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Why Does a Black Hole Look Bigger Than It Really Is · Night Science Stories

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  Speaker 2: And welcome to the Sleepy Scientist. It's lovely to have
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  Speaker 2: a small thing, but, rather like tonight's subject, it's bigger
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  Speaker 2: than it appears, and to everyone who has already done so,
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  Speaker 2: thank you. It's wonderful knowing so many of us share
• 1:50
  Speaker 2: these rather unusual bedtime conversations. Now, a black hole is
• 1:54
  Speaker 2: already one of the strangest things in the universe. But
• 1:58
  Speaker 2: tonight there's an extra twist. You may have seen the
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  Speaker 2: famous pictures. A glowing ring with a deep, dark circle
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  Speaker 2: sitting in the middle. It's natural to assume that dark
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  Speaker 2: circle is the black hole itself. It isn't. Not quite.
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  Speaker 2: The shadow we see is actually far larger than the
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  Speaker 2: black hole's true edge, the point beyond which nothing comes back.
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  Speaker 2: And the reason comes down to the peculiar way gravity
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  Speaker 2: bends light around it, like a lens made out of
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  Speaker 2: space itself. Which raises a slightly awkward question. If we've
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  Speaker 2: finally managed to photograph something that was meant to be invisible,
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  Speaker 2: and it turns out to look much bigger than it
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  Speaker 2: really is, then what exactly is in the picture? A
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  Speaker 2: rather nice puzzle to settle into this evening. So, let's begin.
• 2:47
  Speaker 2: Why does a black hole look bigger than it really is?
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  Speaker 2: There is something rather extraordinary about the first photograph of
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  Speaker 2: a black hole. At the centre of the picture sits
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  Speaker 2: an enormous region of darkness, surrounded by a glowing ring
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  Speaker 2: of light. It looks almost like a great circular hole
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  Speaker 2: cut into the universe, with the surrounding material illuminating its edges.
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  Speaker 2: And yet, the strangest thing about this image is that
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  Speaker 2: the darkness we see is considerably larger than the black
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  Speaker 2: hole itself. In April 2019… The Event Horizon Telescope collaboration
• 3:23
  Speaker 2: revealed an image that astronomers had spent decades hoping to produce.
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  Speaker 2: It showed the region immediately surrounding a supermassive black hole
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  Speaker 2: at the centre of a distant galaxy called Messier 87,
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  Speaker 2: approximately 55 million light-years from Earth. The black hole contains
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  Speaker 2: around 6.5 billion times the mass of our Sun, making
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  Speaker 2: it one of the most remarkable objects ever examined in
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  Speaker 2: such detail. The image itself was surprisingly simple. An uneven
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  Speaker 2: ring of orange-coloured emission surrounded a darker central region. There
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  Speaker 2: were no visible stars disappearing into a whirlpool, no dramatic
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  Speaker 2: streams of matter stretching across the photograph. Just an almost
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  Speaker 2: circular patch of darkness, enclosed by radiation from material moving
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  Speaker 2: through an extraordinarily powerful gravitational field. But appearances can be deceptive.
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  Speaker 2: especially when the object being photographed is capable of bending
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  Speaker 2: the very light used to observe it. To understand the problem,
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  Speaker 2: it helps to distinguish between a black hole's actual size
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  Speaker 2: and the size it appears to have. The physical boundary
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  Speaker 2: of a black hole is called the event horizon. This
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  Speaker 2: is the boundary beyond which light can no longer escape
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  Speaker 2: to reach distant observers. Anything crossing it becomes unable to
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  Speaker 2: send a signal back to the outside universe. The event
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  Speaker 2: horizon is not a solid surface. There is no shell
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  Speaker 2: of material marking its location and no enormous black sphere
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  Speaker 2: sitting inside space like a rather unfortunate planet. Instead, it
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  Speaker 2: is a boundary defined by the geometry of spacetime itself.
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  Speaker 2: When physicists describe the size of a simple, non-rotating black hole,
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  Speaker 2: they often use the Schwarzschild radius, named after the German
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  Speaker 2: physicist Karl Schwarzschild. This radius describes the distance from the
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  Speaker 2: centre to the event horizon, as measured using the standard
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  Speaker 2: coordinates of the Schwarzschild solution. It seems reasonable to imagine
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  Speaker 2: that the dark circular region in a photograph should correspond
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  Speaker 2: directly to that boundary. After all, when we look at
• 5:34
  Speaker 2: an ordinary object blocking a light source, its silhouette generally
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  Speaker 2: tells us something about its dimensions. A black hole, however,
• 5:42
  Speaker 2: does not behave like an ordinary object. According to Albert
• 5:46
  Speaker 2: Einstein's general theory of relativity, gravity is a manifestation of
• 5:51
  Speaker 2: curved spacetime. Massive objects influence the geometry around them, and
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  Speaker 2: light follows paths through that geometry. Even though photons have
• 6:01
  Speaker 2: no rest mass, their trajectories can be profoundly affected by gravity.
• 6:06
  Speaker 2: Near an ordinary star, this bending is relatively modest. Near
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  Speaker 2: a black hole, it can become extreme. A beam of
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  Speaker 2: light passing close to a black hole may be redirected
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  Speaker 2: through a considerable angle. Another beam passing slightly closer might
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  Speaker 2: travel partway around the black hole before escaping, and a
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  Speaker 2: beam following a sufficiently close-incoming trajectory may never escape at all.
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  Speaker 2: This last possibility is especially important. Some of the light
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  Speaker 2: that disappears into a black hole would have travelled safely
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  Speaker 2: past the event horizon if spacetime were flat. but the
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  Speaker 2: black hole's gravitational field changes its path, redirecting it towards
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  Speaker 2: the region from which escape is impossible. As a result,
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  Speaker 2: the black hole can remove light from a wider range
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  Speaker 2: of viewing directions than we might expect from the horizon's
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  Speaker 2: physical dimensions alone. This produces what astronomers call the black
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  Speaker 2: hole shadow. The shadow is not simply the event horizon
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  Speaker 2: photographed from a distance. It is a region of missing light,
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  Speaker 2: with its geometrical boundary determined by which photon trajectories can
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  Speaker 2: escape and which become captured. One of the most important
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  Speaker 2: features controlling those trajectories is the photon sphere. For an
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  Speaker 2: ideal non-rotating black hole, the photon sphere is a spherical
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  Speaker 2: region outside the event horizon, where light can theoretically travel
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  Speaker 2: along circular orbits. These orbits are unstable, meaning that the
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  Speaker 2: smallest departure from the required trajectory can eventually send a
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  Speaker 2: photon outwards, or towards the black hole. The photon sphere
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  Speaker 2: therefore plays a central role in separating escaping light from
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  Speaker 2: captured light. It also helps explain why the observed shadow
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  Speaker 2: extends well beyond the event horizon, and here the numbers
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  Speaker 2: become particularly interesting. For a distant observer looking towards an
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  Speaker 2: ideal non-rotating black hole, with suitable background illumination, the geometrical
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  Speaker 2: shadow has an apparent diameter approximately 2.6 times the diameter
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  Speaker 2: of the event horizon. That is a substantial difference. Imagine
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  Speaker 2: a black hole whose event horizon measures 60 kilometres across.
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  Speaker 2: Its apparent shadow, expressed on the same distant observer scale,
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  Speaker 2: would measure roughly 150 kilometres across. The black hole has
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  Speaker 2: not physically grown. Its event horizon remains exactly where the
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  Speaker 2: geometry places it. What has changed is the apparent region
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  Speaker 2: of the sky from which background light fails to reach us.
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  Speaker 2: This phenomenon is closely connected to gravitational lensing, the bending
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  Speaker 2: of light by gravity. Gravitational lensing can distort the appearance
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  Speaker 2: of distant objects, shift their apparent positions, and sometimes produce
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  Speaker 2: several images of the same source. Around a black hole,
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  Speaker 2: those effects become particularly dramatic There is another distinction worth
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  Speaker 2: making when looking at the famous photographs. The bright ring
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  Speaker 2: surrounding the central darkness is not the photon sphere itself,
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  Speaker 2: nor is it a glowing outline painted around the event horizon.
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  Speaker 2: Much of the observed radiation comes from hot plasma moving
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  Speaker 2: in the black hole's surroundings. Its light is redirected by
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  Speaker 2: gravity before reaching Earth, producing the luminous structure that makes
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  Speaker 2: the central darkness recognisable. The observed emission ring and the
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  Speaker 2: theoretical shadow boundary are related, but they are not necessarily identical.
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  Speaker 2: The appearance also depends upon the distribution and behaviour of
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  Speaker 2: the emitting material. So the famous image contains several different
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  Speaker 2: physical ideas within one deceptively simple picture. There is the
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  Speaker 2: event horizon, defining the black hole. There are the strongly
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  Speaker 2: curved paths followed by photons. There is the shadow, created
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  Speaker 2: by the capture of light, and there is the glowing
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  Speaker 2: surrounding material, providing the radiation that our instruments can detect.
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  Speaker 2: What makes the image so remarkable is that the apparent
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  Speaker 2: enlargement is not a photographic mistake or an illusion caused
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  Speaker 2: by an imperfect telescope. It is a genuine prediction of
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  Speaker 2: general relativity. The black hole looks bigger than it really
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  Speaker 2: is because gravity changes the paths light can take around it.
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  Speaker 2: We are not merely looking at an invisible object surrounded
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  Speaker 2: by glowing matter. We are seeing the consequences of space-time
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  Speaker 2: curvature expressed through the presence and absence of light, and
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  Speaker 2: perhaps that is the most remarkable part of all. Even
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  Speaker 2: the darkness surrounding a black hole contains information about how
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  Speaker 2: the universe works. One of the more unusual things about
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  Speaker 2: a black hole is that it has a boundary, but
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  Speaker 2: not a surface. We are accustomed to objects having an
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  Speaker 2: outside that we can touch, photograph, or at least imagine
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  Speaker 2: standing upon. The Earth has its rocky surface, the Sun
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  Speaker 2: has a visible layer from which most of its light escapes,
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  Speaker 2: and even a cloud has a recognisable shape. Although its
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  Speaker 2: edges are rather less definite, a black hole is different.
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  Speaker 2: Its defining boundary is not made from matter at all.
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  Speaker 2: It is a feature of spacetime itself. This boundary is
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  Speaker 2: called the event horizon, and it marks the point beyond
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  Speaker 2: which nothing can escape to the outside universe. Not a spacecraft,
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  Speaker 2: not a particle, and not even a beam of light.
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  Speaker 2: Once something crosses the event horizon, every possible future path
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  Speaker 2: it can follow remains inside the black hole. There is
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  Speaker 2: no sufficiently powerful engine that could reverse the journey, because
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  Speaker 2: escaping would require something more fundamental than simply travelling faster.
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  Speaker 2: The event horizon is therefore not a barrier that light
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  Speaker 2: struggles to overcome. It is a boundary separating regions of
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  Speaker 2: spacetime according to whether signals can ever reach distant observers.
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  Speaker 2: Outside it, light can still escape along suitable trajectories. Inside it,
• 12:14
  Speaker 2: no future-directed light signal can reach the distant universe. The
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  Speaker 2: mathematical description of this boundary emerged from Albert Einstein's general
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  Speaker 2: theory of relativity. In 1915, Einstein presented a new understanding
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  Speaker 2: of gravity in which matter and energy influence the geometry
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  Speaker 2: of spacetime. Rather than treating gravity simply as a force
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  Speaker 2: pulling objects towards one another, General relativity describes how objects
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  Speaker 2: and light move through the curved geometry surrounding massive bodies.
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  Speaker 2: Only a short time later, in 1916, the German physicist
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  Speaker 2: Karl Schwarzschild found an exact solution to Einstein's equations. Schwarzschild
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  Speaker 2: was serving in the German army during the First World
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  Speaker 2: War when he carried out this work, an extraordinary achievement
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  Speaker 2: under circumstances that were hardly ideal for mathematical contemplation. His
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  Speaker 2: solution described the spacetime geometry outside an idealised spherical, non-rotating mass.
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  Speaker 2: When interpreted as a black hole, it provides one of
• 13:20
  Speaker 2: the simplest and most important models in gravitational physics. From
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  Speaker 2: this description comes the Schwarzschild radius. For an uncharged, non-rotating
• 13:29
  Speaker 2: black hole, the Schwarzschild radius gives the radial coordinate of
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  Speaker 2: its event horizon. Its value depends upon three quantities. the
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  Speaker 2: mass of the black hole, the gravitational constant, and the
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  Speaker 2: speed of light. The relationship is wonderfully straightforward. The more
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  Speaker 2: massive the black hole, the larger its Schwarzschild radius. The
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  Speaker 2: Schwarzschild radius equals twice the gravitational constant, multiplied by the
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  Speaker 2: black hole's mass, divided by the square of the speed
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  Speaker 2: of light. This means that a black hole with twice
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  Speaker 2: the mass has twice the Schwarzschild radius. A black hole
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  Speaker 2: with 10 times the mass has 10 times the radius.
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  Speaker 2: Unlike many complicated relationships in astrophysics, this one behaves rather politely.
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  Speaker 2: For every solar mass, the Schwarzschild radius is approximately 3 kilometres.
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  Speaker 2: A non-rotating black hole containing 10 times the mass of
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  Speaker 2: our Sun would therefore have an event horizon radius of
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  Speaker 2: approximately 30 kilometres. Its event horizon would measure around 60
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  Speaker 2: kilometres across. that is an astonishing concentration of mass. Ten
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  Speaker 2: suns worth of matter and energy would be associated with
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  Speaker 2: a black hole whose event horizon could fit comfortably inside
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  Speaker 2: the boundaries of a large city. At the other extreme
• 14:50
  Speaker 2: are supermassive black holes containing millions or billions of solar masses.
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  Speaker 2: Consider Sagittarius A star, the supermassive black hole at the
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  Speaker 2: centre of the Milky Way. its mass is approximately 4
• 15:05
  Speaker 2: million times that of the Sun. If we use the
• 15:08
  Speaker 2: non-rotating Schwarzschild model for an object of that mass, its
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  Speaker 2: event horizon radius would be roughly 12 million kilometres. The
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  Speaker 2: corresponding diameter would be approximately 24 million kilometres, vastly larger
• 15:22
  Speaker 2: than the diameter of the Sun. These comparisons reveal why
• 15:26
  Speaker 2: black holes cannot all be thought of as tiny objects.
• 15:29
  Speaker 2: Some have horizons measuring only tens of kilometres across, while
• 15:33
  Speaker 2: others possess horizons larger than the orbits of planets. There
• 15:37
  Speaker 2: is also an important distinction between radius, diameter, surface area
• 15:42
  Speaker 2: and apparent size. The radius describes the distance from the
• 15:46
  Speaker 2: centre to the horizon in the chosen coordinate system. The
• 15:50
  Speaker 2: diameter is twice that value. The horizon's area is a
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  Speaker 2: geometrical quantity describing its two-dimensional extent. even though the horizon
• 15:59
  Speaker 2: is not a material surface. For a Schwarzschild black hole,
• 16:04
  Speaker 2: the horizon's area increases with the square of its radius.
• 16:08
  Speaker 2: Doubling the mass doubles the radius, but increases the horizon
• 16:12
  Speaker 2: area fourfold. Its apparent size in an astronomical image is
• 16:16
  Speaker 2: something different again. That depends upon the black hole's distance,
• 16:20
  Speaker 2: the observer's position, and the paths taken by light through
• 16:24
  Speaker 2: the surrounding gravitational field. Physicists also use a related quantity
• 16:29
  Speaker 2: called the gravitational radius. This is the gravitational constant multiplied
• 16:34
  Speaker 2: by the black hole's mass, divided by the square of
• 16:37
  Speaker 2: the speed of light. For a Schwarzschild black hole, the
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  Speaker 2: gravitational radius is exactly half the Schwarzschild radius. These two
• 16:46
  Speaker 2: names are easily confused, particularly when scientific discussions switch between
• 16:52
  Speaker 2: them without much warning. Yet they describe different reference lengths.
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  Speaker 2: and the distinction becomes important when comparing an event horizon
• 17:00
  Speaker 2: with the photon sphere or an observed shadow. Of course,
• 17:05
  Speaker 2: real black holes are not necessarily described perfectly by Schwarzschild's
• 17:09
  Speaker 2: idealised solution. Most astrophysical black holes are expected to possess
• 17:14
  Speaker 2: angular momentum, meaning they rotate. Rotation changes the spacetime geometry
• 17:20
  Speaker 2: and alters the relationship between mass and the location of
• 17:23
  Speaker 2: the event horizon. Nevertheless, the Schwarzschild model provides a remarkably
• 17:28
  Speaker 2: useful foundation for understanding what an event horizon actually is.
• 17:34
  Speaker 2: And perhaps its strangest feature is that crossing it need
• 17:37
  Speaker 2: not involve encountering anything immediately visible or tangible. For someone
• 17:42
  Speaker 2: falling freely into a sufficiently large black hole, there would
• 17:46
  Speaker 2: not necessarily be a sudden flash, a solid obstruction, or
• 17:51
  Speaker 2: a dramatic sensation at the precise moment of crossing. Local
• 17:56
  Speaker 2: physical laws would continue to operate normally. Depending on the
• 17:59
  Speaker 2: black hole's mass and the circumstances of the fall, tidal
• 18:03
  Speaker 2: gravitational forces might even be relatively modest at the horizon.
• 18:08
  Speaker 2: The significance of crossing would be enormous, but it would
• 18:11
  Speaker 2: concern the traveller's possible future rather than the appearance of
• 18:15
  Speaker 2: a physical boundary. This is why identifying an event horizon
• 18:20
  Speaker 2: in an astronomical image is not as simple as finding
• 18:23
  Speaker 2: the edge of a dark circle. The horizon itself does
• 18:26
  Speaker 2: not provide an ordinary visible outline. What reaches our telescopes
• 18:31
  Speaker 2: has been shaped by the surrounding spacetime, sometimes along paths
• 18:35
  Speaker 2: that bend considerably before arriving. The true boundary of a
• 18:39
  Speaker 2: black hole is therefore both mathematically precise and remarkably elusive
• 18:44
  Speaker 2: to direct observation. It has a definable location and measurable
• 18:49
  Speaker 2: geometrical properties, yet it is not something that can be
• 18:52
  Speaker 2: illuminated and photographed like the surface of a planet. The
• 18:56
  Speaker 2: darkness we observe around a black hole belongs to a
• 18:59
  Speaker 2: much larger story about the journey of light. The event
• 19:03
  Speaker 2: horizon determines where escape becomes impossible, but the image arriving
• 19:07
  Speaker 2: at our telescopes is shaped by the gravitational field extending
• 19:11
  Speaker 2: far beyond that boundary. And that difference between where a
• 19:15
  Speaker 2: black hole actually begins and where its apparent darkness begins
• 19:18
  Speaker 2: is precisely what makes its appearance so extraordinary. Light has
• 19:23
  Speaker 2: a reputation for travelling in straight lines. Shine a torch
• 19:27
  Speaker 2: across a dark room and the beam appears to follow
• 19:30
  Speaker 2: a perfectly direct route. Sunlight passing through a narrow opening
• 19:34
  Speaker 2: produces a familiar pattern of illumination, and distant stars seem
• 19:39
  Speaker 2: to occupy fixed positions in the night sky. Under ordinary
• 19:44
  Speaker 2: circumstances there is little reason to imagine that light might
• 19:47
  Speaker 2: take anything other than the most straightforward journey. between two places,
• 19:51
  Speaker 2: but the straightness of a path depends upon the geometry
• 19:55
  Speaker 2: of the space through which it travels. In empty, flat space-time,
• 19:59
  Speaker 2: light follows straight trajectories. There is nothing to redirect its motion,
• 20:04
  Speaker 2: and its direction remains unchanged unless it encounters matter or
• 20:08
  Speaker 2: some other influence. yet the universe is not everywhere flat. Stars, planets,
• 20:15
  Speaker 2: galaxies and black holes all influence the geometry surrounding them,
• 20:19
  Speaker 2: and this changes what a straight path can mean. The
• 20:23
  Speaker 2: explanation comes from Albert Einstein's General Theory of Relativity, completed
• 20:28
  Speaker 2: in 1915. Einstein proposed that gravity is not simply an
• 20:32
  Speaker 2: invisible force pulling objects towards one another. Instead, Matter and
• 20:38
  Speaker 2: energy influence the curvature of spacetime and freely moving objects
• 20:42
  Speaker 2: follow trajectories determined by that geometry. Spacetime combines the three
• 20:47
  Speaker 2: dimensions of space with the dimension of time into a
• 20:51
  Speaker 2: single four-dimensional structure. Its curvature describes how distances, durations and
• 20:58
  Speaker 2: possible paths differ from those in the flat spacetime of
• 21:01
  Speaker 2: special relativity. The distinction may sound rather abstract but it
• 21:07
  Speaker 2: leads to a beautiful way of understanding gravity. Consider the
• 21:11
  Speaker 2: Earth travelling around the Sun. In the traditional Newtonian description,
• 21:16
  Speaker 2: the Sun exerts a gravitational force that continually changes Earth's direction.
• 21:22
  Speaker 2: In general relativity, Earth's freely falling motion follows a natural
• 21:27
  Speaker 2: trajectory through the curved spacetime surrounding the Sun. The mathematical
• 21:32
  Speaker 2: name for such a trajectory is a geodesic, A geodesic
• 21:36
  Speaker 2: is the generalisation of a straight line to curved geometry.
