monopoly on imagination, and physicists have explored many other
• 1:47:39
  Speaker 1: possibilities in which the centre of a black hole remains
• 1:47:42
  Speaker 1: extreme without becoming literally singular. One influential example dates back
• 1:47:49
  Speaker 1: to 1968. The physicist James Bardeen introduced a mathematical black
• 1:47:53
  Speaker 1: hole model. looked broadly like an ordinary black hole from
• 1:47:57
  Speaker 1: far away, but avoided the usual central curvature singularity. Today,
• 1:48:03
  Speaker 1: models of this general type are often called regular black holes.
• 1:48:07
  Speaker 1: The word regular does not mean ordinary. A regular black
• 1:48:11
  Speaker 1: hole would still possess enormously strong gravity, an event horizon,
• 1:48:16
  Speaker 1: under suitable conditions, and a highly unusual interior, Regular simply
• 1:48:21
  Speaker 1: means that the spacetime avoids the divergence and curvature associated
• 1:48:25
  Speaker 1: with the classical singularity. Instead of the geometry becoming infinite
• 1:48:30
  Speaker 1: at the centre, it remains finite. This can be achieved
• 1:48:34
  Speaker 1: in different ways depending on the model. Some regular black
• 1:48:38
  Speaker 1: holes contain a central region where the effective behaviour of
• 1:48:42
  Speaker 1: matter or space-time differs radically from that described by ordinary
• 1:48:46
  Speaker 1: classical gravity. Others introduce corrections that become important only when
• 1:48:51
  Speaker 1: curvature is extremely large. Far from the centre, those corrections
• 1:48:56
  Speaker 1: become negligible. The black hole then approaches the familiar behaviour
• 1:49:01
  Speaker 1: predicted by general relativity. That last point is essential. Any
• 1:49:06
  Speaker 1: proposed modification of a black hole interior has to explain
• 1:49:09
  Speaker 1: why Einstein's theory works so well everywhere that physicists have
• 1:49:14
  Speaker 1: actually tested it. If an alternative theory dramatically changed the
• 1:49:19
  Speaker 1: orbit of a planet or the bending of light around
• 1:49:22
  Speaker 1: an ordinary star, it would already be in trouble. The
• 1:49:25
  Speaker 1: new behaviour must usually hide itself until conditions become extreme.
• 1:49:30
  Speaker 1: A regular black hole attempts to do exactly that. From
• 1:49:34
  Speaker 1: a safe distance, it can look remarkably conventional. Deep inside, however,
• 1:49:39
  Speaker 1: the singularity is replaced by a finite high curvature region.
• 1:49:44
  Speaker 1: The difficulty is explaining what physical mechanism produces that regular interior.
• 1:49:50
  Speaker 1: A mathematical model can be constructed without necessarily telling us
• 1:49:54
  Speaker 1: what fundamental theory of nature generates it. James Bardeen's example, therefore,
• 1:49:59
  Speaker 1: became important. partly because it demonstrated that a black hole-like
• 1:50:03
  Speaker 1: spacetime without a conventional singularity was mathematically possible. It did
• 1:50:08
  Speaker 1: not establish that astrophysical black holes actually possess that structure.
• 1:50:14
  Speaker 1: Another proposal moves even farther away from the usual picture.
• 1:50:18
  Speaker 1: In the early 21st century, Powell, Mazur and Emil Motula
• 1:50:22
  Speaker 1: explored an object known as a gravistar. The name is
• 1:50:26
  Speaker 1: short for gravitational vacuum star. A gravistar is not simply
• 1:50:31
  Speaker 1: a black hole whose centre has been repaired. It is
• 1:50:34
  Speaker 1: an alternative compact object model. In the simplest version, collapse
• 1:50:38
  Speaker 1: does not produce the familiar black hole interior ending in
• 1:50:42
  Speaker 1: a singularity. Instead, the object contains an unusual vacuum-like interior
• 1:50:48
  Speaker 1: surrounded by a thin layer of matter. The internal pressure
• 1:50:53
  Speaker 1: associated with that vacuum state acts very differently from ordinary
• 1:50:57
  Speaker 1: material pressure. From sufficiently far away, a gravistar could in
• 1:51:02
  Speaker 1: principle resemble a black hole quite closely. Its gravitational field
• 1:51:06
  Speaker 1: could be extremely strong. Its surface could lie very near
• 1:51:10
  Speaker 1: the location where a black hole event horizon would ordinarily form.
• 1:51:15
  Speaker 1: But the internal structure would be completely different. There would
• 1:51:19
  Speaker 1: be no conventional singularity hidden at the centre. And depending
• 1:51:23
  Speaker 1: on the precise model, there might not be a true
• 1:51:26
  Speaker 1: event horizon in exactly the usual sense either. Gravastars remain speculative.
• 1:51:33
  Speaker 1: No observation has established that the compact objects astronomers identify
• 1:51:38
  Speaker 1: as black holes are actually gravastars. The models are valuable
• 1:51:42
  Speaker 1: because they provide a concrete example of just how differently
• 1:51:46
  Speaker 1: gravitational collapse might behave if new physics intervened before the
• 1:51:50
  Speaker 1: classical singularity formed. A very different possibility comes from work
• 1:51:56
  Speaker 1: by Carlo Rovelli and Francesca Vidotto. They introduced the idea
• 1:52:00
  Speaker 1: of a Planck star within a quantum gravity-inspired framework. The
• 1:52:05
  Speaker 1: basic suggestion is that collapse might eventually encounter a regime
• 1:52:10
  Speaker 1: where quantum effects of gravity become strong enough to oppose
• 1:52:13
  Speaker 1: further compression. At ordinary densities, gravity dominates and collapse continues.
• 1:52:21
  Speaker 1: But as matter approaches an extraordinarily extreme density associated with
• 1:52:25
  Speaker 1: the Planck scale, the classical description may no longer apply.
• 1:52:30
  Speaker 1: Quantum gravitational effects could then become effectively repulsive or otherwise
• 1:52:35
  Speaker 1: prevent the unlimited collapse predicted by general relativity. The result
• 1:52:40
  Speaker 1: would not be an ordinary star. The densities involved would
• 1:52:43
  Speaker 1: be far beyond anything familiar. But the central region could
• 1:52:48
  Speaker 1: remain finite rather than collapsing into a classical singularity. Some
• 1:52:54
  Speaker 1: versions of the idea involve a transition between black-hole-like and
• 1:52:58
  Speaker 1: white-hole-like behaviour. A white hole is essentially the time-reverse counterpart
• 1:53:03
  Speaker 1: of a black hole in certain solutions of general relativity.
• 1:53:08
  Speaker 1: Matter can emerge from it but cannot enter from outside.
• 1:53:12
  Speaker 1: Whether anything resembling such a transition actually occurs in nature
• 1:53:17
  Speaker 1: remains completely unsettled. The Planck star proposal is therefore an
• 1:53:21
  Speaker 1: intriguing theoretical possibility, not an established description of what happens
• 1:53:26
  Speaker 1: inside real black holes. The wider landscape is even larger.
• 1:53:31
  Speaker 1: Some researchers investigate non-singular cores in which spacetime curvature reaches
• 1:53:36
  Speaker 1: a maximum value rather than diverging. Others study phase transitions
• 1:53:41
  Speaker 1: in the geometry itself. In these scenarios, spacetime inside the
• 1:53:46
  Speaker 1: deepest region of a black hole might enter a new
• 1:53:49
  Speaker 1: state in much the same way that matter can change
• 1:53:51
  Speaker 1: phase from liquid to solid. The comparison is only an analogy,
• 1:53:56
  Speaker 1: but it captures an important possibility. The laws governing spacetime
• 1:54:01
  Speaker 1: under ordinary conditions may not describe its behaviour under the
• 1:54:04
  Speaker 1: most extreme conditions imaginable. Other models introduce modified gravity. These
• 1:54:11
  Speaker 1: theories alter Einstein's equations in ways designed to become noticeable
• 1:54:15
  Speaker 1: only at very high curvature. Some produce effective repulsive behaviour
• 1:54:20
  Speaker 1: in the deepest interior. Others prevent physical quantities from exceeding
• 1:54:25
  Speaker 1: certain bounds. Still others replace the central singularity with a
• 1:54:30
  Speaker 1: region that connects to a different spacetime geometry. The sheer
• 1:54:34
  Speaker 1: number of possibilities can initially sound like physicists are simply
• 1:54:38
  Speaker 1: making increasingly elaborate guesses. But there is a reason the
• 1:54:42
  Speaker 1: landscape is so broad. The classical theory presents a genuine problem.
