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Every Mystery Physics Still Can_t Explain... Almost · Night Science Stories

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Speaker 1: reach a little further. If you're already part of that,
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Speaker 1: thank you. Having you here, evening after evening means more
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Speaker 1: than you'd think. Tonight's a little different though. Instead of
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Speaker 1: exploring something physics has worked out rather well, we're going
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Speaker 1: to sit with the things it still hasn't, and there
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Speaker 1: turn out to be more of those than you might imagine.
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Speaker 1: It can predict how particles behave with astonishing precision, describe
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Speaker 1: the way spacetime bends around a black hole and trace
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Speaker 1: the universe's history back billions of years. And yet it
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Speaker 1: still can't tell us what most of the universe is
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Speaker 1: actually made of, why matter exists at all, what really
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Speaker 1: happens the moment a quantum measurement is made, why time
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Speaker 1: only ever seems to move forward or whether consciousness is
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Speaker 1: even a question physics is equipped to answer. I'll try
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Speaker 1: not to make any of that sound too unsettling. Think
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Speaker 1: of it less as a catalogue of scientific failure and
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Speaker 1: more as a quiet walk along the boundary of what
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Speaker 1: we currently understand, places where the equations hold up perfectly
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Speaker 1: until suddenly they don't, places where experiments have handed us
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Speaker 1: clues rather than answers, and places where physics' next big
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Speaker 1: idea might already be sitting, waiting to be noticed. So
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Speaker 1: how does physics manage to explain so much of the
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Speaker 1: universe and still leave some of its biggest questions completely open?
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Speaker 1: Physics is remarkably good at explaining things. It can tell
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Speaker 1: us why planets follow curved paths around stars, why light
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Speaker 1: bends as it passes near a massive object, why atoms
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Speaker 1: form the structures they do, and why the sun has
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Speaker 1: been shining steadily for billions of years. It can describe
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Speaker 1: events taking place at distances so small that no ordinary
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Speaker 1: microscope could ever reveal them, and it can follow the
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Speaker 1: evolution of the universe across distances so enormous that light
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Speaker 1: itself requires billions of years to cross them. Sometimes physics
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Speaker 1: has even predicted things before anyone had seen them. Albert
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Speaker 1: Einstein's general theory of relativity predicted phenomena that later became observable,
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Speaker 1: including gravitational waves. Paul Dirac's attempt to reconcile quantum mechanics
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Speaker 1: with special relativity led to the prediction of antimatter before
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Speaker 1: the positron was discovered. The standard model of particle physics
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Speaker 1: predicted the existence of particles whose experimental confirmation came years,
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Speaker 1: or sometimes decades, later. There is something almost unnerving about
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Speaker 1: how well some of these ideas work. Measurements of certain
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Speaker 1: quantum phenomena agree with theoretical calculations to extraordinary precision. Spacecraft
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Speaker 1: can be guided across the solar system using gravitational theories
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Speaker 1: developed generations before those spacecraft existed. Experiments involving individual particles
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Speaker 1: repeatedly reproduce probabilities described by equations written on paper. And yet,
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Speaker 1: after all of that success, physics remains surrounded by things
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Speaker 1: it cannot explain. This is not quite the contradiction it
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Speaker 1: first appears to be, A scientific theory does not have
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Speaker 1: to explain everything in order to explain something extremely well.
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Speaker 1: Newton's theory of gravity remains excellent for calculating many everyday
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Speaker 1: motions even though Albert Einstein later showed that it was
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Speaker 1: not the deepest description of gravity. In much the same way,
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Speaker 1: today's most successful theories can be extraordinarily accurate while still
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Speaker 1: leaving important parts of reality outside their reach. Two frameworks
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Speaker 1: dominate much of modern fundamental physics. The first is the
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Speaker 1: standard model of particle physics. It describes the known elementary
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Speaker 1: particles and three of the fundamental interactions between them– electromagnetism,
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Speaker 1: the strong nuclear interaction and the weak nuclear interaction. The
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Speaker 1: second is Albert Einstein's general theory of relativity, our best
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Speaker 1: established description of gravity. In general relativity, gravity is not
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Speaker 1: treated simply as an invisible pulling force between objects. matter
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Speaker 1: and energy influence the geometry of spacetime, and objects move
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Speaker 1: through that curved geometry. Together, these theories explain an astonishing amount.
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Speaker 1: But they do not explain everything. General relativity does not
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Speaker 1: provide a complete quantum description of gravity. The Standard Model
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Speaker 1: does not tell us what dark matter is, neither gives
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Speaker 1: us a satisfactory explanation for dark energy. Our established particle
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Speaker 1: physics does not explain why the observable universe contains far
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Speaker 1: more matter than antimatter, and the scale of what remains
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Speaker 1: unexplained is not small. According to the standard cosmological picture,
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Speaker 1: ordinary matter, the material from which stars, planets, gas clouds
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Speaker 1: and human beings are made, contributes only about 5% of
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Speaker 1: the present cosmic energy budget. Dark matter contributes roughly 27%.
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Speaker 1: Dark energy contributes roughly 68%. so the material we understand
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Speaker 1: most directly makes up only a small fraction of the universe.
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Speaker 1: That does not mean physicists know nothing about the remaining majority.
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Speaker 1: Quite the opposite. Dark matter, for example, has been constrained
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Speaker 1: through the motions of stars and galaxies, the behaviour of
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Speaker 1: galaxy clusters, gravitational lensing and the large-scale structure of the cosmos.
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Speaker 1: We know a great deal about what dark matter must
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Speaker 1: do gravitationally. We know many things it probably cannot be.
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Speaker 1: What we do not know is what it actually is.
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Speaker 1: That distinction matters. An unsolved mystery in physics is rarely
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Speaker 1: an empty patch marked simply with the words nobody knows.
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Speaker 1: Quite often it is surrounded by measurements. Scientists may know
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Speaker 1: how strongly an unexplained effect operates, where it appears, which
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Speaker 1: proposed explanations have failed, and what any successful theory will
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Speaker 1: eventually need to reproduce. Sometimes the boundaries around the unknown
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Speaker 1: are remarkably precise. In that sense, ignorance in science can
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Speaker 1: become surprisingly sophisticated. A century of experiments might reduce an
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Speaker 1: enormous range of possibilities to a narrow collection of surviving
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Speaker 1: ideas without revealing which one is correct. That may sound frustrating,
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Speaker 1: but it is progress. Discovering that an attractive explanation is
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Speaker 1: wrong is still discovering something about nature. Science is unusually
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Speaker 1: comfortable with this kind of admission. There is no requirement
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Speaker 1: that every question receive an immediate answer. There is no
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Speaker 1: embarrassment in writing down the limits of a theory and
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Speaker 1: saying that beyond this point something is missing. In fact,
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Speaker 1: knowing precisely where an explanation fails is often the beginning
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Speaker 1: of the next explanation. The strange motions of planets helped
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Speaker 1: expose limitations in older astronomical models. Problems involving light and
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Speaker 1: electromagnetism helped lead towards relativity. Difficulties explaining atoms and radiation
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Speaker 1: helped open the door to quantum mechanics. The uncomfortable edges
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Speaker 1: of physics have repeatedly turned out to be productive places.
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Speaker 1: And the edges that remain today are extraordinary. There are
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Speaker 1: galaxies behaving as though they contain invisible mass. The expansion
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Speaker 1: of the universe is accelerating for reasons we do not understand.
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Speaker 1: Matter somehow survived the early universe when matter and antimatter
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Speaker 1: might have been expected to destroy almost all of one another.
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Speaker 1: Gravity and quantum mechanics refuse to fit comfortably into one
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Speaker 1: complete framework. Quantum theory predicts experiments beautifully, while leaving physicists
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Speaker 1: arguing about what its mathematics ultimately means. The constants of
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Speaker 1: nature possess particular values, without any established deeper explanation for
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Speaker 1: why those values were chosen. Time has an obvious direction
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Speaker 1: in everyday life, even though many fundamental equations are far
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Speaker 1: less particular about which direction time should run. And finally,
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Speaker 1: there is the peculiar fact that somewhere inside the physical universe,
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Speaker 1: Collections of particles have become capable of experiencing that universe.
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Speaker 1: None of these questions erase what physics already knows. They
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Speaker 1: sharpen the boundary around it. They remind us that scientific
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Speaker 1: knowledge is not a finished building with a few decorative
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Speaker 1: details left to complete. It is more like an illuminated
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Speaker 1: island whose coastline keeps growing as we explore it. Each
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Speaker 1: new stretch of land reveals a little more territory, but
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Speaker 1: it also creates a longer border with the darkness beyond.
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Speaker 1: which is perhaps one of the nicest things about physics.
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Speaker 1: The better we become at understanding the universe, the better
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Speaker 1: we become at seeing exactly how much of it is
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Speaker 1: still waiting for us. Galaxies look peaceful from a distance.
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Speaker 1: Their stars seem to drift in great, graceful patterns, their
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Speaker 1: spiral arms winding slowly through space, their gas clouds glowing
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Speaker 1: softly between billions of suns. But once astronomers began measuring
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Speaker 1: how galaxies actually move, Something did not add up. The
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Speaker 1: problem was gravity. Or, more precisely, there seemed to be
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Speaker 1: far more gravity than the visible matter could explain. One
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Speaker 1: of the earliest clues appeared not inside a single galaxy,
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Speaker 1: but in a cluster of galaxies. During the 1930s, the
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Speaker 1: Swiss astronomer Fritz Zwicky studied the Coma Cluster, a vast
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Speaker 1: collection of galaxies more than 300 million light-years away. He
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Speaker 1: measured how rapidly some of those galaxies were moving within
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Speaker 1: the cluster and compared their speeds with the amount of
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Speaker 1: visible matter he could see. The galaxies were moving surprisingly quickly.
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Speaker 1: If the cluster contained only the stars and luminous gas
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Speaker 1: visible through telescopes, its gravity should not have been strong
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Speaker 1: enough to hold everything together. At those speeds, many of
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Speaker 1: the galaxies should simply have escaped. Zwicky concluded that additional
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Speaker 1: mass must be present. He referred to it as dark matter.
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Speaker 1: At the time, the idea attracted relatively little attention. Astronomy
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Speaker 1: was still developing the tools needed to measure galaxies precisely,
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Speaker 1: and there were plenty of uncertainties in estimating their masses.
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Speaker 1: But decades later, the mystery returned much more clearly. During
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Speaker 1: the 1960s and 1970s, astronomer Vera Rubin and instrument maker
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Speaker 1: Kent Ford made careful measurements of the rotation of spiral galaxies.
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Speaker 1: The basic expectation seems straightforward. Most of the visible matter
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Speaker 1: in a spiral galaxy is concentrated towards its centre. If
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Speaker 1: that visible material represented most of the galaxy's mass, then
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Speaker 1: stars orbiting far from the centre should behave somewhat like
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Speaker 1: planets orbiting the Sun. Mercury moves around the Sun faster
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Speaker 1: than Earth. Earth moves faster than Neptune. As distance increases,
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Speaker 1: orbital speed generally falls because most of the solar system's
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Speaker 1: mass is concentrated near the centre. Something similar was expected
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Speaker 1: in galaxies. Instead, Rubin and Ford found that stars in
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Speaker 1: the outer regions of spiral galaxies were moving much faster
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Speaker 1: than expected. Their orbital speeds often remained roughly constant even
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Speaker 1: far beyond the bright central regions. Astronomers described this behaviour
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Speaker 1: using galactic rotation curves. A rotation curve shows how orbital
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Speaker 1: speed changes with distance from the centre of a galaxy.
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Speaker 1: If the visible stars and gas provided nearly all the mass,
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Speaker 1: the curve should rise near the centre and then fall
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Speaker 1: as the distance increases. But in many galaxies, the outer
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Speaker 1: part of the curve stays surprisingly flat. The stars keep
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Speaker 1: moving quickly, and this creates an immediate gravitational problem. If
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Speaker 1: those stars really are orbiting at the measured speeds, then
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Speaker 1: something must provide enough gravity to keep them from flying away.
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Speaker 1: The simplest explanation is that a galaxy contains far more
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Speaker 1: mass than we can see. Instead of ending near the
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Speaker 1: visible edge of the spiral disc, the mass appears to
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Speaker 1: continue outward in a vast, roughly spherical region known as
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Speaker 1: a dark matter halo. This halo may extend far beyond
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Speaker 1: the bright stars that make the galaxy easy to photograph.
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Speaker 1: The visible galaxy is therefore not necessarily the whole galaxy
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Speaker 1: in a gravitational sense. What we see may be only
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Speaker 1: the luminous centre of a much larger structure, and rotation
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Speaker 1: curves are not the only reason physicists believe this. There
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Speaker 1: is another way to detect mass, even when the mass
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Speaker 1: itself produces no light. Albert Einstein's general theory of relativity
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Speaker 1: tells us that matter and energy curve spacetime. Light travelling
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Speaker 1: through that curved spacetime follows a bent path. This effect
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Speaker 1: is called gravitational lensing. A massive galaxy or galaxy cluster
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Speaker 1: can therefore distort the light coming from more distant objects
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Speaker 1: behind it. Sometimes this produces stretched arcs, multiple images, or
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Speaker 1: subtle changes in the apparent shapes of background galaxies. By
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Speaker 1: measuring those distortions, astronomers can estimate how mass is distributed
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Speaker 1: in the foreground object. Crucially, the mass does not need
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Speaker 1: to glow. Gravity reveals it anyway. One of the most
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Speaker 1: famous examples is the bullet cluster. formerly known as 1E0657-558.
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Speaker 1: The bullet cluster is actually the result of two galaxy
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Speaker 1: clusters colliding. Much of the ordinary matter in galaxy clusters
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Speaker 1: exists not in the galaxies themselves, but in enormous clouds
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Speaker 1: of extremely hot gas between them. When the two clusters
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Speaker 1: passed through each other, those gas clouds interacted strongly. They
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Speaker 1: were slowed by collisions and pressure, producing intense X-ray emission
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Speaker 1: observed by the Chandra X-ray Observatory. The galaxies themselves pass
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Speaker 1: through more easily, because the distances between individual stars are enormous.
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Speaker 1: But the most interesting result came from gravitational lensing. The
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Speaker 1: regions containing most of the gravitational mass were found to
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Speaker 1: be offset from much of the hot gas. In other words,
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Speaker 1: the majority of the ordinary matter traced by X-rays was
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Speaker 1: in one place, while much of the mass inferred through
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Speaker 1: gravity appeared somewhere else. That separation is difficult to explain
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Speaker 1: if the only important mass is the ordinary matter we
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Speaker 1: can already see. The Bullet Cluster therefore became one of
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Speaker 1: the strongest observational examples supporting the idea that an additional,
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Speaker 1: largely collisionless component of matter exists. And the evidence does
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Speaker 1: not stop there. The motions of galaxies inside clusters point
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Speaker 1: towards missing mass. Gravitational lensing points towards missing mass. Patterns
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Speaker 1: in the cosmic microwave background point towards missing mass. The
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Speaker 1: way large-scale structure formed across the universe also works far
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Speaker 1: better when a substantial dark matter component is included. These
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Speaker 1: are very different observations, made using different instruments and different
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Speaker 1: physical effects, yet they converge on a remarkably similar picture.