• 21:40
  Speaker 2: On the surface of a sphere, for example, the shortest
• 21:43
  Speaker 2: route between two locations follows part of a great circle.
• 21:48
  Speaker 2: When represented on certain flat maps, that route can appear curved,
• 21:52
  Speaker 2: even though it is the natural shortest path across the
• 21:54
  Speaker 2: spherical surface. Space-time is more complicated than the surface of
• 21:59
  Speaker 2: a globe, particularly because time is part of its geometry. Nevertheless,
• 22:04
  Speaker 2: the underlying principle remains useful. An object can follow its
• 22:08
  Speaker 2: natural path through curved geometry without requiring a conventional force
• 22:13
  Speaker 2: to continually steer it. Light follows a special kind of
• 22:17
  Speaker 2: geodesic called a null geodesic. Locally, in a freely falling
• 22:21
  Speaker 2: reference frame, its trajectory is straight and it always travels
• 22:26
  Speaker 2: at the speed of light in a vacuum. Yet when
• 22:28
  Speaker 2: that same trajectory is examined across a larger region of
• 22:31
  Speaker 2: curved spacetime, it can appear to bend. This also answers
• 22:36
  Speaker 2: a question that often arises whenever gravity and light are discussed.
• 22:40
  Speaker 2: If photons have no rest mass, why should gravity affect
• 22:44
  Speaker 2: them at all? The answer is that general relativity does
• 22:47
  Speaker 2: not require light to possess rest mass in order to
• 22:50
  Speaker 2: respond to gravity. Photons travel through space-time and their trajectories
• 22:55
  Speaker 2: are governed by its geometry. They are therefore affected by
• 22:59
  Speaker 2: the gravitational fields of massive objects despite having no rest mass.
• 23:04
  Speaker 2: This prediction provided one of the earliest opportunities to test
• 23:08
  Speaker 2: Einstein's theory. In May 1919, a total solar eclipse allowed
• 23:13
  Speaker 2: astronomers to observe stars appearing close to the Sun in
• 23:17
  Speaker 2: the sky. Ordinarily, the Sun's brightness makes those stars impossible
• 23:21
  Speaker 2: to see, but during totality, the Moon temporarily blocks the
• 23:25
  Speaker 2: solar disc. British astronomical expeditions organised under the leadership of
• 23:31
  Speaker 2: Arthur Stanley Eddington and Frank Watson Dyson travelled to the
• 23:35
  Speaker 2: island of Principe, off the western coast of Africa and
• 23:39
  Speaker 2: Sobral in Brazil. Their objective was to measure the apparent
• 23:43
  Speaker 2: positions of stars whose light had passed close to the Sun.
• 23:47
  Speaker 2: According to general relativity, the Sun's gravitational field should bend
• 23:52
  Speaker 2: this incoming starlight, causing the stars to appear slightly displaced
• 23:56
  Speaker 2: from their usual positions. Einstein's prediction for light passing close
• 24:01
  Speaker 2: to the sun's visible edge was a deflection of approximately 1.75 arcseconds.
• 24:08
  Speaker 2: An arcsecond is 1 3600th of a degree. The predicted
• 24:13
  Speaker 2: displacement was therefore extraordinarily small, requiring careful astronomical measurements. The
• 24:19
  Speaker 2: eclipse observations were announced as supporting Einstein's prediction. Although the
• 24:24
  Speaker 2: original measurements had uncertainties and their interpretation has received considerable
• 24:29
  Speaker 2: historical scrutiny, later observations using more precise techniques have firmly
• 24:34
  Speaker 2: confirmed gravitational light deflection. The discovery revealed something remarkable. Even
• 24:40
  Speaker 2: the Sun, whose gravity seems relatively ordinary compared with that
• 24:44
  Speaker 2: of a black hole, changes the paths of light travelling
• 24:47
  Speaker 2: past it. Near the Sun, the effect is small, A
• 24:51
  Speaker 2: passing ray changes direction by only a tiny angle. Near
• 24:55
  Speaker 2: a black hole the situation can become dramatically different. The
• 24:59
  Speaker 2: curvature of spacetime becomes increasingly significant as light approaches the
• 25:04
  Speaker 2: black hole and the amount of deflection depends upon the
• 25:07
  Speaker 2: trajectory it follows. A ray passing relatively far away may
• 25:11
  Speaker 2: experience only a modest change in direction. Another approaching much
• 25:15
  Speaker 2: closer can be redirected through a large angle. while certain
• 25:19
  Speaker 2: trajectories can carry light around the black hole before it escapes.
• 25:24
  Speaker 2: This behaviour is called gravitational lensing. The name is useful,
• 25:28
  Speaker 2: although it can create a slightly misleading impression. An ordinary
• 25:32
  Speaker 2: glass lens bends light through refraction because light propagates differently
• 25:37
  Speaker 2: through the material than through the surrounding air or vacuum.
• 25:41
  Speaker 2: A black hole does not need a transparent material to
• 25:44
  Speaker 2: act as a gravitational lens. The bending arises from the
• 25:48
  Speaker 2: geometry of spacetime itself. Nor does the process require light
• 25:52
  Speaker 2: to slow below its usual vacuum speed in its immediate surroundings.
• 25:57
  Speaker 2: Every freely falling local observer measuring a passing photon obtains
• 26:02
  Speaker 2: the same fundamental speed of light. The curved trajectory emerges
• 26:07
  Speaker 2: when the journey is considered across a larger region of spacetime.
• 26:11
  Speaker 2: This has an extraordinary consequence for astronomy. When we look
• 26:15
  Speaker 2: towards a distant object, we naturally imagine that the light
• 26:18
  Speaker 2: reaching our eyes has travelled directly from its source. Under
• 26:22
  Speaker 2: ordinary circumstances, that approximation works extremely well. But in a
• 26:27
  Speaker 2: strongly curved region, the light arriving from one direction may
• 26:31
  Speaker 2: have originated somewhere entirely different from where a straight-line reconstruction
• 26:36
  Speaker 2: would suggest. A distant star might appear displaced. A background
• 26:40
  Speaker 2: galaxy might appear stretched into an arc. Light emitted from
• 26:44
  Speaker 2: behind a black hole might travel around its gravitational field
• 26:48
  Speaker 2: and reach an observer who could never have seen it
• 26:51
  Speaker 2: along a direct route. The apparent position of something is
• 26:55
  Speaker 2: therefore not always its actual position. And when the gravitational
• 26:59
  Speaker 2: field becomes strong enough to capture some of those incoming
• 27:02
  Speaker 2: rays completely, even the outline of darkness begins to change.
• 27:07
  Speaker 2: The black hole has not altered the nature of light.
• 27:10
  Speaker 2: Photons still follow the physical laws governing their motion. What
• 27:14
  Speaker 2: has changed is the geometry through which they travel. That
• 27:18
  Speaker 2: is why a black hole can appear substantially larger than
• 27:21
  Speaker 2: its event horizon. The image is not determined simply by
• 27:25
  Speaker 2: where the physical boundary lies, but by which paths through
• 27:29
  Speaker 2: curved spacetime can carry light towards us. What we see
• 27:33
  Speaker 2: is shaped not only by the objects producing or blocking
• 27:36
  Speaker 2: the light, but by the entire journey the light has
• 27:39
  Speaker 2: taken to reach our eyes. There is a rather curious
• 27:42
  Speaker 2: problem with looking at the universe. We tend to assume
• 27:46
  Speaker 2: that whatever appears in a particular direction must actually be
• 27:49
  Speaker 2: located there. A distant galaxy occupies a small patch of
• 27:54
  Speaker 2: the night sky, and we naturally imagine a straight line
• 27:57
  Speaker 2: connecting our eyes to that galaxy. Yet sometimes, the light
• 28:02
  Speaker 2: reaching us has followed a very different route. It may
• 28:05
  Speaker 2: have travelled around an enormous concentration of matter, changed direction
• 28:10
  Speaker 2: several times, or arrived alongside other light that began its
• 28:14
  Speaker 2: journey at exactly the same source. This remarkable behaviour is
• 28:18
  Speaker 2: known as gravitational lensing, and it means that gravity can
• 28:21
  Speaker 2: change not only how objects move, but how the universe appears.
• 28:26
  Speaker 2: The basic principle follows directly from Albert Einstein's general theory
• 28:30
  Speaker 2: of relativity. Matter and energy curve spacetime, and light travels
• 28:35
  Speaker 2: along paths determined by that curvature. When a beam passes
• 28:39
  Speaker 2: near a massive object, its direction can change. To a
• 28:43
  Speaker 2: distant observer, the source of that light may consequently appear
• 28:46
  Speaker 2: somewhere other than its true position. Einstein investigated the gravitational
• 28:51
  Speaker 2: deflection of light during the development of general relativity. In 1936,
• 28:57
  Speaker 2: he also published a calculation describing how the gravity of
• 29:00
  Speaker 2: one star could produce a ring-shaped image of another star
• 29:04
  Speaker 2: lying almost directly behind it. The idea was elegant. although
• 29:08
  Speaker 2: the tiny angular separations involved made such stellar lensing extremely
• 29:14
  Speaker 2: difficult to observe with the instruments available at the time. Today,
• 29:18
  Speaker 2: gravitational lensing has become one of astronomy's most useful methods
• 29:22
  Speaker 2: for studying distant objects. It allows scientists to investigate the
• 29:26
  Speaker 2: distribution of matter in galaxies, examine enormous clusters of galaxies,
• 29:32
  Speaker 2: and sometimes observe objects that would otherwise be too faint
• 29:35
  Speaker 2: to detect. The effect is easiest to understand by imagining
• 29:40
  Speaker 2: a distant galaxy with another massive galaxy positioned between it
• 29:44
  Speaker 2: and Earth. Light from the background galaxy travels towards us,
• 29:49
  Speaker 2: but its path passes through the curved spacetime surrounding the
• 29:52
  Speaker 2: foreground galaxy. Different rays encounter different amounts of gravitational deflection.
• 29:59
  Speaker 2: As a result, the image arriving at our telescopes may
• 30:01
  Speaker 2: appear displaced, stretched, enlarged, or divided into several separate images.
• 30:07
  Speaker 2: The foreground galaxy has not physically changed the shape of
• 30:11
  Speaker 2: the background galaxy. It has changed the routes by which
• 30:15
  Speaker 2: its light reaches us. Astronomers distinguish between two particularly important
• 30:20
  Speaker 2: forms of gravitational lensing. The first is weak gravitational lensing,
• 30:26
  Speaker 2: This occurs when the distortion is comparatively subtle. A distant
• 30:30
  Speaker 2: galaxy may appear slightly stretched or sheared, but the change
• 30:34
  Speaker 2: can be so small that it is difficult to identify
• 30:37
  Speaker 2: from a single image. By examining the shapes of large
• 30:42
  Speaker 2: numbers of galaxies, however, astronomers can detect statistical patterns in
• 30:47
  Speaker 2: these distortions. Such measurements help reveal how matter is distributed
• 30:52
  Speaker 2: across enormous regions of space. including dark matter that does
• 30:57
  Speaker 2: not emit detectable light of its own. The second form
• 31:01
  Speaker 2: is strong gravitational lensing. Here, the bending becomes substantial enough
• 31:06
  Speaker 2: to produce unmistakable changes in appearance. A background galaxy may
• 31:11
  Speaker 2: be stretched into a long luminous arc. Several images of
• 31:15
  Speaker 2: the same distant object may appear around a foreground galaxy
• 31:18
  Speaker 2: or cluster. Under particularly favourable circumstances, the light can form
• 31:24
  Speaker 2: something resembling a complete circle. This is called an Einstein ring.
• 31:29
  Speaker 2: An Einstein ring occurs when the background source, the gravitational
• 31:33
  Speaker 2: lens and the observer are sufficiently well aligned. Light travelling
• 31:37
  Speaker 2: around different sides of the intervening mass is bent towards
• 31:41
  Speaker 2: the observer, producing a ring-shaped image centred approximately on the lens.
• 31:47
  Speaker 2: Perfect alignment and sufficient symmetry can produce a complete ring,
• 31:51
  Speaker 2: while less precise arrangements often create partial rings or curved arcs.
• 31:57
  Speaker 2: The remarkable point is that the background source has not
• 32:00
  Speaker 2: become circular. A galaxy containing billions of stars may be
• 32:04
  Speaker 2: transformed into an apparent ring simply because its light has
• 32:08
  Speaker 2: travelled through curved spacetime. Gravitational lensing can also increase the
• 32:13
  Speaker 2: apparent angular size of an object. Astronomers call this magnification.
• 32:18
  Speaker 2: Magnification does not mean that the source has physically expanded.
• 32:23
  Speaker 2: Its stars have not moved farther apart and the galaxy
• 32:26
  Speaker 2: itself has not suddenly grown. Instead, the lens changes the
• 32:31
  Speaker 2: angular area occupied by the image in the observer's sky.
• 32:35
  Speaker 2: This can make a distant object easier to study because
• 32:38
  Speaker 2: gravitational lensing may increase the amount of its light received
• 32:41
  Speaker 2: per unit solid angle of the original source image's apparent
• 32:45
  Speaker 2: extent While conserving surface brightness in the ideal case, there
• 32:50
  Speaker 2: is another unusual consequence. Light from the same source can
• 32:54
  Speaker 2: reach Earth along several different trajectories. Imagine a distant quasar,
• 32:59
  Speaker 2: an exceptionally luminous galactic centre powered by matter accreting towards
• 33:04
  Speaker 2: a supermassive black hole. If a massive galaxy lies between
• 33:08
  Speaker 2: the quasar and Earth, some of the quasar's light may
• 33:11
  Speaker 2: travel around one side of the intervening galaxy. while other
• 33:15
  Speaker 2: light follows a different route. Both paths can lead to
• 33:18
  Speaker 2: the same observer. The telescope may therefore record two or
• 33:22
  Speaker 2: more separate images of a single quasar. Each appears in
• 33:26
  Speaker 2: a different position, even though all the light originated from
• 33:29
  Speaker 2: the same astronomical object. Because the paths have different lengths
• 33:34
  Speaker 2: and pass through different gravitational environments, the light may also
• 33:38
  Speaker 2: arrive at different times. If the quasar changes brightness, the
• 33:42
  Speaker 2: same variation can appear in its separate images with a
• 33:45
  Speaker 2: measurable delay. The precise appearance depends upon several factors. The
• 33:50
  Speaker 2: mass of the intervening object matters, but so does the
• 33:53
  Speaker 2: distribution of that mass. A compact spherical lens produces different
• 33:58
  Speaker 2: patterns from an extended galaxy or an irregular cluster. The
• 34:02
  Speaker 2: distances between source, lens and observer are also important, as
• 34:07
  Speaker 2: is their alignment. This variety makes gravitational lensing a powerful
• 34:12
  Speaker 2: scientific tool, although it also reminds us that the appearance
• 34:15
  Speaker 2: of an astronomical object is not always a simple picture
• 34:18
  Speaker 2: of its physical structure. Around a black hole, these principles
• 34:23
  Speaker 2: become especially dramatic. The gravitational field near the event horizon
• 34:28
  Speaker 2: is much stronger than the fields responsible for most familiar
• 34:31
  Speaker 2: galaxy-scale lensing effects. Light passing sufficiently close can be redirected
• 34:37
  Speaker 2: through enormous angles. Some rays may travel around a substantial
• 34:41
  Speaker 2: portion of the black hole before escaping, while others can
• 34:44
  Speaker 2: circle it more than once under extremely precise conditions, and
• 34:48
  Speaker 2: some light never escapes at all. This introduces an important
• 34:53
  Speaker 2: distinction between gravitational magnification and the black hole shadow. An
• 34:58
  Speaker 2: Einstein ring is formed from light that successfully reaches the
• 35:02
  Speaker 2: observer after being redirected by gravity. A black hole shadow,
• 35:06
  Speaker 2: by contrast, is associated with directions from which background light
• 35:10
  Speaker 2: cannot reach the observer because the corresponding photon trajectories are captured.
• 35:16
  Speaker 2: Both phenomena arise from curved spacetime, but one concerns the
• 35:20
  Speaker 2: redistribution of arriving light, while the other concerns its absence.
• 35:25
  Speaker 2: The shadow is therefore not simply a magnified picture of
• 35:28
  Speaker 2: a dark sphere. Its outline is determined by the boundary
• 35:32
  Speaker 2: between photon trajectories that can connect distant sources to an
• 35:37
  Speaker 2: observer and those that terminate at the event horizon when
• 35:40
  Speaker 2: traced backwards. Looking towards a black hole means looking into
• 35:44
  Speaker 2: a complicated arrangement of possible light paths. Some photons arrive
• 35:49
  Speaker 2: after modest deflections, others have travelled around the central mass.
• 35:53
  Speaker 2: Still others are missing because their trajectories lead into the
• 35:56
  Speaker 2: black hole. What appears in the final image is the
• 35:59
  Speaker 2: combined result of all these possibilities, And that is why
• 36:03
  Speaker 2: the apparent size of a black hole cannot be understood
• 36:06
  Speaker 2: simply by measuring its physical horizon. Gravity changes the routes
• 36:10
  Speaker 2: by which light reaches us, and in doing so, changes
• 36:14
  Speaker 2: the dimensions of the darkness we see. The universe has
• 36:18
  Speaker 2: not moved the black hole or enlarged its event horizon.
• 36:21
  Speaker 2: It has altered the geometry of the light travelling around it. Sometimes,
• 36:26
  Speaker 2: what we see tells us as much about the space
• 36:28
  Speaker 2: between ourselves and an object as it does about the
• 36:31
  Speaker 2: object itself. Imagine a beam of light travelling through the
• 36:36
  Speaker 2: darkness of space towards a black hole. From a great distance,
• 36:40
  Speaker 2: its journey might appear entirely ordinary. The photon moves through
• 36:44
  Speaker 2: the vacuum at the speed of light, following a path
• 36:47
  Speaker 2: that would ordinarily carry it onwards through the universe. Yet
• 36:51
  Speaker 2: as it approaches the black hole, the geometry of spacetime
• 36:55
  Speaker 2: begins to influence its direction. Depending upon the path it follows,
• 37:00
  Speaker 2: that photon may experience one of several very different outcomes.
• 37:04
  Speaker 2: Some light passes relatively far from the black hole and
• 37:07
  Speaker 2: continues onwards, experiencing only a small change in direction. Other
• 37:12
  Speaker 2: photons approach more closely and are deflected through much larger angles.
• 37:18
  Speaker 2: A few may follow extraordinary curved trajectories around the central
• 37:21
  Speaker 2: mass before eventually escaping, and some cross a boundary from
• 37:26
  Speaker 2: which they can never return. The difference between these outcomes
• 37:30
  Speaker 2: depends not upon the photon becoming tired, slowing down, or
• 37:34
  Speaker 2: running out of energy, but upon the geometry of its trajectory.
• 37:39
  Speaker 2: To describe these different journeys, physicists use a quantity called
• 37:43
  Speaker 2: the impact parameter. Imagine a photon approaching a black hole
• 37:47
  Speaker 2: from a very great distance, where the gravitational field is
• 37:50
  Speaker 2: comparatively weak. If spacetime were flat, the photon would continue
• 37:55
  Speaker 2: along a straight line. The perpendicular distance between that imagined
• 37:59
  Speaker 2: straight line and the centre of the black hole provides
• 38:02
  Speaker 2: a useful picture of the impact parameter. More precisely, for
• 38:06
  Speaker 2: light approaching a stationary, non-rotating black hole from infinity, the
• 38:11
  Speaker 2: impact parameter is related to the photon's angular momentum divided
• 38:15
  Speaker 2: by its energy. It tells us how the initial trajectory
• 38:18
  Speaker 2: is positioned relative to the central mass. A large impact
• 38:22
  Speaker 2: parameter means the photon approaches along a route that would
• 38:25
  Speaker 2: miss the black hole by a considerable distance. A small
• 38:29
  Speaker 2: impact parameter means its trajectory is directed much closer to
• 38:32
  Speaker 2: the centre, and this distinction has remarkable consequences. Consider a
• 38:38
  Speaker 2: photon passing far outside the event horizon. The black hole's
• 38:42
  Speaker 2: gravity bends its path slightly, but the photon retains trajectory
• 38:47
  Speaker 2: that carries it away into distant space. Its direction has changed.
• 38:51
  Speaker 2: yet it remains free to continue travelling through the universe.
• 38:56
  Speaker 2: Now imagine another photon approaching more closely. The curvature of
• 39:00
  Speaker 2: spacetime becomes more important, and its path bends through a
• 39:04
  Speaker 2: greater angle. Instead of continuing almost straight ahead, it may
• 39:09
  Speaker 2: sweep around the black hole before heading outwards again. The
• 39:12
  Speaker 2: closer the incoming trajectory approaches the critical conditions for capture,
• 39:17
  Speaker 2: the more dramatic this deflection can become. There is, however,
• 39:21
  Speaker 2: a limit separating light that escapes from light that falls
• 39:24
  Speaker 2: into the black hole. This dividing value is called the
• 39:27
  Speaker 2: critical impact parameter. For an ideal non-rotating Schwarzschild black hole,
• 39:33
  Speaker 2: its value is determined by the black hole's mass, the
• 39:36
  Speaker 2: gravitational constant, and the speed of light. The critical impact
• 39:41
  Speaker 2: parameter equals three times the square root of three, multiplied
• 39:45
  Speaker 2: by the gravitational constant and the black hole's mass, divided
• 39:49
  Speaker 2: by the square of the speed of light. Although that
• 39:52
  Speaker 2: sounds rather formidable when spoken aloud, its physical meaning is straightforward.