• 1:54:47
  Speaker 1: General relativity predicts singular behaviour, while most physicists expect that
• 1:54:52
  Speaker 1: a complete quantum description should modify the situation before literal
• 1:54:56
  Speaker 1: infinities become physically meaningful. What remains uncertain is how. A
• 1:55:04
  Speaker 1: finite core is possible. A bounce is possible in some models.
• 1:55:09
  Speaker 1: A phase transition is possible. An exotic vacuum interior is possible.
• 1:55:14
  Speaker 1: A string theory microstate description is possible. a discrete quantum
• 1:55:19
  Speaker 1: geometry is possible, and nature may be doing something that
• 1:55:22
  Speaker 1: does not fit neatly into any of these pictures. Distinguishing
• 1:55:26
  Speaker 1: between them is extremely difficult. Suppose two models produce almost
• 1:55:31
  Speaker 1: identical gravitational fields outside the event horizon. One contains a
• 1:55:36
  Speaker 1: classical singularity, another contains a finite quantum core. To an
• 1:55:42
  Speaker 1: astronomer watching from far away, The two black holes might
• 1:55:46
  Speaker 1: appear virtually indistinguishable. The most dramatic differences could be hidden
• 1:55:50
  Speaker 1: entirely inside the horizon, where ordinary signals cannot return to
• 1:55:55
  Speaker 1: report what has happened. That creates an unusual observational problem.
• 1:56:00
  Speaker 1: Science normally advances by comparing competing explanations with measurements. Black
• 1:56:06
  Speaker 1: hole interiors make that process awkward, because the region containing
• 1:56:10
  Speaker 1: the clearest differences may be causally disconnected from outside observers.
• 1:56:15
  Speaker 1: Researchers therefore search for indirect consequences. Perhaps different interiors subtly
• 1:56:20
  Speaker 1: alter gravitational waves. Perhaps they modify the way a black
• 1:56:24
  Speaker 1: hole evaporates. Perhaps quantum effects near the horizon leave some trace.
• 1:56:29
  Speaker 1: Perhaps mergers produce signatures that classical black holes cannot reproduce.
• 1:56:35
  Speaker 1: So far, however, no such observation has produced an experimentally
• 1:56:39
  Speaker 1: confirmed picture of the central region. And that is precisely
• 1:56:43
  Speaker 1: why there are so many serious proposals. The variety is
• 1:56:47
  Speaker 1: not evidence that physicists know what is at the centre.
• 1:56:51
  Speaker 1: It is evidence that they do not. What they do
• 1:56:54
  Speaker 1: know is that the classical singularity creates a fundamental challenge.
• 1:56:59
  Speaker 1: They know that general relativity can describe the exterior of
• 1:57:02
  Speaker 1: black holes extraordinarily well. They know that singular behaviour appears
• 1:57:07
  Speaker 1: robustly when the classical equations are continued inward. and they
• 1:57:11
  Speaker 1: know that several different theoretical frameworks can replace that singularity
• 1:57:16
  Speaker 1: with something finite. What remains missing is nature's verdict. The
• 1:57:21
  Speaker 1: centre might contain a tiny high curvature core. It might
• 1:57:26
  Speaker 1: undergo a quantum transition. It might resemble nothing that deserves
• 1:57:30
  Speaker 1: to be called a centre in the ordinary sense. For now,
• 1:57:35
  Speaker 1: these possibilities remain open. The infinity predicted by classical relativity
• 1:57:40
  Speaker 1: may not survive in the final theory, but if it disappears,
• 1:57:44
  Speaker 1: physicists still do not know what takes its place. There
• 1:57:48
  Speaker 1: is another problem hidden inside black holes that is closely
• 1:57:52
  Speaker 1: connected to the mystery of their centres. It begins with
• 1:57:56
  Speaker 1: a surprisingly simple question. What happens to information when something
• 1:58:01
  Speaker 1: falls in? In everyday language, information might mean the words
• 1:58:06
  Speaker 1: in a book, or the files stored on a computer.
• 1:58:10
  Speaker 1: In physics, the idea is broader. The complete quantum state
• 1:58:14
  Speaker 1: of a physical system contains information about how that system
• 1:58:18
  Speaker 1: is arranged. If a collection of particles forms a star,
• 1:58:22
  Speaker 1: then in principle its quantum state contains enormously detailed information
• 1:58:27
  Speaker 1: about those particles. their relationships, their energies, and the way
• 1:58:31
  Speaker 1: the system developed. When that star collapses into a black hole,
• 1:58:35
  Speaker 1: the material does not simply stop having a history. The
• 1:58:40
  Speaker 1: difficult question is where that information ultimately goes. This problem
• 1:58:44
  Speaker 1: became much sharper through the work of Jacob Bekenstein. In
• 1:58:48
  Speaker 1: the early 1970s, Bekenstein proposed that black holes should possess entropy.
• 1:58:53
  Speaker 1: Entropy is often associated with disorder. but more precisely it
• 1:58:57
  Speaker 1: is connected with the number of microscopic states that can
• 1:59:00
  Speaker 1: correspond to the same large-scale appearance. A black hole looks
• 1:59:05
  Speaker 1: remarkably simple from the outside. Its external gravitational behaviour can
• 1:59:10
  Speaker 1: be described largely by quantities such as mass, electric charge
• 1:59:14
  Speaker 1: and angular momentum. Yet Bekenstein argued that a black hole
• 1:59:18
  Speaker 1: must somehow account for the enormous amount of information carried
• 1:59:22
  Speaker 1: by everything that formed it, and later fell inside. His
• 1:59:27
  Speaker 1: remarkable conclusion was that black hole entropy should be proportional
• 1:59:30
  Speaker 1: to the area of the event horizon, not the volume
• 1:59:34
  Speaker 1: enclosed by the horizon, the area. That result was strange
• 1:59:38
  Speaker 1: enough on its own. Then Stephen Hawking made the story
• 1:59:42
  Speaker 1: even stranger. In the 1970s, Hawking studied quantum fields around
• 1:59:48
  Speaker 1: black holes and discovered that black holes should not be
• 1:59:51
  Speaker 1: completely black. Quantum effects cause them to emit thermal radiations.
• 1:59:56
  Speaker 1: This is now known as Hawking radiation. The effect is
• 2:00:00
  Speaker 1: often explained using a little story in which pairs of
• 2:00:03
  Speaker 1: virtual particles appear near the event horizon. One falls into
• 2:00:07
  Speaker 1: the black hole and the other escapes. That picture can
• 2:00:10
  Speaker 1: be useful as a rough piece of intuition, but it
• 2:00:13
  Speaker 1: should not be mistaken for the actual derivation. Hawking radiation
• 2:00:18
  Speaker 1: arises from the behaviour of quantum fields in curved spacetime.
• 2:00:22
  Speaker 1: The meaning of a particle depends partly on how quantum
• 2:00:25
  Speaker 1: field modes are defined by observers in different regions of
• 2:00:28
  Speaker 1: that spacetime. When a black hole forms, the relationship between
• 2:00:33
  Speaker 1: those field modes leads a distant observer to detect radiation
• 2:00:36
  Speaker 1: with a thermal spectrum. The important consequence is that a
• 2:00:40
  Speaker 1: black hole can slowly lose energy. As it radiates, its
• 2:00:44
  Speaker 1: mass decreases. Given enough time, an isolated black hole could
• 2:00:49
  Speaker 1: in principle evaporate, For an astrophysical black hole, this process
• 2:00:53
  Speaker 1: is fantastically slow. A black hole with roughly the mass
• 2:00:57
  Speaker 1: of the Sun would take vastly longer than the present
• 2:00:59
  Speaker 1: age of the universe to evaporate. But the timescale is
• 2:01:03
  Speaker 1: not the deepest problem. The nature of the radiation is.
• 2:01:08
  Speaker 1: In Hawking's original semi-classical calculation, the outgoing radiation is thermal.
• 2:01:13
  Speaker 1: Thermal radiation appears not to contain the detailed information required
• 2:01:17
  Speaker 1: to reconstruct everything that entered the black hole. Imagine forming
• 2:01:22
  Speaker 1: a black hole from one particular arrangement of matter. Then
• 2:01:26
  Speaker 1: imagine forming another black hole with the same final mass
• 2:01:29
  Speaker 1: and other large-scale properties, but from a completely different microscopic arrangement.
• 2:01:35
  Speaker 1: If both eventually evaporate into exactly the same kind of
• 2:01:39
  Speaker 1: thermal radiation, then the detailed distinction between those original quantum
• 2:01:44
  Speaker 1: states seems to disappear. That creates the black hole information paradox.