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Speaker 1: In the modern cosmological model, dark matter contributes roughly 27%
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Speaker 1: of the universe's total mass-energy content. Among matter alone, dark
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Speaker 1: matter is dominant. It makes up roughly 85% of all matter,
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Speaker 1: while ordinary atoms account for only the remaining fraction. That
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Speaker 1: is an extraordinary thought. Everything familiar from planets and oceans
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Speaker 1: to stars and human bodies belongs to the minority form
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Speaker 1: of matter. Most matter seems to be something else. But
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Speaker 1: this is where an important distinction appears. There is strong
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Speaker 1: evidence that an unseen gravitational component exists. That is not
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Speaker 1: the same as knowing what it is. Astronomers can map
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Speaker 1: where dark matter appears to be. They can estimate how
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Speaker 1: much of it exists. they can watch its gravity influence
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Speaker 1: galaxies and bend light. But none of that tells us
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Speaker 1: directly what dark matter is made from. It does not
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Speaker 1: tell us whether it is a new kind of particle,
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Speaker 1: some unfamiliar field, a population of objects we have overlooked,
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Speaker 1: or a sign that gravity behaves differently from what we
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Speaker 1: currently believe. So dark matter occupies a peculiar place in physics.
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Speaker 1: It is invisible, but not undetectable, unknown, but not unconstrained.
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Speaker 1: And all across the universe, Galaxies appear to be quietly
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Speaker 1: advertising its presence through the simple fact that they are
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Speaker 1: moving far too quickly for the matter we can see.
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Speaker 1: Knowing that something is there is not the same as
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Speaker 1: knowing what it is. That is the uncomfortable position physicists
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Speaker 1: have reached with dark matter. The gravitational evidence is widespread.
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Speaker 1: Galaxies rotate as though they contain more mass than we
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Speaker 1: can see. Galaxy clusters bend light more strongly than their
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Speaker 1: visible contents seem able to explain. The large-scale structure of
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Speaker 1: the universe fits naturally into models containing a substantial dark
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Speaker 1: matter component. But when physicists ask the next question, the
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Speaker 1: answer becomes much less satisfying. What is dark matter actually
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Speaker 1: made of? For many years, one of the most important
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Speaker 1: possibilities was a new kind of particle, known as a
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Speaker 1: weakly interacting massive particle. The name is unusually descriptive. Such
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Speaker 1: a particle would have mass, would interact through gravity, and
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Speaker 1: would interact only very weakly with ordinary matter. That would
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Speaker 1: explain why enormous amounts of it could apparently surround galaxies
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Speaker 1: without glowing, absorbing much light, or constantly colliding with the
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Speaker 1: atoms around us. Weakly interacting massive particles also became attractive
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Speaker 1: for another reason. Some theories extending the standard model of
17:22
Speaker 1: particle physics naturally produced new stable particles, with properties that
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Speaker 1: could make them suitable dark matter candidates. Even more intriguingly,
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Speaker 1: Calculations of the early universe suggested that a particle interacting
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Speaker 1: with roughly the strength expected from the weak nuclear interaction
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Speaker 1: could freeze out of the primordial cosmic plasma with approximately
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Speaker 1: the right abundance to account for the dark matter measured today.
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Speaker 1: For a time, this coincidence seemed almost suspiciously convenient. It
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Speaker 1: became known as the Weakly Interacting Massive Particle Miracle. Unfortunately,
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Speaker 1: nature has so far declined to cooperate with the name.
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Speaker 1: If weakly interacting massive particles fill the dark matter halo
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Speaker 1: surrounding the Milky Way, then enormous numbers of them should
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Speaker 1: be moving through Earth continuously. They would pass through buildings, mountains, oceans,
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Speaker 1: laboratories and human bodies almost without noticing any of it.
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Speaker 1: That sounds inconvenient if you are trying to find one.
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Speaker 1: But weak interaction does not necessarily mean no interaction at all.
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Speaker 1: In principle, a dark matter particle might very occasionally strike
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Speaker 1: the nucleus of an ordinary atom and cause that nucleus
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Speaker 1: to recoil. Direct detection experiments are designed to catch precisely
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Speaker 1: such an event. The basic idea is almost comically patient.
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Speaker 1: Build an extremely sensitive detector. Remove as many sources of
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Speaker 1: ordinary interference as possible, then wait. The difficulty is that
18:42
Speaker 1: nature produces plenty of other particles capable of creating similar signals.
18:46
Speaker 1: Cosmic rays constantly strike Earth's atmosphere. Radioactive materials exist naturally
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Speaker 1: in rock, construction materials, and even tiny traces of contamination
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Speaker 1: inside experimental equipment, neutrons and neutrinos can also produce interactions
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Speaker 1: that resemble the kind of event a dark matter detector
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Speaker 1: is searching for. So modern detectors are placed in exceptionally
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Speaker 1: quiet environments. One of the largest is the Lux Zeppelin
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Speaker 1: experiment at the Sanford Underground Research Facility in South Dakota.
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Speaker 1: The detector operates almost a mile underground. The rock above
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Speaker 1: it provides an enormous natural shield against cosmic rays. At Ida,
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Speaker 1: its centre is a chamber containing several tonnes of extremely
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Speaker 1: pure liquid xenon. If a particle collides with the xenon nucleus,
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Speaker 1: the interaction can produce tiny flashes of light and free electrons.
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Speaker 1: Sensitive instruments record those signals and reconstruct properties of the event.
19:40
Speaker 1: Around the central detector are additional layers designed to identify
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Speaker 1: ordinary particles that might otherwise imitate dark matter. The entire
19:48
Speaker 1: apparatus is therefore less like a telescope staring into space
19:51
Speaker 1: and more like an exquisitely quiet trap waiting for something
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Speaker 1: invisible to bump into it, and physicists have been building
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Speaker 1: increasingly sensitive traps of this kind for decades. Experiments have
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Speaker 1: repeatedly searched through regions of particle mass and interaction strength
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Speaker 1: that once looked promising. Again and again, the expected dark
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Speaker 1: matter signal has failed to appear convincingly. Those null results matter.
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Speaker 1: They do not prove that weakly interacting massive particles do
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Speaker 1: not exist. Instead, they remove possibilities. A particle with certain
20:22
Speaker 1: masses and interaction strengths might once have been allowed by
20:24
Speaker 1: theory and experiment. After a sufficiently sensitive search finds nothing,
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Speaker 1: that region of parameter space becomes much harder to defend.
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Speaker 1: Year by year, the net tightens. And then, in September 2026,
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Speaker 1: the Lux Zeppelin collaboration reported something unusual. During a search
20:42
Speaker 1: extending into higher energy nuclear recoils, the detector recorded one
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Speaker 1: event that looked consistent with a xenon nucleus receiving a
20:50
Speaker 1: recoil energy of roughly 250,000 electron volts. The collaboration had
20:55
Speaker 1: expected very little known background in that region. The event
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Speaker 1: survived detailed scrutiny and did not immediately reveal itself as
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Speaker 1: an obvious detector problem or familiar background interaction. That made
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Speaker 1: it interesting. But interesting is not the same as discovered.
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Speaker 1: After accounting for the fact that researchers had examined a
21:14
Speaker 1: range of possible models and energies, the event corresponded to
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Speaker 1: a global statistical significance of about 2.6 standard deviations. That
21:23
Speaker 1: is far below the level physicists normally require before announcing
21:26
Speaker 1: a new particle. The collaboration itself has been careful about this.
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Speaker 1: The event is difficult to explain with the known backgrounds
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Speaker 1: they have studied. and if it were eventually connected to
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Speaker 1: dark matter it would be remarkable. But one anomalous event
21:39
Speaker 1: is not enough. More data could strengthen the signal, or
21:42
Speaker 1: it could reveal the event to have been an unusual
21:44
Speaker 1: background fluctuation after all. Physics has seen many intriguing bumps
21:49
Speaker 1: disappear with more data, so for now the particle nobody
21:52
Speaker 1: has managed to find remains unfound, and weakly interacting massive
21:57
Speaker 1: particles are only one possibility. Another candidate is the axiom,
22:02
Speaker 1: Axions emerge from a completely different problem in particle physics.
22:05
Speaker 1: The theory describing the strong nuclear interaction, known as quantum chromodynamics,
22:10
Speaker 1: appears to permit a certain kind of violation involving charge
22:13
Speaker 1: and spatial reflection. Yet experiments indicate that the strong interaction
22:18
Speaker 1: respects this symmetry to an extraordinary degree. One proposed solution
22:23
Speaker 1: introduces a new field associated with an extremely light particle
22:26
Speaker 1: called the axion. Physicists later realised that if axions exist
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Speaker 1: in sufficient abundance, they could also provide the dark matter.
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Speaker 1: Axions would behave very differently from the heavy particles sought
22:38
Speaker 1: by many xenon detectors. Experiments such as the axion dark
22:42
Speaker 1: matter experiment instead attempt to convert axions into detectable electromagnetic
22:47
Speaker 1: signals using strong magnetic fields and resonant cavities. So far,
22:52
Speaker 1: no universally accepted axion detection has appeared either, and the
22:56
Speaker 1: list of alternatives keeps growing. Dark matter could consist of
23:00
Speaker 1: ultralight fields whose behaviour is better described as waves spread
23:03
Speaker 1: across enormous distances. It could involve sterile neutrinos, hypothetical particles
23:09
Speaker 1: that interact even more weakly than the neutrinos already known.
23:12
Speaker 1: Some fraction might conceivably consist of primordial black holes formed
23:16
Speaker 1: in the very early universe, although observations severely restrict how
23:20
Speaker 1: much dark matter such objects can provide across many possible masses.
23:25
Speaker 1: There may even be an entire hidden sector of particles
23:27
Speaker 1: and forces largely disconnected from ordinary matter except through gravity
23:31
Speaker 1: or extremely faint interactions. The wider the search becomes, the
23:35
Speaker 1: stranger the possibilities can sound. There is also a more
23:39
Speaker 1: radical option. Perhaps the missing ingredient is not matter at all.
23:43
Speaker 1: Modified gravity theories attempt to explain at least some dark
23:46
Speaker 1: matter phenomena by changing how gravity behaves over very large
23:50
Speaker 1: distances or at extremely low accelerations. Mordechai Milgram proposed modified
23:55
Speaker 1: Newtonian dynamics during the early 1980s. showing that an alteration
24:00
Speaker 1: to the effective gravitational behaviour at small accelerations can reproduce
24:04
Speaker 1: aspects of many galactic rotation curves. That success makes modified
24:08
Speaker 1: gravity scientifically interesting, but reproducing individual galaxy rotation curves is
24:13
Speaker 1: not the entire problem. Any alternative must also confront galaxy clusters,
24:18
Speaker 1: gravitational lensing, the cosmic microwave background, the growth of structure
24:23
Speaker 1: and systems such as the bullet cluster. Creating one theory
24:26
Speaker 1: that matches all of those observations has proved extremely difficult.
24:30
Speaker 1: And so, the dark matter mystery remains balanced in an
24:33
Speaker 1: unusual way. On astronomical scales, something behaves with extraordinary consistency.
24:39
Speaker 1: Its gravity helps shape galaxies. It influences clusters. It leaves
24:43
Speaker 1: signatures in the oldest observable light in the universe. It
24:47
Speaker 1: appears to dominate the total amount of matter in existence,
24:50
Speaker 1: yet place a detector beneath nearly a mile of rock.
24:53
Speaker 1: fill it with tons of carefully purified xenon, shield it
24:57
Speaker 1: from almost everything else nature can throw at it, and
24:59
Speaker 1: wait for the dark matter itself to announce what it is.
25:03
Speaker 1: So far, it has remained almost perfectly quiet. For most
25:06
Speaker 1: of the 20th century, the expanding universe seemed to come
25:10
Speaker 1: with a fairly intuitive expectation. It might be expanding now,
25:14
Speaker 1: but gravity should be trying to slow that expansion down.
25:17
Speaker 1: After all, gravity pulls matter together, galaxies attract other galaxies,
25:21
Speaker 1: Clusters of galaxies attract one another. On the largest possible scale,
25:25
Speaker 1: it therefore seemed reasonable that all of the matter in
25:27
Speaker 1: the universe should gradually act as a break. The real
25:30
Speaker 1: question appeared to be how strong that breaking would be.
25:33
Speaker 1: Perhaps the expansion would slow forever but never quite stop.
25:37
Speaker 1: Perhaps enough matter existed to halt it completely. In some models,
25:41
Speaker 1: the universe might eventually reverse direction and begin collapsing. What
25:46
Speaker 1: almost nobody expected was for the expansion to speed up.
25:50
Speaker 1: The story began much earlier. During the 1920s, Georges Lemaître
25:55
Speaker 1: used Albert Einstein's general theory of relativity to describe a
25:59
Speaker 1: universe that could expand. Around the same period, observations by
26:03
Speaker 1: Edwin Hubble and others established a relationship between the distances
26:07
Speaker 1: of galaxies and the redshifts of their light. Distant galaxies,
26:11
Speaker 1: on average, appeared to be receding from us faster than
26:14
Speaker 1: nearby ones. This was not interpreted as Earth, sitting at
26:18
Speaker 1: the centre of a gigantic explosion. Instead, space itself was expanding.
26:23
Speaker 1: As the universe grows, sufficiently distant galaxies become farther apart
26:28
Speaker 1: because the space between them stretches. Once expansion had been established,
26:32
Speaker 1: physicists naturally wanted to know how that expansion had changed
26:35
Speaker 1: over cosmic history. Doing that requires two kinds of information.
26:40
Speaker 1: Astronomers need to know how quickly an object appears to
26:42
Speaker 1: be receding, which can be estimated from its redshift, and
26:45
Speaker 1: they need to know how far away it is. The
26:48
Speaker 1: second part is difficult. A distant object can look faint
26:51
Speaker 1: because it is genuinely far away, or simply because it
26:54
Speaker 1: was never very bright to begin with. What astronomers needed
26:57
Speaker 1: was something whose intrinsic brightness could be estimated reliably. One
27:02
Speaker 1: particularly useful object is a Type Ia supernova. These explosions
27:06
Speaker 1: are associated with white dwarfs in binary systems. Although the
27:10
Speaker 1: exact evolutionary routes can vary, Type Ia supernovae show regular
27:14
Speaker 1: relationships between features of their light curves and their intrinsic luminosities.
27:18
Speaker 1: That allows astronomers to standardise them. They are therefore often
27:21
Speaker 1: described as standardisable candles. If you can estimate how luminous
27:25
Speaker 1: an explosion truly was, and then measure how faint it
27:28
Speaker 1: appears from Earth, you can infer its distance. And because
27:31
Speaker 1: Type Ia supernovae are extremely bright, they can be observed
27:35
Speaker 1: across enormous stretches of the universe. During the 1990s, two
27:40
Speaker 1: international teams began using these explosions to measure the history
27:43
Speaker 1: of cosmic expansion. One was the High Redshift Supernova Search Team.
27:48
Speaker 1: led by Brian Schmidt, and including Adam Rees. The other
27:51
Speaker 1: was the Supernova Cosmology Project, led by Saul Perlmutter. Both
27:56
Speaker 1: teams were looking for distant Type Ie supernovae. The expectation
28:00
Speaker 1: was broadly that the universe's expansion should have been slowing
28:03
Speaker 1: under gravity. If that were true, supernovae at a given
28:06
Speaker 1: redshift should appear at certain predicted distances. But the observations
28:10
Speaker 1: produced something unexpected. The distant supernovae were fainter than expected.