• 39:58
  Speaker 2: The critical impact parameter is approximately 2.6 times the Schwarzschild radius.
• 40:04
  Speaker 2: For photons approaching from a great distance, an impact parameter
• 40:07
  Speaker 2: greater than this critical value allows the light to escape again.
• 40:11
  Speaker 2: A smaller impact parameter leads to capture, with the photon
• 40:15
  Speaker 2: eventually crossing the event horizon. At the exact critical value,
• 40:20
  Speaker 2: the idealised photon trajectory approaches an unstable circular orbit rather
• 40:25
  Speaker 2: than escaping or crossing the horizon in the usual way.
• 40:29
  Speaker 2: This produces a particularly surprising result. A photon does not
• 40:33
  Speaker 2: have to be travelling directly towards the event horizon to
• 40:37
  Speaker 2: fall into a black hole. Its original straight-line path might
• 40:40
  Speaker 2: have missed the horizon completely. Nevertheless, the curvature of spacetime
• 40:45
  Speaker 2: can redirect that photon sufficiently it to become captured. A
• 40:50
  Speaker 2: black hole therefore intercepts light arriving along a wider range
• 40:54
  Speaker 2: of trajectories than its physical horizon might initially suggest. The
• 40:58
  Speaker 2: distinction becomes clearer with a numerical example. Consider a non-rotating
• 41:04
  Speaker 2: black hole containing 10 solar masses. Its Schwarzschild radius is
• 41:08
  Speaker 2: approximately 30 kilometres, giving its event horizon a diameter of
• 41:12
  Speaker 2: roughly 60 kilometres. Yet the critical impact parameter for incoming
• 41:17
  Speaker 2: light is approximately 77 kilometres. This means that photons arriving
• 41:22
  Speaker 2: from far away can be captured even when their imagined
• 41:25
  Speaker 2: straight-line paths would have missed the horizon by several tens
• 41:28
  Speaker 2: of kilometres. The effect is not caused by the event
• 41:32
  Speaker 2: horizon extending outwards. It arises because the light itself follows
• 41:36
  Speaker 2: a different path through curved spacetime. And once a photon
• 41:40
  Speaker 2: crosses the event horizon, the situation changes fundamentally. Inside a
• 41:45
  Speaker 2: black hole, every future-directed light-like path remains within the horizon.
• 41:50
  Speaker 2: There is no outward trajectory capable of carrying a signal
• 41:54
  Speaker 2: back to distant observers. This does not mean that photons
• 41:58
  Speaker 2: suddenly stop moving. Light continues to travel at the speed
• 42:01
  Speaker 2: of light when measured locally by a freely falling observer.
• 42:05
  Speaker 2: The familiar physical law governing its speed remains valid. What
• 42:09
  Speaker 2: changes is the causal structure of spacetime. Inside the event horizon...
• 42:14
  Speaker 2: All physically possible future-directed paths lead towards the black hole's
• 42:18
  Speaker 2: interior rather than escaping to the outside universe. Travelling faster
• 42:22
  Speaker 2: would not solve the problem, because no physical signal can
• 42:26
  Speaker 2: exceed the local speed of light. The restriction is built
• 42:29
  Speaker 2: into the geometry of spacetime itself. Astronomers can examine these
• 42:33
  Speaker 2: trajectories from another perspective, using a technique called gravitational ray tracing.
• 42:40
  Speaker 2: Instead of following a photon forwards from a distant source,
• 42:44
  Speaker 2: Imagine tracing a possible light ray backwards from a telescope
• 42:48
  Speaker 2: towards the black hole. For some viewing directions, the backwards
• 42:52
  Speaker 2: trajectory curves around the central mass and eventually reaches a
• 42:56
  Speaker 2: distant region of space. Light from a suitable source could
• 43:01
  Speaker 2: travel along that path and arrive at the telescope. For
• 43:06
  Speaker 2: other directions, the backwards trajectory reaches the event horizon. Those
• 43:11
  Speaker 2: paths cannot connect the observer to a distant source behind
• 43:14
  Speaker 2: the black hole. The boundary between these two possibilities determines
• 43:19
  Speaker 2: the geometrical edge of the black hole shadow. Real astronomical
• 43:23
  Speaker 2: images are more complicated because glowing gas in front of
• 43:26
  Speaker 2: the black hole can still contribute light within the shadow's
• 43:29
  Speaker 2: apparent outline. Nevertheless, the underlying separation between escaping and captured
• 43:35
  Speaker 2: trajectories remains fundamental. This is why a black hole's shadow
• 43:39
  Speaker 2: can be substantially larger than its event horizon. The darkness
• 43:44
  Speaker 2: does not simply outline the place where the black hole begins.
• 43:48
  Speaker 2: It reflects the range of directions from which light cannot
• 43:51
  Speaker 2: reach us because the corresponding trajectories are captured. What appears
• 43:56
  Speaker 2: to be the edge of a dark object is therefore
• 43:58
  Speaker 2: something considerably more interesting. It is a boundary drawn not
• 44:03
  Speaker 2: by matter but by the possible journeys of light through
• 44:06
  Speaker 2: curved spacetimes. There is a region surrounding a black hole
• 44:10
  Speaker 2: where light can do something rather extraordinary. Instead of simply
• 44:14
  Speaker 2: passing by, escaping into the distance, or disappearing across the
• 44:19
  Speaker 2: event horizon, a photon can theoretically travel in a complete
• 44:23
  Speaker 2: circle around the black hole. It can follow an orbit,
• 44:27
  Speaker 2: returning to the same place again and again while continuing
• 44:31
  Speaker 2: to move at the speed of light. This unusual region
• 44:34
  Speaker 2: is known as the photon sphere. and it is one
• 44:37
  Speaker 2: of the most remarkable predictions of Albert Einstein's general theory
• 44:41
  Speaker 2: of relativity. To understand how it works, consider an ordinary
• 44:45
  Speaker 2: planet orbiting a star. The Earth follows a nearly circular
• 44:50
  Speaker 2: path around the Sun because of the Sun's gravitational influence.
• 44:54
  Speaker 2: Its orbital motion continually changes direction, preventing it from travelling
• 44:59
  Speaker 2: away along a straight line. Light behaves differently from a planet,
• 45:03
  Speaker 2: but the curved spacetime surrounding a black hole produce something
• 45:07
  Speaker 2: that looks surprisingly similar. In the idealised Schwarzschild description of
• 45:13
  Speaker 2: a non-rotating, uncharged black hole, there is a particular distance
• 45:17
  Speaker 2: from the centre at which photons can follow circular trajectories.
• 45:23
  Speaker 2: That distance is exactly one and a half times the
• 45:26
  Speaker 2: Schwarzschild radius. The Schwarzschild radius marks the position of the
• 45:31
  Speaker 2: event horizon, the boundary beyond which escape becomes impossible. The
• 45:36
  Speaker 2: photon sphere lies outside that horizon, in a region where
• 45:40
  Speaker 2: light is still capable of escaping under the right circumstances.
• 45:44
  Speaker 2: For a black hole containing 10 times the mass of
• 45:47
  Speaker 2: the Sun, the Schwarzschild radius would be approximately 30 kilometres.
• 45:52
  Speaker 2: Its photon sphere would therefore lie at a radius of
• 45:55
  Speaker 2: approximately 45 kilometres. Only 15 kilometres separate these two characteristic
• 46:01
  Speaker 2: radii in this particular example. Yet they describe very different
• 46:05
  Speaker 2: features of spacetime. The event horizon is a causal boundary.
• 46:10
  Speaker 2: Once light crosses it, no future-directed trajectory can carry that
• 46:14
  Speaker 2: light back into the outside universe. The photon sphere, by contrast,
• 46:20
  Speaker 2: is not a boundary separating the inside from the outside.
• 46:24
  Speaker 2: It identifies the location of a special collection of circular
• 46:27
  Speaker 2: light trajectories. A photon following one of these trajectories has
• 46:31
  Speaker 2: not become stationary. nor has it slowed down. It continues
• 46:36
  Speaker 2: travelling at the speed of light, as measured locally, while
• 46:39
  Speaker 2: the geometry of spacetime continually changes its direction. The result
• 46:45
  Speaker 2: is an orbit. For an ordinary observer, the idea of
• 46:48
  Speaker 2: something moving at the speed of light while continually going
• 46:52
  Speaker 2: around in a circle may sound rather improbable. But there
• 46:56
  Speaker 2: is no contradiction. Speed describes how quickly something moves, while
• 47:01
  Speaker 2: its trajectory describes the direction in which it travels. A
• 47:05
  Speaker 2: photon can maintain its local speed while following a curved path,
• 47:10
  Speaker 2: just as a moving object can change direction without necessarily
• 47:13
  Speaker 2: changing its speed. There is, however, an important complication. The
• 47:18
  Speaker 2: circular photon orbits surrounding a Schwarzschild black hole are unstable.
• 47:23
  Speaker 2: This means that a photon must follow an exceptionally precise
• 47:27
  Speaker 2: trajectory to remain in such an orbit. Even a minute
• 47:30
  Speaker 2: deviation from the exact required conditions causes it to move
• 47:34
  Speaker 2: away from the circular path. A small outward disturbance can
• 47:39
  Speaker 2: eventually allow the photon to escape towards distant space. A
• 47:43
  Speaker 2: small inward disturbance can send it towards the event horizon,
• 47:47
  Speaker 2: where it may become permanently captured. The photon sphere is
• 47:51
  Speaker 2: therefore very different from a stable planetary orbit. Earth can
• 47:55
  Speaker 2: remain in its orbit around the Sun for billions of years,
• 47:59
  Speaker 2: despite small gravitational disturbances from other planets. Its orbit can
• 48:04
  Speaker 2: change gradually without immediately becoming unstable. A circular photon orbit
• 48:10
  Speaker 2: around a Schwarzschild black hole offers no comparable stability. The
• 48:14
  Speaker 2: slightest departure from the exact orbit grows rather than remaining
• 48:18
  Speaker 2: harmlessly confined. This is why the photon sphere should not
• 48:23
  Speaker 2: be imagined as a permanent collection of light, endlessly circling
• 48:27
  Speaker 2: the black hole in a brilliant spherical shell. The photon
• 48:32
  Speaker 2: sphere is a mathematical and geometrical feature of spacetime. It
• 48:36
  Speaker 2: describes where certain possible trajectories exist, not a solid surface
• 48:41
  Speaker 2: or a luminous object. In an ideal mathematical model, a
• 48:46
  Speaker 2: photon placed on precisely the correct circular orbit could continue
• 48:50
  Speaker 2: circling indefinitely. In realistic circumstances, however, Photons approaching the black
• 48:56
  Speaker 2: hole are not generally placed on those exact trajectories. Instead,
• 49:00
  Speaker 2: some pass extremely close to the critical conditions and follow
• 49:04
  Speaker 2: paths that resemble portions of circular orbits. They may travel
• 49:09
  Speaker 2: around the black hole through a large angle, or even
• 49:11
  Speaker 2: complete one or more revolutions before eventually escaping or falling inward.
• 49:17
  Speaker 2: The closer a trajectory comes to the critical conditions, the
• 49:20
  Speaker 2: longer the photon can remain near the photon sphere. This
• 49:25
  Speaker 2: makes the region especially important when considering the appearance of
• 49:28
  Speaker 2: a black hole. Imagine tracing incoming light rays towards a
• 49:32
  Speaker 2: non-rotating black hole. Rays approaching along sufficiently distant trajectories are
• 49:37
  Speaker 2: deflected and escape. Those arriving on closer trajectories may be captured.
• 49:43
  Speaker 2: Between these outcomes lies a critical dividing trajectory associated with
• 49:48
  Speaker 2: the unstable circular photon orbit. The photon sphere therefore governs
• 49:53
  Speaker 2: the boundary between incoming light that can return to distant
• 49:57
  Speaker 2: space and light that disappears through the event horizon. That
• 50:02
  Speaker 2: division is fundamental to the formation of the black hole shadow.
• 50:06
  Speaker 2: but the shadow does not have the same apparent radius
• 50:09
  Speaker 2: as the photon sphere itself. Gravitational bending changes how the
• 50:13
  Speaker 2: photon sphere's critical trajectories appear to a distant observer. Although
• 50:18
  Speaker 2: the photon sphere lies at one and a half times
• 50:21
  Speaker 2: the Schwarzschild radius, the corresponding critical impact parameter is approximately 2.6
• 50:26
  Speaker 2: times that radius. This difference helps explain why the apparent
• 50:32
  Speaker 2: shadow extends so much farther across the observer's sky the
• 50:36
  Speaker 2: event horizon. It is also important not to confuse the
• 50:40
  Speaker 2: photon sphere with the glowing ring seen in astronomical images.
• 50:45
  Speaker 2: The bright emission surrounding a black hole generally comes from
• 50:48
  Speaker 2: hot plasma, with its radiation altered by gravitational lensing and
• 50:53
  Speaker 2: other relativistic effects. The photon sphere is not itself a
• 50:57
  Speaker 2: visible shell of light. Some photons contributing to extremely narrow
• 51:02
  Speaker 2: higher order images may have travelled close to the photon sphere,
• 51:07
  Speaker 2: but the broad luminous ring seen by the Event Horizon
• 51:10
  Speaker 2: Telescope is not simply a photograph of that spherical region.
• 51:14
  Speaker 2: There is one further limitation to this simple picture. The
• 51:19
  Speaker 2: photon sphere at 1.5 Schwarzschild radii belongs specifically to the
• 51:24
  Speaker 2: idealised non-rotating Schwarzschild black hole. Real astrophysical black holes are
• 51:30
  Speaker 2: expected to rotate. Their surrounding spacetime is described more accurately
• 51:35
  Speaker 2: by the Kerr solution, in which frame dragging changes the
• 51:38
  Speaker 2: possible photon trajectories. Instead of one universal spherical radius containing
• 51:44
  Speaker 2: all circular photon orbits, rotating black holes possess a more
• 51:48
  Speaker 2: complicated photon region, with trajectories depending upon their direction and
• 51:53
  Speaker 2: angular momentum. Nevertheless, the underlying phenomena remains. Strongly curved spacetime
• 52:00
  Speaker 2: can redirect light so dramatically that photons may travel around
• 52:04
  Speaker 2: a black hole before escaping or being captured. The photon
• 52:08
  Speaker 2: sphere reveals just how far ordinary intuition about straight line
• 52:12
  Speaker 2: motion can fail near an intense gravitational field. And it
• 52:17
  Speaker 2: explains something essential about the darkness we observe. The apparent
• 52:21
  Speaker 2: outline of a black hole is not simply the edge
• 52:24
  Speaker 2: of the event horizon. It is shaped by the extraordinary
• 52:27
  Speaker 2: trajectories available to light outside that boundary. Even where there
• 52:32
  Speaker 2: is no physical surface, the geometry of space-time can create
• 52:36
  Speaker 2: an unmistakable outline against the surrounding universe. A photon travelling
• 52:42
  Speaker 2: near a black hole can experience something far stranger than
• 52:46
  Speaker 2: an ordinary change in direction. Under the right circumstances, its
• 52:51
  Speaker 2: path can curve so dramatically that it travels almost all
• 52:54
  Speaker 2: the way around the black hole before escaping. If its
• 52:57
  Speaker 2: trajectory is particularly close to a certain critical condition, it
• 53:02
  Speaker 2: may complete an entire revolution. And with even more precise conditions,
• 53:07
  Speaker 2: the same photon can circle the black hole several times
• 53:10
  Speaker 2: before finally continuing its journey through space. This is one
• 53:14
  Speaker 2: of the more extraordinary consequences of gravitational lensing. Light does
• 53:19
  Speaker 2: not simply bend slightly as it passes a black hole.
• 53:22
  Speaker 2: It can follow a path that would seem almost impossible
• 53:25
  Speaker 2: in the familiar geometry of everyday life. The explanation begins
• 53:29
  Speaker 2: with the unstable photon sphere surrounding a non-rotating black hole.
• 53:34
  Speaker 2: At a radius of one and a half times the
• 53:36
  Speaker 2: Schwarzschild radius, light can theoretically travel along circular orbits. These
• 53:42
  Speaker 2: orbits are perfectly possible within general relativity, but they are
• 53:46
  Speaker 2: extraordinarily sensitive to even the smallest deviations. Most photons approaching
• 53:51
  Speaker 2: from distant space do not settle into perfectly circular orbits. Instead,
• 53:56
  Speaker 2: their trajectories either carry them past the black hole or
• 54:00
  Speaker 2: eventually lead them through the event horizon. Between these two
• 54:04
  Speaker 2: outcomes lies a critical boundary. For light approaching a Schwarzschild
• 54:09
  Speaker 2: black hole from a great distance, the critical impact parameter
• 54:12
  Speaker 2: determines whether the photon escapes or becomes captured. A photon
• 54:17
  Speaker 2: travelling along a trajectory just outside this critical value can
• 54:21
  Speaker 2: approach the photon sphere, curve around the black hole, and
• 54:25
  Speaker 2: eventually escape towards the distant universe. As its impact parameter
• 54:30
  Speaker 2: approaches the critical value, the amount of gravitational deflection increases dramatically.
• 54:36
  Speaker 2: A photon passing comfortably outside the critical region may change
• 54:40
  Speaker 2: direction by only a modest angle. Another passing closer can
• 54:44
  Speaker 2: turn through half a circle. a photon approaching the critical
• 54:48
  Speaker 2: trajectory even more precisely may bend through an angle greater
• 54:52
  Speaker 2: than a complete revolution. It can travel around the black
• 54:55
  Speaker 2: hole and emerge in a direction that bears little resemblance
• 54:58
  Speaker 2: to its original course. To an observer watching from far away,
• 55:03
  Speaker 2: the arriving light may appear to have come from somewhere
• 55:06
  Speaker 2: entirely different from its actual source, and there is no
• 55:09
  Speaker 2: fundamental limit of one revolution. In the mathematical description of
• 55:14
  Speaker 2: a Schwarzschild black hole, Photons can follow trajectories involving two, three,
• 55:20
  Speaker 2: or many more near orbits before escaping. Each additional revolution
• 55:25
  Speaker 2: requires the incoming trajectory to be tuned increasingly close to
• 55:29
  Speaker 2: the critical impact parameter. The relationship becomes especially fascinating because
• 55:35
  Speaker 2: the required precision increases extremely rapidly. A photon completing several
• 55:40
  Speaker 2: near orbits must approach the unstable photon sphere with extraordinary accuracy.
• 55:46
  Speaker 2: Even a tiny difference in its trajectory can determine whether
• 55:49
  Speaker 2: it escapes after one revolution, continues around again, or disappears
• 55:54
  Speaker 2: through the horizon. This is rather like trying to balance
• 55:57
  Speaker 2: something at an unstable equilibrium point, except that the balancing
• 56:02
  Speaker 2: act involves light travelling at nearly 300,000 km per second
• 56:06
  Speaker 2: through curved spacetime. The photon itself experiences no need to
• 56:11
  Speaker 2: slow down. Its locally measured speed remains constant. It is
• 56:15
  Speaker 2: the curvature of spacetime that redirects its motion. There is
• 56:19
  Speaker 2: also an important difference between temporarily circling a black hole
• 56:24
  Speaker 2: and occupying an exactly circular photon orbit. An ideal photon
• 56:28
  Speaker 2: following the precise circular trajectory could remain there indefinitely in
• 56:33
  Speaker 2: the mathematical model. But a photon arriving from outside generally
• 56:37
  Speaker 2: follows a slightly different path. It spends some time close
• 56:41
  Speaker 2: to the unstable orbit before departing, either outwards towards distant
• 56:45
  Speaker 2: space or inwards towards the event horizon. These temporary loops
• 56:50
  Speaker 2: have remarkable consequences for what a distant observer can see.
• 56:55
  Speaker 2: Imagine a luminous source positioned behind a black hole. Under
• 56:59
  Speaker 2: ordinary circumstances, the black hole would obstruct the direct view.
• 57:03
  Speaker 2: Yet light from that source can sometimes travel around the
• 57:06
  Speaker 2: black hole and reach an observer on the opposite side.
• 57:10
  Speaker 2: Some photons may take a comparatively direct curved route. Others
• 57:15
  Speaker 2: can pass around another side of the black hole, following
• 57:18
  Speaker 2: a different path before arriving. Still others may travel almost
• 57:22
  Speaker 2: completely around the black hole, or perform additional revolutions before
• 57:26
  Speaker 2: reaching the same observer. The result is that one source
• 57:30
  Speaker 2: can produce several distinct images. Astronomers call these higher order
• 57:35
  Speaker 2: lensed images when they arise from trajectories involving increasingly large
• 57:40
  Speaker 2: gravitational deflections. An observer might therefore receive light from the
• 57:45
  Speaker 2: same region of hot gas along several different paths. One
• 57:49
  Speaker 2: image comes from relatively direct radiation, while additional images contain
• 57:54
  Speaker 2: photons that have travelled considerably farther around the black hole.
• 57:58
  Speaker 2: The geometry effectively allows certain regions normally hidden from direct
• 58:02
  Speaker 2: view to become visible through gravitational lensing. In theoretical images
• 58:07
  Speaker 2: of black holes surrounded by luminous material, these different photon
• 58:12
  Speaker 2: paths can produce a remarkable sequence of narrow structures. They
• 58:16
  Speaker 2: are often described as photon rings or photon sub-rings. Each
• 58:21
  Speaker 2: successive higher-order sub-ring is associated with light that has spent
• 58:25
  Speaker 2: more of its journey close to the unstable photon orbits.