• 2:01:49
  Speaker 1: Quantum theory normally obeys a principle called unitarity. In broad terms,
• 2:01:55
  Speaker 1: unitarity means that quantum evolution preserves the information contained in
• 2:01:59
  Speaker 1: the quantum state. The state may become extraordinarily complicated. Information
• 2:02:05
  Speaker 1: can become scrambled so thoroughly that recovering it is effectively impossible,
• 2:02:10
  Speaker 1: but it is not supposed to be fundamentally destroyed. Burn
• 2:02:14
  Speaker 1: a book and the words are no longer conveniently readable.
• 2:02:18
  Speaker 1: Yet in principle, the resulting smoke, heat, ash and radiation,
• 2:02:23
  Speaker 1: still depend on the precise physical state of the original book.
• 2:02:27
  Speaker 1: The information has been dispersed into a horrendously complicated form.
• 2:02:32
  Speaker 1: It has not simply vanished from the laws of physics.
• 2:02:35
  Speaker 1: A black hole appears to threaten something more serious. If
• 2:02:39
  Speaker 1: matter carries information through the horizon, and the black hole
• 2:02:42
  Speaker 1: later disappears while emitting purely thermal radiation, then the original
• 2:02:47
  Speaker 1: quantum information seems to have been erased from the universe.
• 2:02:51
  Speaker 1: Stephen Hawking initially argued that genuine information loss could occur.
• 2:02:56
  Speaker 1: Many other physicists found that conclusion deeply uncomfortable because it
• 2:03:00
  Speaker 1: would require a fundamental change to ordinary quantum mechanics. This
• 2:03:05
  Speaker 1: is where the problem of the black hole interior becomes
• 2:03:07
  Speaker 1: difficult to separate from the problem of information. Perhaps the
• 2:03:11
  Speaker 1: information somehow remains encoded inside. But if the black hole
• 2:03:15
  Speaker 1: completely evaporates, where does that interior eventually go? Perhaps the
• 2:03:20
  Speaker 1: information emerges in the radiation. If so, the radiation cannot
• 2:03:25
  Speaker 1: be exactly thermal in the simplest sense. It must contain
• 2:03:29
  Speaker 1: subtle correlations capable of carrying information outward. This expectation leads
• 2:03:35
  Speaker 1: to an important idea known as the Page Curve. The
• 2:03:38
  Speaker 1: concept is associated with the physicist Don Page. Imagine tracking
• 2:03:43
  Speaker 1: the quantum entropy of the Hawking radiation as a black
• 2:03:47
  Speaker 1: hole slowly evaporates. If information is preserved, the entropy should
• 2:03:52
  Speaker 1: initially rise as more radiation becomes entangled with the remaining
• 2:03:56
  Speaker 1: black hole. Eventually, however, the trend should change. After roughly
• 2:04:01
  Speaker 1: the point where the black hole has radiated a substantial
• 2:04:04
  Speaker 1: fraction of its original entropy … Later radiation must begin
• 2:04:08
  Speaker 1: carrying correlations that allow the overall quantum state to remain
• 2:04:12
  Speaker 1: consistent with unitarity. The entropy of the radiation should then
• 2:04:17
  Speaker 1: decline as evaporation approaches completion. The resulting rise and fall
• 2:04:21
  Speaker 1: is the Page curve. For many years, reproducing this behaviour
• 2:04:26
  Speaker 1: directly from gravitational calculations was extremely difficult. More recently, theoretical
• 2:04:32
  Speaker 1: work involving quantum extremal surfaces Gravitational path integrals in regions
• 2:04:38
  Speaker 1: known as islands has produced page curve behaviour in important
• 2:04:42
  Speaker 1: simplified gravitational models. These calculations have attracted enormous attention because
• 2:04:48
  Speaker 1: they suggest that gravity may be able to preserve information
• 2:04:51
  Speaker 1: after all. They also hint at something deeply surprising about
• 2:04:55
  Speaker 1: how the black hole interior is encoded. In some of
• 2:04:59
  Speaker 1: these calculations, a portion of what would ordinarily be regarded
• 2:05:03
  Speaker 1: as the black hole interior becomes included in the quantum
• 2:05:06
  Speaker 1: description associated with the outgoing radiation. This does not mean
• 2:05:11
  Speaker 1: that a little piece of the interior physically flies through
• 2:05:14
  Speaker 1: the event horizon. Rather, it suggests that the division between
• 2:05:18
  Speaker 1: inside information and outside information may be subtler in quantum
• 2:05:22
  Speaker 1: gravity than classical spacetime makes it appear. Another important clue
• 2:05:26
  Speaker 1: comes from the holographic principle. The holographic principle suggests that
• 2:05:31
  Speaker 1: the information describing a region of space containing gravity may
• 2:05:35
  Speaker 1: sometimes be represented by degrees of freedom associated with a
• 2:05:39
  Speaker 1: lower dimensional boundary. One of the most powerful examples was
• 2:05:44
  Speaker 1: introduced by Juan Maldacena in 1997. Maldacena proposed a correspondence
• 2:05:50
  Speaker 1: between certain gravitational theories in a spacetime known as anti-de
• 2:05:54
  Speaker 1: Sitter space and certain quantum field theories defined on its boundary.
• 2:06:00
  Speaker 1: This is known as the anti-de Sitter and conformal field
• 2:06:03
  Speaker 1: theory correspondence. In suitable cases, the gravitational system and the
• 2:06:07
  Speaker 1: boundary quantum theory provide two different mathematical descriptions of the
• 2:06:12
  Speaker 1: same underlying physics. The quantum theory is unitary. That strongly
• 2:06:17
  Speaker 1: suggests that black holes within such descriptions should not fundamentally
• 2:06:21
  Speaker 1: destroy information. This is an extraordinary result, but it does
• 2:06:26
  Speaker 1: not mean the information paradox has been completely solved for
• 2:06:30
  Speaker 1: every black hole in our universe. anti-de-sitter space-time used in
• 2:06:37
  Speaker 1: the best understood versions of the correspondence. The microscopic details
• 2:06:42
  Speaker 1: of realistic rotating astrophysical black holes remain much harder, and
• 2:06:46
  Speaker 1: knowing that information is preserved in principle is not the
• 2:06:50
  Speaker 1: same as possessing a complete account of what an infalling
• 2:06:53
  Speaker 1: observer experiences deep inside. This distinction is important because the
• 2:06:59
  Speaker 1: information paradox and the singularity problem are related without being identical.
• 2:07:05
  Speaker 1: The singularity problem asks what replaces the failure of classical
• 2:07:09
  Speaker 1: spacetime in the deepest interior. The information problem asks how
• 2:07:14
  Speaker 1: quantum information behaves when black holes form, radiate, and perhaps
• 2:07:20
  Speaker 1: eventually disappear. A successful theory of quantum gravity may have
• 2:07:25
  Speaker 1: to answer both, but solving one does not automatically provide
• 2:07:29
  Speaker 1: the complete physical picture of the other. information might be
• 2:07:33
  Speaker 1: preserved without physicists yet knowing the detailed fate of matter
• 2:07:37
  Speaker 1: near the central high curvature region. A theory might remove
• 2:07:41
  Speaker 1: the singularity while still leaving difficult questions about how information
• 2:07:44
  Speaker 1: returns to the outside universe. The two problems constrain each other,
• 2:07:49
  Speaker 1: but they do not collapse into one question. This is
• 2:07:52
  Speaker 1: why the interior of a black hole remains such a
• 2:07:55
  Speaker 1: powerful testing ground for fundamental physics. Gravity says that an
• 2:07:59
  Speaker 1: event horizon divides the causal structure of spacetime. Quantum theory
• 2:08:04
  Speaker 1: says information should not simply disappear. Thermodynamics says black holes
• 2:08:09
  Speaker 1: possess entropy and temperature. Hawking radiation says black holes can
• 2:08:13
  Speaker 1: lose mass. And the singularity tells us that classical spacetime
• 2:08:18
  Speaker 1: eventually stops being enough. Somewhere within this collection of ideas
• 2:08:22
  Speaker 1: lies the deeper description. Modern calculations have provided increasingly strong
• 2:08:27
  Speaker 1: theoretical reasons to think that quantum information may ultimately be preserved.
• 2:08:33
  Speaker 1: But they have not supplied astronomers with a confirmed picture
• 2:08:36
  Speaker 1: of what physically occupies the deepest region of a real
• 2:08:39
  Speaker 1: black hole. The information may survive, the singularity may not.
• 2:08:45
  Speaker 1: And yet the actual interior remains hidden. leaving one of
• 2:08:48
  Speaker 1: the strangest possibilities in physics still open. The black hole
• 2:08:53
  Speaker 1: may preserve everything that falls into it while forcing us
• 2:08:56
  Speaker 1: to abandon our familiar ideas about where that information is
• 2:08:59
  Speaker 1: located at all. There is a rather frustrating problem with
• 2:09:03
  Speaker 1: trying to find out what happens inside a black hole.