28:15
Speaker 1: After researchers carefully considered possible complications, the simplest interpretation was
28:21
Speaker 1: that the supernovae were farther away than they would have
28:23
Speaker 1: been in a universe whose expansion had simply been slowing
28:26
Speaker 1: in the expected way. The implication was remarkable. The expansion
28:30
Speaker 1: of the universe had accelerated. The result emerged from the
28:34
Speaker 1: two teams independently and was reported in landmark work around
28:37
Speaker 1: 1998 and 1999. It was one of those moments in
28:41
Speaker 1: science where the universe appeared to have answered a reasonable
28:44
Speaker 1: question with an entirely different question. Physicists had been asking
28:48
Speaker 1: how quickly gravity was slowing the expansion. The answer seemed
28:52
Speaker 1: to be that, during the relatively recent cosmic past, the
28:55
Speaker 1: expansion had started speeding up instead. That does not mean
28:59
Speaker 1: individual galaxies are being pushed apart against every local gravitational force.
29:04
Speaker 1: Galaxies can remain gravitationally bound. The solar system is not
29:08
Speaker 1: being steadily stretched apart by cosmic acceleration. Neither is your bedroom,
29:12
Speaker 1: which is probably reassuring at this hour. The effect becomes
29:15
Speaker 1: important on enormous cosmological scales, where the expansion of space
29:19
Speaker 1: dominates over the gravitational binding of individual structures. In 2011,
29:24
Speaker 1: the Nobel Prize in Physics was awarded to Saul Perlmutter,
29:28
Speaker 1: Brian Schmidt and Adam Ries for the discovery of the
29:31
Speaker 1: accelerating expansion of the universe. But giving the discovery a
29:35
Speaker 1: prize did not explain its cause. For that, physicists use
29:39
Speaker 1: the term dark energy. The name can sound more informative
29:42
Speaker 1: than it really is. Dark energy is not the name
29:45
Speaker 1: of a substance that physicists have captured in a laboratory
29:48
Speaker 1: and studied directly. It is a broad label for whatever
29:51
Speaker 1: is responsible for the observed accelerated expansion or for whatever
29:55
Speaker 1: ingredient in our cosmological equations describes it correctly. The simplest
30:01
Speaker 1: current model uses a cosmological constant. Albert Einstein had originally
30:05
Speaker 1: introduced such a term into his gravitational equations for completely
30:09
Speaker 1: different reasons. In modern cosmology, a positive cosmological constant behaves
30:14
Speaker 1: like an energy associated with empty space itself and can
30:17
Speaker 1: drive accelerated expansion. Whether that is the complete explanation remains unknown,
30:23
Speaker 1: and dark energy is not a minor correction. In the
30:26
Speaker 1: standard cosmological model, It accounts for roughly 68% of the
30:30
Speaker 1: present total mass energy content of the universe. Dark matter
30:34
Speaker 1: contributes roughly 27%. Ordinary matter contributes only around 5%. So
30:40
Speaker 1: the largest single component of the cosmic inventory is also
30:43
Speaker 1: the one whose physical nature is least understood. The evidence
30:46
Speaker 1: no longer depends on supernovae alone. Measurements of the cosmic
30:50
Speaker 1: microwave background help constrain the geometry and contents of the universe.
30:55
Speaker 1: Baryon acoustic oscillations preserve a characteristic scale in the distribution
30:59
Speaker 1: of galaxies, allowing astronomers to trace how cosmic expansion changed
31:04
Speaker 1: over time. Surveys of large-scale structure examine how galaxies and
31:08
Speaker 1: clusters formed under the competing influences of gravity and expansion. Together,
31:13
Speaker 1: these techniques have made the accelerated expansion of the universe
31:17
Speaker 1: part of the standard cosmological picture. But they have not
31:20
Speaker 1: told us what dark energy actually is. That creates an
31:23
Speaker 1: odd reversal. With dark matter, physicists infer an unseen component
31:29
Speaker 1: because there appears to be more gravity than visible matter
31:32
Speaker 1: can provide. With dark energy, the mystery appears because cosmic
31:36
Speaker 1: expansion is behaving as though something is counteracting the tendency
31:40
Speaker 1: of gravity to slow it. In both cases, the effect
31:44
Speaker 1: is measurable, the underlying cause is not, and dark energy
31:47
Speaker 1: is perhaps the stranger of the two. It appears to
31:50
Speaker 1: dominate the universe today. influencing the evolution of space on
31:54
Speaker 1: the larger scales imaginable. Yet we know it mainly through
31:57
Speaker 1: the behaviour it produces. We can watch galaxies reveal the
32:01
Speaker 1: expansion history. We can measure ancient light. We can map
32:04
Speaker 1: enormous structures across billions of light years. And somewhere inside
32:09
Speaker 1: all of those measurements is the unmistakable message that the
32:12
Speaker 1: universe is not merely expanding, it is accelerating. Why it
32:17
Speaker 1: is doing so remains one of the deepest unanswered questions
32:20
Speaker 1: in physics. The accelerated expansion of the universe presents an
32:24
Speaker 1: awkward problem. There is actually a very simple way to
32:27
Speaker 1: put something into Albert Einstein's general theory of relativity that
32:31
Speaker 1: can make the expansion of space accelerate. The difficulty is
32:34
Speaker 1: explaining why that something has the value we observe. The
32:39
Speaker 1: relevant ingredient is called the cosmological constant. Albert Einstein introduced
32:43
Speaker 1: the cosmological constant into his gravitational equations in the early
32:47
Speaker 1: 20th century. At the time, he was trying to construct
32:50
Speaker 1: a model of a static universe before cosmic expansion had
32:54
Speaker 1: been firmly established observationally. The cosmological constant later acquired a
33:00
Speaker 1: very different role. In general relativity, it can be interpreted
33:04
Speaker 1: as a property of empty space itself. A positive cosmological
33:08
Speaker 1: constant remains present even as the universe expands, and on
33:12
Speaker 1: sufficiently large scales it can produce exactly the kind of
33:15
Speaker 1: accelerated expansion astronomers observe. This makes it the simplest explanation
33:21
Speaker 1: for dark energy. Unfortunately, another part of physics makes the
33:25
Speaker 1: simplicity rather uncomfortable. Quantum field theory describes particles as excitations
33:30
Speaker 1: of underlying quantum fields. An electron is associated with an
33:34
Speaker 1: electron field. Photons arise from the electromagnetic field. Similar fields
33:39
Speaker 1: exist for the other known fundamental particles. And in quantum physics,
33:44
Speaker 1: the lowest possible energy state of a field is not
33:47
Speaker 1: necessarily equivalent to the classical idea of absolute nothingness. Even
33:51
Speaker 1: when there are no ordinary particles present, the quantum fields remain.
33:55
Speaker 1: This lowest energy state is called the vacuum state. That
33:59
Speaker 1: immediately suggests an intriguing possibility. Perhaps the energy associated with
34:03
Speaker 1: the quantum vacuum is what appears cosmologically as the cosmological constant.
34:08
Speaker 1: At first glance, this sounds wonderfully economical. General relativity says
34:12
Speaker 1: that an energy associated with empty space could accelerate the universe.
34:16
Speaker 1: Quantum field theory says that empty space is not simply
34:19
Speaker 1: an absence of fields or energy. Perhaps these are two
34:22
Speaker 1: descriptions of the same thing. And then the numbers are calculated.
34:27
Speaker 1: This is where the elegance rather quickly disappears. Simple estimates
34:31
Speaker 1: of quantum vacuum energy can produce values enormously larger than
34:35
Speaker 1: the dark energy density inferred from cosmological observations. Under one
34:40
Speaker 1: commonly quoted estimate, Using the Planck scale as an upper
34:43
Speaker 1: energy scale in a straightforward calculation of quantum zero-point contributions,
34:47
Speaker 1: the mismatch can reach roughly 120 orders of magnitude. An
34:52
Speaker 1: order of magnitude means a factor of 10. So a
34:54
Speaker 1: difference of 120 orders of magnitude is not a slightly
34:58
Speaker 1: inaccurate prediction. It is the difference between two quantities separated
35:02
Speaker 1: by something like a 1 followed by 120 zeros. This
35:06
Speaker 1: is why the cosmological constant problem sometimes described as the
35:10
Speaker 1: worst prediction in theoretical physics. There is an important qualification, however.
35:15
Speaker 1: The famous 120 orders of magnitude should not be understood
35:18
Speaker 1: as though quantum field theory contains one perfectly defined prediction
35:22
Speaker 1: for dark energy. Physicists insert the numbers and nature simply
35:26
Speaker 1: disagrees by precisely that amount. The real problem is subtler.
35:31
Speaker 1: Vacuum energy calculations depend on how the theory is regulated
35:34
Speaker 1: and renormalised. In quantum field theory, Naive sums of zero-point
35:39
Speaker 1: energies are not directly measurable quantities, and an arbitrary high-energy
35:43
Speaker 1: cut-off should not itself be treated as a physical prediction.
35:46
Speaker 1: Different assumptions can therefore produce different versions of the discrepancy.
35:51
Speaker 1: But the deeper problem survives. Known quantum fields naturally contribute
35:55
Speaker 1: to vacuum energy, and there is no established mechanism explaining
35:58
Speaker 1: why the total gravitational effect should be so extraordinarily small
36:03
Speaker 1: while apparently remaining slightly greater than zero. If the contributions
36:07
Speaker 1: are somehow cancelling, the cancellation must be astonishingly precise. And
36:12
Speaker 1: if the cosmological constant really is simply a fundamental constant
36:16
Speaker 1: of nature with the value we observe, physics still does
36:20
Speaker 1: not explain why it has that particular value. So perhaps
36:23
Speaker 1: dark energy is not constant at all. One possibility is quintessence.
36:27
Speaker 1: Instead of empty space containing a perfectly fixed energy density,
36:31
Speaker 1: quintessence proposes a dynamical field whose properties can evolve as
36:35
Speaker 1: the universe ages. Such a field could still drive accelerated expansion,
36:40
Speaker 1: but the strength or character of dark energy might change
36:43
Speaker 1: over cosmic time. Another possibility is that general relativity itself
36:47
Speaker 1: needs modification across enormous distances. General relativity has passed an
36:51
Speaker 1: extraordinary range of tests, from the solar system to gravitational waves,
36:56
Speaker 1: so any modification would have to reproduce those successes while
36:59
Speaker 1: behaving differently, only under particular cosmological conditions. This is one
37:04
Speaker 1: reason measurements of the expansion history have become so important.
37:08
Speaker 1: The Dark Energy Spectroscopic Instrument was built specifically to map
37:11
Speaker 1: their history with remarkable precision. Mounted on the Nicholas Ulrich
37:15
Speaker 1: Mayall Telescope at Kitt Peak National Observatory in Arizona, the
37:19
Speaker 1: instrument observes enormous numbers of galaxies and quasars, measuring their
37:23
Speaker 1: positions and redshifts. From this vast three-dimensional map, physicists can
37:27
Speaker 1: study a feature called baryon acoustic oscillations. In the Young Universe,
37:32
Speaker 1: Ordinary matter and radiation formed a hot plasma through which
37:35
Speaker 1: pressure waves travelled. When the universe cooled enough for atoms
37:39
Speaker 1: to form, those waves left behind a preferred scale in
37:42
Speaker 1: the distribution of matter. Billions of years later, that scale
37:46
Speaker 1: remains faintly visible in the clustering of galaxies. It effectively
37:49
Speaker 1: provides astronomers with a standard ruler. By measuring how that
37:53
Speaker 1: ruler appears at different distances, and therefore different periods of
37:56
Speaker 1: cosmic history, researchers can reconstruct how the expansion of the
38:00
Speaker 1: universe changed over time. and recently those measurements have become
38:04
Speaker 1: especially interesting. Results based on the first three years of
38:07
Speaker 1: dark energy spectroscopic instrument observations released in 2025 strengthened earlier
38:13
Speaker 1: hints that a model with evolving dark energy could fit
38:16
Speaker 1: combinations of cosmological observations better than the simplest model containing
38:20
Speaker 1: a constant cosmological constant. That did not amount to a
38:24
Speaker 1: discovery that dark energy changes. The evidence depended on combining
38:28
Speaker 1: the instrument's measurements with other cosmological datasets and the statistical
38:32
Speaker 1: preference had not reached the level normally required to declare
38:35
Speaker 1: new physics. Still, it was enough to make cosmologists pay
38:39
Speaker 1: very close attention. Then the picture became more complicated. In
38:44
Speaker 1: July 2026, researchers released a more precise analysis using the
38:48
Speaker 1: Lyman Alpha forest. The Lyman Alpha forest consists of patterns
38:52
Speaker 1: of absorption in the light from extremely distant quasars. As
38:57
Speaker 1: quasar light travels towards us, clouds of hydrogen between galaxies
39:01
Speaker 1: absorb particular wavelengths, leaving a series of features that allow
39:04
Speaker 1: astronomers to probe the distribution of matter when the universe
39:07
Speaker 1: was much younger. The improved dark energy, spectroscopic instrument analysis
39:13
Speaker 1: used more information from these patterns and produced tighter cosmological constraints.
39:18
Speaker 1: Its central results shifted towards the standard model in which
39:21
Speaker 1: dark energy is constant. Rather than cleanly strengthening the case
39:25
Speaker 1: for evolving dark energy, the new measurement complicated it. The collapse.
39:30
Speaker 1: noted that the result could mean the earlier hints of
39:33
Speaker 1: evolution will eventually fade as measurements improve. It could also
39:36
Speaker 1: mean that the true behaviour of dark energy is more
39:39
Speaker 1: complicated than either of the simplest models currently being compared.
39:42
Speaker 1: For now, there is no established discovery that dark energy
39:45
Speaker 1: changes with time. There is a tension worth investigating. And
39:49
Speaker 1: that is almost more interesting. Cosmic acceleration itself is secure.
39:53
Speaker 1: Something in our description of the universe causes expansion on
39:56
Speaker 1: the larger scales to accelerate. But the physical explanation remains
40:00
Speaker 1: remarkably uncertain. It might be Einstein's cosmological constant. It might
40:05
Speaker 1: be quantum vacuum energy, whose tiny observed gravitational effect somehow
40:10
Speaker 1: emerges from physics we do not yet understand. It might
40:13
Speaker 1: be a slowly evolving field. It might reveal something unexpected
40:17
Speaker 1: about gravity. or the eventual explanation might not resemble any
40:20
Speaker 1: of those possibilities. The most abundant component in the present
40:25
Speaker 1: cosmic energy budget is therefore described by a name that is,
40:29
Speaker 1: in some ways, a confession. Dark energy tells us very
40:32
Speaker 1: clearly what the universe is doing. It still refuses to
40:36
Speaker 1: tell us why. Everything familiar is made of matter. The
40:39
Speaker 1: stars overhead, the planets circling them, the gas between galaxies,
40:44
Speaker 1: the atoms in your body and the device playing this
40:46
Speaker 1: voice all belong overwhelmingly the same side of a fundamental
40:49
Speaker 1: division in particle physics. That is stranger than it sounds.
40:53
Speaker 1: For many known particles, nature provides an antiparticle. An electron
40:57
Speaker 1: has the positron, a proton has the antiproton, neutrons have antineutrons.