• 58:29
  Speaker 2: The photons contributing to these structures have followed increasingly elaborate
• 58:33
  Speaker 2: paths before finally reaching the observer. and the rings do
• 58:37
  Speaker 2: not simply appear at arbitrary distances from the black hole.
• 58:41
  Speaker 2: They accumulate extremely close to a particular curve in the
• 58:45
  Speaker 2: observer's image, known as the critical curve. For an ideal
• 58:49
  Speaker 2: non-rotating black hole viewed from far away, this critical curve
• 58:54
  Speaker 2: is circular and corresponds to the boundary between escaping and
• 58:58
  Speaker 2: captured photon trajectories. As the number of near orbits increases,
• 59:03
  Speaker 2: the associated sub-rings become progressively narrower and approach that limiting curve.
• 59:09
  Speaker 2: Their widths decrease extremely rapidly, approximately exponentially with successive orbital
• 59:15
  Speaker 2: order in the idealised geometrical description. Consequently, a theoretical image
• 59:21
  Speaker 2: with unlimited resolution could contain an increasingly fine sequence of
• 59:26
  Speaker 2: light structures clustered around the black hole's shadow boundary. Real observations, however,
• 59:33
  Speaker 2: introduce several complications. The luminous plasma surrounding a black hole
• 59:38
  Speaker 2: does not necessarily produce sharply separated rings, and the amount
• 59:42
  Speaker 2: of radiation following each higher order path depends upon the
• 59:45
  Speaker 2: material's distribution and emission properties. More importantly, these theoretical sub-rings
• 59:51
  Speaker 2: can be extraordinarily narrow. The famous images produced by the
• 59:56
  Speaker 2: Event Horizon Telescope show a comparatively broad ring of radiation
• 1:00:01
  Speaker 2: surrounding a central brightness depression. That broad structure is largely
• 1:00:06
  Speaker 2: associated with synchrotron emission from hot plasma, whose appearance has
• 1:00:10
  Speaker 2: been altered by gravitational lensing. It is not a separately
• 1:00:15
  Speaker 2: resolved collection of all the extremely narrow photon sub-rings predicted
• 1:00:19
  Speaker 2: by theory. Distinguishing those finer structures requires substantially greater observational
• 1:00:25
  Speaker 2: resolution and careful interpretation. Nevertheless, their theoretical existence reveals something
• 1:00:31
  Speaker 2: profound about the appearance of black holes. A camera pointed
• 1:00:36
  Speaker 2: towards one of these objects does not simply receive light,
• 1:00:39
  Speaker 2: travelling directly from the visible material in front of it.
• 1:00:43
  Speaker 2: Some of the radiation may have travelled behind the black hole.
• 1:00:47
  Speaker 2: Some may have curved around its sides. A tiny fraction
• 1:00:51
  Speaker 2: may have followed trajectories bringing it around the central mass repeatedly,
• 1:00:55
  Speaker 2: before finally escaping. The light arriving at the telescope can
• 1:00:59
  Speaker 2: therefore contain several different views of the same surrounding material,
• 1:01:03
  Speaker 2: superimposed or compressed into an extraordinarily small region of the image.
• 1:01:08
  Speaker 2: What looks like a relatively simple ring of light can
• 1:01:11
  Speaker 2: conceal an intricate collection of journeys through space-time. and the
• 1:01:16
  Speaker 2: closer those journeys come to the critical boundary between capture
• 1:01:19
  Speaker 2: and escape, the more dramatically their paths are altered. A
• 1:01:24
  Speaker 2: black hole does not merely hide whatever lies behind it.
• 1:01:28
  Speaker 2: Through gravitational lensing it can redirect light from places that
• 1:01:31
  Speaker 2: would otherwise remain unseen. The resulting image is shaped by
• 1:01:35
  Speaker 2: routes so curved that ordinary ideas of front, back and
• 1:01:38
  Speaker 2: direct line of sight become surprisingly unreliable. Even a single
• 1:01:43
  Speaker 2: point of light in the surrounding universe may reach us
• 1:01:46
  Speaker 2: by more than one path, having travelled around the same
• 1:01:49
  Speaker 2: black hole in several entirely different ways. A black hole
• 1:01:53
  Speaker 2: can appear more than twice as wide as its event horizon,
• 1:01:56
  Speaker 2: even when viewed from an enormous distance. This is not
• 1:02:00
  Speaker 2: an exaggeration caused by an imperfect telescope, nor is it
• 1:02:04
  Speaker 2: simply an artistic interpretation of something that cannot be seen directly.
• 1:02:09
  Speaker 2: It is a precise mathematical consequence of Albert Einstein's general
• 1:02:13
  Speaker 2: theory of relativity, and one of the clearest demonstrations of
• 1:02:16
  Speaker 2: how strongly curved spacetime changes the appearance of an object.
• 1:02:21
  Speaker 2: For the simplest kind of black hole, the relationship is
• 1:02:24
  Speaker 2: especially elegant. Consider an ideal, non-rotating, uncharged black hole, described
• 1:02:31
  Speaker 2: by the Schwarzschild solution to Einstein's gravitational equations. Imagine observing
• 1:02:36
  Speaker 2: it from a very great distance. with a suitable background
• 1:02:40
  Speaker 2: of light illuminating the region behind it. Under these conditions,
• 1:02:44
  Speaker 2: the black hole produces a circular geometrical shadow. Its boundary
• 1:02:48
  Speaker 2: separates directions from which distant light can reach the observer
• 1:02:53
  Speaker 2: from directions whose corresponding rays are captured by the black hole.
• 1:02:57
  Speaker 2: The surprising result is that this shadow has an apparent
• 1:03:00
  Speaker 2: diameter approximately 2.6 times the diameter of the event horizon.
• 1:03:06
  Speaker 2: To understand where that number comes from, it helps to
• 1:03:09
  Speaker 2: consider three different distances associated with the black hole. The
• 1:03:13
  Speaker 2: first is the gravitational radius. This is a convenient reference
• 1:03:18
  Speaker 2: length determined by the mass of the black hole, the
• 1:03:21
  Speaker 2: gravitational constant, and the speed of light. The gravitational radius
• 1:03:26
  Speaker 2: equals the gravitational constant multiplied by the black hole's mass
• 1:03:30
  Speaker 2: divided by the square of the speed of light. For
• 1:03:33
  Speaker 2: a black hole containing one solar mass, The gravitational radius
• 1:03:37
  Speaker 2: is approximately 1.5 kilometres. The second distance is the Schwarzschild radius,
• 1:03:43
  Speaker 2: which identifies the location of the event horizon in this
• 1:03:47
  Speaker 2: idealised model. The Schwarzschild radius equals twice the gravitational radius. Consequently,
• 1:03:53
  Speaker 2: a black hole containing one solar mass would have an
• 1:03:57
  Speaker 2: event horizon radius of approximately 3 kilometres. The third important
• 1:04:02
  Speaker 2: distance is the radius of the photon sphere. where light
• 1:04:05
  Speaker 2: can theoretically follow unstable circular trajectories around the black hole.
• 1:04:10
  Speaker 2: The photon sphere radius equals three times the gravitational radius.
• 1:04:14
  Speaker 2: This places the photon sphere at one and a half
• 1:04:17
  Speaker 2: times the Schwarzschild radius. It would be tempting to conclude
• 1:04:21
  Speaker 2: that the black hole's shadow should therefore appear one and
• 1:04:24
  Speaker 2: a half times larger than its event horizon. After all,
• 1:04:27
  Speaker 2: the photon sphere lies outside the horizon and plays a
• 1:04:31
  Speaker 2: central role in determining which photons escape. but there is
• 1:04:35
  Speaker 2: an important distinction. The radius of the photon sphere describes
• 1:04:39
  Speaker 2: the location of possible circular photon orbits in Schwarzschild coordinates.
• 1:04:44
  Speaker 2: It does not directly describe the apparent radius of the
• 1:04:47
  Speaker 2: shadow in a distant observer's image. That apparent radius is
• 1:04:52
  Speaker 2: determined by a different quantity called the critical impact parameter.
• 1:04:57
  Speaker 2: The impact parameter describes the geometry of an incoming light
• 1:05:00
  Speaker 2: ray relative to the black hole. when considered far from
• 1:05:04
  Speaker 2: the gravitational field. For a Schwarzschild black hole, there is
• 1:05:09
  Speaker 2: a critical value separating photons that escape from those that
• 1:05:12
  Speaker 2: become captured. The critical impact parameter equals three times the
• 1:05:17
  Speaker 2: square root of three multiplied by the gravitational radius. Three
• 1:05:21
  Speaker 2: times the square root of three is approximately 5.2. The
• 1:05:26
  Speaker 2: critical impact parameter is therefore approximately 5.2 gravitational radii. Meanwhile,
• 1:05:32
  Speaker 2: the event horizon radius is only two gravitational radii. Dividing
• 1:05:37
  Speaker 2: the first distance by the second gives approximately 2.6. And
• 1:05:41
  Speaker 2: there is the answer. The apparent radius of the black
• 1:05:44
  Speaker 2: hole's geometrical shadow is about 2.6 times its event horizon radius.
• 1:05:51
  Speaker 2: Because both are circular in this idealised comparison, the same
• 1:05:54
  Speaker 2: factor applies to their diameters. A simple numerical example makes
• 1:05:59
  Speaker 2: this difference easier to appreciate. Imagine a non-rotating black hole
• 1:06:05
  Speaker 2: containing 10 times the mass of the Sun. Its event
• 1:06:08
  Speaker 2: horizon radius would be approximately 30 kilometres, giving it a
• 1:06:12
  Speaker 2: diameter of around 60 kilometres. Its photon sphere would lie
• 1:06:17
  Speaker 2: at a radius of approximately 45 kilometres, corresponding to a
• 1:06:21
  Speaker 2: coordinate diameter of 90 kilometres. But the critical impact parameter
• 1:06:26
  Speaker 2: would be approximately 78 kilometres. The geometrical shadow would therefore
• 1:06:31
  Speaker 2: have an apparent diameter corresponding to roughly 156 kilometres on
• 1:06:36
  Speaker 2: the distant observer's length scale. In other words, a black
• 1:06:40
  Speaker 2: hole with an event horizon only 60 kilometres across could
• 1:06:44
  Speaker 2: produce a shadow appearing more than 150 kilometres across. The
• 1:06:49
  Speaker 2: event horizon has not grown by a single kilometre. Nothing
• 1:06:52
  Speaker 2: has physically expanded into the surrounding space. Instead, the black
• 1:06:57
  Speaker 2: hole's gravitational field captures photons arriving, along trajectories that would
• 1:07:02
  Speaker 2: ordinarily have passed outside the horizon. These missing light paths
• 1:07:06
  Speaker 2: create an apparent dark region, extending well beyond the horizon's
• 1:07:11
  Speaker 2: direct geometrical projection. There is another way to describe the
• 1:07:15
  Speaker 2: same effect. Physicists sometimes consider the capture cross-section of a
• 1:07:19
  Speaker 2: black hole. This represents the effective area presented to incoming
• 1:07:23
  Speaker 2: light arriving from a great distance. For a Schwarzschild black hole,
• 1:07:28
  Speaker 2: That area is determined by the critical impact parameter rather
• 1:07:32
  Speaker 2: than simply by the event horizon radius. Because the area
• 1:07:36
  Speaker 2: of a circle increases with the square of its radius,
• 1:07:39
  Speaker 2: the difference becomes even more substantial. The effective capture cross-section
• 1:07:44
  Speaker 2: for distant photons is approximately 6.75 times the circular area
• 1:07:50
  Speaker 2: calculated using the Schwarzschild radius. This does not mean that
• 1:07:54
  Speaker 2: the physical surface area of the event horizon has increased.
• 1:07:57
  Speaker 2: It means that curved spacetime allows the black hole to
• 1:08:00
  Speaker 2: capture incoming radiation over a much larger range of trajectories
• 1:08:05
  Speaker 2: than a simple flat-space picture would suggest. The distinction is
• 1:08:09
  Speaker 2: essential when interpreting astronomical photographs. The ideal geometrical shadow is
• 1:08:15
  Speaker 2: a mathematical prediction based upon photon trajectories. A real black
• 1:08:19
  Speaker 2: hole image, however, depends upon the light emitted by its surroundings.
• 1:08:24
  Speaker 2: Hot plasma can contribute radiation from regions in front of
• 1:08:28
  Speaker 2: the black hole. Its distribution, optical depth, temperature and motion
• 1:08:33
  Speaker 2: all influence the final appearance. The observed central brightness depression
• 1:08:38
  Speaker 2: may therefore differ from the exact theoretical shadow boundary. Likewise,
• 1:08:43
  Speaker 2: the bright emission ring need not coincide precisely with the
• 1:08:47
  Speaker 2: critical curve. Rotation introduces further complications. A spinning black hole
• 1:08:53
  Speaker 2: is described by Kerr spacetime, and its shadow can depend
• 1:08:56
  Speaker 2: upon the spin and the observer's viewing angle. The familiar
• 1:09:00
  Speaker 2: factor of approximately 2.6 is consequently not a universal measurement
• 1:09:05
  Speaker 2: that applies unchanged to every black hole image. It is
• 1:09:09
  Speaker 2: the precise reference result for a distant observer viewing an
• 1:09:13
  Speaker 2: ideal Schwarzschild black hole. Nevertheless, It reveals the essential reason
• 1:09:18
  Speaker 2: for the apparent enlargement. A black hole's shadow is not
• 1:09:22
  Speaker 2: a direct photograph of its event horizon. It is a
• 1:09:26
  Speaker 2: geometrical silhouette defined by the possible paths of light through
• 1:09:30
  Speaker 2: curved spacetime. The horizon determines where escape becomes impossible. The
• 1:09:35
  Speaker 2: photon sphere helps determine the critical trajectories. And the resulting
• 1:09:40
  Speaker 2: pattern of captured and escaping light establishes a shadow substantially
• 1:09:45
  Speaker 2: wider than the horizon itself. What looks like a larger
• 1:09:48
  Speaker 2: black hole is really a larger region of missing light,
• 1:09:52
  Speaker 2: and that apparent enlargement is not a trick of the camera.
• 1:09:55
  Speaker 2: It is spacetime revealing its geometry through the paths that
• 1:09:59
  Speaker 2: photons can and cannot follow. When we look at a
• 1:10:03
  Speaker 2: photograph of a black hole, the most striking feature is
• 1:10:06
  Speaker 2: usually the darkness at its centre. Surrounded by glowing material,
• 1:10:11
  Speaker 2: it appears as a deep circular opening in the image,
• 1:10:14
  Speaker 2: almost as though a piece of the universe has been removed.
• 1:10:18
  Speaker 2: Yet this darkness is not a substance, a cloud, or
• 1:10:21
  Speaker 2: the visible surface of some enormous black object. It is
• 1:10:25
  Speaker 2: something rather more unusual. It is a region from which
• 1:10:28
  Speaker 2: less light reaches us than from its surroundings. This is
• 1:10:32
  Speaker 2: the essential meaning of a black hole shadow. In astronomy,
• 1:10:36
  Speaker 2: darkness generally indicates an absence or shortage of detectable radiation.
• 1:10:41
  Speaker 2: A dark region might be caused by something blocking light,
• 1:10:45
  Speaker 2: by material absorbing it, or simply by the absence of
• 1:10:49
  Speaker 2: a sufficiently bright source. Around a black hole, however, the
• 1:10:53
  Speaker 2: explanation involves the extraordinary behaviour of light travelling through curved spacetime.
• 1:10:59
  Speaker 2: The shadow is produced because certain light trajectories lead into
• 1:11:02
  Speaker 2: the black hole, rather than connecting a distant source to
• 1:11:06
  Speaker 2: an observer. Imagine a perfectly non-rotating black hole positioned against
• 1:11:11
  Speaker 2: a distant, uniformly glowing background. Without the black hole, Light
• 1:11:17
  Speaker 2: from that background would arrive from every direction across the
• 1:11:20
  Speaker 2: observer's field of view. Introduce the black hole and the
• 1:11:25
  Speaker 2: picture changes. Some rays from the background pass relatively far away,
• 1:11:30
  Speaker 2: experiencing only modest gravitational deflection before reaching the observer. Others
• 1:11:35
  Speaker 2: approach the black hole more closely, travelling along dramatically curved
• 1:11:40
  Speaker 2: paths before escaping. but there is a range of trajectories
• 1:11:44
  Speaker 2: that cannot connect the distant background to the observer at all.
• 1:11:48
  Speaker 2: When followed towards the black hole, those paths cross the
• 1:11:52
  Speaker 2: event horizon. The observer therefore receives no background light along
• 1:11:56
  Speaker 2: those particular directions. Against the surrounding illumination, this missing radiation
• 1:12:02
  Speaker 2: creates a dark region. A useful way to understand its
• 1:12:06
  Speaker 2: shape is to imagine tracing possible light rays backwards from
• 1:12:09
  Speaker 2: a telescope. Each point in an astronomical image corresponds to
• 1:12:14
  Speaker 2: a particular direction from which radiation might arrive. For directions
• 1:12:18
  Speaker 2: outside the black hole's critical curve, a backwards-traced ray can
• 1:12:22
  Speaker 2: curve around the black hole and continue towards the distant background.
• 1:12:27
  Speaker 2: There may be a luminous source somewhere along that trajectory.
• 1:12:30
  Speaker 2: For directions inside the critical curve, the backwards-traced path encounters
• 1:12:35
  Speaker 2: the event horizon instead. There is no corresponding route connecting
• 1:12:39
  Speaker 2: the telescope to the distant background. The boundary between these
• 1:12:43
  Speaker 2: outcomes produces the geometrical black hole shadow. What makes this
• 1:12:48
  Speaker 2: especially interesting is that the shadow extends beyond the event
• 1:12:51
  Speaker 2: horizon's apparent direct projection. The additional darkness does not represent
• 1:12:56
  Speaker 2: an enlarged black hole. It represents directions from which photons
• 1:13:00
  Speaker 2: are missing, because the gravitational field has redirected their possible
• 1:13:05
  Speaker 2: trajectories into the horizon. For an ideal Schwarzschild black hole
• 1:13:10
  Speaker 2: viewed from a great distance, the shadow's apparent diameter is
• 1:13:13
  Speaker 2: approximately 2.6 times the event horizon's diameter. That enlargement emerges
• 1:13:20
  Speaker 2: entirely from the geometry of light propagation. The idea that
• 1:13:24
  Speaker 2: black holes could produce such distinctive appearances was explored long
• 1:13:28
  Speaker 2: before astronomers had the instruments needed to photograph them. In 1979,
• 1:13:34
  Speaker 2: the French astrophysicist Jean-Pierre Luminet published a pioneering computer-generated image
• 1:13:39
  Speaker 2: of a black hole surrounded by a luminous accretion disk.
• 1:13:44
  Speaker 2: Using mathematical calculations and an early computer, Luminet reconstructed how
• 1:13:49
  Speaker 2: light from the disk would travel through the black hole's
• 1:13:52
  Speaker 2: gravitational field. The resulting picture showed a strongly distorted luminous
• 1:13:57
  Speaker 2: structure with light from the far side of the disk
• 1:14:00
  Speaker 2: bent into view. It was a striking demonstration that a
• 1:14:03
  Speaker 2: black hole would not simply resemble a dark circle surrounded
• 1:14:06
  Speaker 2: by an ordinary flat disk. Gravity itself would reshape the
• 1:14:10
  Speaker 2: entire image. Luminet's work helped establish the visual consequences of
• 1:14:15
  Speaker 2: gravitational lensing around black holes decades before the Event Horizon
• 1:14:19
  Speaker 2: Telescope produced its famous observations. There is, however, an important
• 1:14:24
  Speaker 2: difference between a perfectly black theoretical shadow and the darker
• 1:14:28
  Speaker 2: central region seen in a real astronomical image. Actual black
• 1:14:33
  Speaker 2: holes are often surrounded by hot gas and plasma, which
• 1:14:36
  Speaker 2: can emit radiation from many different locations. Some of this
• 1:14:41
  Speaker 2: material lies between the black hole and the observer. Light
• 1:14:44
  Speaker 2: emitted by that foreground plasma can reach the telescope even
• 1:14:48
  Speaker 2: along viewing directions, whose backwards trace trajectories would eventually intersect
• 1:14:53
  Speaker 2: the event horizon. Consequently, the central shadow region need not
• 1:14:58
  Speaker 2: appear completely black. Instead, astronomers often observe a brightness depression,
• 1:15:04
  Speaker 2: meaning a region where the detected intensity is substantially lower
• 1:15:08
  Speaker 2: than in the surrounding emission. The darkness is relative rather
• 1:15:12
  Speaker 2: than necessarily absolute. The optical properties of the surrounding plasma
• 1:15:17
  Speaker 2: are particularly important. If the material is optically thin at
• 1:15:22
  Speaker 2: the observing wavelength, radiation can travel through much of it
• 1:15:26
  Speaker 2: without being absorbed or scattered away. This allows light emitted
• 1:15:30
  Speaker 2: at different depths to contribute to the final image. Along
• 1:15:34
  Speaker 2: some directions, the telescope receives radiation from an extended region
• 1:15:38
  Speaker 2: of emitting gas. Along directions associated with photon capture, part
• 1:15:43
  Speaker 2: of that potential emitting path is effectively cut short by
• 1:15:47
  Speaker 2: the event horizon. The resulting shortage of radiation helps produce
• 1:15:51
  Speaker 2: a recognisable central depression. The exact appearance depends upon how
• 1:15:56
  Speaker 2: the plasma is distributed, how brightly it radiates, and how
• 1:16:00
  Speaker 2: much of the emission is absorbed or scattered before reaching
• 1:16:03
  Speaker 2: the observer. The telescope itself also affects what can be distinguished.