• 2:09:06
  Speaker 1: The most obvious experiment is impossible. We cannot simply send
• 2:09:10
  Speaker 1: something across the event horizon, ask it to have a
• 2:09:13
  Speaker 1: look around and wait for the results. Once a signal
• 2:09:17
  Speaker 1: originates inside an event horizon, there is no future-directed path
• 2:09:21
  Speaker 1: that allows that signal to return to the outside universe.
• 2:09:24
  Speaker 1: Light cannot escape. Radio waves cannot escape. A probe cannot
• 2:09:29
  Speaker 1: transmit photographs home. This is not a limitation of telescope technology.
• 2:09:34
  Speaker 1: Building a larger telescope does not solve it. The event
• 2:09:37
  Speaker 1: horizon is a causal boundary built into the geometry of spacetime.
• 2:09:42
  Speaker 1: So astronomers have developed increasingly sophisticated ways of examining everything
• 2:09:47
  Speaker 1: that happens around black holes instead. And those observations have
• 2:09:51
  Speaker 1: become astonishingly precise. One of the most famous examples comes
• 2:09:55
  Speaker 1: from the Event Horizon Telescope collaboration. The Event Horizon Telescope
• 2:10:01
  Speaker 1: is not one telescope in the ordinary sense. It links
• 2:10:04
  Speaker 1: radio observatories distributed across the Earth, allowing them to work
• 2:10:09
  Speaker 1: together as though they formed parts of a telescope. almost
• 2:10:12
  Speaker 1: as wide as the planet. This technique is called very
• 2:10:16
  Speaker 1: long baseline interferometry. In 2019, the collaboration released the first
• 2:10:22
  Speaker 1: horizon-scale image of the black hole at the centre of
• 2:10:26
  Speaker 1: the giant elliptical galaxy Messier 87. The black hole is
• 2:10:30
  Speaker 1: commonly known as Messier 87 star. Its mass is roughly 6.5
• 2:10:35
  Speaker 1: billion times that of the Sun. The image showed a
• 2:10:38
  Speaker 1: bright ring surrounding a dark central region. A few years later,
• 2:10:43
  Speaker 1: the collaboration released an image of Sagittarius Astar, the supermassive
• 2:10:47
  Speaker 1: black hole at the centre of our own Milky Way.
• 2:10:50
  Speaker 1: Sagittarius Astar is much less massive, at around 4 million
• 2:10:54
  Speaker 1: times the mass of the Sun, and lies roughly 26,000
• 2:10:58
  Speaker 1: light-years from Earth. These images are often described as photographs
• 2:11:02
  Speaker 1: of black holes. That is reasonable in everyday conversation, but
• 2:11:06
  Speaker 1: physically something subtler is happening. The telescope is detecting radio
• 2:11:11
  Speaker 1: emission from hot material around the black hole. The dark
• 2:11:15
  Speaker 1: central region is the black hole shadow. It is not
• 2:11:17
  Speaker 1: a photograph of the singularity. It is not a photograph
• 2:11:21
  Speaker 1: of matter sitting inside the event horizon. And the edge
• 2:11:24
  Speaker 1: of the shadow is not simply the event horizon drawn
• 2:11:27
  Speaker 1: as a black circle. The shadow is produced by the
• 2:11:30
  Speaker 1: way strongly curved space-time redirects and captures light near the
• 2:11:34
  Speaker 1: black hole. Some radiation manages to escape toward us, Other
• 2:11:39
  Speaker 1: radiation follows paths that carry it into the black hole.
• 2:11:42
  Speaker 1: The result is a dark, apparent region whose size is
• 2:11:45
  Speaker 1: influenced by the black hole's gravity, and is generally larger
• 2:11:49
  Speaker 1: than the event horizon itself. This means the Event Horizon
• 2:11:53
  Speaker 1: Telescope can test the geometry immediately around a black hole
• 2:11:57
  Speaker 1: without seeing the interior directly. The measured sizes and structures
• 2:12:01
  Speaker 1: of the shadows have been broadly consistent, with expectations from
• 2:12:05
  Speaker 1: general relativity. and with the space-time expected around rotating black holes.
• 2:12:11
  Speaker 1: That is a remarkable achievement. Human beings have effectively measured
• 2:12:15
  Speaker 1: the behaviour of light on scales comparable to the horizon
• 2:12:19
  Speaker 1: of objects millions or billions of times more massive than
• 2:12:22
  Speaker 1: the Sun. But even this does not show what happens
• 2:12:25
  Speaker 1: at the centre. A completely different observational tool comes from
• 2:12:29
  Speaker 1: gravitational waves. When black holes orbit one another, they disturb space-time.
• 2:12:35
  Speaker 1: As their orbit shrinks, they radiate energy through ripples in
• 2:12:38
  Speaker 1: the geometry known as gravitational waves. Eventually they collide and merge.
• 2:12:44
  Speaker 1: The Laser Interferometer Gravitational Wave Observatory made the first direct
• 2:12:50
  Speaker 1: detection of gravitational waves from a black hole merger in 2015.
• 2:12:55
  Speaker 1: Since then, the Laser Interferometer, Gravitational Wave Observatory, Virgo and
• 2:13:00
  Speaker 1: CAGRA collaborations have observed many compact object merger signals. These
• 2:13:05
  Speaker 1: events allow physicists to test gravity under conditions far more
• 2:13:09
  Speaker 1: extreme than those available within the solar system. A typical
• 2:13:13
  Speaker 1: black hole merger can be divided conceptually into several stages.
• 2:13:17
  Speaker 1: During the inspiral, the two black holes orbit one another
• 2:13:20
  Speaker 1: while gradually moving closer together. Their gravitational wave signal rises
• 2:13:25
  Speaker 1: in frequency and strength. Near the merger itself, spacetime becomes
• 2:13:29
  Speaker 1: highly dynamical as two horizons combine into one distorted remnant.
• 2:13:35
  Speaker 1: Then comes the ringdown. The newly formed black hole is
• 2:13:38
  Speaker 1: initially deformed. It settles toward a stable rotating state by
• 2:13:42
  Speaker 1: emitting gravitational waves. The oscillations involved are described using characteristic
• 2:13:48
  Speaker 1: patterns called quasi-normal modes. The name is slightly less alarming
• 2:13:52
  Speaker 1: than it sounds. They are essentially the natural ringing frequencies
• 2:13:56
  Speaker 1: of a disturbed black hole. A bell has characteristic tones
• 2:14:00
  Speaker 1: determined by its structure. A black hole has characteristic gravitational
• 2:14:05
  Speaker 1: oscillations determined primarily by properties such as its mass and spin.
• 2:14:10
  Speaker 1: By measuring ring-down signals, physicists can test whether the final
• 2:14:14
  Speaker 1: object behaves as the rotating black hole predicted by general relativity.
• 2:14:19
  Speaker 1: As detectors improve, increasingly precise measurements of these modes may
• 2:14:24
  Speaker 1: provide even stronger tests of the Kerr description. This is
• 2:14:28
  Speaker 1: sometimes called black hole spectroscopy. The idea is to use
• 2:14:32
  Speaker 1: several characteristic modes to probe the nature of the remnant
• 2:14:35
  Speaker 1: in much the same spirit. The ordinary spectroscopy uses different
• 2:14:39
  Speaker 1: frequencies of light to learn about atoms and molecules. But
• 2:14:44
  Speaker 1: there is a fundamental limitation. Suppose the exterior of a
• 2:14:47
  Speaker 1: black hole follows the Kerr solution almost perfectly. That does
• 2:14:52
  Speaker 1: not necessarily tell us what replaces the singularity deep inside.
• 2:14:57
  Speaker 1: One possible black hole might contain the classical singular behaviour
• 2:15:00
  Speaker 1: predicted by general relativity. Another hypothetical model might replace the
• 2:15:05
  Speaker 1: deepest region with a quantum core. If both possess almost
• 2:15:09
  Speaker 1: identical external geometry, observations made outside the event horizon may
• 2:15:14
  Speaker 1: struggle to distinguish between them. The exterior can tell us
• 2:15:18
  Speaker 1: a great deal without telling us everything. This has led
• 2:15:21
  Speaker 1: physicists to search for subtle deviations. Some proposed alternatives to
• 2:15:26
  Speaker 1: ordinary black holes might possess surfaces very close to where
• 2:15:29
  Speaker 1: an event horizon would otherwise be. Others could alter the
• 2:15:33
  Speaker 1: late-time gravitational wave signal. There have been theoretical suggestions that
• 2:15:38
  Speaker 1: unusual structures near a horizon might produce delayed gravitational wave
• 2:15:42
  Speaker 1: features known as echoes after a merger. Such echoes have
• 2:15:46
  Speaker 1: been searched for, but no accepted detection has established that
• 2:15:50
  Speaker 1: black holes possess this kind of exotic structure. Researchers also
• 2:15:54
  Speaker 1: study whether extremely precise measurements of black hole shadows, accretion
• 2:15:59
  Speaker 1: flows and gravitational wave ring downs could reveal small departures
• 2:16:03
  Speaker 1: from general relativity. Future observatories should make these tests considerably
• 2:16:08
  Speaker 1: more sensitive. Space-based gravitational wave detectors could observe different black
• 2:16:13
  Speaker 1: hole populations and frequencies. Larger networks of ground-based detectors could
• 2:16:19
  Speaker 1: measure mergers with much greater precision Improved horizon scale radio
• 2:16:23
  Speaker 1: observations could reveal finer structures around supermassive black holes. These
• 2:16:28
  Speaker 1: experiments may eliminate some possible alternatives. They may tell us
• 2:16:32
  Speaker 1: that certain exotic compact objects cannot reproduce what nature is doing.