41:02
Speaker 1: These counterparts have the same mass as their ordinary partners,
41:05
Speaker 1: but possess opposite electric charge where charge is present, along
41:09
Speaker 1: with reversed values of certain other quantum properties. Antimatter is
41:13
Speaker 1: not hypothetical. It has been produced, measured and studied in
41:16
Speaker 1: laboratories for decades. Its existence first emerged from theory. In 1928,
41:22
Speaker 1: the British physicist Paul Dirac developed a relativistic quantum equation
41:26
Speaker 1: describing the electron. The mathematics produced solutions that seemed to
41:30
Speaker 1: imply the existence of a counterpart to the electron with
41:33
Speaker 1: the same mass but opposite electric charge. A few years later,
41:37
Speaker 1: Nature provided one. In 1932, The American physicist Carl Anderson
41:41
Speaker 1: was studying tracks produced by cosmic rays when he identified
41:44
Speaker 1: a particle with the mass of an electron but a
41:46
Speaker 1: positive electric charge. It was the positron. Antimatter had gone
41:51
Speaker 1: from an unsettling feature of an equation to an observable
41:54
Speaker 1: part of nature. An antimatter behaves in a particularly dramatic
41:57
Speaker 1: way when it encounters ordinary matter. When a particle meets
42:01
Speaker 1: its corresponding antiparticle, the pair can annihilate. Their mass and
42:05
Speaker 1: energy do not simply disappear, they are transformed into other forms,
42:09
Speaker 1: often including photons or other particles. While the relevant conservation
42:13
Speaker 1: laws remain satisfied, an electron encountering a positron, for example,
42:18
Speaker 1: can annihilate and produce photons. The reverse can also occur
42:22
Speaker 1: under suitable conditions. Sufficiently energetic radiation can produce particle and
42:27
Speaker 1: antiparticle pairs. This exchange between radiation and matter was especially
42:32
Speaker 1: important in the very young universe. Shortly after the Big Bang,
42:36
Speaker 1: conditions were enormously hotter and denser than anything familiar today.
42:41
Speaker 1: Energy was abundant enough for particles and antiparticles to be
42:44
Speaker 1: continuously created and destroyed. Pairs formed. Pairs annihilated. New pairs appeared.
42:51
Speaker 1: As long as temperatures remained high enough, this cosmic population
42:55
Speaker 1: was constantly being reshuffled. And that leads to a remarkably
42:58
Speaker 1: simple question. Why was anything left? The known laws of
43:02
Speaker 1: particle physics treat matter and antimatter almost symmetrically in many respects.
43:07
Speaker 1: In a hot early universe, one therefore expects matter and
43:10
Speaker 1: antimatter to have been produced in almost equal amounts. If
43:14
Speaker 1: the numbers had been exactly equal and remained equal, then
43:17
Speaker 1: as the universe expanded and cooled, particles and antiparticles would
43:21
Speaker 1: increasingly have found one another and annihilated. Eventually almost all
43:26
Speaker 1: of them should have vanished into radiation. There would still
43:29
Speaker 1: have been a universe, it just would not have contained
43:32
Speaker 1: much of the kind of matter required to build galaxies, stars,
43:35
Speaker 1: planets or people. Clearly that is not what happened. The
43:38
Speaker 1: observable universe is overwhelmingly made from matter. The stars of
43:42
Speaker 1: the Milky Way are matter. The gas between those stars
43:46
Speaker 1: is matter. Other galaxies, as far as observations can determine,
43:50
Speaker 1: are also predominantly matter. Antimatter certainly exists, but mostly in
43:55
Speaker 1: small quantities produced by energetic natural processes or deliberately created
44:00
Speaker 1: in particle accelerators. There is no evidence that nearby regions
44:04
Speaker 1: of the observable universe contain enormous populations of antistars or
44:08
Speaker 1: antigalaxies balancing the matter around us. If large neighbouring domains
44:13
Speaker 1: of matter and antimatter existed, the boundaries between them should
44:17
Speaker 1: produce powerful annihilation signatures. Astronomers do not observe the widespread
44:22
Speaker 1: gamma radiation that such boundaries would be expected to generate,
44:26
Speaker 1: so something appears to have given matter an advantage. Not
44:29
Speaker 1: a large advantage, an astonishingly small one. common way of
44:34
Speaker 1: illustrating the required imbalance is to imagine roughly a billion
44:37
Speaker 1: matter particles, accompanied by roughly a billion antimatter particles, plus
44:42
Speaker 1: perhaps one additional matter particle. Almost every matter particle then
44:47
Speaker 1: finds an antiparticle and annihilates. The paired particles disappear into radiation,
44:52
Speaker 1: but that tiny excess remains. One particle here, one particle there.
44:57
Speaker 1: And after the annihilation is over, those leftovers become essentially
45:00
Speaker 1: all the ordinary matter that survives. The stars are made
45:03
Speaker 1: from the leftovers. The planets are made from the leftovers.
45:07
Speaker 1: We are made from the leftovers. The exact numerical description
45:11
Speaker 1: depends on which quantities and epoch are being discussed, but
45:14
Speaker 1: measurements of the early universe indicate an asymmetry of roughly
45:17
Speaker 1: this extraordinary scale. Only a tiny excess of matter over
45:21
Speaker 1: antimatter was necessary. The mystery is why that excess existed.
45:25
Speaker 1: Physicists call the present imbalance between baryonic matter and antimatter
45:30
Speaker 1: the baryon asymmetry of the universe. baryons are particles made
45:33
Speaker 1: from three quarks, including familiar protons and neutrons. The unknown
45:38
Speaker 1: physical process that created an excess of baryons over antibaryons
45:42
Speaker 1: is generally called baryogenesis, and baryogenesis remains unexplained. We have
45:47
Speaker 1: several clues that help constrain what happened. The cosmic microwave background,
45:52
Speaker 1: the ancient radiation released when the universe became transparent, preserves
45:56
Speaker 1: information about the density of ordinary matter in the early cosmos.
46:00
Speaker 1: The primordial abundances of light elements such as hydrogen and
46:03
Speaker 1: helium provide another independent test, because the amounts produced during
46:08
Speaker 1: the first minutes depended sensitively on the density of baryonic matter.
46:12
Speaker 1: These observations agree remarkably well with a universe containing a
46:16
Speaker 1: small but definite abundance of ordinary matter. At the same time,
46:21
Speaker 1: searches for large cosmic reservoirs of antimatter have not revealed
46:25
Speaker 1: anything capable of restoring a simple matter-antimatter balance. on observable scales.
46:31
Speaker 1: The conclusion is difficult to avoid. At some point in
46:35
Speaker 1: the early history of the universe, matter acquired a slight advantage.
46:39
Speaker 1: Perhaps matter was produced a little more readily. Perhaps antimatter
46:44
Speaker 1: was destroyed a little more efficiently. Perhaps processes occurred that
46:48
Speaker 1: treated the two differently in ways that the familiar everyday
46:51
Speaker 1: laws of physics barely reveal. Whatever happened, the imbalance had
46:55
Speaker 1: to be tiny. and yet almost everything visible today depended
47:00
Speaker 1: upon it. Every galaxy filled with stars can be regarded
47:04
Speaker 1: as evidence that matter won an extraordinarily close contest in
47:07
Speaker 1: the first moments of cosmic history. The victory was so
47:11
Speaker 1: narrow that for every enormous population of particles that vanished
47:15
Speaker 1: with their antimatter partners, only a minute residue remained. But
47:20
Speaker 1: that residue was enough. Enough for hydrogen. Enough for stars.
47:25
Speaker 1: Enough for carbon. oxygen and planets. Enough, eventually, for something
47:31
Speaker 1: made of matter to look back at the universe and
47:33
Speaker 1: wonder why there was any matter left at all. If
47:37
Speaker 1: matter survived because the early universe treated matter and antimatter differently,
47:43
Speaker 1: then somewhere in the laws of physics there must be
47:45
Speaker 1: a mechanism capable of creating that difference. Physicists do know
47:50
Speaker 1: of such asymmetries. The difficulty is that the ones we
47:53
Speaker 1: have discovered do not appear to be nearly strong enough
47:56
Speaker 1: In 1967, the Soviet physicist Andrei Sakharov described three broad
48:01
Speaker 1: conditions that could allow an initially almost symmetric universe to
48:05
Speaker 1: develop an excess of matter. The first requirement is baryon
48:09
Speaker 1: number violation. Baryon number is a quantum bookkeeping rule that
48:14
Speaker 1: in ordinary particle reactions effectively keeps track of particles such
48:18
Speaker 1: as protons and neutrons. If the universe is to begin
48:21
Speaker 1: with equal amounts of matter and antimatter, and somehow finish
48:25
Speaker 1: with more baryonic matter, then some process must be capable
48:29
Speaker 1: of changing that balance. The second requirement involves violations of
48:34
Speaker 1: particular symmetries between matter and antimatter. One of these is
48:39
Speaker 1: charge symmetry. In very broad terms, charge symmetry asks what
48:43
Speaker 1: happens if every particle in a physical process is replaced
48:46
Speaker 1: with its antiparticle. Another is charge parity symmetry, which combines
48:51
Speaker 1: that exchange with a reversal of spatial coordinates. rather like
48:55
Speaker 1: viewing the process in a mirror while simultaneously replacing matter
48:58
Speaker 1: with antimatter. If the laws of nature treated those transformed
49:02
Speaker 1: situations perfectly identically, there would be no obvious way for
49:06
Speaker 1: the early universe to favour matter. So some asymmetry is required.
49:10
Speaker 1: Sakharov's third condition is a departure from thermal equilibrium. In
49:15
Speaker 1: perfect thermal equilibrium, forward and reverse reactions balance one another
49:19
Speaker 1: to a Any developing preference for matter can simply be
49:24
Speaker 1: undone by the reverse process. The Young Universe, however, was
49:28
Speaker 1: expanding and cooling rapidly. Under the right circumstances, reactions could
49:33
Speaker 1: fall out of equilibrium, providing an opportunity for a small
49:36
Speaker 1: imbalance to become permanently preserved. These three ideas are now
49:40
Speaker 1: known as the Sakharov conditions, and rather intriguingly, nature does
49:45
Speaker 1: provide at least part of what they require. Matter and
49:48
Speaker 1: antimatter do not always behave identically. One of the earliest
49:53
Speaker 1: major clues appeared in 1964. James Cronin and Val Fitch,
49:58
Speaker 1: working with their colleagues, studied particles called neutral kaons. Kaons
50:03
Speaker 1: are mesons, particles built from a quark and an antiquark.
50:07
Speaker 1: The expectation had been that the combined operation of replacing
50:10
Speaker 1: particles with antiparticles and reflecting the spatial arrangement should preserve
50:15
Speaker 1: the relevant behaviour. Instead, the neutral kaons occasionally decayed in
50:20
Speaker 1: a way showing that this combined charge parity symmetry was
50:23
Speaker 1: not exact. Matter and antimatter really could behave slightly differently.
50:29
Speaker 1: The discovery was profound. Cronin and Fitch later received the
50:33
Speaker 1: Nobel Prize in Physics for it. Since then, charge parity
50:37
Speaker 1: violation has been measured in other systems. It has been
50:39
Speaker 1: observed in particles containing beauty quarks and, much later, in
50:44
Speaker 1: particles containing charm quarks. These results have allowed physicists to
50:48
Speaker 1: examine the asymmetry with increasing precision. Within the standard model
50:53
Speaker 1: of particle physics, an important explanation was developed by Makoto
50:56
Speaker 1: Kobayashi and Toshihide Muskawa. In 1973, They showed that charge
51:02
Speaker 1: parity violation could arise naturally if quarks existed in at
51:05
Speaker 1: least three generations and their quantum states mixed with one
51:08
Speaker 1: another through the weak interaction. At the time, not all
51:12
Speaker 1: of the required quarks had even been discovered. Nature eventually
51:16
Speaker 1: supplied them. The framework became part of the modern description
51:19
Speaker 1: of quark mixing, and Kobayashi and Muskawa were jointly awarded
51:23
Speaker 1: part of the 2008 Nobel Prize in Physics for explaining
51:26
Speaker 1: the origin of this broken symmetry. So physics does contain
51:30
Speaker 1: a mechanism that distinguishes matter from antimatter. That sounds encouraging.
51:34
Speaker 1: But it does not solve the cosmic problem. The amount
51:37
Speaker 1: of charge parity violation produced by the known standard model
51:40
Speaker 1: mechanisms appears to be far too small to explain why
51:43
Speaker 1: the observable universe contains such an overwhelming excess of matter.
51:48
Speaker 1: The difference exists. It simply does not seem remotely large enough.
51:52
Speaker 1: That has made the search for additional forms of charge
51:54
Speaker 1: parity violation especially important. One of the places where physicists
51:59
Speaker 1: look is the Large Hadron Collider at the European Organization
52:03
Speaker 1: for Nuclear Research near Geneva. The Large Hadron Collider beauty
52:07
Speaker 1: experiment is designed with particular sensitivity to particles containing beauty
52:12
Speaker 1: and charm quarks. These particles are unstable and survive for
52:16
Speaker 1: only tiny fractions of a second before decaying. But the
52:19
Speaker 1: patterns of their decay can reveal subtle differences between matter
52:23
Speaker 1: and antimatter. Physicists can produce enormous numbers of these particles,
52:27
Speaker 1: reconstruct their decay products, and compare how frequently a particle
52:31
Speaker 1: follows a particular path with how frequently its antiparticle follows
52:35
Speaker 1: the corresponding antimatter path. Tiny differences matter. And in 2025,
52:41
Speaker 1: the Large Hadron Collider Beauty Collaboration announced an important result.
52:45
Speaker 1: For the first time, charge parity violation was observed with
52:48
Speaker 1: discovery-level statistical significance in the decay of a baryon. The
52:52
Speaker 1: particle involved was a Beauty Lambda baryon a short-lived relative
52:56
Speaker 1: of familiar baryons such as protons and neutrons. Unlike mesons,
53:01
Speaker 1: which consist of a quark and an antiquark, baryons contain
53:04
Speaker 1: three quarks. Physicists had expected charge parity violation to occur
53:09
Speaker 1: in baryons as well, but demonstrating it conclusively had taken
53:12
Speaker 1: decades of increasingly large datasets. The result opened another window
53:16
Speaker 1: onto differences between matter and antimatter. But it did not
53:20
Speaker 1: provide the missing explanation for the universe. the observed behaviour
53:24
Speaker 1: is broadly part of the kind of asymmetry allowed within
53:27
Speaker 1: known particle physics, and the overall amount of charge parity
53:30
Speaker 1: violation available in the Standard Model remains many orders of
53:33
Speaker 1: magnitude too small to generate the matter-antimatter imbalance seen across
53:37
Speaker 1: the cosmos. So theorists have explored additional possibilities. One is
53:42
Speaker 1: electroweak baryogenesis. In the early universe, the electromagnetic and weak
53:46
Speaker 1: nuclear interactions were connected through the electroweak theory. If the
53:50
Speaker 1: transition associated with electroweak symmetry breaking occurred under suitable conditions,
53:55
Speaker 1: and if additional sources of charge parity violation existed beyond
53:58
Speaker 1: those currently known, it might have produced a lasting matter excess.