• 1:16:09
  Speaker 2: Finite angular resolution blurs extremely fine structures together. Instrumental processing
• 1:16:15
  Speaker 2: introduces additional uncertainties While scattering by material between Earth and
• 1:16:20
  Speaker 2: the black hole can further alter the observed image, this
• 1:16:24
  Speaker 2: is particularly relevant for Sagittarius A star, the black hole
• 1:16:28
  Speaker 2: at the centre of the Milky Way, whose radiation travels
• 1:16:31
  Speaker 2: through interstellar material before reaching Earth. There is another reason
• 1:16:36
  Speaker 2: not to imagine every black hole as an easily recognisable
• 1:16:40
  Speaker 2: dark circle, a black hole in an otherwise dark region
• 1:16:44
  Speaker 2: of space. without a sufficiently bright background or surrounding emitting material,
• 1:16:49
  Speaker 2: might be almost impossible to distinguish directly. The theoretical shadow
• 1:16:54
  Speaker 2: would still be defined by its gravitational geometry, but there
• 1:16:57
  Speaker 2: might be no suitable illumination to reveal it. A shadow,
• 1:17:01
  Speaker 2: after all, requires some contrast. And even when luminous material
• 1:17:05
  Speaker 2: is present, there can be more than one physically meaningful
• 1:17:09
  Speaker 2: dark boundary. The conventional black hole shadow is associated with
• 1:17:14
  Speaker 2: a critical division, between escaping and captured light trajectories. The
• 1:17:19
  Speaker 2: event horizon itself lies farther inward in the Schwarzschild model,
• 1:17:23
  Speaker 2: and its direct projected dimensions are smaller. In certain models
• 1:17:27
  Speaker 2: involving a thin luminous accretion disk extending towards the horizon,
• 1:17:33
  Speaker 2: another feature called the inner shadow can arise. This inner
• 1:17:36
  Speaker 2: shadow is associated with the lensed image of the event horizon,
• 1:17:40
  Speaker 2: as it appears against the particular geometry of the emitting disk.
• 1:17:45
  Speaker 2: Its boundary can differ from the familiar critical curve and
• 1:17:49
  Speaker 2: its visibility depends upon the structure of the accretion flow.
• 1:17:53
  Speaker 2: These distinctions are important because a dark region in a
• 1:17:56
  Speaker 2: black hole image does not automatically identify one unique physical boundary.
• 1:18:01
  Speaker 2: The observed darkness is shaped by the relationship between spacetime curvature,
• 1:18:06
  Speaker 2: photon capture, emitting material and the observer's viewing direction. What
• 1:18:11
  Speaker 2: appears to be a simple circular silhouette is therefore a
• 1:18:15
  Speaker 2: remarkably intricate optical phenomenon. The black hole is not surrounded
• 1:18:19
  Speaker 2: by a shell of darkness. Nor does its event horizon
• 1:18:23
  Speaker 2: cast a conventional shadow in quite the same way as
• 1:18:26
  Speaker 2: an ordinary opaque object. Instead, its gravity determines which paths
• 1:18:31
  Speaker 2: can deliver light to us and which paths cannot. The
• 1:18:34
  Speaker 2: resulting absence of radiation creates an apparent outline that may
• 1:18:38
  Speaker 2: extend far beyond the horizon itself, and in that sense,
• 1:18:42
  Speaker 2: The darkness at the centre of a black hole photograph
• 1:18:45
  Speaker 2: is not merely an empty part of the picture. It
• 1:18:48
  Speaker 2: is a visible consequence of the extraordinary limits that curved
• 1:18:51
  Speaker 2: spacetime places upon the journey of light. Perhaps the most
• 1:18:56
  Speaker 2: recognisable feature of a black hole photograph is the bright
• 1:18:59
  Speaker 2: ring surrounding its dark centre. It gives the impression that
• 1:19:03
  Speaker 2: the black hole is glowing around its edges almost as
• 1:19:06
  Speaker 2: though an enormous circular object has been illuminated from behind.
• 1:19:11
  Speaker 2: The contrast is striking. At the centre, a region where
• 1:19:15
  Speaker 2: light seems to disappear. Around it, a luminous structure revealing
• 1:19:20
  Speaker 2: one of the most energetic environments in the universe. But
• 1:19:23
  Speaker 2: the ring is not part of the black hole itself.
• 1:19:27
  Speaker 2: What we are actually seeing is radiation produced by matter
• 1:19:31
  Speaker 2: moving through the black hole's gravitational field. That matter can
• 1:19:36
  Speaker 2: become extraordinarily hot, and the light it emits follows curved
• 1:19:40
  Speaker 2: paths through spacetime before reaching our telescopes. To understand where
• 1:19:45
  Speaker 2: the glowing material comes from, it helps to consider the
• 1:19:48
  Speaker 2: environments in which black holes exist. Black holes are not
• 1:19:52
  Speaker 2: always isolated objects drifting through otherwise empty space. Many are
• 1:19:57
  Speaker 2: surrounded by gas, dust, stars and other material. A stellar-mass
• 1:20:02
  Speaker 2: black hole may draw matter from a companion star, While
• 1:20:05
  Speaker 2: a supermassive black hole can receive gas from the central
• 1:20:08
  Speaker 2: regions of its host galaxy, some of this material gradually
• 1:20:12
  Speaker 2: moves inwards under the influence of gravity. The resulting inward
• 1:20:16
  Speaker 2: movement of matter is called accretion. It might seem reasonable
• 1:20:20
  Speaker 2: to imagine that gas approaching a black hole simply falls
• 1:20:22
  Speaker 2: directly towards the event horizon. But most astronomical material possesses
• 1:20:28
  Speaker 2: angular momentum, which means its motion includes some rotation around
• 1:20:32
  Speaker 2: the central mass. angular momentum makes a considerable difference. Rather
• 1:20:38
  Speaker 2: than falling straight in, the gas can circulate around the
• 1:20:41
  Speaker 2: black hole, following orbital paths as it gradually moves closer.
• 1:20:45
  Speaker 2: When the material has sufficient angular momentum and the surrounding
• 1:20:49
  Speaker 2: conditions permit, it can form an accretion disk. An accretion
• 1:20:53
  Speaker 2: disk is a rotating structure of matter whose particles move
• 1:20:57
  Speaker 2: around the central object, while gradually transferring angular momentum outwards.
• 1:21:02
  Speaker 2: This allows some material to spiral inwards towards the black hole.
• 1:21:07
  Speaker 2: The disc is not necessarily a perfectly flat or orderly arrangement.
• 1:21:12
  Speaker 2: Gas can become turbulent, magnetic fields can develop complicated structures,
• 1:21:17
  Speaker 2: and different regions may move at substantially different speeds. These
• 1:21:22
  Speaker 2: processes are important because they allow gravitational energy to be
• 1:21:26
  Speaker 2: converted into heat. As matter moves deeper into the black
• 1:21:30
  Speaker 2: hole's gravitational field, energy becomes available for conversion into other forms. Turbulence,
• 1:21:37
  Speaker 2: magnetic stresses and interactions between different parts of the accretion
• 1:21:41
  Speaker 2: flow can increase the internal energy of the gas. One
• 1:21:45
  Speaker 2: important mechanism is magnetorotational instability, which allows magnetic fields to
• 1:21:50
  Speaker 2: drive turbulence and transport angular momentum through differentially rotating material.
• 1:21:57
  Speaker 2: Without sufficiently effective angular momentum transport, much of the gas
• 1:22:02
  Speaker 2: would have difficulty moving inwards. The resulting heating can be extraordinary.
• 1:22:08
  Speaker 2: In some accretion environments, temperatures reach millions of degrees or
• 1:22:12
  Speaker 2: considerably higher, producing radiation across much of the electromagnetic spectrum.
• 1:22:18
  Speaker 2: Closer to many black holes, the gas becomes an ionised plasma.
• 1:22:23
  Speaker 2: Plasma is matter containing freely moving charged particles, including electrons
• 1:22:29
  Speaker 2: and positively charged ions. Because these particles respond to electric
• 1:22:33
  Speaker 2: and magnetic fields, their behaviour can be considerably more complicated
• 1:22:38
  Speaker 2: than that of an ordinary neutral gas. They can also
• 1:22:41
  Speaker 2: produce substantial electromagnetic radiation. One particularly important mechanism near supermassive
• 1:22:47
  Speaker 2: black holes is synchrotron radiation. Synchrotron radiation occurs when charged particles,
• 1:22:55
  Speaker 2: often relativistic electrons, are accelerated as they move through magnetic fields.
• 1:23:01
  Speaker 2: Their changing direction causes them to emit electromagnetic radiation. The
• 1:23:06
  Speaker 2: emitted wavelengths depend upon the particles' energies, the magnetic field
• 1:23:10
  Speaker 2: strengths and other conditions within the plasma. Near the black
• 1:23:14
  Speaker 2: holes in Messier 87 and at the centre of our
• 1:23:17
  Speaker 2: own galaxy, synchrotron radiation is a major contributor to the
• 1:23:21
  Speaker 2: emission detected at radio and millimetre wavelengths. The Event Horizon
• 1:23:26
  Speaker 2: Telescope observes at a wavelength of approximately 1.3 millimetres, allowing
• 1:23:31
  Speaker 2: astronomers to study radiation originating extremely close to these supermassive
• 1:23:36
  Speaker 2: black holes. The famous orange ring surrounding the black hole
• 1:23:41
  Speaker 2: in Messier 87 is a visual representation of this detected radiation.
• 1:23:46
  Speaker 2: It is worth remembering that the orange colour was chosen
• 1:23:49
  Speaker 2: for displaying the image. The telescopes were detecting millimetre wave radiation,
• 1:23:54
  Speaker 2: which human eyes cannot see directly. The black hole itself
• 1:23:58
  Speaker 2: is not producing an enormous orange flame, nor is the
• 1:24:02
  Speaker 2: event horizon glowing like the surface of a star. An
• 1:24:06
  Speaker 2: event horizon is a boundary in spacetime, not a hot
• 1:24:10
  Speaker 2: physical shell. The luminous material lies outside it, where charged
• 1:24:15
  Speaker 2: particles and magnetic fields can produce radiation capable of escaping
• 1:24:20
  Speaker 2: towards distant observers. But the presence of glowing plasma alone
• 1:24:25
  Speaker 2: does not fully explain the ring's appearance. Gravity also changes
• 1:24:29
  Speaker 2: where the light seems to originate. Radiation emitted from the
• 1:24:32
  Speaker 2: far side of the accretion flow can bend around the
• 1:24:35
  Speaker 2: black hole before reaching Earth. Light originating beside or behind
• 1:24:40
  Speaker 2: the central object may therefore contribute to the observed brightness
• 1:24:44
  Speaker 2: around its apparent outline. This gravitational lensing can make parts
• 1:24:48
  Speaker 2: of the accretion flow visible, in directions where ordinary straight-line
• 1:24:52
  Speaker 2: geometry would suggest they should be hidden. It also redistributes
• 1:24:57
  Speaker 2: the apparent positions and brightness of the emitting material. Consequently,
• 1:25:02
  Speaker 2: the observed ring is not necessarily a direct view of
• 1:25:05
  Speaker 2: a simple circular band of gas. It is a gravitationally
• 1:25:08
  Speaker 2: distorted image of radiation, emitted throughout a complicated three-dimensional environment.
• 1:25:15
  Speaker 2: Some photons reach the observer along relatively direct paths. Others
• 1:25:19
  Speaker 2: travel closer to the black hole and experience much stronger deflection.
• 1:25:24
  Speaker 2: A small contribution can come from photons that approach unstable
• 1:25:28
  Speaker 2: orbital trajectories and travel partway around the central mass before escaping.
• 1:25:33
  Speaker 2: These highly bent trajectories are associated with the theoretical photon
• 1:25:37
  Speaker 2: sub-rings predicted by general relativity. However, the broad luminous rings
• 1:25:44
  Speaker 2: seen in current event horizons, telescope images should not be
• 1:25:47
  Speaker 2: confused with those extremely narrow theoretical structures. Much of the
• 1:25:52
  Speaker 2: broad ring's emission comes from surrounding plasma, whose apparent shape
• 1:25:56
  Speaker 2: is influenced by gravitational lensing, magnetic fields and the distribution
• 1:26:00
  Speaker 2: of emitting matter. The photon subrings are much finer features,
• 1:26:05
  Speaker 2: associated with light that has completed increasing numbers of near
• 1:26:09
  Speaker 2: orbits around the black hole. The diameter of the observed
• 1:26:13
  Speaker 2: emission ring is closely related to the characteristic scale of
• 1:26:16
  Speaker 2: the black hole shadow, but the two are not automatically identical.
• 1:26:21
  Speaker 2: Different arrangements of plasma, different optical depths and different viewing
• 1:26:25
  Speaker 2: angles can alter the relationship between the theoretical shadow boundary
• 1:26:29
  Speaker 2: and the measured brightness distribution. Observing wavelength matters as well.
• 1:26:34
  Speaker 2: In particularly hot accretion environments, material can emit powerful X-rays.
• 1:26:40
  Speaker 2: These observations may reveal structures and physical processes different from
• 1:26:44
  Speaker 2: those dominating millimetre wave images. A black hole can therefore
• 1:26:49
  Speaker 2: look quite different depending upon which part of the electromagnetic
• 1:26:52
  Speaker 2: spectrum astronomers examine. The familiar glowing ring is the result
• 1:26:56
  Speaker 2: of several remarkable processes acting together. Gravity draws material towards
• 1:27:02
  Speaker 2: the black hole. Angular momentum causes much of that material
• 1:27:06
  Speaker 2: to circulate. Magnetic stresses and turbulence help heat the plasma,
• 1:27:11
  Speaker 2: which produces electromagnetic radiation. Spacetime curvature bends the escaping light
• 1:27:18
  Speaker 2: into the pattern recorded by our telescopes. What appears to
• 1:27:22
  Speaker 2: be a bright outline around a dark object is really
• 1:27:24
  Speaker 2: an image of energetic matter moving through an extraordinary gravitational environment.
• 1:27:31
  Speaker 2: The light reveals the presence of the black hole without
• 1:27:33
  Speaker 2: coming from its event horizon, and the resulting ring shows
• 1:27:37
  Speaker 2: us something rather beautiful about astronomy. Even when an object
• 1:27:41
  Speaker 2: cannot be seen directly, the behaviour of the material and
• 1:27:45
  Speaker 2: light surrounding it can reveal its existence with astonishing clarity.
• 1:27:50
  Speaker 2: One of the most intriguing details in the first photograph
• 1:27:53
  Speaker 2: of a black hole is that the glowing ring is
• 1:27:55
  Speaker 2: not equally bright all the way around. In the image
• 1:27:59
  Speaker 2: of Messier 87, released by the Event Horizon Telescope collaboration
• 1:28:04
  Speaker 2: in April 2019, the lower part of the ring appears
• 1:28:07
  Speaker 2: noticeably brighter than the upper part. Instead of a perfectly
• 1:28:11
  Speaker 2: uniform circle of light, we see something resembling a luminous crescent,
• 1:28:16
  Speaker 2: wrapped around an enormous region of darkness. At first, this
• 1:28:20
  Speaker 2: might seem rather peculiar. If the black hole's gravitational field
• 1:28:25
  Speaker 2: is responsible for bending light into a roughly circular structure,
• 1:28:29
  Speaker 2: why should one part of that structure shine so much
• 1:28:32
  Speaker 2: more brightly than another? Much of the answer lies in
• 1:28:35
  Speaker 2: the extraordinary speeds of the material surrounding the black hole.
• 1:28:39
  Speaker 2: Gas approaching a black hole rarely falls directly towards its centre.
• 1:28:44
  Speaker 2: Because the material possesses angular momentum, much of it circulates
• 1:28:47
  Speaker 2: around the black hole in an accretion flow. Close to
• 1:28:51
  Speaker 2: the event horizon, the gas can move at a substantial
• 1:28:54
  Speaker 2: fraction of the speed of light. This motion has a
• 1:28:57
  Speaker 2: remarkable effect upon the radiation received by a distant observer.
• 1:29:01
  Speaker 2: Consider a rotating ring of luminous material viewed from an angle.
• 1:29:06
  Speaker 2: On one side, the gas has a component of its
• 1:29:08
  Speaker 2: velocity directed towards us. On the opposite side, its motion
• 1:29:12
  Speaker 2: has a component directed away. Although both regions might produce
• 1:29:16
  Speaker 2: similar amounts of radiation in their own local surroundings, they
• 1:29:20
  Speaker 2: need not appear equally bright from Earth. The reason is
• 1:29:24
  Speaker 2: the relativistic Doppler effect. The familiar Doppler effect occurs whenever
• 1:29:28
  Speaker 2: a source of waves moves relative to an observer. An
• 1:29:32
  Speaker 2: approaching ambulance siren sounds higher in pitch, while the same
• 1:29:36
  Speaker 2: siren sounds lower as the vehicle moves away. Light also
• 1:29:40
  Speaker 2: experiences Doppler shifts, although the precise relationship is governed by
• 1:29:45
  Speaker 2: Albert Einstein's special theory of relativity. When a luminous source
• 1:29:49
  Speaker 2: approaches an observer, the frequency of its radiation can increase.
• 1:29:54
  Speaker 2: This is called a blue shift. When the source moves away,
• 1:29:58
  Speaker 2: the observed frequency can decrease, producing a redshift. Near a
• 1:30:02
  Speaker 2: black hole, the orbital velocities can be so enormous that
• 1:30:06
  Speaker 2: these changes become particularly significant. But the observed effect involves
• 1:30:11
  Speaker 2: more than a change in colour or frequency. Relativistic Doppler
• 1:30:16
  Speaker 2: boosting can also make approaching material appear considerably brighter than
• 1:30:20
  Speaker 2: receding material. Radiation emitted by rapidly approaching plasma is shifted
• 1:30:26
  Speaker 2: in frequency, while the observed intensity is enhanced. Meanwhile, radiation
• 1:30:31
  Speaker 2: from material moving away can appear comparatively weaker. Even if
• 1:30:35
  Speaker 2: two regions of gas have similar intrinsic emission, their different
• 1:30:39
  Speaker 2: velocities can produce strikingly different appearances. This helps explain why
• 1:30:44
  Speaker 2: a black hole's luminous surroundings often resemble a crescent, rather
• 1:30:48
  Speaker 2: than a uniformly glowing ring. Another important contribution comes from
• 1:30:53
  Speaker 2: relativistic aberration. Aberration describes how the apparent directions of light
• 1:30:58
  Speaker 2: change between observers moving relative to one another. When an
• 1:31:03
  Speaker 2: emitting particle travels at a speed approaching that of light,
• 1:31:06
  Speaker 2: its radiation can become strongly concentrated in its forward direction
• 1:31:11
  Speaker 2: as measured in another reference frame. This concentration is sometimes
• 1:31:15
  Speaker 2: called relativistic beaming. For the rapidly moving electrons and plasma
• 1:31:20
  Speaker 2: near a black hole, relativistic aberration and Doppler boosting work
• 1:31:25
  Speaker 2: together to influence how much radiation reaches a distant telescope.
• 1:31:30
  Speaker 2: The result is that the approaching side of an accretion
• 1:31:32
  Speaker 2: flow can look substantially brighter than the receding side, and
• 1:31:36
  Speaker 2: the faster the material moves, the more pronounced these relativistic
• 1:31:40
  Speaker 2: effects can become. However, motion is not the only influence
• 1:31:44
  Speaker 2: on the observed brightness. There is also gravitational redshift. According
• 1:31:49
  Speaker 2: to general relativity, Radiation travelling outwards from deep within a
• 1:31:53
  Speaker 2: gravitational field is measured at a lower frequency by distant
• 1:31:58
  Speaker 2: observers than by suitable local observers near its source. Near
• 1:32:02
  Speaker 2: a black hole, where spacetime curvature is exceptionally strong, gravitational
• 1:32:07
  Speaker 2: redshift can become substantial. Light escaping from different regions of
• 1:32:12
  Speaker 2: the accretion flow may therefore undergo different gravitational frequency shifts,
• 1:32:17
  Speaker 2: depending upon where it was emitted and the path it follows.
• 1:32:21
  Speaker 2: These shifts interact with the Doppler effects produced by the
• 1:32:24
  Speaker 2: movement of the plasma. Gravitational lensing adds another layer of complexity.
• 1:32:30
  Speaker 2: Light emitted behind the black hole may bend around the
• 1:32:33
  Speaker 2: central mass before reaching Earth. Other rays can travel along
• 1:32:37
  Speaker 2: strongly curved paths, changing the apparent positions of their sources. Consequently,
• 1:32:43
  Speaker 2: a bright region in the image does not necessarily correspond
• 1:32:46
  Speaker 2: to the part of the accretion flow physically closest to
• 1:32:49
  Speaker 2: the observer It represents radiation arriving from particular directions after
• 1:32:54
  Speaker 2: its journey through curved spacetime. The plasma itself is equally important. Temperature, density,
• 1:33:02
  Speaker 2: magnetic field strength, and the distribution of energetic electrons all
• 1:33:06
  Speaker 2: influence how brightly different regions emit. The observed crescent therefore
• 1:33:11
  Speaker 2: emerges from several effects acting together, rather than from Doppler
• 1:33:16
  Speaker 2: boosting alone. There is also a subtle distinction between the
• 1:33:19
  Speaker 2: rotation of the surrounding gas and the rotation of the
• 1:33:22
  Speaker 2: black hole. A black hole can possess angular momentum, producing
• 1:33:27
  Speaker 2: the frame dragging effects described by the Kerr solution to
• 1:33:31
  Speaker 2: Einstein's equations. Yet an asymmetric emission ring does not, by itself,
• 1:33:36
  Speaker 2: prove that the black hole rotates in a particular direction.