• 2:16:37
  Speaker 1: They may reveal that the exterior geometry agrees with general
• 2:16:40
  Speaker 1: relativity to extraordinary accuracy. What they cannot currently provide is
• 2:16:45
  Speaker 1: a direct image of the central region after the horizon
• 2:16:49
  Speaker 1: has been crossed. That is what makes this problem unusual. Normally,
• 2:16:53
  Speaker 1: when physics reaches a disagreement between competing ideas, scientists try
• 2:16:58
  Speaker 1: to examine the place where the predictions differ most strongly.
• 2:17:02
  Speaker 1: With black holes, that place may be hidden behind a
• 2:17:05
  Speaker 1: boundary that prevents information from escaping. We can watch matter
• 2:17:09
  Speaker 1: approach the horizon. We can study the light bending around it.
• 2:17:14
  Speaker 1: We can detect black holes moving. We can hear the
• 2:17:17
  Speaker 1: gravitational waves produced when they collide. We can examine the
• 2:17:21
  Speaker 1: way the final object rings as it settles. And all
• 2:17:24
  Speaker 1: of these observations can test the surrounding spacetime with extraordinary precision.
• 2:17:29
  Speaker 1: Yet the final destination of matter falling deep inside remains
• 2:17:33
  Speaker 1: inaccessible to direct external observation. So black holes offer an
• 2:17:37
  Speaker 1: almost peculiar arrangement. The boundary can be studied. The exterior
• 2:17:42
  Speaker 1: can be tested. The theory can be pushed into some
• 2:17:45
  Speaker 1: of the strongest gravitational fields found anywhere in the universe.
• 2:17:49
  Speaker 1: And still, somewhere beyond that horizon lies a region that
• 2:17:53
  Speaker 1: no ordinary telescope can ever simply look into. We may
• 2:17:58
  Speaker 1: eventually infer far more about it than we know today.
• 2:18:01
  Speaker 1: We may discover indirect fingerprints of quantum gravity, but for
• 2:18:05
  Speaker 1: now the centre remains hidden not because it is too
• 2:18:08
  Speaker 1: faint or too distant, but because space-time itself prevents its
• 2:18:12
  Speaker 1: story from travelling back to us. Imagine for a moment
• 2:18:15
  Speaker 1: that instead of watching a black hole from a safe distance,
• 2:18:19
  Speaker 1: you are falling toward one. Not because this is an
• 2:18:22
  Speaker 1: especially sensible travel plan, but because following a physical object
• 2:18:26
  Speaker 1: inward helps separate what general relativity predicts confidently from what
• 2:18:30
  Speaker 1: remains unknown. Far from the black hole, you might initially
• 2:18:34
  Speaker 1: feel very little. If you're freely falling … You're following
• 2:18:38
  Speaker 1: a natural path through curved space-time. Locally, you would feel weightless,
• 2:18:43
  Speaker 1: much as an astronaut orbiting Earth does. The danger comes
• 2:18:47
  Speaker 1: not simply from gravity being strong, but from gravity changing
• 2:18:51
  Speaker 1: across the length of your body. This difference is called
• 2:18:54
  Speaker 1: tidal gravity. Suppose your feet are pointing toward the black
• 2:18:58
  Speaker 1: hole while your head is slightly farther away. Your feet
• 2:19:01
  Speaker 1: experience a stronger gravitational effect than your head. At the
• 2:19:05
  Speaker 1: same time, the geometry tends to squeeze you sideways. The
• 2:19:09
  Speaker 1: result is stretching in one direction and compression in the others.
• 2:19:12
  Speaker 1: This is the process commonly known as spaghettification. The name
• 2:19:16
  Speaker 1: sounds almost playful. The effect is not. Whether spaghettification becomes
• 2:19:21
  Speaker 1: severe before or after the event horizon depends strongly on
• 2:19:24
  Speaker 1: the mass of the black hole. For a relatively small
• 2:19:28
  Speaker 1: stellar mass black hole, the horizon is compact. and the
• 2:19:32
  Speaker 1: gravitational field changes very rapidly across short distances. The tidal
• 2:19:37
  Speaker 1: difference between your head and feet could become destructive before
• 2:19:41
  Speaker 1: you ever reach the event horizon. A much larger black
• 2:19:45
  Speaker 1: hole behaves differently. A supermassive black hole with millions or
• 2:19:50
  Speaker 1: billions of times the mass of the Sun has an
• 2:19:53
  Speaker 1: enormous event horizon. At that horizon, the overall gravity is extreme.
• 2:19:59
  Speaker 1: but the change in gravity across a human-sized distance can
• 2:20:03
  Speaker 1: be comparatively gentle. So if the black hole was sufficiently massive,
• 2:20:08
  Speaker 1: you could cross the event horizon without being torn apart
• 2:20:11
  Speaker 1: at that moment. There would be no surface, no wall,
• 2:20:16
  Speaker 1: no sudden flash. Locally, the crossing might be remarkably uneventful.
• 2:20:22
  Speaker 1: This can make supermassive black holes sound almost hospitable. They
• 2:20:26
  Speaker 1: are not. Surviving the horizon only means that the most
• 2:20:30
  Speaker 1: violent tidal effects have not reached you yet. Once inside,
• 2:20:35
  Speaker 1: the geometry still carries every future-directed path farther inward. For
• 2:20:41
  Speaker 1: an ideal, non-rotating Schwarzschild black hole, a freely falling observer
• 2:20:46
  Speaker 1: reaches the singular region after a finite amount of proper time.
• 2:20:51
  Speaker 1: How long that takes depends on the mass of the
• 2:20:53
  Speaker 1: black hole and on the details of the trajectory. For
• 2:20:57
  Speaker 1: a stellar-mass black hole, the remaining time could be extraordinarily short.
• 2:21:01
  Speaker 1: For a supermassive black hole, it could be much longer,
• 2:21:04
  Speaker 1: potentially measured in seconds, minutes, or more, for sufficiently enormous
• 2:21:10
  Speaker 1: masses and suitable trajectories. But it is never infinite for
• 2:21:14
  Speaker 1: the infalling observer in the classical Schwarzschild picture. The journey ends.
• 2:21:20
  Speaker 1: As the radius decreases, tidal forces grow. At first, a
• 2:21:24
  Speaker 1: large object might simply be stretched. then structures held together
• 2:21:28
  Speaker 1: by ordinary material strength would fail. Biological tissue, machinery and
• 2:21:34
  Speaker 1: larger molecules would no longer remain intact. As conditions became
• 2:21:38
  Speaker 1: more extreme, the scale of destruction would continue downward. Atoms
• 2:21:44
  Speaker 1: abound primarily by electromagnetic interactions. Nuclei are held together by
• 2:21:49
  Speaker 1: the strong nuclear interactions. These forces are enormously stronger than
• 2:21:54
  Speaker 1: the chemical bonds holding everyday matter together. But sufficiently extreme
• 2:21:59
  Speaker 1: tidal gradients can, in principle, overwhelm even very strongly bound systems.
• 2:22:06
  Speaker 1: The important caution is that we should not pretend to
• 2:22:08
  Speaker 1: know the exact sequence all the way to the deepest region.
• 2:22:13
  Speaker 1: Classical general relativity tells us how curvature and tidal effects grow.