54:03
Speaker 1: The simplest version of the standard model does not seem
54:06
Speaker 1: capable of making this mechanism work strongly enough, but extensions
54:10
Speaker 1: of the theory could change that. Another possibility is leptogenesis.
54:15
Speaker 1: Leptons form another family of fundamental particles that includes electrons
54:19
Speaker 1: and neutrinos. In leptogenesis, processes in the very early universe
54:23
Speaker 1: first create an imbalance between leptons and antileptons. Other known
54:28
Speaker 1: high-temperature processes could then convert part of that lepton asymmetry
54:31
Speaker 1: into an excess of baryons. Some versions involve very heavy
54:35
Speaker 1: hypothetical neutrino-like particles, whose properties might also help explain why
54:40
Speaker 1: ordinary neutrinos have such extraordinarily small masses. None of these
54:44
Speaker 1: ideas has yet been established as the answer. There may
54:47
Speaker 1: be undiscovered particles, there may be new interactions. there may
54:51
Speaker 1: be additional sources of charge parity violation that appear only
54:54
Speaker 1: at energies or through processes we have not yet reached,
54:57
Speaker 1: or the mechanism responsible may belong to a deeper theory
55:01
Speaker 1: that changes the way we understand the earliest fractions of
55:03
Speaker 1: cosmic history. This leaves ordinary matter in a rather remarkable position.
55:08
Speaker 1: We often think of new physics as something that must
55:10
Speaker 1: be discovered by building a larger machine or finding an
55:13
Speaker 1: exotic particle, but the evidence that our present theory is
55:17
Speaker 1: incomplete may already be sitting everywhere around us, every atom
55:20
Speaker 1: of hydrogen, every rocky planet, every cloud of interstellar gas,
55:25
Speaker 1: every star burning quietly in the night. All of them
55:28
Speaker 1: exist because, very early in cosmic history, matter somehow gained
55:32
Speaker 1: an advantage that the physics we currently know cannot adequately explain.
55:36
Speaker 1: The mystery is not why antimatter exists. We understand that
55:40
Speaker 1: it does. The mystery is why matter won. And the
55:43
Speaker 1: fact that anything is here to ask the question suggests
55:46
Speaker 1: that nature once made a very small distinction between with
55:49
Speaker 1: consequences large enough to fill an entire universe. Modern physics
55:53
Speaker 1: rests on two extraordinarily successful foundations. One describes gravity, planets, stars, galaxies,
56:01
Speaker 1: and the structure of spacetime itself. The other describes atoms, particles, radiation,
56:07
Speaker 1: and the microscopic behaviour of nature. Both work. Both have
56:11
Speaker 1: survived astonishingly demanding tests, and yet they do not fit together.
56:16
Speaker 1: Albert Einstein's general theory of relativity, is our best established
56:20
Speaker 1: description of gravity. Before Einstein, gravity was usually treated in
56:24
Speaker 1: the spirit of Isaac Newton, as a force acting between masses.
56:29
Speaker 1: Einstein changed the picture. In general relativity, matter and energy
56:33
Speaker 1: alter the geometry of spacetime, and objects move through that
56:36
Speaker 1: curved geometry. The Earth orbits the Sun not because an
56:40
Speaker 1: invisible force is reaching across empty space in the old
56:42
Speaker 1: classical sense, but because the Sun changes the geometry around it,
56:47
Speaker 1: and the Earth follows the resulting path through spacetime. The
56:50
Speaker 1: same theory explains the bending of light by massive objects,
56:54
Speaker 1: the slow shift of Mercury's orbit, the existence of black holes,
56:58
Speaker 1: and the propagation of gravitational waves. It has been tested
57:02
Speaker 1: from the solar system to systems containing neutron stars and
57:05
Speaker 1: black holes, and it continues to work remarkably well. Quantum
57:09
Speaker 1: mechanics is equally successful, but in a very different domain.
57:13
Speaker 1: At microscopic scales, Particles do not behave like tiny classical
57:17
Speaker 1: objects following perfectly definite paths. They are described using quantum states, probabilities,
57:23
Speaker 1: interference and superposition. Quantum mechanics explains atomic structure, chemical bonding, radioactivity, lasers,
57:30
Speaker 1: semiconductors and much of modern technology. Its more complete relativistic form,
57:36
Speaker 1: quantum field theory, treats particles as excitations of underlying fields.
57:41
Speaker 1: Within this framework, electromagnetism, The strong nuclear interaction and the
57:46
Speaker 1: weak nuclear interaction can all be described quantum mechanically. Predictions
57:52
Speaker 1: made using quantum field theory have been confirmed to extraordinary precision.
57:56
Speaker 1: So the problem is not that physicists have one successful
57:58
Speaker 1: theory and one unsuccessful theory. That would be much easier.
58:03
Speaker 1: The problem is that they have two astonishingly successful theories
58:06
Speaker 1: that describe nature in fundamentally different ways. Most of the time,
58:10
Speaker 1: this causes no practical difficulty. If you are calculating the
58:14
Speaker 1: motion of a planet, quantum effects are negligible. If you
58:17
Speaker 1: are calculating the behaviour of an electron inside an atom,
58:21
Speaker 1: the curvature of spacetime is negligible. Each theory can quietly
58:24
Speaker 1: dominate its own territory. Trouble begins when both should matter
58:28
Speaker 1: at the same time. General relativity treats spacetime as dynamical.
58:33
Speaker 1: Spacetime bends, stretches and evolves. Its geometry responds to matter
58:38
Speaker 1: and energy. Ordinary quantum field theory, by contrast, is usually
58:41
Speaker 1: formulated on top of some spacetime background. The quantum fields
58:45
Speaker 1: fluctuate and interact within that arena. But if spacetime itself
58:49
Speaker 1: must also obey quantum rules, the usual separation begins to
58:53
Speaker 1: break down. Now the stage is part of the performance,
58:56
Speaker 1: and nobody has yet produced a complete experimentally confirmed theory
59:00
Speaker 1: describing that situation. Black holes provide one of the clearest
59:04
Speaker 1: places where the problem appears. According to classical general relativity
59:08
Speaker 1: … enough matter compressed into a sufficiently small region can
59:11
Speaker 1: form a black hole. Inside the event horizon, the theory
59:15
Speaker 1: predicts continued gravitational collapse. Under broad assumptions, that collapse leads
59:20
Speaker 1: towards a singularity. A singularity is not best understood as
59:23
Speaker 1: a tiny physical object sitting at the centre. It is
59:27
Speaker 1: a sign that the classical theory has reached a boundary
59:29
Speaker 1: where quantities such as spacetime curvature become unbounded and the
59:33
Speaker 1: description stops being physically complete. Roger Penrose showed during the
59:37
Speaker 1: 1960s that singular behaviour could arise, generically, in gravitational collapse
59:43
Speaker 1: rather than being an artefact of one unusually idealised black
59:46
Speaker 1: hole model. Stephen Hawking later extended related methods to cosmology. Together,
59:51
Speaker 1: the singularity theorems associated with Penrose and Hawking demonstrated that
59:56
Speaker 1: singularities are difficult to avoid within classical general relativity under
1:00:00
Speaker 1: broad physical conditions. That creates an obvious warning. If a
1:00:04
Speaker 1: theory predicts a regime in which its own quantities diverge
1:00:07
Speaker 1: without limit, that is often a clue that some deeper
1:00:10
Speaker 1: description is required. The same difficulty appears when general relativity
1:00:14
Speaker 1: is extrapolated backwards towards the beginning of the universe. As
1:00:18
Speaker 1: the cosmic scale shrinks, temperatures and densities rise. Eventually, the
1:00:23
Speaker 1: classical equations lead towards conditions associated with the Big Bang singularity.
1:00:29
Speaker 1: But at sufficiently extreme densities and tiny scales, quantum effects
1:00:33
Speaker 1: should become unavoidable. A complete account of the earliest moments
1:00:37
Speaker 1: of cosmic history therefore requires something more than classical spacetime alone.
1:00:42
Speaker 1: Physicists often use the Planck scale to indicate where this
1:00:45
Speaker 1: deeper theory might become essential. The Planck length is roughly
1:00:48
Speaker 1: 10 to the power of negative 35 metres. That is
1:00:52
Speaker 1: an almost absurdly small distance. A proton is already tiny,
1:00:56
Speaker 1: with a characteristic size around 10 to the power of
1:00:59
Speaker 1: negative 15 metres. The Planck length is roughly 20 orders
1:01:03
Speaker 1: of magnitude smaller again. At scales of this order, many
1:01:06
Speaker 1: physicists expect our familiar description of smooth spacetime to break down.
1:01:11
Speaker 1: The corresponding Planck energy is unimaginably high, compared with the
1:01:15
Speaker 1: energies produced in present-day particle accelerators. This is one reason
1:01:19
Speaker 1: quantum gravity is so difficult to investigate experimentally. There is
1:01:23
Speaker 1: another clue hiding around black holes. In the 1970s, Stephen
1:01:28
Speaker 1: Hawking combined ideas from quantum field theory with curved spacetime
1:01:32
Speaker 1: and found that black holes should not be perfectly black.
1:01:35
Speaker 1: Quantum fields near a black hole allow it to emit
1:01:38
Speaker 1: thermal radiation. This is now known as Hawking radiation. A
1:01:42
Speaker 1: black hole therefore possesses temperature. Given enough time, an isolated
1:01:46
Speaker 1: black hole can lose energy and shrink. That result is
1:01:50
Speaker 1: remarkable because it brings quantum theory, gravity and thermodynamics together
1:01:55
Speaker 1: in one calculation. But it also creates a serious puzzle.
1:01:58
Speaker 1: If a black hole forms from matter in some particular
1:02:01
Speaker 1: quantum state and later evaporates into apparently thermal radiation, what
1:02:06
Speaker 1: happens to the detailed quantum information describing the original state?
1:02:11
Speaker 1: Ordinary quantum theory strongly suggests that information encoded in a
1:02:14
Speaker 1: quantum state should not simply be destroyed. Classical black hole
1:02:19
Speaker 1: evaporation seems to threaten that principle. This tension is known
1:02:23
Speaker 1: as the black hole information problem. It has occupied physicists
1:02:26
Speaker 1: for decades, and once again it appears exactly where gravity
1:02:30
Speaker 1: and quantum mechanics are forced into the same room. What
1:02:33
Speaker 1: is needed is a theory of quantum gravity. Such a
1:02:36
Speaker 1: theory would have to describe spacetime itself in quantum terms.
1:02:40
Speaker 1: It would need to reproduce ordinary quantum mechanics where gravity
1:02:43
Speaker 1: is weak. It would also need to reproduce general relativity
1:02:46
Speaker 1: where quantum effects of spacetime are negligible, and in the
1:02:50
Speaker 1: extreme regimes where both matter, it would need to tell
1:02:53
Speaker 1: us what actually replaces the singularities predicted by classical relativity.
1:02:58
Speaker 1: The challenge is severe because direct experiments at the Planck
1:03:01
Speaker 1: scale are far beyond current technology. The Large Hadron Collider
1:03:05
Speaker 1: reaches enormous energies by human standards, but they remain fantastically
1:03:09
Speaker 1: below the Planck energy, so physicists must search indirectly. They
1:03:14
Speaker 1: look to black holes. They examine the early universe. They
1:03:17
Speaker 1: study gravitational waves. They search for subtle departures from known
1:03:21
Speaker 1: physics that might reveal how spacetime behaves at its deepest level.
1:03:25
Speaker 1: For now, the two great frameworks remain side by side.
1:03:29
Speaker 1: General relativity describes the architecture of the cosmos. Quantum theory
1:03:33
Speaker 1: describes the microscopic ingredients inside it. Both are astonishingly successful.
1:03:39
Speaker 1: Neither can simply replace the other. And somewhere inside black holes,
1:03:43
Speaker 1: somewhere near the earliest moments of the universe, nature requires
1:03:46
Speaker 1: them to become one description. We simply do not yet
1:03:50
Speaker 1: know what that description is. If gravity must eventually become
1:03:53
Speaker 1: quantum mechanical, then an even stranger question appears. What exactly
1:03:58
Speaker 1: is spacetime? In everyday life, space seems like the place
1:04:02
Speaker 1: where things happen, while time seems like the sequence in
1:04:05
Speaker 1: which they happen. Albert Einstein's general theory of relativity combined
1:04:09
Speaker 1: those ideas into spacetime, four-dimensional geometry that can stretch, curve
1:04:15
Speaker 1: and evolve. But general relativity still treats spacetime as something smooth.
1:04:21
Speaker 1: You can imagine magnifying a region again and again and, mathematically,
1:04:25
Speaker 1: the geometry remains continuous. Quantum physics encourages a very different suspicion.
1:04:31
Speaker 1: At sufficiently small scales, many quantities do not behave as
1:04:34
Speaker 1: smooth classical variables. Energy comes in discrete amounts. Quantum states fluctuate.
1:04:40
Speaker 1: Particles are described by fields and probabilities, so perhaps spacetime
1:04:44
Speaker 1: itself is not fundamentally smooth. Perhaps what looks like continuous
1:04:48
Speaker 1: space and time emerges from something deeper. That possibility changes
1:04:53
Speaker 1: the question completely. Instead of asking what matter does inside spacetime,
1:04:58
Speaker 1: physicists begin asking what spacetime itself might be made from.
1:05:02
Speaker 1: One of the most famous attempts to answer this is
1:05:04
Speaker 1: string theory. In conventional particle physics, Elementary particles such as
1:05:09
Speaker 1: electrons are treated as point-like objects with no known internal size.
1:05:13
Speaker 1: String theory replaces those points with extraordinarily tiny one-dimensional objects
1:05:18
Speaker 1: called strings. A string can vibrate in different ways, and
1:05:22
Speaker 1: just as different vibration patterns on a musical string correspond
1:05:25
Speaker 1: to different notes, different quantum vibration states of a fundamental
1:05:29
Speaker 1: string can appear as different particles. One vibration might behave
1:05:33
Speaker 1: like an electron. another might possess properties associated with another particle.
1:05:38
Speaker 1: Most importantly for quantum gravity, the mathematics naturally contains a
1:05:42
Speaker 1: massless particle with spin 2. That is exactly the sort
1:05:46
Speaker 1: of particle expected for the hypothetical quantum carrier of gravity
1:05:50
Speaker 1: known as the graviton. Gravity therefore appears within string theory
1:05:54
Speaker 1: rather than having to be added afterwards. That feature is
1:05:57
Speaker 1: one reason the theory attracted enormous attention. But string theory
1:06:02
Speaker 1: comes with consequences that are much harder to see. The
1:06:05
Speaker 1: mathematics of conventional superstring theory requires more spatial dimensions than
1:06:10
Speaker 1: the three we experience. Rather than length, width and height
1:06:14
Speaker 1: being the complete spatial picture, additional dimensions would exist as well.