• 1:33:40
  Speaker 2: The gas has its own orbital motion, and the relationship
• 1:33:43
  Speaker 2: between that motion The black hole's spin and the observed
• 1:33:47
  Speaker 2: brightness requires careful physical modelling. Viewing angle matters enormously as well.
• 1:33:54
  Speaker 2: If an accretion flow is observed almost directly along its
• 1:33:57
  Speaker 2: rotation axis, much of the orbital motion occurs across the
• 1:34:01
  Speaker 2: observer's line of sight. The approaching and receding velocity components
• 1:34:05
  Speaker 2: are relatively small, so the resulting Doppler brightness contrast may
• 1:34:10
  Speaker 2: be less pronounced. From a more inclined perspective, those line-of-sight
• 1:34:15
  Speaker 2: velocity components become greater, potentially producing a stronger crescent-shaped asymmetry.
• 1:34:21
  Speaker 2: The appearance also depends upon the flow's thickness, turbulence and
• 1:34:24
  Speaker 2: magnetic structure. This is why astronomers use detailed computer simulations
• 1:34:29
  Speaker 2: when interpreting black hole images. The observed ring contains information
• 1:34:34
  Speaker 2: not only about the central gravitational field, but also about
• 1:34:38
  Speaker 2: the behaviour of matter moving through it. In Messier 87,
• 1:34:42
  Speaker 2: the uneven brightness provides valuable evidence about the emitting plasma
• 1:34:46
  Speaker 2: and its relativistic motion. Yet the characteristic scale of the
• 1:34:50
  Speaker 2: central shadow remains more directly connected to the geometry of
• 1:34:54
  Speaker 2: spacetime around the black hole. The two features reveal different
• 1:34:58
  Speaker 2: aspects of the same extraordinary environment. The darkness tells us
• 1:35:02
  Speaker 2: about the paths from which light cannot escape. The glowing
• 1:35:06
  Speaker 2: crescent tells us about matter moving at remarkable speeds. producing
• 1:35:10
  Speaker 2: radiation that is transformed by both motion and gravity. And
• 1:35:15
  Speaker 2: perhaps the most fascinating detail is that the black hole
• 1:35:18
  Speaker 2: need not emit any light of its own for this
• 1:35:20
  Speaker 2: pattern to appear. The surrounding material provides the radiation. Relativity
• 1:35:26
  Speaker 2: determines much of how that radiation reaches us. What looks
• 1:35:30
  Speaker 2: like a slightly uneven ring is therefore a remarkably detailed
• 1:35:33
  Speaker 2: record of matter, motion and curved space-time. all contained within
• 1:35:39
  Speaker 2: one distant astronomical image. An accretion disk is usually imagined
• 1:35:43
  Speaker 2: as a great flat structure of glowing material rotating around
• 1:35:47
  Speaker 2: a central object. It resembles an enormous ring or a
• 1:35:52
  Speaker 2: flattened whirlpool with gas gradually spiralling inwards as gravity draws
• 1:35:57
  Speaker 2: it towards the centre. Seen from directly above, such a
• 1:36:01
  Speaker 2: disk might appear almost circular. Viewed from the side, it
• 1:36:04
  Speaker 2: would seem much thinner, perhaps resembling a narrow band of
• 1:36:08
  Speaker 2: light stretching across the darkness. But place that same disk
• 1:36:12
  Speaker 2: around a black hole, and its appearance can become extraordinarily strange.
• 1:36:17
  Speaker 2: The reason is gravitational lensing. Light emitted by the disk
• 1:36:21
  Speaker 2: does not necessarily travel towards an observer along a straight path. Instead,
• 1:36:26
  Speaker 2: it moves through the strongly curved spacetime surrounding the black hole,
• 1:36:31
  Speaker 2: sometimes bending through such large angles. the resulting image bears
• 1:36:35
  Speaker 2: little resemblance to the physical arrangement of the material. One
• 1:36:39
  Speaker 2: of the most surprising consequences is that an observer can
• 1:36:42
  Speaker 2: see parts of the disk that would normally be hidden
• 1:36:45
  Speaker 2: behind the black hole. Imagine looking towards an accretion disk
• 1:36:49
  Speaker 2: from a position slightly above its plane. The nearer portion
• 1:36:53
  Speaker 2: of the disk lies between the observer and the black hole,
• 1:36:56
  Speaker 2: while the far side extends behind the central region. In
• 1:37:00
  Speaker 2: ordinary geometry, Much of the far side would be concealed
• 1:37:03
  Speaker 2: by the object in the middle. Near a black hole, however,
• 1:37:06
  Speaker 2: light emitted from that hidden material can travel upwards and
• 1:37:10
  Speaker 2: curve around the central mass before reaching the observer. The
• 1:37:13
  Speaker 2: result can be a bright arc appearing above the black hole,
• 1:37:17
  Speaker 2: even though the emitting gas is physically located behind it.
• 1:37:21
  Speaker 2: Light taking different trajectories can also produce an additional image
• 1:37:25
  Speaker 2: beneath the central darkness. The disc can therefore seem to
• 1:37:29
  Speaker 2: bend upwards and downwards around the black hole, creating a
• 1:37:32
  Speaker 2: strange arrangement of luminous bands surrounding its shadow. What appears
• 1:37:37
  Speaker 2: to be a warped or folded structure may actually be
• 1:37:39
  Speaker 2: a comparatively flat disc whose light has been dramatically redirected.
• 1:37:45
  Speaker 2: This distinction is particularly important. The apparent distortion does not
• 1:37:49
  Speaker 2: necessarily mean that the gas itself has physically climbed above
• 1:37:52
  Speaker 2: the black hole or folded around it. The geometry of
• 1:37:56
  Speaker 2: spacetime has changed the apparent location from which its radiation arrives.
• 1:38:01
  Speaker 2: In some theoretical images, a relatively thin accretion disc appears
• 1:38:06
  Speaker 2: almost to wrap around the black hole, with the far
• 1:38:09
  Speaker 2: side visible both above and below the central region. These
• 1:38:13
  Speaker 2: effects become particularly striking when the disc is viewed at
• 1:38:16
  Speaker 2: a substantial inclination. A disc observed almost face-on presents a
• 1:38:21
  Speaker 2: relatively symmetrical arrangement. Much of its orbital structure appears circular,
• 1:38:27
  Speaker 2: although gravitational lensing and the motion of the emitting material
• 1:38:30
  Speaker 2: can still modify its brightness. As the viewing angle moves
• 1:38:34
  Speaker 2: closer to the plane of the disk, the image becomes
• 1:38:37
  Speaker 2: increasingly complicated. The nearer side may stretch across the foreground,
• 1:38:42
  Speaker 2: while radiation from the far side is redirected into apparent
• 1:38:46
  Speaker 2: arcs above and below the shadow. Some portions of the
• 1:38:50
  Speaker 2: material can even appear more than once. This happens because
• 1:38:54
  Speaker 2: photons emitted from the same region may reach an observer
• 1:38:57
  Speaker 2: along different trajectories. One group of photons might follow a
• 1:39:01
  Speaker 2: comparatively direct route. Another may pass closer to the black
• 1:39:05
  Speaker 2: hole and experience much greater deflection. Under particularly precise conditions,
• 1:39:10
  Speaker 2: additional photons can travel around a substantial portion of the
• 1:39:14
  Speaker 2: black hole before escaping. The observer therefore receives several gravitationally
• 1:39:19
  Speaker 2: lensed views of the same physical material. It is a
• 1:39:22
  Speaker 2: rather extraordinary arrangement. A single accretion disc can produce an
• 1:39:26
  Speaker 2: image containing structures that appear to occupy several different positions,
• 1:39:31
  Speaker 2: even though they originate from the same rotating flow. The
• 1:39:35
  Speaker 2: appearance also depends upon how easily radiation can travel through
• 1:39:39
  Speaker 2: the material. Astronomers distinguish between optically thick and optically thin gas.
• 1:39:45
  Speaker 2: An optically thick region absorbs or scatters enough radiation that
• 1:39:49
  Speaker 2: light from behind it cannot travel freely through the material.
• 1:39:52
  Speaker 2: Such a region can obscure parts of the disk or
• 1:39:56
  Speaker 2: prevent an observer from seeing more distant emitting layers. An
• 1:40:00
  Speaker 2: optically thin plasma, by contrast, allows a substantial fraction of
• 1:40:05
  Speaker 2: radiation to pass through it. This means that light emitted
• 1:40:09
  Speaker 2: from several different positions along the same line of sight
• 1:40:12
  Speaker 2: can contribute to the observed image. An optically thin accretion
• 1:40:17
  Speaker 2: flow may therefore appear more diffuse, with radiation originating from
• 1:40:21
  Speaker 2: a complicated three-dimensional distribution, rather than one clearly defined luminous surface.
• 1:40:27
  Speaker 2: To predict these appearances, scientists use gravitational ray tracing. This
• 1:40:32
  Speaker 2: computational technique follows possible photon trajectories through the curved spacetime
• 1:40:37
  Speaker 2: surrounding a black hole. Researchers calculate how light travels from
• 1:40:41
  Speaker 2: emitting material towards an observer, taking account of gravitational bending,
• 1:40:46
  Speaker 2: frequency shifts and the motion of the plasma. Sophisticated simulations
• 1:40:51
  Speaker 2: can also incorporate magnetic fields, turbulent gas dynamics, radiative transfer
• 1:40:57
  Speaker 2: and the effects of black hole rotation. The results can
• 1:41:00
  Speaker 2: be remarkably detailed, but the foundations of this work go
• 1:41:04
  Speaker 2: back many decades. In the early 1970s, the American physicist
• 1:41:09
  Speaker 2: James Bardeen investigated how rotating black holes would appear to
• 1:41:14
  Speaker 2: distant observers, helping establish the mathematical description of their apparent silhouettes. Then,
• 1:41:20
  Speaker 2: in 1979, the French astrophysicist Jean-Pierre Lumine produced a pioneering
• 1:41:26
  Speaker 2: computer-generated image of a black hole surrounded by a thin
• 1:41:30
  Speaker 2: luminous accretion disk. His calculation revealed the striking distortions produced
• 1:41:35
  Speaker 2: by gravitational lensing, including the remarkable appearance of the far
• 1:41:39
  Speaker 2: side of the disk above the central dark region. These
• 1:41:43
  Speaker 2: theoretical images were developed long before telescopes could resolve structures
• 1:41:47
  Speaker 2: on the scale of black hole event horizons. They demonstrated
• 1:41:51
  Speaker 2: that photographing a black hole would never be as straightforward
• 1:41:55
  Speaker 2: as taking a picture of an ordinary astronomical object. There is, however,
• 1:42:00
  Speaker 2: an important distinction between these dramatic simulations and the photographs
• 1:42:04
  Speaker 2: obtained by the event horizon telescopes. The familiar theoretical images
• 1:42:10
  Speaker 2: often show relatively thin, sharply defined disks viewed from particular angles.
• 1:42:15
  Speaker 2: Actual observations of Messier 87 and Sagittarius A star at
• 1:42:19
  Speaker 2: millimetre wavelengths reveal radiation from hot plasma distributed throughout more
• 1:42:25
  Speaker 2: complicated accretion environments. The emitting gas can be geometrically thick,
• 1:42:30
  Speaker 2: turbulent and partially transparent. The finite angular resolution of the
• 1:42:35
  Speaker 2: telescopes also blends extremely fine structures together. Consequently, the observed
• 1:42:41
  Speaker 2: images display broad luminous rings rather than every delicate arc
• 1:42:45
  Speaker 2: and overlapping image predicted in idealised models. Both representations are valuable,
• 1:42:51
  Speaker 2: but they describe different observational circumstances. What remains consistent is
• 1:42:56
  Speaker 2: the underlying influence of curved spacetime. An accretion disc can
• 1:43:00
  Speaker 2: appear larger than its physical dimensions might suggest, material behind
• 1:43:05
  Speaker 2: the black hole can become visible. A single region of
• 1:43:08
  Speaker 2: gas can produce several images and a relatively flat structure
• 1:43:12
  Speaker 2: can appear to fold around the central darkness. The observer
• 1:43:15
  Speaker 2: is not seeing an ordinary projection of the material's physical shape. Instead,
• 1:43:20
  Speaker 2: the final picture records radiation that has travelled along the
• 1:43:24
  Speaker 2: possible paths permitted by the black hole's gravitational field. And
• 1:43:28
  Speaker 2: this is why even the luminous surroundings of a black
• 1:43:31
  Speaker 2: hole can be so deceptive. The glowing material may be
• 1:43:35
  Speaker 2: perfectly real, but its apparent position, shape and extent have
• 1:43:38
  Speaker 2: been transformed by the geometry of spacetime. Around a black hole,
• 1:43:43
  Speaker 2: seeing something does not necessarily mean seeing it where it
• 1:43:47
  Speaker 2: actually is. A black hole might seem like the last
• 1:43:50
  Speaker 2: thing in the universe that could have an interesting relationship
• 1:43:52
  Speaker 2: with rotation. After all, it has no solid surface to
• 1:43:56
  Speaker 2: watch spinning, no visible markings moving around its edges, and
• 1:44:00
  Speaker 2: no familiar physical shape that would reveal its motions. Yet
• 1:44:04
  Speaker 2: black holes can rotate, and that rotation changes the geometry
• 1:44:08
  Speaker 2: of the spacetime surrounding them in some rather extraordinary ways.
• 1:44:12
  Speaker 2: It can influence how matter moves, how light travels, and
• 1:44:16
  Speaker 2: even the apparent outline of the darkness seen by a
• 1:44:19
  Speaker 2: distant observer. The simplest mathematical description of a black hole
• 1:44:24
  Speaker 2: assumes that it is perfectly spherical, electrically uncharged, and not rotating.
• 1:44:30
  Speaker 2: This is the Schwarzschild black hole based upon the solution
• 1:44:33
  Speaker 2: discovered by the German physicist Karl Schwarzschild in 1916. In
• 1:44:38
  Speaker 2: this idealised case, the geometry depends upon a single fundamental property,
• 1:44:43
  Speaker 2: the black hole's mass. The event horizon is located at
• 1:44:46
  Speaker 2: the Schwarzschild radius, while the photon sphere lies farther out,
• 1:44:50
  Speaker 2: at one and a half times that radius. For a
• 1:44:54
  Speaker 2: distant observer, the resulting geometrical shadow is perfectly circular, But
• 1:44:59
  Speaker 2: real black holes are unlikely to remain completely motionless in
• 1:45:02
  Speaker 2: this rotational sense. Stars rotate, gas clouds possess angular momentum,
• 1:45:08
  Speaker 2: and matter falling towards a black hole can carry its
• 1:45:11
  Speaker 2: rotation with it. When a black hole forms from a
• 1:45:14
  Speaker 2: collapsing star, or grows by accreting surrounding material, it can
• 1:45:19
  Speaker 2: acquire considerable angular momentum, and once rotation enters the picture,
• 1:45:24
  Speaker 2: the geometry becomes more complicated. In 1963, the New Zealand
• 1:45:30
  Speaker 2: mathematician Roy Kerr discovered an exact solution to Albert Einstein's
• 1:45:34
  Speaker 2: equations describing the spacetime surrounding an ideal, rotating, uncharged black hole.
• 1:45:40
  Speaker 2: This solution is known as the Kerr metric. Unlike the
• 1:45:43
  Speaker 2: Schwarzschild description, the Kerr metric depends upon both the black
• 1:45:47
  Speaker 2: hole's mass and its angular momentum. That additional property changes
• 1:45:52
  Speaker 2: the possible trajectories of particles and photons moving nearby. One
• 1:45:57
  Speaker 2: of the most remarkable consequences is called frame dragging. In
• 1:46:01
  Speaker 2: general relativity, a rotating massive object influences the surrounding spacetime
• 1:46:06
  Speaker 2: in a way that affects the motion of nearby objects.
• 1:46:10
  Speaker 2: Around a rapidly spinning black hole, this effect can become
• 1:46:13
  Speaker 2: exceptionally strong. It is sometimes described as the black hole
• 1:46:17
  Speaker 2: dragging spacetime around with its rotation, although this is an
• 1:46:21
  Speaker 2: analogy rather than a literal movement of some physical substance.
• 1:46:26
  Speaker 2: The important point is that rotation changes the relationship between
• 1:46:29
  Speaker 2: nearby motion and the reference frame of a distant observer.
• 1:46:33
  Speaker 2: Light attempting to travel around the black hole in one
• 1:46:36
  Speaker 2: direction can encounter different orbital conditions from light travelling in
• 1:46:40
  Speaker 2: the opposite direction. This introduces a kind of directional distinction
• 1:46:44
  Speaker 2: that does not exist in the perfectly symmetric Schwarzschild model.
• 1:46:49
  Speaker 2: There is also a particularly unusual region surrounding a rotating
• 1:46:52
  Speaker 2: black hole called the Ergosphere. The ergosphere lies outside the
• 1:46:56
  Speaker 2: event horizon, although the two boundaries meet at the rotational
• 1:46:59
  Speaker 2: poles in the ideal Kerr description. Within this region, frame
• 1:47:03
  Speaker 2: dragging becomes so strong that no physical observer can remain
• 1:47:07
  Speaker 2: stationary relative to distant non-rotating reference directions. An object can
• 1:47:12
  Speaker 2: still move and under suitable circumstances escape from the ergosphere,
• 1:47:17
  Speaker 2: but it cannot remain completely stationary in the required sense.
• 1:47:20
  Speaker 2: This makes the ergosphere fundamentally different from the event horizon.
• 1:47:25
  Speaker 2: Crossing the event horizon prevents any future escape to the
• 1:47:28
  Speaker 2: outside universe. Entering the ergosphere does not. The ergosphere is
• 1:47:32
  Speaker 2: also distinct from the region containing the black hole's unstable
• 1:47:35
  Speaker 2: photon trajectories. Rotation changes the event horizon itself. For a
• 1:47:41
  Speaker 2: Kerr black hole, the horizon's radial coordinate depends upon both
• 1:47:45
  Speaker 2: mass and spin. In the standard coordinates used to describe
• 1:47:49
  Speaker 2: Kerr spacetime, Increasing the rotation reduces this coordinate radius relative
• 1:47:54
  Speaker 2: to the Schwarzschild value for a black hole of the
• 1:47:56
  Speaker 2: same mass. That does not mean the black hole simply
• 1:48:00
  Speaker 2: shrinks like an ordinary rotating ball. Its horizon geometry is
• 1:48:04
  Speaker 2: more complicated, and coordinate radii must not be confused with
• 1:48:08
  Speaker 2: straightforward physical measurements. Nevertheless, rotation introduces genuine change in the
• 1:48:14
  Speaker 2: horizon's geometry. The behaviour of light becomes even more interesting.
• 1:48:19
  Speaker 2: Around a non black hole, the familiar photon sphere lies
• 1:48:23
  Speaker 2: at one particular radius. Around a rotating black hole, there
• 1:48:27
  Speaker 2: is no equivalent single spherical radius containing every relevant photon orbit. Instead,
• 1:48:34
  Speaker 2: Kerr spacetime possesses a more complicated photon region. containing unstable
• 1:48:39
  Speaker 2: spherical photon trajectories whose properties depend upon their direction and
• 1:48:43
  Speaker 2: relationship to the black hole's rotation. Some photons travel in
• 1:48:48
  Speaker 2: the same general direction as the spin, while others travel
• 1:48:51
  Speaker 2: against it. Their allowed orbital conditions differ because of frame dragging. Consequently,
• 1:48:58
  Speaker 2: the critical trajectories separating escaping light from captured light are
• 1:49:03
  Speaker 2: no longer arranged with exactly the same symmetry as in
• 1:49:06
  Speaker 2: the Schwarzschild case. This can affect the shape of the
• 1:49:09
  Speaker 2: black hole shadow. For a distant observer, a rotating black
• 1:49:13
  Speaker 2: hole may produce a shadow whose outline is displaced or
• 1:49:16
  Speaker 2: somewhat asymmetric rather than a perfectly centred circle. The precise
• 1:49:21
  Speaker 2: appearance depends upon the black hole's spin and the observer's
• 1:49:25
  Speaker 2: inclination relative to its rotation axis. A black hole viewed
• 1:49:29
  Speaker 2: approximately along that axis can retain a highly circular looking shadow,
• 1:49:33
  Speaker 2: even when rotating rapidly. At greater viewing inclinations, spin-related distortions
• 1:49:39
  Speaker 2: can become more apparent, but there is an important qualification.
• 1:49:44
  Speaker 2: Many rotating black holes still produce shadows that are remarkably
• 1:49:47
  Speaker 2: close to circular. The effects of spin upon the shadow
• 1:49:51
  Speaker 2: outline can be comparatively subtle, and the shape alone does
• 1:49:55
  Speaker 2: not necessarily reveal the rotation rate or direction. Astronomers therefore
• 1:50:00
  Speaker 2: cannot simply inspect a photograph, notice a roughly circular shadow,
• 1:50:04
  Speaker 2: and conclude that the black hole is not spinning. Nor
• 1:50:08
  Speaker 2: can they assume that an unevenly bright ring proves a
• 1:50:11
  Speaker 2: particular spin direction. The luminous material surrounding the black hole
• 1:50:15
  Speaker 2: has its own motion, temperature, magnetic fields and density structure.