• 2:22:18
  Speaker 1: Known particle physics tells us a great deal about how
• 2:22:21
  Speaker 1: matter behaves under extreme energy and density. But once the
• 2:22:25
  Speaker 1: curvature approaches the regime where quantum gravity should matter, our
• 2:22:29
  Speaker 1: ordinary descriptions can no longer be trusted without qualification. So
• 2:22:34
  Speaker 1: it is reasonable to say that familiar matter would eventually
• 2:22:37
  Speaker 1: be destroyed as the classical tidal field becomes extreme. It
• 2:22:41
  Speaker 1: is not reasonable to describe with confidence exactly What remains
• 2:22:46
  Speaker 1: at the final quantum gravitational stage? That stage is precisely
• 2:22:51
  Speaker 1: what physics has not yet settled. There is another popular
• 2:22:54
  Speaker 1: idea worth correcting. Sometimes black holes are described as places where,
• 2:22:59
  Speaker 1: while falling inward, you would watch the entire future history
• 2:23:03
  Speaker 1: of the external universe unfold. Stars would burn out. Galaxies
• 2:23:08
  Speaker 1: would fade. Perhaps the universe itself would age infinitely while
• 2:23:13
  Speaker 1: you continued inward. That is not a general prediction for
• 2:23:17
  Speaker 1: an observer falling into a realistic black hole. Gravitational time
• 2:23:22
  Speaker 1: dilation does mean that clocks at different gravitational potentials are
• 2:23:26
  Speaker 1: compared differently, and light from the outside universe can be
• 2:23:30
  Speaker 1: shifted in frequency and arrive in unusual ways. But an
• 2:23:34
  Speaker 1: infalling observer does not simply receive the entire infinite future
• 2:23:38
  Speaker 1: of the universe before reaching the centre. The causal structure
• 2:23:43
  Speaker 1: does not provide an unlimited window onto eternity. Only signals
• 2:23:47
  Speaker 1: that can actually intersect the observer's worldline before the observer
• 2:23:52
  Speaker 1: reaches the interior endpoint can be seen. The rest of
• 2:23:56
  Speaker 1: the universe does not politely fast-forward through infinite cosmic history
• 2:24:00
  Speaker 1: for your benefit. Rotating black holes make the story even
• 2:24:05
  Speaker 1: less straightforward. A realistic astrophysical black hole is expected to
• 2:24:10
  Speaker 1: possess angular momentum. so the ideal Kerr solution is more
• 2:24:14
  Speaker 1: relevant than the perfectly non-rotating Schwarzschild picture. In the exact
• 2:24:19
  Speaker 1: mathematical Kerr geometry, the interior contains an inner horizon and
• 2:24:24
  Speaker 1: a ring-shaped singularity. The equations can even be extended into
• 2:24:29
  Speaker 1: regions with very unusual causal properties. On paper, some trajectories
• 2:24:35
  Speaker 1: appear capable of passing into further regions of spacetime, but
• 2:24:39
  Speaker 1: the ideal Kerr interior is probably not stable. Near the
• 2:24:43
  Speaker 1: inner horizon, radiation can become enormously blue-shifted. The mass inflation
• 2:24:49
  Speaker 1: instability suggests that tiny disturbances can grow into extremely large curvature.
• 2:24:55
  Speaker 1: A real black hole contains matter, radiation and gravitational perturbations,
• 2:25:01
  Speaker 1: so it would be unsafe to imagine an actual traveller
• 2:25:05
  Speaker 1: gliding calmly through the textbook Kerr geometry and emerging neatly
• 2:25:10
  Speaker 1: into another universe. The pristine extension is a property of
• 2:25:14
  Speaker 1: an ideal solution. The interior of a physically formed rotating
• 2:25:19
  Speaker 1: black hole is expected to be much messier, and once again,
• 2:25:23
  Speaker 1: as curvature becomes extreme, classical relativity eventually reaches the same boundary.
• 2:25:30
  Speaker 1: Quantum gravity should matter. This is the point where the
• 2:25:34
  Speaker 1: distinction between prediction and speculation becomes most important. General relativity
• 2:25:40
  Speaker 1: can tell us a remarkable amount about the fall. It
• 2:25:43
  Speaker 1: can describe the event horizon. It can describe the causal structure.
• 2:25:48
  Speaker 1: It can calculate tidal forces. It can show why escape
• 2:25:52
  Speaker 1: becomes impossible. It can tell us that classical trajectories inside
• 2:25:56
  Speaker 1: a non-rotating black hole reach the singular region after finite
• 2:26:00
  Speaker 1: proper time. It can also reveal instabilities in the interiors
• 2:26:04
  Speaker 1: of rotating black holes. but it cannot complete the journey
• 2:26:08
  Speaker 1: into the final regime with a trustworthy physical description. There,
• 2:26:13
  Speaker 1: the familiar equations point toward their own limitation. The observer
• 2:26:17
  Speaker 1: can be followed mathematically almost all the way down, the
• 2:26:21
  Speaker 1: stretching can be calculated, the geometry can be mapped, the
• 2:26:25
  Speaker 1: remaining proper time can be determined within the classical model,
• 2:26:29
  Speaker 1: and then the theory reaches a place where the question changes.
• 2:26:34
  Speaker 1: It is no longer simply a matter of asking what
• 2:26:36
  Speaker 1: general relativity predicts. It becomes a matter of asking what
• 2:26:41
  Speaker 1: replaces general relativity when spacetime itself enters the quantum regime.
• 2:26:46
  Speaker 1: That is the final boundary. We can say with confidence
• 2:26:50
  Speaker 1: that falling into a black hole is not an endless
• 2:26:52
  Speaker 1: suspension at the horizon. We can say that tidal forces
• 2:26:56
  Speaker 1: eventually become severe. We can say that ordinary matter cannot
• 2:27:00
  Speaker 1: remain intact indefinitely. but the exact physical fate of matter
• 2:27:05
  Speaker 1: in the deepest interior is still unknown. Classical relativity carries
• 2:27:10
  Speaker 1: us astonishingly far. It is the last part of the
• 2:27:13
  Speaker 1: fall that remains hidden. Black holes are no longer merely
• 2:27:17
  Speaker 1: mathematical possibilities. They are objects that astronomers observe through their
• 2:27:22
  Speaker 1: effects on nearby stars, gas and light. They reveal themselves
• 2:27:27
  Speaker 1: through powerful accretion flows, through the motion of companion stars,
• 2:27:32
  Speaker 1: through horizon-scale radio observations and through the gravitational waves produced
• 2:27:37
  Speaker 1: when two of them collide. The evidence that black holes
• 2:27:41
  Speaker 1: exist is overwhelming. The evidence that general relativity describes their
• 2:27:46
  Speaker 1: exterior behaviour extremely well is also impressive. Stars orbit the
• 2:27:51
  Speaker 1: compact object at the centre of the Milky Way in
• 2:27:54
  Speaker 1: ways consistent with a supermassive black hole containing roughly 4
• 2:27:59
  Speaker 1: million times the mass of the Sun. The Event Horizon
• 2:28:04
  Speaker 1: Telescope has observed dark shadows surrounded by glowing material around
• 2:28:09
  Speaker 1: Sagittarius Astar and the much larger black hole in Messier 87.
• 2:28:14
  Speaker 1: Gravitational wave observatories have detected black holes spiralling together, merging
• 2:28:20
  Speaker 1: and settling into new rotating objects. Across all of these observations,
• 2:28:25
  Speaker 1: Albert Einstein's theory continues to perform remarkably well. So there
• 2:28:30
  Speaker 1: is a great deal that physicists do know. They know
• 2:28:33
  Speaker 1: how black holes curve spacetime around them. They know how
• 2:28:37
  Speaker 1: light behaves near their horizons. They know that rotating black
• 2:28:41
  Speaker 1: holes produce frame dragging. They know that matter can form
• 2:28:45
  Speaker 1: hodocretion disks outside them. They know that black holes can
• 2:28:49
  Speaker 1: merge and radiate gravitational waves. And they know that an
• 2:28:53
  Speaker 1: event horizon is not a solid surface but a causal
• 2:28:57
  Speaker 1: boundary beyond which ordinary signals cannot return to the external universe.
• 2:29:03
  Speaker 1: The uncertainty begins much farther in. Classical general relativity predicts
• 2:29:08
  Speaker 1: that sufficiently complete gravitational collapse leads towards singular behaviour, This
• 2:29:14
  Speaker 1: conclusion is not based only on one perfectly spherical model.
• 2:29:19
  Speaker 1: The Schwarzschild solution demonstrates the problem in its simplest form,
• 2:29:24
  Speaker 1: but the singularity theorems developed by Roger Penrose and later
• 2:29:28
  Speaker 1: expanded with Stephen Hawking showed that spacetime incompleteness appears under
• 2:29:33
  Speaker 1: much broader circumstances. The classical problem cannot simply be dismissed
• 2:29:38
  Speaker 1: by saying that real stars are uneven, rotating or surrounded
• 2:29:42
  Speaker 1: by disturbances. General relativity itself drives the geometry toward a
• 2:29:47
  Speaker 1: regime where the classical spacetime description becomes incomplete. For a
• 2:29:52
  Speaker 1: non-rotating black hole, future-directed trajectories lead toward the singular region.