1:06:19
Speaker 1: The obvious question is why nobody has noticed them. One
1:06:22
Speaker 1: proposed answer is compactification. The extra dimensions may be curled
1:06:26
Speaker 1: up into structures so small that they are effectively invisible
1:06:29
Speaker 1: at ordinary scales. A simple analogy is a garden hose
1:06:33
Speaker 1: seen from far away. From a distance, it may look
1:06:36
Speaker 1: like a one-dimensional line running across the ground. Move close enough,
1:06:40
Speaker 1: and another dimension becomes apparent because an ant could also
1:06:43
Speaker 1: walk around the circumference. Extra dimensions in string theory would
1:06:47
Speaker 1: be far more complicated and far smaller. But the general
1:06:50
Speaker 1: idea is similar. During the 1980s, major advances by theorists
1:06:55
Speaker 1: including Michael Green and John Schwartz helped establish the consistency
1:06:59
Speaker 1: of superstring theories. Later work revealed relationships between several apparently
1:07:04
Speaker 1: different versions. During the 1990s, Edward Witten and others helped
1:07:09
Speaker 1: develop the idea that these theories may be different limits
1:07:12
Speaker 1: of a deeper framework commonly called M-theory. The achievement was
1:07:16
Speaker 1: mathematically impressive, but it introduced another problem. There does not
1:07:20
Speaker 1: appear to be one obvious string theory producing one inevitable universe. Instead,
1:07:24
Speaker 1: the theory seems capable of allowing an enormous landscape of
1:07:27
Speaker 1: possible solutions. Different ways of compactifying the extra dimensions can
1:07:32
Speaker 1: produce different particles, constants, and low-energy physics. The challenge is
1:07:37
Speaker 1: therefore not merely constructing a mathematically consistent quantum theory containing gravity.
1:07:42
Speaker 1: It is explaining why that theory should produce this universe,
1:07:45
Speaker 1: the one with the particles we observe, the forces we measure,
1:07:48
Speaker 1: and the particular cosmological history that surrounds us. String theory
1:07:53
Speaker 1: is not the only attempt to quantify spacetime. Another major
1:07:57
Speaker 1: program is loop quantum gravity. Its development is closely associated
1:08:01
Speaker 1: with physicists including Abhay Ashtakar, Carlo Rovelli and Lee Smolin.
1:08:06
Speaker 1: Rather than beginning with strings moving through a higher dimensional background,
1:08:10
Speaker 1: loop quantum gravity tries to quantize the geometry of space itself.
1:08:14
Speaker 1: In this framework, quantities such as area and volume can
1:08:17
Speaker 1: acquire discrete values. At extremely small scales, geometry may therefore
1:08:22
Speaker 1: come in something resembling indivisible quantum units. Space would appear
1:08:26
Speaker 1: smooth only when viewed on scales enormously larger than those units.
1:08:31
Speaker 1: The mathematics uses structures known as spin networks. A spin
1:08:35
Speaker 1: network can be pictured loosely as a web of links
1:08:38
Speaker 1: and nodes carrying quantum information about geometry. Its evolution can
1:08:42
Speaker 1: be described using related structures known as spin foams. These
1:08:46
Speaker 1: should not be imagined as literal physical threads floating inside
1:08:49
Speaker 1: some pre-existing empty room. The network is intended to describe
1:08:54
Speaker 1: the geometry itself, there may be no deeper background space
1:08:57
Speaker 1: underneath it. If that idea is correct, asking what lies
1:09:01
Speaker 1: between the fundamental pieces of space could be rather like
1:09:04
Speaker 1: asking what lies north of the North Pole. The question
1:09:08
Speaker 1: may assume a background that does not exist, and there
1:09:10
Speaker 1: are still more approaches. Causal set theory explores the possibility
1:09:15
Speaker 1: that spacetime is fundamentally a discrete collection of events linked
1:09:19
Speaker 1: by causal relationships. Causal dynamical triangulations attempt to construct large-scale
1:09:25
Speaker 1: spacetime from simpler quantum building blocks while preserving a meaningful
1:09:29
Speaker 1: distinction between cause and effect. Asymptotic safety investigates whether gravity
1:09:34
Speaker 1: might remain mathematically well-behaved at arbitrarily high energies because its
1:09:38
Speaker 1: interactions approach a special quantum state known as a fixed point.
1:09:42
Speaker 1: None of these programs has yet provided the experimentally confirmed
1:09:46
Speaker 1: theory physicists are looking for, another family of ideas, goes
1:09:50
Speaker 1: in an even more surprising direction. Perhaps spacetime emerges from
1:09:55
Speaker 1: quantum information. One clue comes from the holographic principle. The
1:09:59
Speaker 1: basic suggestion is that the physics inside a region may,
1:10:02
Speaker 1: under certain circumstances, be completely represented by information associated with
1:10:07
Speaker 1: a lower dimensional boundary. This is not simply the claim
1:10:11
Speaker 1: that the universe is a hologram in the everyday sense.
1:10:14
Speaker 1: It is a precise mathematical idea inspired partly by the
1:10:18
Speaker 1: strange relationship between black hole entropy and the area of
1:10:22
Speaker 1: a black hole's horizon. A major breakthrough came from Juan
1:10:26
Speaker 1: Maldacina in 1997. He proposed a remarkable correspondence in which
1:10:32
Speaker 1: a gravitational theory in a particular kind of higher-dimensional spacetime
1:10:36
Speaker 1: can be mathematically equivalent to a quantum field theory without
1:10:40
Speaker 1: gravity living on its lower-dimensional boundary. This relationship is known
1:10:44
Speaker 1: as gauge-gravity duality. The two descriptions appear completely different. One
1:10:50
Speaker 1: contains gravity and an extra spatial dimension. The other does not.
1:10:56
Speaker 1: Yet in certain settings, they can encode the same physics.
1:10:59
Speaker 1: That result has encouraged a striking possibility. Space-time itself may
1:11:04
Speaker 1: emerge from more fundamental quantum relationships. Work connecting geometry with
1:11:09
Speaker 1: quantum entanglement has strengthened this suspicion. Entanglement describes correlations between
1:11:14
Speaker 1: quantum systems that cannot be reproduced by ordinary classical probabilities.
1:11:19
Speaker 1: In some theoretical models, changing the pattern of the entanglement
1:11:23
Speaker 1: changes the effective geometry that emerges. Reduce the appropriate connections
1:11:28
Speaker 1: and space-time can begin to separate, strengthen them and a
1:11:32
Speaker 1: connected geometry can appear. This raises the possibility that distance
1:11:36
Speaker 1: itself is not fundamental. Two things may appear close together
1:11:40
Speaker 1: because of the way underlying quantum degrees of freedom are related. Location,
1:11:45
Speaker 1: separation and geometry could be collective properties, rather like temperature.
1:11:49
Speaker 1: A single molecule does not possess a temperature in quite
1:11:52
Speaker 1: the same way that a glass of water does. Temperature
1:11:55
Speaker 1: emerges from the behaviour of many microscopic constituents. Perhaps spacetime
1:12:00
Speaker 1: is similar. The difficulty is deciding which, if any, of
1:12:04
Speaker 1: these beautiful mathematical ideas describes nature. String theory has produced
1:12:08
Speaker 1: deep connections across mathematics and theoretical physics. but it has
1:12:12
Speaker 1: not yet supplied decisive experimental evidence showing that fundamental strings exist.
1:12:18
Speaker 1: Loop quantum gravity has developed a detailed mathematical description of
1:12:22
Speaker 1: quantum geometry, but no observation has established that spacetime possesses
1:12:26
Speaker 1: the discrete structure it predicts. The same caution applies to
1:12:30
Speaker 1: the other approaches. Physicists search for indirect clues. The early
1:12:34
Speaker 1: universe may preserve signatures of quantum gravity. Black holes may
1:12:39
Speaker 1: reveal departures from classical spacetime. Gravitational waves might someday expose
1:12:43
Speaker 1: tiny effects produced by deeper geometry. High-energy astrophysical observations can
1:12:49
Speaker 1: test whether particles travelling across immense distances behave exactly as
1:12:53
Speaker 1: ordinary relativity predicts. Precision laboratory experiments may eventually probe whether
1:12:59
Speaker 1: gravity can place quantum systems into unmistakably quantum states, but
1:13:03
Speaker 1: there remains an enormous distance between an intriguing possible signal
1:13:07
Speaker 1: and a confirmed theory of quantum spacetime. And so the
1:13:10
Speaker 1: simplest question remains unanswered. What is space? What is time?
1:13:15
Speaker 1: At ordinary scales, they feel like the most basic ingredients imaginable.
1:13:20
Speaker 1: Everything appears to exist somewhere, and every event seems to
1:13:23
Speaker 1: happen sometime. But that familiarity may be misleading. Deep enough
1:13:28
Speaker 1: beneath the smooth geometry described by Albert Einstein, space and
1:13:32
Speaker 1: time may dissolve into strings, networks, causal relations, quantum information, entanglement,
1:13:40
Speaker 1: or something no existing theory has yet captured. The stage
1:13:44
Speaker 1: on which the universe appears to perform may turn out
1:13:46
Speaker 1: not to be a stage at all. It may be
1:13:48
Speaker 1: something the performance itself creates. Quantum mechanics works extraordinarily well,
1:13:54
Speaker 1: that is, what makes one of its deepest problems so peculiar.
1:13:58
Speaker 1: The theory can predict the outcomes of experiments with remarkable accuracy.
1:14:02
Speaker 1: It explains atoms, lasers, semiconductors, chemical bonds and much of
1:14:07
Speaker 1: modern technology. And yet there is still no universally accepted
1:14:11
Speaker 1: answer to a very simple question. What exactly happens when
1:14:15
Speaker 1: a quantum system is measured? At the centre of the
1:14:18
Speaker 1: problem is quantum superposition. In classical physics, an object usually
1:14:23
Speaker 1: has definite properties whether or not anyone is checking them.
1:14:26
Speaker 1: A ball is in one place rather than another. A
1:14:29
Speaker 1: planet follows one particular orbit. A coin lying on a
1:14:33
Speaker 1: table is either heads up or tails up. Quantum systems
1:14:36
Speaker 1: are described differently. A particle can be represented mathematically as
1:14:40
Speaker 1: a combination of several possible states at once. It may
1:14:44
Speaker 1: be in a superposition of different positions, or different energies,
1:14:48
Speaker 1: or different values of some other measurable property. This does
1:14:51
Speaker 1: not simply mean that the particle secretly has one definite value,
1:14:56
Speaker 1: and we have not yet discovered which one. Experiments show
1:14:59
Speaker 1: that the different possibilities can interfere with one another. That
1:15:03
Speaker 1: interference is one of the most distinctly quantum features of nature.
1:15:07
Speaker 1: The mathematical object used to describe a quantum system is
1:15:10
Speaker 1: called the wave function. The wave function contains information about
1:15:14
Speaker 1: the possible outcomes of measurements and the relationships between them.
1:15:18
Speaker 1: By itself, however, it does not simply tell you which
1:15:22
Speaker 1: outcome will occur. That connection is provided by a rule
1:15:25
Speaker 1: introduced by Max Born. Born's rule says, roughly speaking, that
1:15:30
Speaker 1: the squared magnitude of the relevant part of the wave
1:15:33
Speaker 1: function determines the probability of obtaining a particular measurement result.
1:15:38
Speaker 1: Quantum mechanics therefore makes extremely precise predictions. But often they
1:15:42
Speaker 1: are predictions about probabilities. One of the clearest demonstrations is
1:15:46
Speaker 1: the double-slit experiment. Imagine particles such as electrons being sent
1:15:51
Speaker 1: one at a time towards a barrier containing two narrow openings.
1:15:55
Speaker 1: Behind the barrier is a detector screen. If the particles
1:15:58
Speaker 1: behaved like tiny classical pellets, each one might be expected
1:16:02
Speaker 1: to pass through one slit or the other and land
1:16:04
Speaker 1: somewhere behind it. Over time, two broad concentrations should form. Instead,
1:16:10
Speaker 1: under the right conditions, the impacts gradually build up an
1:16:13
Speaker 1: interference pattern. The strange part is that the particles can
1:16:16
Speaker 1: be sent individually. Each electron arrives at the detector at
1:16:20
Speaker 1: one definite point. But after many electrons have been recorded,
1:16:24
Speaker 1: the pattern resembles the interference produced by waves passing through
1:16:28
Speaker 1: both slits. The alternatives appear to interfere. That behaviour cannot
1:16:32
Speaker 1: be explained by imagining every electron as following one ordinary
1:16:36
Speaker 1: classical path known in advance. The quantum state must somehow
1:16:40
Speaker 1: include the possibilities together, and then measurement enters. Before detection,
1:16:45
Speaker 1: the wavefunction may describe a superposition of possible outcomes. After detection,
1:16:50
Speaker 1: the apparatus records one result. One spot appears on the screen,
1:16:55
Speaker 1: not several. This creates the measurement problem. When the wavefunction
1:16:59
Speaker 1: is left alone, Its evolution is described by Erwin Schrödinger' s equation.
1:17:03
Speaker 1: That evolution is smooth and deterministic. If the wave function
1:17:07
Speaker 1: is known at one moment, the equation tells you how
1:17:10
Speaker 1: it develops. But the traditional account of measurement introduces something
1:17:14
Speaker 1: very different. When a measurement occurs, the wave function is
1:17:17
Speaker 1: said to collapse. The range of possible outcomes is replaced
1:17:21
Speaker 1: by one actual result. And that result appears probabilistically rather
1:17:26
Speaker 1: than deterministically. the mathematics works, the conceptual question is what
1:17:31
Speaker 1: this collapse actually means. Is wavefunction collapse a genuine physical event?
1:17:36
Speaker 1: Does something real happen to the quantum state, or is
1:17:39
Speaker 1: collapse merely a convenient way of describing a change in
1:17:42
Speaker 1: what an observer knows? Perhaps no physical collapse occurs at all,
1:17:46
Speaker 1: and the appearance of one emerges from some deeper description.
1:17:50
Speaker 1: Quantum mechanics itself does not provide one universally accepted answer.
1:17:55
Speaker 1: The problem becomes even more difficult when we ask where
1:17:58
Speaker 1: measurement begins. John von Neumann developed a careful mathematical treatment
1:18:02
Speaker 1: of quantum measurement during the early 20th century. One of
1:18:06
Speaker 1: the difficulties exposed by this framework is the lack of
1:18:08
Speaker 1: an obvious fundamental boundary between the quantum system and the
1:18:11
Speaker 1: measuring device. Suppose an electron interacts with a detector. The
1:18:15
Speaker 1: electron is quantum mechanical, but the detector is made from atoms.
1:18:19
Speaker 1: Those atoms are also quantum mechanical. The detector might then
1:18:22
Speaker 1: trigger an electrical signal. that signal moves through more physical equipment,
1:18:27
Speaker 1: eventually the information might reach a screen, a storage device
1:18:30
Speaker 1: or a human nervous system. At what exact point did
1:18:33
Speaker 1: the quantum possibilities become one definite outcome? If quantum mechanics
1:18:37
Speaker 1: applies universally, then simply placing a large measuring device next
1:18:40
Speaker 1: to a small particle does not automatically explain why one
1:18:44
Speaker 1: part should follow quantum superposition while the other part should
1:18:47
Speaker 1: possess a definite classical state. This tension was made famous
1:18:51
Speaker 1: by Erwin Schrödinger in 1935. Schrödinger imagined a cat sealed
1:18:55
Speaker 1: inside a box together with a quantum event connected to
1:18:58
Speaker 1: a mechanism capable of killing the animal. If the quantum
1:19:01
Speaker 1: event exists in a superposition of occurring and not occurring,
1:19:05
Speaker 1: then applying the same logic to the entire apparatus seems
1:19:08
Speaker 1: to place the cat into a superposition of being alive
1:19:11
Speaker 1: and dead. Schrödinger did not propose this because he thought
1:19:14
Speaker 1: cats literally spend their afternoons comfortably half alive. The thought
1:19:19
Speaker 1: experiment was designed to expose how strange quantum mechanics becomes
1:19:23
Speaker 1: when microscopic superpositions are extended directly into the macroscopic world.