• 1:50:21
  Speaker 2: Relativistic Doppler boosting can make approaching plasma appear brighter than
• 1:50:26
  Speaker 2: receding plasma, while gravitational lensing and gravitational redshift modify the
• 1:50:31
  Speaker 2: radiation reaching the observer. These effects influence the emission ring
• 1:50:36
  Speaker 2: independently of and sometimes in combination with the central black
• 1:50:41
  Speaker 2: hole's rotation. The underlying shadow geometry and the brightness distribution
• 1:50:46
  Speaker 2: of the surrounding plasma must therefore be considered separately. Despite
• 1:50:50
  Speaker 2: these complications, the most important principle remains unchanged. A rotating
• 1:50:56
  Speaker 2: black hole still possesses an event horizon and its gravitational
• 1:51:00
  Speaker 2: field still captures light arriving along certain trajectories. The geometrical
• 1:51:06
  Speaker 2: shadow remains a feature of photon capture and curved spacetime,
• 1:51:10
  Speaker 2: not a direct photograph of a solid surface. Rotation modifies
• 1:51:15
  Speaker 2: the details of that geometry, changing the paths available to
• 1:51:18
  Speaker 2: photons and potentially altering the apparent outline, but it does
• 1:51:22
  Speaker 2: not remove the fundamental difference between the event horizon and
• 1:51:26
  Speaker 2: the larger dark region seen by an observer. Whether a
• 1:51:30
  Speaker 2: black hole rotates slowly, rapidly or not at all, the
• 1:51:34
  Speaker 2: light travelling around it reveals much more than the object's
• 1:51:37
  Speaker 2: physical boundary. Its appearance is shaped by spacetime itself, and
• 1:51:42
  Speaker 2: rotation makes that geometry even more extraordinary. On the 10th
• 1:51:47
  Speaker 2: of April 2019, the world was shown an astronomical image,
• 1:51:52
  Speaker 2: unlike anything that had come before. At its centre was
• 1:51:55
  Speaker 2: a dark, almost circular region. surrounded by a glowing ring
• 1:52:00
  Speaker 2: that appeared brighter along its lower edge. The picture was
• 1:52:03
  Speaker 2: slightly blurred, its details were surprisingly simple, and yet it
• 1:52:08
  Speaker 2: represented one of the most extraordinary achievements in the history
• 1:52:11
  Speaker 2: of astronomy. For the first time, scientists had produced an
• 1:52:16
  Speaker 2: image resolving structures on the scale of a black hole's
• 1:52:18
  Speaker 2: event horizon. The object was located at the centre of
• 1:52:23
  Speaker 2: Messier 87. a giant elliptical galaxy approximately 55 million light-years
• 1:52:29
  Speaker 2: from Earth. Its central black hole contains around 6.5 billion
• 1:52:34
  Speaker 2: times the mass of our Sun. That extraordinary mass is
• 1:52:37
  Speaker 2: an important part of the story. Although Messier 87 is
• 1:52:41
  Speaker 2: immensely distant, its black hole is so enormous that the
• 1:52:45
  Speaker 2: characteristic dimensions of its shadow occupy a measurable angle in
• 1:52:50
  Speaker 2: the sky. Even so, the challenge was formidable. From Earth,
• 1:52:54
  Speaker 2: the observed luminous ring has an angular diameter of approximately
• 1:52:59
  Speaker 2: 42 micro arcseconds. A micro arcsecond is one millionth of
• 1:53:03
  Speaker 2: an arcsecond, and an arcsecond is one three thousand six
• 1:53:07
  Speaker 2: hundredth of a degree. The ring therefore occupies an astonishingly
• 1:53:11
  Speaker 2: small portion of the sky, far beyond the resolving power
• 1:53:15
  Speaker 2: of ordinary astronomical telescopes. To observe something on this scale,
• 1:53:20
  Speaker 2: astronomers needed an instrument with an effective diameter comparable to
• 1:53:24
  Speaker 2: that of Earth itself. Building a single telescope of that
• 1:53:28
  Speaker 2: size would present a few practical difficulties, not least the
• 1:53:31
  Speaker 2: inconvenient presence of oceans and continents. Instead, researchers connected radio
• 1:53:37
  Speaker 2: telescopes at widely separated locations around the planet, creating a
• 1:53:42
  Speaker 2: coordinated observing network called the Event Horizon Telescope. The technique
• 1:53:47
  Speaker 2: is known as very long baseline interferometry. Rather than physically
• 1:53:52
  Speaker 2: combining the telescopes into one enormous structure, the method records
• 1:53:56
  Speaker 2: radio signals at individual observatories with extremely precise timing. The
• 1:54:01
  Speaker 2: measurements are later combined to recover information about the structure
• 1:54:05
  Speaker 2: of the distant radio source. The greater the separation between
• 1:54:08
  Speaker 2: participating telescopes, the finer the angular details that the network
• 1:54:13
  Speaker 2: can potentially resolve. During April 2017, the Event Horizon Telescope
• 1:54:18
  Speaker 2: observed Messier 87 using radio telescopes distributed across several continents
• 1:54:24
  Speaker 2: and observing sites. The instruments recorded radiation at a wavelength
• 1:54:29
  Speaker 2: of approximately 1.3 millimetres. This is important because the famous
• 1:54:33
  Speaker 2: image was not produced using an ordinary optical camera. The
• 1:54:37
  Speaker 2: telescopes were detecting millimetre wave radiation emitted by energetic plasma
• 1:54:43
  Speaker 2: close to the black hole. not visible light of the
• 1:54:45
  Speaker 2: kind detected by the human eye. The observations were collected
• 1:54:50
  Speaker 2: in 2017, but producing the published image required extensive calibration,
• 1:54:55
  Speaker 2: analysis and reconstruction. It was not until April 2019 that
• 1:55:00
  Speaker 2: the results were publicly revealed. The achievement involved an international
• 1:55:05
  Speaker 2: collaboration of hundreds of researchers, engineers and technical specialists. Shepard
• 1:55:10
  Speaker 2: Doleman played a leading role in the Event Horizon Telescope project.
• 1:55:14
  Speaker 2: while Catherine Buhmann contributed to the development of computational imaging
• 1:55:19
  Speaker 2: methods used in the broader effort. But the result depended
• 1:55:22
  Speaker 2: upon far more than any individual contribution. Telescope operators, instrument designers,
• 1:55:29
  Speaker 2: calibration specialists, theoretical physicists and imaging researchers all played essential roles.
• 1:55:36
  Speaker 2: The challenge was not simply gathering enough radiation. Very long
• 1:55:40
  Speaker 2: baseline interferometry does not provide a complete photograph in the
• 1:55:44
  Speaker 2: conventional sense. Instead, it measures selected components of the source's
• 1:55:49
  Speaker 2: spatial structure, leaving astronomers to reconstruct an image consistent with
• 1:55:54
  Speaker 2: the available information. Different computational approaches can produce somewhat different
• 1:56:00
  Speaker 2: detailed appearances, particularly when the observations are incomplete. To test
• 1:56:05
  Speaker 2: the reliability of their result, the collaboration used independent imaging
• 1:56:09
  Speaker 2: teams and multiple reconstruction methods. Researchers also examined simulated observations
• 1:56:15
  Speaker 2: of theoretical black hole environments, checking which structures could be
• 1:56:20
  Speaker 2: recovered reliably from data resembling the actual measurements. Across these investigations,
• 1:56:25
  Speaker 2: one feature remained especially consistent. A broad, asymmetric ring surrounded
• 1:56:31
  Speaker 2: a substantially darker central region. The ring's measured diameter was
• 1:56:36
  Speaker 2: approximately 42 microarcseconds, with its lower portion brighter than the
• 1:56:41
  Speaker 2: upper portion. But what exactly had the telescopes photographed? The
• 1:56:46
  Speaker 2: radiation came primarily from hot plasma moving through the gravitational
• 1:56:50
  Speaker 2: environment surrounding the black hole. Charged particles interacting with magnetic
• 1:56:55
  Speaker 2: fields produced synchrotron radiation, some of which escaped towards Earth.
• 1:57:01
  Speaker 2: As that radiation travelled through curved spacetime, Gravitational lensing altered
• 1:57:06
  Speaker 2: its apparent position and distribution. Meanwhile, photon capture created a
• 1:57:11
  Speaker 2: central deficit of radiation, associated with the black hole shadow.
• 1:57:16
  Speaker 2: The resulting picture was therefore not a direct photograph of
• 1:57:19
  Speaker 2: the event horizon, as though it were a solid black surface.
• 1:57:24
  Speaker 2: It was an image of luminous matter and missing light,
• 1:57:27
  Speaker 2: shaped by the geometry of spacetime. The orange colour was
• 1:57:31
  Speaker 2: also a visual choice. Because the measured radiation lay outside
• 1:57:35
  Speaker 2: the visible spectrum, the image's colours were assigned to represent
• 1:57:39
  Speaker 2: the detected intensity. They were not the natural colours that
• 1:57:43
  Speaker 2: a human observer would necessarily perceive while looking at the
• 1:57:46
  Speaker 2: black hole. The most important scientific question concerned the size
• 1:57:50
  Speaker 2: of the observed structure. General relativity predicts that the apparent
• 1:57:55
  Speaker 2: shadow of a black hole should be substantially larger than
• 1:57:58
  Speaker 2: the event horizon itself. for a black hole with the
• 1:58:02
  Speaker 2: independently estimated mass and distance of the object in Messier 87,
• 1:58:07
  Speaker 2: astronomers could calculate the expected angular scale of the gravitationally
• 1:58:11
  Speaker 2: lensed emission and central shadow. The measured ring diameter was
• 1:58:15
  Speaker 2: consistent with those predictions, within the uncertainties of the observations
• 1:58:20
  Speaker 2: and models. This provided strong evidence that the image was
• 1:58:24
  Speaker 2: revealing the characteristic gravitational environment of a supermassive black hole.
• 1:58:29
  Speaker 2: and the story did not end with the original photograph.
• 1:58:32
  Speaker 2: In January 2024, the Event Horizon Telescope Collaboration released reconstructed
• 1:58:39
  Speaker 2: images based upon observations collected in April 2018. These newer
• 1:58:44
  Speaker 2: images showed a luminous ring with approximately the same diameter
• 1:58:48
  Speaker 2: as the original. However, the brightest region had shifted around
• 1:58:52
  Speaker 2: the ring by roughly 30 degrees. This was an important distinction.
• 1:58:57
  Speaker 2: The surrounding plasma is turbulent and constantly changing. Its brightness
• 1:59:02
  Speaker 2: distribution can evolve as material moves through the black hole's
• 1:59:06
  Speaker 2: gravitational field, but the characteristic gravitational scale of a supermassive
• 1:59:11
  Speaker 2: black hole should remain essentially unchanged over such a short period.
• 1:59:17
  Speaker 2: The persistent ring diameter, together with the changing brightness pattern,
• 1:59:22
  Speaker 2: was consistent with that expectation. The first black hole image
• 1:59:26
  Speaker 2: was therefore much more than a remarkable astronomical photograph. It
• 1:59:31
  Speaker 2: brought together the predictions of general relativity, the physics of
• 1:59:36
  Speaker 2: energetic plasma, sophisticated radio astronomy and computational imaging. Most importantly,
• 1:59:43
  Speaker 2: it demonstrated that the enormous darkness surrounding a black hole
• 1:59:48
  Speaker 2: is not simply the visible outline of its physical boundary.
• 1:59:52
  Speaker 2: The event horizon remains hidden from direct view. What the
• 1:59:56
  Speaker 2: telescopes recorded was the behaviour of light travelling through the
• 2:00:00
  Speaker 2: extraordinary spacetime surrounding it. And after more than a century
• 2:00:04
  Speaker 2: of theoretical investigation, astronomers finally had an observational image of
• 2:00:09
  Speaker 2: that remarkable geometry. At the centre of our own galaxy,
• 2:00:14
  Speaker 2: hidden behind enormous distances of stars, gas and interstellar dust,
• 2:00:19
  Speaker 2: lies one of the most remarkable objects in the Milky Way.
• 2:00:23
  Speaker 2: It cannot be seen directly through an ordinary telescope, and
• 2:00:26
  Speaker 2: yet its presence has been established with extraordinary confidence. Millions
• 2:00:32
  Speaker 2: of times more massive than the Sun, it occupies a
• 2:00:35
  Speaker 2: region of space where gravity becomes so powerful that light
• 2:00:39
  Speaker 2: itself can be captured. This object is called Sagittarius Astar,
• 2:00:45
  Speaker 2: the supermassive black hole at the centre of the Milky Way.
• 2:00:49
  Speaker 2: It lies approximately 27,000 light-years from Earth, considerably closer than
• 2:00:55
  Speaker 2: the black hole in Messier 87. Nevertheless, even this distance
• 2:01:00
  Speaker 2: is difficult to comprehend. The light arriving from the galactic
• 2:01:04
  Speaker 2: centre tonight began its journey thousands of years before the
• 2:01:07
  Speaker 2: earliest human civilisations developed. Sagittarius A star contains approximately 4
• 2:01:15
  Speaker 2: million times the mass of our Sun. And that makes
• 2:01:18
  Speaker 2: it an extraordinarily massive object by almost any familiar standard.
• 2:01:23
  Speaker 2: Yet compared with the black hole in Messier 87, which
• 2:01:26
  Speaker 2: contains approximately 6.5 billion solar masses, it is relatively modest.
• 2:01:32
  Speaker 2: The difference in mass is more than a thousandfold. For
• 2:01:36
  Speaker 2: an ideal non-rotating black hole with the mass of Sagittarius
• 2:01:40
  Speaker 2: A star, the Schwarzschild radius would be roughly 12 million kilometres.
• 2:01:45
  Speaker 2: Its event horizon would therefore have a diameter of approximately
• 2:01:49
  Speaker 2: 24 million kilometres. That is an enormous region of space,
• 2:01:54
  Speaker 2: but still dramatically smaller than the event horizon associated with
• 2:01:58
  Speaker 2: the much more massive black hole in Messier 87. For decades,
• 2:02:04
  Speaker 2: astronomers investigated the galactic centre without possessing a direct image
• 2:02:08
  Speaker 2: of its central black hole. Much of the strongest early
• 2:02:12
  Speaker 2: evidence came from observing the movements of stars. Two research programmes,
• 2:02:17
  Speaker 2: led by Reinhard Genzel and Andrea Ghez, spent years studying
• 2:02:22
  Speaker 2: stars travelling around an apparently invisible central mass. Using powerful
• 2:02:27
  Speaker 2: telescopes and increasingly sophisticated infrared observations, their teams tracked individual
• 2:02:33
  Speaker 2: stars moving through the crowded central region of the Milky Way.
• 2:02:37
  Speaker 2: Some followed remarkably tight rapid orbits. One particularly important star,
• 2:02:43
  Speaker 2: known as S2, travels around the central object in approximately
• 2:02:46
  Speaker 2: 16 years. Its motion reveals the presence of an extraordinary
• 2:02:51
  Speaker 2: amount of mass confined within a very small region. By
• 2:02:55
  Speaker 2: measuring these stellar trajectories, researchers could determine the location and
• 2:03:00
  Speaker 2: approximate mass of the unseen object. The results strongly supported
• 2:03:04
  Speaker 2: the existence of a supermassive black hole. In 2020, Reinhard
• 2:03:09
  Speaker 2: Genzel and Andrea Gies shared part of the Nobel Prize
• 2:03:12
  Speaker 2: in Physics for the discovery of a supermassive compact object
• 2:03:16
  Speaker 2: at the centre of our galaxy. But establishing the presence
• 2:03:20
  Speaker 2: of a black hole through stellar motion is different from
• 2:03:23
  Speaker 2: resolving its immediate surroundings. That achievement came on the 12th
• 2:03:27
  Speaker 2: of May, 2022, when the Event Horizon Telescope collaboration released
• 2:03:33
  Speaker 2: the first event horizon scale image of Sagittarius a star.
• 2:03:37
  Speaker 2: The observations had been collected during April 2017 using the
• 2:03:41
  Speaker 2: same global telescope network responsible for the historic image of
• 2:03:45
  Speaker 2: Messier 87. The results showed a luminous ring surrounding a
• 2:03:50
  Speaker 2: darker central region. Its apparent diameter was approximately 52 microarcseconds.
• 2:03:56
  Speaker 2: A microarcsecond is an extraordinarily small angular measurement, and resolving
• 2:04:01
  Speaker 2: an object of this apparent size requires the exceptional angular
• 2:04:05
  Speaker 2: resolution provided by very long baseline interferometry. The image was
• 2:04:11
  Speaker 2: broadly consistent with the expected appearance of a black hole
• 2:04:14
  Speaker 2: containing approximately 4 million solar masses at the measured distance
• 2:04:19
  Speaker 2: of the galactic centre. There was, however, something particularly surprising
• 2:04:24
  Speaker 2: about comparing the two famous images. The ring surrounding Sagittarius
• 2:04:29
  Speaker 2: Astar appeared slightly larger in angular diameter than the ring
• 2:04:33
  Speaker 2: around the black hole in Messier 87. Yet Sagittarius A
• 2:04:37
  Speaker 2: star is more than a thousand times less massive. The
• 2:04:41
  Speaker 2: explanation is distance. The black hole in Messier 87 lies
• 2:04:45
  Speaker 2: approximately 55 million light-years away, while Sagittarius A star is
• 2:04:50
  Speaker 2: only about 27,000 light-years from Earth. An object's apparent angular
• 2:04:55
  Speaker 2: size depends upon both its physical dimensions and its distance
• 2:05:00
  Speaker 2: from the observer. The enormous black hole in Messier 87
• 2:05:05
  Speaker 2: is much larger physically, but it is also vastly farther away. Consequently,
• 2:05:11
  Speaker 2: the two objects produce ring images occupying remarkably similar angles
• 2:05:15
  Speaker 2: in Earth's sky. Their similarities are especially interesting because the
• 2:05:19
  Speaker 2: environments surrounding them are not identical. One major difference concerns
• 2:05:24
  Speaker 2: the speed at which the emitting material changes. The characteristic
• 2:05:29
  Speaker 2: physical timescales close to a black hole increase approximately in
• 2:05:32
  Speaker 2: proportion to its mass. Around Sagittarius, a star, the hot
• 2:05:37
  Speaker 2: plasma can undergo noticeable changes within minutes. Around the much
• 2:05:41
  Speaker 2: more massive black hole in Messier 87, comparable changes in
• 2:05:45
  Speaker 2: the innermost flow generally occur over much longer periods, often
• 2:05:49
  Speaker 2: days rather than minutes. This made imaging Sagittarius A star
• 2:05:54
  Speaker 2: particularly challenging. The Event Horizon Telescope collects measurements over several
• 2:06:01
  Speaker 2: hours using Earth's rotation to help obtain different views of
• 2:06:04
  Speaker 2: the source's spatial structure. But during those hours, the plasma
• 2:06:09
  Speaker 2: surrounding Sagittarius A star can change considerably. The researchers were
• 2:06:14
  Speaker 2: therefore attempting to reconstruct an image of something that was
• 2:06:17
  Speaker 2: not remaining perfectly still while they observed it. This complicated
• 2:06:22
  Speaker 2: the analysis, and required imaging methods capable of accounting for
• 2:06:27
  Speaker 2: variability in the emission. There was another challenge as well.
• 2:06:31
  Speaker 2: Between Earth and the galactic centre lies a substantial amount
• 2:06:35
  Speaker 2: of interstellar material, including ionised gas. This material can scatter
• 2:06:41
  Speaker 2: incoming radio waves, changing their apparent distribution and introducing additional
• 2:06:46
  Speaker 2: distortions into the measurements. Astronomers must account for this interstellar
• 2:06:51
  Speaker 2: scattering when reconstructing the emission close to Sagittarius A star.
• 2:06:56
  Speaker 2: The resulting image is therefore the product of careful observational analysis,
• 2:07:01
  Speaker 2: rather than a conventional photograph taken in a single instant.
• 2:07:06
  Speaker 2: Researchers compared the measurements with extensive computer simulations of accretion
• 2:07:10
  Speaker 2: flows around black holes. Different models could reproduce aspects of
• 2:07:15
  Speaker 2: the observed brightness distribution, although they varied in their assumptions
• 2:07:19
  Speaker 2: about plasma behaviour, magnetic fields and viewing geometry. Despite these
• 2:07:25
  Speaker 2: uncertainties the characteristic ring diameter remained an important and comparatively
• 2:07:30
  Speaker 2: robust observational result. It was consistent with the expected scale
• 2:07:35
  Speaker 2: of strong gravitational lensing and photon capture around a black
• 2:07:41
  Speaker 2: hole of the independently measured mass. The darker interior however
• 2:07:45
  Speaker 2: should not be mistaken for a direct photograph of the
• 2:07:48
  Speaker 2: event horizon. As with Messier 87 The reconstructed image shows
• 2:07:54
  Speaker 2: radiation from surrounding plasma shaped by gravitational lensing relatively motion,
• 2:08:01
  Speaker 2: and the capture of photons. The central brightness depression is
• 2:08:05
  Speaker 2: related to the expected geometrical shadow, but its detailed appearance
• 2:08:10
  Speaker 2: also depends upon the emitting material and the limitations of observation.