• 2:29:59
  Speaker 1: For an ideal rotating Kerr black hole, the interior structure
• 2:30:03
  Speaker 1: becomes more complicated, with an inner horizon and a ring-shaped
• 2:30:07
  Speaker 1: singularity in the exact mathematical solution. And when realistic disturbances
• 2:30:13
  Speaker 1: are included, Phenomena such as mass inflation suggest that even
• 2:30:17
  Speaker 1: this elegant interior can become violently unstable. The central difficulty
• 2:30:22
  Speaker 1: therefore survives every attempt to make the problem slightly less tidy.
• 2:30:27
  Speaker 1: But there is an equally important fact. Physicists do not
• 2:30:31
  Speaker 1: know that a literal point or ring of infinite density
• 2:30:34
  Speaker 1: physically exists inside real black holes. That statement goes beyond
• 2:30:39
  Speaker 1: what observations have established. The singularity is a prediction of
• 2:30:43
  Speaker 1: classical general relativity. It is where quantities such as space-time
• 2:30:47
  Speaker 1: curvature diverge and where geodesics become incomplete. Those features are
• 2:30:52
  Speaker 1: mathematically serious. They tell us that something is wrong with
• 2:30:56
  Speaker 1: simply extending the classical theory indefinitely. What they do not
• 2:31:00
  Speaker 1: tell us is what a complete description of nature places
• 2:31:03
  Speaker 1: there instead. This is why the singularity is often treated
• 2:31:08
  Speaker 1: as a sign that classical general relativity has reached the
• 2:31:11
  Speaker 1: edge of its domain. General relativity contains no quantum description
• 2:31:15
  Speaker 1: of spacetime. Yet the conditions predicted near the singular regime
• 2:31:20
  Speaker 1: become so extreme that quantum gravitational effects are expected to matter.
• 2:31:25
  Speaker 1: And here lies the central gap. There is currently no
• 2:31:28
  Speaker 1: experimentally verified theory of quantum gravity that tells physicists exactly
• 2:31:33
  Speaker 1: what happens in that region. Several possibilities remain open. Perhaps
• 2:31:38
  Speaker 1: some form of singular boundary survives in the final theory,
• 2:31:42
  Speaker 1: although its interpretation may differ from the classical infinity. Perhaps
• 2:31:46
  Speaker 1: quantum gravity replaces the singularity with a finite region of
• 2:31:49
  Speaker 1: enormous but bounded curvature. Perhaps collapsing matter enters a new phase.
• 2:31:55
  Speaker 1: Perhaps spacetime undergoes a transition. Perhaps the familiar distinction between
• 2:31:59
  Speaker 1: inside and outside must be revised once gravity becomes fully quantum.
• 2:32:04
  Speaker 1: Or perhaps the correct theory contains something that current approaches
• 2:32:07
  Speaker 1: have not yet imagined. Loop quantum gravity, string theory, regular
• 2:32:12
  Speaker 1: black hole models, Planck star proposals, fuzzballs, gravastars and other
• 2:32:17
  Speaker 1: ideas explore different versions of this territory. Some replace the
• 2:32:21
  Speaker 1: classical singularity. Some alter the interior much earlier. Some preserve
• 2:32:26
  Speaker 1: much of classical space-time before new physics appears only in
• 2:32:29
  Speaker 1: the deepest region. They cannot all be treated as confirmed descriptions.
• 2:32:34
  Speaker 1: At present, none has received the decisive experimental evidence required
• 2:32:39
  Speaker 1: to identify as the interior of real astrophysical black holes.
• 2:32:44
  Speaker 1: That does not mean observation has nothing more to contribute.
• 2:32:47
  Speaker 1: Quite the opposite. Every improvement in black hole astronomy narrows
• 2:32:52
  Speaker 1: the room available for competing theories. More precise images of
• 2:32:56
  Speaker 1: black hole shadows test the geometry near event horizons. More
• 2:33:00
  Speaker 1: sensitive gravitational wave observations test how black holes spiral together,
• 2:33:05
  Speaker 1: merge and ring down. Measurements of quasi-normal modes can test
• 2:33:09
  Speaker 1: whether merger remnants behave like the rotating Kerr black holes
• 2:33:13
  Speaker 1: predicted by general relativity. If future observations reveal a clear
• 2:33:17
  Speaker 1: departure from those predictions, they could provide an important clue
• 2:33:21
  Speaker 1: about new physics. Even the absence of such deviations matters.
• 2:33:26
  Speaker 1: If general relativity continues to survive increasingly precise tests, then
• 2:33:30
  Speaker 1: alternative models must reproduce its predictions with corresponding accuracy wherever
• 2:33:36
  Speaker 1: observations can reach. The difficulty is that the most dramatic
• 2:33:40
  Speaker 1: differences may remain hidden behind the horizon. A black hole
• 2:33:44
  Speaker 1: with a classical singularity and one with a tiny quantum
• 2:33:47
  Speaker 1: core could potentially look almost identical from far away. If
• 2:33:52
  Speaker 1: the modifications occur only in the deepest interior, the external
• 2:33:56
  Speaker 1: universe may receive very little direct evidence of them. This
• 2:33:59
  Speaker 1: leaves physicists in an unusual position. They can describe the
• 2:34:03
  Speaker 1: route inward with extraordinary confidence. They can calculate the horizon.
• 2:34:09
  Speaker 1: They can model the curvature. They can follow falling matter
• 2:34:12
  Speaker 1: through much of the interior. They can identify where classical
• 2:34:15
  Speaker 1: trajectories become incomplete. They can show why the problem is
• 2:34:19
  Speaker 1: not merely a mathematical mistake caused by one special coordinate system.
• 2:34:24
  Speaker 1: And yet, at the point where the question becomes most fundamental,
• 2:34:27
  Speaker 1: the trusted description runs out. So when we say that
• 2:34:30
  Speaker 1: physicists still do not know what happens at the centre
• 2:34:33
  Speaker 1: of a black hole, this should not be mistaken for
• 2:34:36
  Speaker 1: complete ignorance. It is almost the opposite. Physics knows enough
• 2:34:40
  Speaker 1: to identify exactly where the problem begins. General relativity tells
• 2:34:45
  Speaker 1: us that the classical interior cannot remain ordinary. Quantum theory
• 2:34:49
  Speaker 1: tells us that information and microscopic physics cannot simply be ignored.
• 2:34:54
  Speaker 1: Thermodynamics tells us that black holes possess entropy and temperature.
• 2:34:59
  Speaker 1: The information paradox tells us that gravity and quantum mechanics
• 2:35:02
  Speaker 1: are deeply entangled in ways that are still being understood,
• 2:35:06
  Speaker 1: and every one of these paths points toward the same
• 2:35:08
  Speaker 1: unfinished piece of physics. Quantum gravity. The centre of a
• 2:35:13
  Speaker 1: black hole therefore sits at the meeting point of some
• 2:35:15
  Speaker 1: of the most successful ideas humans have ever developed. General
• 2:35:19
  Speaker 1: relativity describes the shape of space-time. Quantum mechanics describes microscopic reality.
• 2:35:26
  Speaker 1: Thermodynamics describes entropy and temperature. Information theory asks what can
• 2:35:32
  Speaker 1: be preserved, lost or reconstructed. Black holes force all of
• 2:35:36
  Speaker 1: them into the same problem, and that is why the
• 2:35:38
  Speaker 1: unanswered question remains so valuable. The mystery at the centre
• 2:35:42
  Speaker 1: is not an empty space in scientific knowledge. It is
• 2:35:45
  Speaker 1: a boundary drawn with unusual precision. We know where classical
• 2:35:49
  Speaker 1: physics leads. We know why that path cannot simply be
• 2:35:52
  Speaker 1: trusted forever. What we still do not know is what
• 2:35:56
  Speaker 1: nature does once the path reaches its end. So, what
• 2:36:00
  Speaker 1: actually exists at the centre of a black hole? The
• 2:36:03
  Speaker 1: most accurate answer is not that physicists know there is
• 2:36:06
  Speaker 1: definitely an infinitely dense point sitting there, But it is
• 2:36:10
  Speaker 1: also not true that physics has no idea what happens.
• 2:36:13
  Speaker 1: The reality lies somewhere in between. General relativity gives us
• 2:36:17
  Speaker 1: a remarkably detailed description of the journey inward. It tells
• 2:36:21
  Speaker 1: us how spacetime curves around a black hole, how light
• 2:36:24
  Speaker 1: behaves near it, where the event horizon forms, and why
• 2:36:28
  Speaker 1: crossing that horizon does not mean immediately striking some hidden surface.