1:19:28
Speaker 1: Where does ordinary definite reality enter? The word observation can
1:19:32
Speaker 1: make the issue even more confusing. It sometimes gives the
1:19:34
Speaker 1: impression that quantum mechanics requires a conscious person to look
1:19:38
Speaker 1: at something before physical reality makes up its mind. That
1:19:41
Speaker 1: is not how ordinary quantum experiments are understood. A detector
1:19:45
Speaker 1: can register an event while nobody is in the room.
1:19:48
Speaker 1: A computer can store the result. An atom can interact
1:19:51
Speaker 1: with another atom without waiting politely for a physicist to arrive.
1:19:55
Speaker 1: In quantum physics, measurement generally refers to a physical interaction
1:19:59
Speaker 1: capable of correlating a system with a measuring apparatus, or
1:20:02
Speaker 1: environment and producing a record. Human awareness is not required
1:20:06
Speaker 1: for a detector to function. But removing consciousness from the
1:20:09
Speaker 1: immediate process does not remove the measurement problem. The question
1:20:13
Speaker 1: still remains. Why do measurements produce definite outcomes at all?
1:20:17
Speaker 1: Why does the formalism that permits superpositions lead to a
1:20:20
Speaker 1: world in which experiments appear to have single results? Physicists
1:20:24
Speaker 1: can calculate the probabilities beautifully. They can build experiments around them.
1:20:28
Speaker 1: They can use quantum theory to design technology with extraordinary reliability.
1:20:33
Speaker 1: But they still disagree about what the underlying process physically represents.
1:20:37
Speaker 1: That is perhaps the most unusual feature of the measurement problem.
1:20:40
Speaker 1: It is not a mystery caused by a theory failing experimentally.
1:20:44
Speaker 1: It is a mystery caused by a theory succeeding almost
1:20:46
Speaker 1: too well. Quantum mechanics tells us exactly how to calculate
1:20:50
Speaker 1: what we are likely to observe. What it does not
1:20:53
Speaker 1: tell us with equal clarity is what reality is doing
1:20:56
Speaker 1: in order for one of those possibilities to become the
1:20:58
Speaker 1: thing that actually happens. The measurement problem leaves quantum physics
1:21:01
Speaker 1: in a slightly unusual position. The mathematics tells us what
1:21:04
Speaker 1: to calculate. Experiments tell us that those calculations work, but
1:21:09
Speaker 1: neither tells us, in a way everyone agrees upon, what
1:21:11
Speaker 1: the wavefunction actually represents. Is it a real physical thing?
1:21:15
Speaker 1: Is it merely a mathematical tool for predicting observations? Does
1:21:18
Speaker 1: it describe many possible realities? Only one hidden reality, or
1:21:22
Speaker 1: information about a reality that cannot be described in ordinary,
1:21:26
Speaker 1: classical terms? More than a century after quantum mechanics began
1:21:30
Speaker 1: taking shape, physicists still disagree. One of the oldest families
1:21:33
Speaker 1: of approaches is associated particularly with Niels Bohr and Werner
1:21:37
Speaker 1: Heisenberg and is usually grouped under the name Copenhagen Interpretation.
1:21:41
Speaker 1: There was never one perfectly unified Copenhagen doctrine. Bohr and
1:21:45
Speaker 1: Heisenberg themselves emphasised somewhat different ideas. But Copenhagen-style approaches generally
1:21:51
Speaker 1: resist the temptation to imagine that quantum systems must possess
1:21:54
Speaker 1: all the familiar classical properties before those properties are measured.
1:21:58
Speaker 1: An electron, for example, should not necessarily be pictured as
1:22:01
Speaker 1: a miniature ball quietly carrying a perfectly definite position and
1:22:05
Speaker 1: momentum while we simply remain ignorant of them. Quantum mechanics
1:22:09
Speaker 1: places fundamental limits on assigning such classical descriptions. Werner Heisenberg's
1:22:15
Speaker 1: Uncertainty Principle expresses one aspect of this. Position and momentum
1:22:20
Speaker 1: cannot simultaneously possess arbitrarily precise, experimentally accessible values in the
1:22:25
Speaker 1: classical manner. Within Copenhagen-style thinking, The wavefunction is therefore closely
1:22:30
Speaker 1: connected to the probabilities of possible measurement outcomes. When a
1:22:34
Speaker 1: measurement occurs, one outcome is obtained, and the mathematical description
1:22:38
Speaker 1: is updated accordingly. This approach works extremely well operationally, but
1:22:44
Speaker 1: asking what physically happens during that update leads straight back
1:22:47
Speaker 1: to the problem we were trying to solve. A radically
1:22:50
Speaker 1: different response was proposed by Hugh Everett III in 1957.
1:22:55
Speaker 1: Everett asked what would happen if the wavefunction never collapsed
1:22:58
Speaker 1: at all. Suppose Erwin Schrödinger' s equation applies universally, not merely
1:23:03
Speaker 1: to electrons, not merely to atoms, but to measuring devices, scientists, laboratories,
1:23:09
Speaker 1: and eventually the entire universe. In Everett's relative state formulation,
1:23:13
Speaker 1: a measurement does not select one outcome by destroying the others. Instead,
1:23:18
Speaker 1: the measuring system becomes entangled with the different possible outcomes.
1:23:22
Speaker 1: The idea later developed into what is commonly called the
1:23:24
Speaker 1: many worlds interpretation. In the simplest broad picture, different outcomes
1:23:29
Speaker 1: become associated with effectively separate branches of the universal quantum state.
1:23:34
Speaker 1: A detector records one result in one branch and another
1:23:37
Speaker 1: result in another. An observer inside either branch experiences only
1:23:41
Speaker 1: the outcome correlated with that branch. There is no fundamental collapse.
1:23:46
Speaker 1: The ordinary quantum evolution continues. This has an appealing simplicity
1:23:50
Speaker 1: in one respect because it does not require an additional
1:23:52
Speaker 1: collapse process, but it comes with an extraordinary picture of reality.
1:23:57
Speaker 1: All outcomes represented in the quantum state remain part of
1:24:00
Speaker 1: the universal description. Questions then arise about what exactly defines
1:24:04
Speaker 1: a branch, why observers should use the familiar probability rule
1:24:07
Speaker 1: if all outcomes occur, and how the apparent uniqueness of
1:24:11
Speaker 1: experience emerges. Many Worlds removes one mystery partly by accepting
1:24:15
Speaker 1: a very large reality. Another possibility takes almost the opposite approach.
1:24:20
Speaker 1: David Bohm developed a version of an earlier idea proposed
1:24:23
Speaker 1: by Louis de Broglie, into what is now often called
1:24:26
Speaker 1: pilot wave theory, or Bohmian mechanics. Here, particles have definite configurations.
1:24:32
Speaker 1: An electron really can have an actual position, but its
1:24:36
Speaker 1: motion is guided by a quantum wave. The wave evolves
1:24:39
Speaker 1: according to the usual quantum dynamics, while the particles follow
1:24:43
Speaker 1: trajectories determined by it. The theory is deterministic. If the
1:24:47
Speaker 1: complete configuration were known exactly, its later behaviour would in
1:24:51
Speaker 1: principle be fixed by the dynamics, but there is a price.
1:24:55
Speaker 1: Bohmian mechanics is explicitly non-local. What happens to one particle
1:24:59
Speaker 1: can depend on the configuration of distant particles in a
1:25:02
Speaker 1: way that cannot be reduced to ordinary local influences travelling
1:25:05
Speaker 1: between neighbouring points. That sounds alarming until we remember that
1:25:08
Speaker 1: quantum mechanics itself already contains remarkably non-classical correlations. This became
1:25:15
Speaker 1: especially clear through the work of John Bell. During the 1960s,
1:25:19
Speaker 1: Bell considered whether quantum behaviour could be explained by hidden
1:25:22
Speaker 1: variables or assigning definite properties to particles while preserving locality.
1:25:28
Speaker 1: He derived what are now called Bell inequalities. Certain broad
1:25:32
Speaker 1: classes of local hidden variable theories must obey these inequalities.
1:25:36
Speaker 1: Quantum mechanics predicts that suitably entangled particles can violate them.
1:25:41
Speaker 1: Experiments repeatedly found violations consistent with quantum predictions. Later generations
1:25:46
Speaker 1: of Bell tests progressively closed important experimental loopholes, and the
1:25:51
Speaker 1: overall conclusion is profound. nature cannot be explained by the
1:25:55
Speaker 1: broad class of local, hidden variable models constrained by Bell's theorem.
1:26:00
Speaker 1: This does not prove that one particular interpretation of quantum
1:26:03
Speaker 1: mechanics is correct. Bohmian mechanics, for example, remains logically possible
1:26:08
Speaker 1: precisely because it accepts non-locality. But Bell's theorem tells us
1:26:13
Speaker 1: that any deeper account cannot simply restore an entirely ordinary
1:26:16
Speaker 1: local classical world beneath quantum mechanics. Another family of ideas
1:26:21
Speaker 1: takes wavefunction, collapse much more literally. Perhaps collapse really happens.
1:26:27
Speaker 1: In objective collapse models, the wave function follows quantum evolution
1:26:31
Speaker 1: most of the time, but possesses an additional physical mechanism
1:26:35
Speaker 1: that occasionally drives superpositions into definite states. Giancarlo Girardi, Alberto
1:26:41
Speaker 1: Rimini and Tulio Weber developed one influential model in which
1:26:44
Speaker 1: spontaneous collapses occur randomly. For microscopic systems, these events would
1:26:49
Speaker 1: be extremely rare, preserving familiar quantum behaviour. For macroscopic objects
1:26:54
Speaker 1: containing enormous numbers of particles, the probability of localisation would
1:26:58
Speaker 1: become much greater. Large objects would therefore appear definite without
1:27:03
Speaker 1: requiring a special boundary called measurement. Roger Penrose has explored
1:27:08
Speaker 1: a different possibility in which gravity may play a role
1:27:10
Speaker 1: in destabilising superpositions involving sufficiently different spacetime geometries. These proposals
1:27:17
Speaker 1: are particularly interesting because some objective collapse theories can, at
1:27:21
Speaker 1: least in principle, make predictions that differ slightly from ordinary
1:27:24
Speaker 1: quantum mechanics. That moves part of the argument from interpretation
1:27:28
Speaker 1: towards experimental physics. There is also an essential ingredient called decoherence.
1:27:34
Speaker 1: Quantum decoherence was developed through work by physicists including Heinz-Dieter
1:27:38
Speaker 1: See and Wojciech Zurek. A perfectly isolated quantum system can
1:27:43
Speaker 1: maintain delicate interference between different parts of a superposition. But
1:27:47
Speaker 1: real macroscopic systems are almost never perfectly isolated. They interact
1:27:52
Speaker 1: with air molecules. Photons scatter from them. Heat flows through them.
1:27:56
Speaker 1: They become entangled with countless degrees of freedom in their surroundings.
1:28:00
Speaker 1: Information about the relative quantum phases becomes dispersed into the environment.
1:28:06
Speaker 1: As this happens, interference between macroscopically different alternatives becomes extraordinarily
1:28:11
Speaker 1: difficult to observe. Decoherence can occur extremely rapidly for large objects.
1:28:17
Speaker 1: This helps explain why tables, cats and measuring devices do
1:28:20
Speaker 1: not normally display obvious interference between wildly different macroscopic states,
1:28:25
Speaker 1: even though they are built entirely from quantum constituents. It
1:28:29
Speaker 1: provides a powerful account of how apparently classical behaviour emerges
1:28:32
Speaker 1: from quantum physics. But decoherence does not automatically settle every
1:28:37
Speaker 1: version of the measurement problem. It explains why different alternatives
1:28:41
Speaker 1: can effectively stop interfering. It explains why certain stable states
1:28:45
Speaker 1: become the ones that behave classically. What it does not
1:28:48
Speaker 1: obviously explain by itself is why one particular outcome is
1:28:52
Speaker 1: experienced rather than a decohered collection of alternatives remaining in
1:28:57
Speaker 1: the complete quantum description. and so the interpretations remain. Copenhagen-style
1:29:03
Speaker 1: approaches focus strongly on what can be meaningfully said about measurements.
1:29:07
Speaker 1: Everett's approach removes fundamental collapse and retains the entire evolving
1:29:12
Speaker 1: wave function. Bohmian mechanics adds definite particle configurations guided by
1:29:17
Speaker 1: a non-local quantum wave. Objective collapse theories modify the dynamic
1:29:22
Speaker 1: so that collapse becomes an actual physical event. Other interpretations
1:29:27
Speaker 1: propose still different accounts. Remarkably, many of them reproduce essentially
1:29:31
Speaker 1: the same familiar experimental predictions across the regimes tested so far.
1:29:36
Speaker 1: They can therefore agree about what the detector will record
1:29:39
Speaker 1: while disagreeing profoundly about what exists before, during and after
1:29:42
Speaker 1: the detection. That is why the argument has survived for
1:29:46
Speaker 1: so long. Usually competing theories are separated by experiment. One
1:29:50
Speaker 1: predicts that a planet will be here, another predicts that
1:29:53
Speaker 1: it will be somewhere else. You look, one loses. Quantum
1:29:58
Speaker 1: interpretation is less accommodating. Different pictures of reality can often
1:30:02
Speaker 1: share the same successful mathematics. So the wave function sits
1:30:06
Speaker 1: at the centre of one of the strangest disputes in science.
1:30:09
Speaker 1: Everyone agrees that it is indispensable to quantum theory. Everyone
1:30:13
Speaker 1: agrees that calculations made with it work extraordinarily well. But
1:30:17
Speaker 1: whether it represents knowledge, physical reality, many branching realities, a
1:30:22
Speaker 1: guiding field, or something requiring a genuine collapse remains unsettled.
1:30:27
Speaker 1: quantum mechanics has not failed. Quite the opposite. It has
1:30:30
Speaker 1: succeeded for so long and with such precision that physicists
1:30:34
Speaker 1: have spent more than a century arguing over what this
1:30:36
Speaker 1: astonishingly successful theory is actually telling us exists. Physics does
1:30:43
Speaker 1: not only contain laws, it also contains numbers. Some of
1:30:47
Speaker 1: those numbers describe how strongly particles interact. Others determine the
1:30:51
Speaker 1: masses of particles, the strengths of forces, or the scale
1:30:55
Speaker 1: at which particular processes occur, and many of them appear
1:30:58
Speaker 1: to have very specific values. That immediately raises an awkward question.
1:31:03
Speaker 1: Why those values? One particularly useful kind of number is
1:31:06
Speaker 1: a dimensionless constant. A dimensionless constant does not depend on
1:31:10
Speaker 1: whether someone measures distance in metres or feet, mass in
1:31:14
Speaker 1: kilograms or pounds, or time in seconds or hours. Changing
1:31:18
Speaker 1: the human system of units does not change the number itself.