• 2:08:16
  Speaker 2: What makes the comparison especially powerful is that the same
• 2:08:20
  Speaker 2: fundamental gravitational principles appear to describe black holes with dramatically
• 2:08:26
  Speaker 2: different masses. One lies at the centre of our own galaxy.
• 2:08:30
  Speaker 2: The other resides in a distant giant elliptical galaxy. Their
• 2:08:35
  Speaker 2: event horizons differ enormously in physical size, and their surrounding
• 2:08:39
  Speaker 2: plasma evolves on very different timescales. Yet both reveal luminous
• 2:08:44
  Speaker 2: structures on the scale predicted by general relativity. The light
• 2:08:48
  Speaker 2: surrounding each black hole follows paths through curved spacetime, while
• 2:08:53
  Speaker 2: other possible trajectories disappear across the event horizon, and in
• 2:08:57
  Speaker 2: both cases the dark central region appears substantially larger than
• 2:09:01
  Speaker 2: the physical horizon itself. Sagittarius A-star therefore provides a remarkable
• 2:09:07
  Speaker 2: example of how the same laws of gravity operate across
• 2:09:11
  Speaker 2: very different astronomical environments. At the centre of the Milky Way,
• 2:09:15
  Speaker 2: the darkness is not simply concealing an invisible object. It
• 2:09:19
  Speaker 2: is revealing the extraordinary geometry of the space surrounding it.
• 2:09:24
  Speaker 2: The shadow of a black hole is much more than
• 2:09:26
  Speaker 2: an unusual patch of darkness in an astronomical photograph. Its
• 2:09:30
  Speaker 2: size and shape contain information about the extraordinary gravitational field
• 2:09:35
  Speaker 2: surrounding one of the most extreme objects in the universe.
• 2:09:39
  Speaker 2: What initially appears to be an absence of light can
• 2:09:43
  Speaker 2: actually become a way of measuring gravity itself. This is
• 2:09:47
  Speaker 2: one of the most remarkable achievements of modern black hole astronomy.
• 2:09:51
  Speaker 2: Scientists cannot place instruments beside an event horizon or measure
• 2:09:56
  Speaker 2: its diameter with an ordinary ruler, but they can observe
• 2:10:00
  Speaker 2: the radiation travelling through the surrounding spacetime and use its
• 2:10:04
  Speaker 2: behaviour to investigate the geometry responsible for shaping the image.
• 2:10:09
  Speaker 2: The basic relationship begins with the mass of the black hole.
• 2:10:13
  Speaker 2: In Albert Einstein's general theory of relativity, the characteristic dimensions
• 2:10:18
  Speaker 2: of a black hole increase with its mass. For an
• 2:10:22
  Speaker 2: ideal non-rotating Schwarzschild black hole, both the event horizon radius
• 2:10:27
  Speaker 2: and the apparent shadow radius are directly proportional to that mass.
• 2:10:32
  Speaker 2: A more massive black hole therefore produces a physically larger shadow,
• 2:10:36
  Speaker 2: but the angular size measured from Earth depends upon distance
• 2:10:40
  Speaker 2: as well. a relatively small black hole nearby can occupy
• 2:10:44
  Speaker 2: a similar angle in the sky to a much larger
• 2:10:47
  Speaker 2: black hole far away. This explains why Sagittarius A star
• 2:10:52
  Speaker 2: and the black hole in Messier 87 produce emission rings
• 2:10:56
  Speaker 2: of comparable apparent dimensions despite their enormous difference in mass.
• 2:11:02
  Speaker 2: If astronomers know the distance to a black hole and
• 2:11:05
  Speaker 2: can accurately determine its shadow scale, they can use that
• 2:11:08
  Speaker 2: measurement to constrain its mass. Conversely, if the mass is
• 2:11:12
  Speaker 2: already known, the expected shadow size provides a prediction that
• 2:11:16
  Speaker 2: can be compared with observations. This comparison is particularly valuable
• 2:11:21
  Speaker 2: because black hole masses can be estimated through entirely different methods.
• 2:11:25
  Speaker 2: At the centre of the Milky Way, researchers have followed
• 2:11:29
  Speaker 2: stars orbiting Sagittarius A star, measuring their trajectories to determine
• 2:11:34
  Speaker 2: the mass concentrated in the central region. In other galaxies,
• 2:11:39
  Speaker 2: astronomers investigate stellar motions, gas dynamics and other gravitational effects
• 2:11:45
  Speaker 2: to estimate the masses of supermassive black holes. These approaches
• 2:11:49
  Speaker 2: do not depend upon directly measuring the black hole shadow.
• 2:11:53
  Speaker 2: When independent mass estimates and shadow scale observations agree with
• 2:11:57
  Speaker 2: the predictions of general relativity, the agreement provides evidence that
• 2:12:02
  Speaker 2: the same gravitational description explains both the movements of nearby
• 2:12:06
  Speaker 2: matter and the behaviour of light. The Event Horizon Telescope
• 2:12:10
  Speaker 2: observations of Messier 87 and Sagittarius A Star have made
• 2:12:15
  Speaker 2: these comparisons possible. Their reconstructed images reveal luminous structures with
• 2:12:20
  Speaker 2: characteristic dimensions broadly consistent with the expected gravitational scales of
• 2:12:25
  Speaker 2: the central black holes. This does not mean that every
• 2:12:29
  Speaker 2: detail has been measured perfectly. The telescope's record interferometric information
• 2:12:34
  Speaker 2: about radiation arriving from extremely distant sources. Researchers must then
• 2:12:40
  Speaker 2: reconstruct images and compare them with physical models to infer
• 2:12:44
  Speaker 2: the underlying black hole geometry. The event horizon itself is
• 2:12:48
  Speaker 2: not measured directly. Instead, its properties are constrained through the
• 2:12:53
  Speaker 2: effects that curved spacetime produces upon surrounding light. One particularly
• 2:12:59
  Speaker 2: important prediction concerns the shape of the shadow. For an
• 2:13:03
  Speaker 2: ideal Schwarzschild black hole, geometrical shadow is circular, a rotating
• 2:13:08
  Speaker 2: black hole described by the Kerr solution can produce a
• 2:13:11
  Speaker 2: somewhat different outline depending upon its angular momentum and the
• 2:13:16
  Speaker 2: observer's viewing angle, yet many realistic rotating black holes still
• 2:13:20
  Speaker 2: produce shadows that remain remarkably close to circular. A significant,
• 2:13:26
  Speaker 2: reliably measured departure from the expected dimensions or shape would
• 2:13:30
  Speaker 2: therefore be scientifically interesting. It might indicate that assumptions about
• 2:13:35
  Speaker 2: the black hole's mass, distance, spin or surrounding plasma need revision.
• 2:13:42
  Speaker 2: It could reveal limitations in the observational reconstruction or the
• 2:13:46
  Speaker 2: models used to interpret it. Under sufficiently well-controlled circumstances, it
• 2:13:52
  Speaker 2: might even suggest that the gravitational geometry differs from Einstein's predictions.
• 2:13:57
  Speaker 2: Such a conclusion would require exceptionally strong evidence because the
• 2:14:02
  Speaker 2: surrounding accretion flow introduces substantial complications. The brightness of the
• 2:14:08
  Speaker 2: observed ring depends upon the temperature, density, magnetic fields and
• 2:14:13
  Speaker 2: motion of the emitting plasma. Black hole rotation and viewing
• 2:14:17
  Speaker 2: inclination also influence the image, while limited resolution makes it
• 2:14:22
  Speaker 2: difficult to separate fine structures. Astronomers must account for these
• 2:14:27
  Speaker 2: uncertainties before interpreting an apparent distortion as a property of
• 2:14:31
  Speaker 2: spacetime itself. One promising possibility involves the extremely narrow photon subrings,
• 2:14:37
  Speaker 2: predicted by gravitational lensing. These structures arise from photons following
• 2:14:43
  Speaker 2: trajectories that pass increasingly close to the unstable photon region,
• 2:14:47
  Speaker 2: sometimes travelling around the black hole before escaping. Their positions
• 2:14:52
  Speaker 2: and shapes depend sensitively upon the surrounding geometry. Resolving them
• 2:14:57
  Speaker 2: could provide more precise measurements of the critical photon trajectories
• 2:15:01
  Speaker 2: and stronger tests of general relativity. However, the broad luminous
• 2:15:07
  Speaker 2: rings visible in current black hole images should not be
• 2:15:10
  Speaker 2: mistaken for separately resolved sequences of these theoretical sub-rings. Identifying
• 2:15:16
  Speaker 2: higher order structures requires considerably greater observational precision and careful
• 2:15:21
  Speaker 2: distinction between genuine photon ring signatures and radiation from the
• 2:15:27
  Speaker 2: wider accretion flow. This is among the scientific motivations for
• 2:15:31
  Speaker 2: the Next Generation Event Horizon Telescope initiative. Researchers aim to
• 2:15:36
  Speaker 2: improve the existing international network by incorporating additional observing stations,
• 2:15:42
  Speaker 2: enhancing sensitivity, extending observations across multiple frequencies, and developing more
• 2:15:48
  Speaker 2: advanced imaging capabilities. A larger and more capable network could
• 2:15:53
  Speaker 2: reveal finer details in the radiation surrounding supermassive black holes.
• 2:15:58
  Speaker 2: It could also make it possible to follow changes in
• 2:16:00
  Speaker 2: their environments with greater precision. That distinction between changing matter
• 2:16:05
  Speaker 2: and comparatively stable geometry is especially valuable. Hot plasma close
• 2:16:11
  Speaker 2: to a black hole can vary over remarkably short timescales.
• 2:16:15
  Speaker 2: Bright regions may shift as gas circulates, Magnetic fields evolve
• 2:16:20
  Speaker 2: and turbulence changes the distribution of radiation. The underlying gravitational scale, however,
• 2:16:26
  Speaker 2: should remain approximately constant over ordinary observing periods. By comparing
• 2:16:31
  Speaker 2: observations obtained at different times, astronomers can investigate which features
• 2:16:36
  Speaker 2: change with the accretion flow and which remain associated with
• 2:16:40
  Speaker 2: the black hole's spacetime geometry. Increasingly detailed measurements may also
• 2:16:45
  Speaker 2: help test the Kerr description of rotating black holes. Examining
• 2:16:49
  Speaker 2: whether real astronomical objects behave as predicted by Einstein's equations,
• 2:16:54
  Speaker 2: yet even the most advanced telescope faces one fundamental limitation.
• 2:16:59
  Speaker 2: The shadow cannot reveal the physical conditions inside the event horizon.
• 2:17:04
  Speaker 2: No light escaping from that interior can reach a distant observer.
• 2:17:08
  Speaker 2: The information contained within the external image therefore comes from
• 2:17:12
  Speaker 2: photon trajectories and radiation accessible outside the horizons. The shadow
• 2:17:18
  Speaker 2: tells us about the geometry surrounding the black hole, not
• 2:17:21
  Speaker 2: the hidden contents of its interior, and perhaps that makes
• 2:17:25
  Speaker 2: its scientific value even more extraordinary. A black hole appears
• 2:17:29
  Speaker 2: larger than its event horizon because gravity changes which paths
• 2:17:33
  Speaker 2: light can follow. What once seemed like an inconvenient distortion
• 2:17:39
  Speaker 2: is actually a measurable prediction of gravitational physics. The apparent
• 2:17:43
  Speaker 2: darkness becomes a tool for investigating mass, motion, and the
• 2:17:47
  Speaker 2: curvature of spacetime. An object whose defining boundary cannot be
• 2:17:52
  Speaker 2: photographed directly can nevertheless reveal remarkable details about the laws
• 2:17:56
  Speaker 2: governing the universe, and all of that information is written,
• 2:18:00
  Speaker 2: in a rather unusual way, into the light that never
• 2:18:03
  Speaker 2: reaches us. There is something rather wonderful about the fact
• 2:18:07
  Speaker 2: that one of the darkest objects in the universe can
• 2:18:10
  Speaker 2: reveal so much about the nature of light. A black
• 2:18:14
  Speaker 2: hole cannot be photographed in the ordinary sense, and yet
• 2:18:17
  Speaker 2: the images astronomers have produced show an unmistakable region of
• 2:18:21
  Speaker 2: darkness surrounded by glowing material. At first glance, it seems
• 2:18:25
  Speaker 2: as though we are looking directly at the black hole itself.
• 2:18:29
  Speaker 2: But the more carefully we examine that picture, the more
• 2:18:32
  Speaker 2: extraordinary it becomes. The darkness is not simply the outline
• 2:18:36
  Speaker 2: of the black hole's physical boundary. It is the result
• 2:18:40
  Speaker 2: of light travelling through spacetime that has been curved so
• 2:18:43
  Speaker 2: strongly that some possible paths lead towards the observer, while
• 2:18:47
  Speaker 2: others lead into the black hole and never return. At
• 2:18:51
  Speaker 2: the centre of this explanation is the event horizon. The
• 2:18:55
  Speaker 2: event horizon marks the boundary beyond which no light or
• 2:18:58
  Speaker 2: other signal can escape to the outside universe. It is
• 2:19:02
  Speaker 2: not a solid surface, an opaque shell or a dark
• 2:19:05
  Speaker 2: material sphere. It is a boundary defined by the causal
• 2:19:09
  Speaker 2: structure of spacetime, separating events that can communicate with distant
• 2:19:13
  Speaker 2: observers from those that cannot. And because it is not
• 2:19:17
  Speaker 2: an ordinary surface, its location cannot be identified simply by
• 2:19:21
  Speaker 2: looking for the edge of a black circle. Albert Einstein's
• 2:19:25
  Speaker 2: general theory of relativity provides the explanation. Matter and energy
• 2:19:30
  Speaker 2: influence the geometry of space-time and light follows paths determined
• 2:19:34
  Speaker 2: by that geometry. Near a black hole, the curvature becomes
• 2:19:39
  Speaker 2: so extreme that the familiar expectation of light travelling along
• 2:19:42
  Speaker 2: an apparently straight route is no longer a useful guide
• 2:19:46
  Speaker 2: to what we see. A photon passing relatively far from
• 2:19:50
  Speaker 2: a black hole may experience only a modest deflection. Another
• 2:19:54
  Speaker 2: passing closer can bend through a much greater angle emerging
• 2:19:58
  Speaker 2: in a direction that seems entirely unrelated to where its
• 2:20:01
  Speaker 2: journey began. Under particularly precise conditions, light can even travel
• 2:20:06
  Speaker 2: around the black hole before escaping. For an ideal non-rotating
• 2:20:11
  Speaker 2: Schwarzschild black hole, there is a remarkable region known as
• 2:20:14
  Speaker 2: the photon sphere. Located at one and a half times
• 2:20:18
  Speaker 2: the event horizon radius, the photon sphere contains possible circular
• 2:20:22
  Speaker 2: trajectories for light. These orbits are unstable. meaning that even
• 2:20:27
  Speaker 2: the smallest departure from the exact conditions causes a photon
• 2:20:31
  Speaker 2: to leave the circular path, eventually escaping or falling towards
• 2:20:35
  Speaker 2: the horizon. The photon sphere is not a glowing shell,
• 2:20:39
  Speaker 2: or a place where light normally remains trapped forever. It
• 2:20:43
  Speaker 2: is a feature of the surrounding geometry, and its unstable
• 2:20:46
  Speaker 2: trajectories play an essential role in establishing the boundary between
• 2:20:50
  Speaker 2: escaping and captured light. That boundary determines the geometrical shadow.
• 2:20:55
  Speaker 2: Imagine looking towards a black hole against a distant luminous background.
• 2:21:00
  Speaker 2: Some of the light from that background successfully travels around
• 2:21:03
  Speaker 2: the central object and reaches the observer. Other photons follow
• 2:21:07
  Speaker 2: trajectories that cross the event horizon. The missing light creates
• 2:21:12
  Speaker 2: a dark region in the image. And here lies the
• 2:21:15
  Speaker 2: numerical answer to the question that makes black hole photographs
• 2:21:19
  Speaker 2: so surprising. For an ideal non-rotating black hole viewed from
• 2:21:23
  Speaker 2: sufficiently far away, The geometrical shadow has an apparent diameter
• 2:21:28
  Speaker 2: approximately 2.6 times the diameter of the event horizon. A
• 2:21:33
  Speaker 2: black hole whose horizon measures 60km across could therefore produce
• 2:21:37
  Speaker 2: a shadow corresponding to more than 150km across on the
• 2:21:42
  Speaker 2: Distant Observer Scale. Nothing about the black hole has physically expanded.
• 2:21:48
  Speaker 2: The event horizon remains at its predicted location. The larger
• 2:21:52
  Speaker 2: shadow exists because the curved spacetime surrounding the black hole
• 2:21:55
  Speaker 2: captures photons from a wider range of incoming trajectories than
• 2:21:59
  Speaker 2: a simple picture of its physical boundary would suggest. It
• 2:22:03
  Speaker 2: is important, then, to distinguish the different features that appear
• 2:22:07
  Speaker 2: in discussions of black hole images. The event horizon marks
• 2:22:10
  Speaker 2: the boundary of no return. The photon sphere describes unstable
• 2:22:15
  Speaker 2: circular light trajectories in the non-rotating model. The shadow represents
• 2:22:20
  Speaker 2: directions associated with missing background light, and the bright emission
• 2:22:25
  Speaker 2: ring comes from radiation produced by matter moving outside the
• 2:22:28
  Speaker 2: black hole. These are related phenomena, but they are not interchangeable.
• 2:22:33
  Speaker 2: The surrounding matter makes the darkness observable. Gas drawn towards
• 2:22:38
  Speaker 2: a black hole can form an accretion flow, where turbulence,
• 2:22:42
  Speaker 2: magnetic fields and gravitational energy help produce extraordinarily hot plasma.
• 2:22:48
  Speaker 2: Charged particles within that plasma emit electromagnetic radiation, including the
• 2:22:53
  Speaker 2: synchrotron radiation detected by the Event Horizon Telescope. Gravitational lensing
• 2:22:59
  Speaker 2: redirects this radiation, sometimes allowing light from behind the black
• 2:23:03
  Speaker 2: hole to reach the observer. Meanwhile, relativistic Doppler boosting can
• 2:23:07
  Speaker 2: make rapidly approaching material appear brighter than receding material, helping
• 2:23:12
  Speaker 2: produce the uneven crescents visible in astronomical images. The famous
• 2:23:17
  Speaker 2: photographs of Messier 87 and Sagittarius A star reveal these
• 2:23:21
  Speaker 2: effects in two remarkably different environments. The black hole in
• 2:23:25
  Speaker 2: Messier 87 contains approximately 6.5 billion solar masses, while the
• 2:23:30
  Speaker 2: black hole at the centre of our Milky Way contains
• 2:23:33
  Speaker 2: approximately 4 million. Yet despite their enormous difference in physical size,
• 2:23:39
  Speaker 2: Their observed emission rings occupy comparable angles in our sky
• 2:23:43
  Speaker 2: because Sagittarius A star is so much closer to Earth.
• 2:23:47
  Speaker 2: Both images show radiation shaped by the same fundamental principles
• 2:23:51
  Speaker 2: of general relativity, and both represent genuine astronomical observations, reconstructed
• 2:23:58
  Speaker 2: from measurements collected by a worldwide network of radio telescopes.
• 2:24:02
  Speaker 2: They are not imaginary illustrations. Although careful computational processing is
• 2:24:07
  Speaker 2: necessary to turn the interferometric measurements into images, the exact
• 2:24:12
  Speaker 2: relationship between each observed emission ring and the underlying geometrical
• 2:24:16
  Speaker 2: shadow depends upon the surrounding plasma and the conditions of observation. Nevertheless,
• 2:24:23
  Speaker 2: their characteristic dimensions agree broadly with the predictions for black
• 2:24:27
  Speaker 2: holes of the independently estimated masses. That agreement is remarkable.
• 2:24:33
  Speaker 2: More than a century after Einstein developed general relativity, astronomers
• 2:24:37
  Speaker 2: can investigate the geometry around an invisible event horizon by
• 2:24:41
  Speaker 2: examining radiation that has travelled through its gravitational field. The
• 2:24:46
  Speaker 2: very effect that makes a black hole look larger than
• 2:24:49
  Speaker 2: it really is has become a scientific measuring tool. Perhaps
• 2:24:53
  Speaker 2: the loveliest part of this story is that the darkness
• 2:24:56
  Speaker 2: itself tells us something. Not because darkness is a substance
• 2:25:00
  Speaker 2: or a physical object. but because its apparent boundary reveals
• 2:25:04
  Speaker 2: which journeys light can complete and which journeys are impossible.
• 2:25:09
  Speaker 2: A black hole does not look larger because its event
• 2:25:12
  Speaker 2: horizon has grown. It looks larger because gravity changes the
• 2:25:16
  Speaker 2: paths available to light, leaving a shadow extending well beyond
• 2:25:20
  Speaker 2: the true boundary of no return. And so, when we
• 2:25:24
  Speaker 2: look at that extraordinary dark circle surrounded by a distant
• 2:25:27
  Speaker 2: glow … We are seeing something more subtle than the
• 2:25:31
  Speaker 2: outline of a black hole. We are seeing the shape
• 2:25:34
  Speaker 2: of the light that cannot reach us. And through that
• 2:25:37
  Speaker 2: absence of light, the universe has found a rather beautiful
• 2:25:40
  Speaker 2: way of revealing the curvature of spacetime.