• 2:36:33
  Speaker 1: The event horizon is not the centre, it is a
• 2:36:36
  Speaker 1: causal boundary. Once crossed, Signals can no longer return to
• 2:36:41
  Speaker 1: the outside universe, but a sufficiently large black hole need
• 2:36:44
  Speaker 1: not produce anything locally dramatic at the exact moment of crossing.
• 2:36:49
  Speaker 1: The deeper changes happen farther in. For a non-rotating black hole,
• 2:36:55
  Speaker 1: Karl Schwarzschild's solution shows how the geometry carries future directed
• 2:36:59
  Speaker 1: paths toward decreasing radius. For a rotating black hole, Roy
• 2:37:03
  Speaker 1: Kerr's solution reveals a much more complicated interior shaped by
• 2:37:07
  Speaker 1: angular momentum frame-dragging, an inner horizon and a classical ring-shaped singularity.
• 2:37:14
  Speaker 1: These are not vague ideas. They are precise consequences of
• 2:37:17
  Speaker 1: Albert Einstein's general theory of relativity. And outside black holes,
• 2:37:23
  Speaker 1: that theory has been tested again and again with extraordinary success.
• 2:37:27
  Speaker 1: We see stars orbiting the supermassive black hole at the
• 2:37:30
  Speaker 1: centre of the Milky Way. We observe the shadows of
• 2:37:33
  Speaker 1: black holes using the Event Horizon Telescope. We detect gravitational
• 2:37:38
  Speaker 1: waves from black holes spiralling together, merging and settling into
• 2:37:42
  Speaker 1: new rotating objects. Across enormous distances and extreme gravitational environments,
• 2:37:48
  Speaker 1: General relativity continues to describe what we observe remarkably well.
• 2:37:54
  Speaker 1: That is what makes the central problem so interesting. The
• 2:37:57
  Speaker 1: theory is not failing everywhere. It is succeeding almost everywhere
• 2:38:01
  Speaker 1: we can test it. And then, when we follow its
• 2:38:05
  Speaker 1: predictions deep enough into a black hole, it leads us
• 2:38:08
  Speaker 1: toward a regime where the classical description becomes incomplete. Roger
• 2:38:13
  Speaker 1: Penrose helped establish just how serious that problem is. His
• 2:38:17
  Speaker 1: singularity theorem showed that singular behaviour could not simply be
• 2:38:20
  Speaker 1: blamed on the unrealistically perfect symmetry of one mathematical model.
• 2:38:25
  Speaker 1: Under broad physical conditions, gravitational collapse within classical general relativity
• 2:38:31
  Speaker 1: leads toward geodesic incompleteness. In simple terms, some path through
• 2:38:36
  Speaker 1: spacetime cannot be continued indefinitely. The classical geometry reaches an edge.
• 2:38:42
  Speaker 1: That result does not prove that a literal infinitely dense
• 2:38:45
  Speaker 1: object exists in nature. It tells us something subtler. Classical
• 2:38:50
  Speaker 1: general relativity eventually stops providing a complete description. And realistic
• 2:38:56
  Speaker 1: rotation makes the situation even more complicated. A real astrophysical
• 2:39:01
  Speaker 1: black hole is expected to spin. Its interior may contain
• 2:39:05
  Speaker 1: an unstable inner horizon. Radiation and gravitational disturbances can be
• 2:39:10
  Speaker 1: enormously blue-shifted there. The phenomenon known as mass inflation can
• 2:39:14
  Speaker 1: drive internal curvature toward extreme values. So even before quantum
• 2:39:19
  Speaker 1: physics is introduced, the interior of a realistic black hole
• 2:39:23
  Speaker 1: may be much messier than the familiar picture of a
• 2:39:25
  Speaker 1: smooth dark sphere surrounding one tiny central dot. And eventually,
• 2:39:31
  Speaker 1: quantum physics cannot be ignored. That is where the possibilities
• 2:39:35
  Speaker 1: begin to branch Loop quantum gravity-inspired models suggest that the
• 2:39:39
  Speaker 1: classical singularity might be replaced by a finite quantum region
• 2:39:43
  Speaker 1: or transition. Some string theory approaches describe black holes in
• 2:39:48
  Speaker 1: terms of enormous numbers of quantum microstates, with fuzzball proposals
• 2:39:52
  Speaker 1: suggesting that the familiar smooth interior may itself be replaced
• 2:39:56
  Speaker 1: by complicated quantum structure. Regular black hole models explore geometries
• 2:40:01
  Speaker 1: where curvature remains finite Planck star proposals suggest collapse could
• 2:40:06
  Speaker 1: be modified when quantum gravitational effects become dominant. Other ideas
• 2:40:10
  Speaker 1: involve phase transitions, non-singular cores or structures that do not
• 2:40:15
  Speaker 1: fit neatly into the classical picture at all. These possibilities
• 2:40:18
  Speaker 1: are scientifically interesting. They are not experimentally established descriptions of
• 2:40:23
  Speaker 1: real black hole interiors. That distinction matters. Physicists have many
• 2:40:28
  Speaker 1: serious proposals precisely because there is not yet one confirmed answer.
• 2:40:33
  Speaker 1: The black hole information problem makes the mystery deeper still.
• 2:40:37
  Speaker 1: Jacob Bekenstein showed that black holes should possess entropy related
• 2:40:41
  Speaker 1: to the area of their horizons. Stephen Hawking discovered that
• 2:40:45
  Speaker 1: quantum effects allow black holes to emit thermal radiation. That
• 2:40:49
  Speaker 1: raises the question of what happens to the quantum information
• 2:40:52
  Speaker 1: carried by everything that falls inside. If a black hole
• 2:40:56
  Speaker 1: eventually evaporates, is the information lost? Is it somehow encoded
• 2:41:01
  Speaker 1: in the radiation? Does the interior have to be understood
• 2:41:04
  Speaker 1: in a fundamentally new way? Modern theoretical work increasingly suggests
• 2:41:09
  Speaker 1: that information may ultimately be preserved, but that still does
• 2:41:13
  Speaker 1: not tell us exactly what an observer encounters in the
• 2:41:16
  Speaker 1: deepest interior. The observational divide remains. The Event Horizon Telescope
• 2:41:22
  Speaker 1: can test the geometry close to black hole horizons. Gravitational
• 2:41:26
  Speaker 1: wave observatories can measure black hole mergers and ringdowns. Future
• 2:41:31
  Speaker 1: instruments may reveal even more precise departures from or confirmations
• 2:41:35
  Speaker 1: of general relativity. But no ordinary signal can leave the
• 2:41:39
  Speaker 1: deep interior after crossing the event horizon. There is no
• 2:41:43
  Speaker 1: photograph from the centre, no radio message, no detector that
• 2:41:47
  Speaker 1: can travel in, send back the result and return home
• 2:41:50
  Speaker 1: with a slightly alarming souvenir. That makes the central region
• 2:41:55
  Speaker 1: one of the strangest frontiers in physics. We can observe
• 2:41:59
  Speaker 1: black holes influencing stars across light years. We can detect
• 2:42:03
  Speaker 1: their collisions from billions of light years away. We can
• 2:42:07
  Speaker 1: measure how they bend light and distort space-time. We can
• 2:42:10
  Speaker 1: test Einstein's predictions under some of the most extreme conditions
• 2:42:15
  Speaker 1: available in the universe. And yet, at the point where
• 2:42:18
  Speaker 1: gravity becomes strongest and space-time reaches its most extreme state,
• 2:42:23
  Speaker 1: our two greatest physical frameworks still do not provide one
• 2:42:27
  Speaker 1: agreed description. General relativity tells us that classical spacetime heads
• 2:42:32
  Speaker 1: towards singular behaviour. Quantum physics tells us that the final
• 2:42:36
  Speaker 1: story cannot ignore quantum principles. What we do not yet
• 2:42:40
  Speaker 1: possess is the experimentally confirmed theory that combines them. So
• 2:42:45
  Speaker 1: the centre of a black hole is not simply a
• 2:42:47
  Speaker 1: place hidden because no light shines from it. It marks
• 2:42:50
  Speaker 1: something deeper. It is a boundary in our understanding. A
• 2:42:54
  Speaker 1: place where spacetime, gravity, quantum mechanics … thermodynamics and information
• 2:43:00
  Speaker 1: theory are all forced into the same problem. We know
• 2:43:03
  Speaker 1: remarkably well how the road leads inward. We know the
• 2:43:07
  Speaker 1: classical equations tell us that the road cannot simply continue
• 2:43:10
  Speaker 1: forever in the ordinary way. What we still do not
• 2:43:14
  Speaker 1: know is what nature places at the end. For now,
• 2:43:18
  Speaker 1: the final part of the calculation remains unwritten.