1:31:22
Speaker 1: That makes dimensionless constants. especially interesting when physicists ask whether
1:31:27
Speaker 1: nature could have been different. One famous example is the
1:31:29
Speaker 1: fine structure constant. The fine structure constant describes the strength
1:31:34
Speaker 1: of the electromagnetic interaction between electrically charged particles. Its value
1:31:39
Speaker 1: is approximately 1 divided by 137. That simple looking number
1:31:44
Speaker 1: appears throughout atomic physics. It helps determine the structure of
1:31:47
Speaker 1: atomic energy levels and the way charged particles interact with light.
1:31:52
Speaker 1: change the strength of electromagnetism sufficiently, and atoms would behave differently.
1:31:58
Speaker 1: Chemical bonds would change. The energies associated with atomic transitions
1:32:02
Speaker 1: would shift. The balance between electromagnetic forces and nuclear forces
1:32:06
Speaker 1: inside atomic nuclei could also be altered. Other numbers matter
1:32:10
Speaker 1: just as much. The electron is dramatically lighter than the proton.
1:32:14
Speaker 1: The proton is roughly 1836 times more massive than the electron.
1:32:20
Speaker 1: That mass ratio helps determine the structure and behaviour of atoms.
1:32:24
Speaker 1: Electrons form extended quantum states around much heavier nuclei, allowing
1:32:29
Speaker 1: atoms to possess the size, stability and chemistry familiar to us.
1:32:34
Speaker 1: If particle masses and interaction strengths were substantially different, matter
1:32:38
Speaker 1: could have developed very differently. The interior of stars provides
1:32:42
Speaker 1: another example. Stars survive because gravity compresses matter while nuclear
1:32:47
Speaker 1: processes release energy. In stars like the Sun, hydrogen is
1:32:51
Speaker 1: gradually converted into helium. In more evolved stars, heavier elements
1:32:56
Speaker 1: can be produced. The details depend delicately on nuclear binding energies,
1:33:00
Speaker 1: particle masses and the strengths of the fundamental interactions. One
1:33:05
Speaker 1: particularly famous case involves carbon. During the 1950s, the British
1:33:10
Speaker 1: astronomer Fred Hoyle considered how stars could manufacture the large
1:33:14
Speaker 1: quantities of carbon known to exist in the universe. Carbon
1:33:17
Speaker 1: can form through a process called the triple alpha process.
1:33:21
Speaker 1: Two helium nuclei can briefly combine to form unstable beryllium.
1:33:25
Speaker 1: If a third helium nucleus arrives quickly enough, carbon can
1:33:28
Speaker 1: be produced. But calculations suggested that this process would not
1:33:32
Speaker 1: occur efficiently enough unless the carbon nucleus possessed an excited
1:33:36
Speaker 1: energy state at approximately the right energy. Hoyle argued that
1:33:40
Speaker 1: such a state must exist. Experiments later confirmed it. That
1:33:45
Speaker 1: excited state is now called the Hoyle state. The story
1:33:49
Speaker 1: is often presented as a striking example of apparent fine-tuning.
1:33:53
Speaker 1: A relatively small change in relevant nuclear energy levels could
1:33:56
Speaker 1: alter the amount of carbon produced in stars. And because
1:33:59
Speaker 1: carbon plays an important role in known chemistry and biology,
1:34:03
Speaker 1: the coincidence naturally attracts attention. But caution is necessary. It
1:34:07
Speaker 1: is tempting to imagine a collection of knobs controlling the
1:34:10
Speaker 1: constants of nature. Turn one slightly, leave every other knob fixed,
1:34:15
Speaker 1: and calculate whether stars, atoms or chemistry still exist. That
1:34:19
Speaker 1: can be scientifically useful. It can reveal how sensitive a
1:34:22
Speaker 1: particular model is to a parameter. But it does not
1:34:25
Speaker 1: automatically tell us what alternative universe is genuinely possible. The
1:34:29
Speaker 1: constants may not be independently adjustable. A deeper theory might
1:34:33
Speaker 1: connect several of them. Changing one could require others to
1:34:36
Speaker 1: change as well. Some combinations that look mathematically imaginable inside
1:34:40
Speaker 1: a simplified calculation might not correspond to any physically consistent theory.
1:34:45
Speaker 1: So showing that a system is sensitive to a constant
1:34:48
Speaker 1: is not the same as proving that nature selected an
1:34:51
Speaker 1: incredibly unlikely value from an enormous menu of equally possible alternatives.
1:34:56
Speaker 1: There is another difficulty. To say that our constants are improbable,
1:35:00
Speaker 1: physicists would need some justified way of assigning probabilities to
1:35:04
Speaker 1: different possible values. At present, no universally accepted probability distribution
1:35:09
Speaker 1: exists for the fundamental constants. We do not know whether
1:35:12
Speaker 1: every conceivable value should be treated as equally likely. We
1:35:16
Speaker 1: do not know the full range of physically possible alternatives,
1:35:19
Speaker 1: and we do not yet know whether the constants really
1:35:21
Speaker 1: are free parameters at the deepest level. This makes claims
1:35:25
Speaker 1: about fine-tuning much more subtle than they sometimes appear. There
1:35:29
Speaker 1: are nevertheless genuine scientific puzzles here. The standard model of
1:35:32
Speaker 1: particle physics contains numerous numerical parameters that must be determined
1:35:36
Speaker 1: from experiment. The theory tells physicists how particles behave once
1:35:41
Speaker 1: those values are supplied. but it does not explain why
1:35:44
Speaker 1: many of those values have the magnitudes they do. Particle
1:35:47
Speaker 1: masses are an obvious example. The electron has one mass.
1:35:51
Speaker 1: The muon has another. Quarks have their own masses. The
1:35:56
Speaker 1: strength of interactions and the pattern of particle mixing also
1:35:59
Speaker 1: require experimentally measured inputs. The standard model organises these quantities
1:36:04
Speaker 1: with extraordinary success. It does not derive all of them
1:36:07
Speaker 1: from a deeper established principle. The cosmological constant provides an
1:36:12
Speaker 1: even more dramatic case. In the simplest description of dark energy,
1:36:16
Speaker 1: the cosmological constant has an extraordinarily small positive value. If
1:36:21
Speaker 1: the effective vacuum energy had been much larger and positive
1:36:24
Speaker 1: in simple cosmological models, accelerated expansion could have begun so
1:36:28
Speaker 1: strongly and so early that matter would have struggled to
1:36:30
Speaker 1: gather into galaxies. If it had taken a sufficiently different
1:36:34
Speaker 1: negative value, the cosmic history could also have changed dramatically. Again,
1:36:38
Speaker 1: that does not prove that the observed value was randomly
1:36:41
Speaker 1: chosen from every number imaginable. But it does highlight how
1:36:45
Speaker 1: sensitive the large-scale structure of the universe can be to
1:36:48
Speaker 1: certain fundamental parameters. This is where the idea of fine-tuning
1:36:52
Speaker 1: enters physics. Fine-tuning does not necessarily mean that someone or
1:36:56
Speaker 1: something deliberately tuned anything. In scientific discussion, it usually refers
1:37:01
Speaker 1: to situations where observed structures or behaviours seem to depend
1:37:05
Speaker 1: sensitively on parameters falling within particular ranges. The interpretation of
1:37:10
Speaker 1: that sensitivity is a separate question. Perhaps the constants simply
1:37:14
Speaker 1: have these values. Perhaps there is no deeper reason. Perhaps
1:37:18
Speaker 1: a more fundamental theory will eventually show that the values
1:37:21
Speaker 1: could not have been otherwise. What currently appears to be
1:37:24
Speaker 1: an arbitrary number might someday be calculated from a deeper
1:37:27
Speaker 1: mathematical structure. Physics has seen something similar before. Johannes Kepler
1:37:32
Speaker 1: once described the distances of the planets using elaborate geometric constructions. Today,
1:37:38
Speaker 1: planetary distances are understood as historical outcomes of the formation
1:37:42
Speaker 1: and evolution of the solar system, rather than fundamental numbers
1:37:45
Speaker 1: written independently into the laws of nature. Something that once
1:37:49
Speaker 1: looks fundamental can later turn out to be derived. The
1:37:53
Speaker 1: same may happen with the constants. Perhaps particle masses, force
1:37:57
Speaker 1: strengths and cosmological parameters emerge from a deeper theory of fields,
1:38:02
Speaker 1: symmetries or quantum gravity? If so, the mystery would change.
1:38:07
Speaker 1: Instead of asking why nature selected these particular numbers, physicists
1:38:11
Speaker 1: might discover that only certain combinations are mathematically or physically possible.
1:38:15
Speaker 1: For now, however, many of the numbers remain inputs. We
1:38:19
Speaker 1: measure them. We place them into our theories. The theories
1:38:22
Speaker 1: then describe nature with remarkable success. But when we ask
1:38:26
Speaker 1: why those numbers, rather than slightly different ones, sit inside
1:38:30
Speaker 1: the equations, physics often becomes quiet. The laws tell us
1:38:34
Speaker 1: how the universe behaves, the constants tell the laws how
1:38:37
Speaker 1: strongly to behave, and one of the deepest unanswered questions
1:38:41
Speaker 1: is whether those constants are accidents, necessities, consequences of something deeper.
1:38:46
Speaker 1: or clues that the theory we currently call fundamental is
1:38:49
Speaker 1: not yet fundamental enough. Once physicists notice that some features
1:38:54
Speaker 1: of the universe seem unusually sensitive to the values of
1:38:56
Speaker 1: certain constants, a deeper question appears. Why do those values
1:39:00
Speaker 1: fall where they do? There is no agreed answer. Instead,
1:39:05
Speaker 1: there are several broad possibilities, each with very different implications.
1:39:09
Speaker 1: The simplest is that there may be no deeper explanation
1:39:12
Speaker 1: at all. The constants could simply have the values they have,
1:39:16
Speaker 1: That possibility can feel unsatisfying, but nature is under no
1:39:20
Speaker 1: obligation to provide a reason for every number. Perhaps the
1:39:23
Speaker 1: fine structure constant, particle masses, the cosmological constant and the
1:39:27
Speaker 1: other measured parameters are simply brute facts. There may be
1:39:31
Speaker 1: no hidden principle beneath them, no deeper mechanism, no cosmic lottery.
1:39:36
Speaker 1: They may just be part of the basic description of reality.
1:39:39
Speaker 1: Physics has occasionally had to accept fundamental inputs of this
1:39:42
Speaker 1: kind before, but history also gives another possibility. What looks
1:39:47
Speaker 1: arbitrary at one level can become necessary at a deeper one.
1:39:50
Speaker 1: A future theory might show that some or all of
1:39:52
Speaker 1: the constants could not consistently have taken other values. Perhaps
1:39:57
Speaker 1: the mathematics of a successful quantum theory of gravity restricts
1:40:00
Speaker 1: what kinds of particles and interactions are possible. Perhaps forces
1:40:04
Speaker 1: that currently look separate emerge from a deeper unified structure.
1:40:08
Speaker 1: Perhaps numbers now inserted into equations by experiment will eventually
1:40:13
Speaker 1: be derived from something more fundamental. If that happens, the
1:40:17
Speaker 1: appearance of fine-tuning could partly disappear. The constants would look
1:40:21
Speaker 1: special only because the deeper theory was missing. This is
1:40:25
Speaker 1: the possibility of necessity. There is also a third line
1:40:28
Speaker 1: of thought and it begins with the observer. Suppose many
1:40:31
Speaker 1: different physical environments are possible. Some might contain long-lived stars
1:40:36
Speaker 1: and stable atoms, others might not. If conscious observers can
1:40:40
Speaker 1: arise only in a small subset, then any observer asking
1:40:43
Speaker 1: about the constants will inevitably find themselves in one of
1:40:46
Speaker 1: the places where conditions allow observers to exist. That is
1:40:49
Speaker 1: the basic idea behind anthropic selection. The word anthropic can
1:40:54
Speaker 1: sound as though human beings are somehow central to the universe.
1:40:57
Speaker 1: That is not what the argument requires. The more general
1:41:01
Speaker 1: point is about observational selection. A fish observing its surroundings
1:41:05
Speaker 1: should not be surprised to discover that it lives somewhere
1:41:07
Speaker 1: containing liquid water. That does not explain why water exists.
1:41:12
Speaker 1: It simply reminds us that the observer is not sampling
1:41:14
Speaker 1: all possible environments at random. The environment must already be
1:41:18
Speaker 1: compatible with the existence of the observer doing the sampling.
1:41:22
Speaker 1: The physicist Brandon Carter helped formalise anthropic reasoning during the
1:41:25
Speaker 1: 20th century. Later, Steven Weinberg used a form of anthropic
1:41:30
Speaker 1: argument in discussions of the cosmological constant. The idea was
1:41:34
Speaker 1: that if the effective cosmological constant varied across some larger
1:41:37
Speaker 1: collection of possible regions, then observers would naturally be found
1:41:41
Speaker 1: only where its value allowed matter to gather into galaxies
1:41:45
Speaker 1: before accelerated expansion became too strong. A very large positive
1:41:50
Speaker 1: cosmological constant could prevent structure from forming efficiently. A sufficiently
1:41:55
Speaker 1: negative one could create a very different cosmic history. So
1:41:59
Speaker 1: the fact that we observe a value compatible with galaxies
1:42:02
Speaker 1: could partly reflect a selection effect. But anthropic reasoning becomes
1:42:06
Speaker 1: much more powerful and much more controversial. if there really
1:42:10
Speaker 1: are many physically different regions to select from. This is
1:42:14
Speaker 1: where the multiverse enters. The multiverse is not one single theory.
1:42:18
Speaker 1: It is a broad label used for several different ideas
1:42:22
Speaker 1: in which the observable universe is only part of a
1:42:24
Speaker 1: much larger reality. One route comes from cosmic inflation. Inflation
1:42:30
Speaker 1: proposes that the very early universe experienced a brief period
1:42:33
Speaker 1: of extremely rapid expansion. Inflation was developed partly to explain
1:42:37
Speaker 1: why the observable cosmos appears so flat, homogeneous, and similar
1:42:42
Speaker 1: in temperature across regions that would otherwise have been difficult
1:42:45
Speaker 1: to place in causal contact. In some models, inflation does
1:42:49
Speaker 1: not end everywhere at once. Certain regions stop inflating and
1:42:53
Speaker 1: develop into universes like our observable region. Elsewhere, inflation continues.
1:42:59
Speaker 1: New regions can then keep forming. This possibility is known
1:43:02
Speaker 1: as eternal inflation. Andre Linde was among the physicists who
1:43:06
Speaker 1: developed influential versions of these ideas. If eternal inflation occurs,
1:43:11
Speaker 1: the larger cosmos could contain an enormous number of causally
1:43:14
Speaker 1: separated regions. These are sometimes called pocket universes. Each region
1:43:19
Speaker 1: may share some underlying laws while developing different low-energy conditions.
1:43:23
Speaker 1: If those conditions include different effective constants or particle properties,
1:43:27
Speaker 1: then apparent fine-tuning could take on a different meaning. Instead
1:43:31
Speaker 1: of asking why one universe happened to receive exactly the
1:43:34
Speaker 1: right constants, we would ask why observers find themselves in
1:43:37
Speaker 1: one member of a huge ensemble. If the ensemble is
1:43:40
Speaker 1: sufficiently varied, most regions might be completely unsuitable for complex structures.
1:43:45
Speaker 1: No stable atoms, no long-lived stars, perhaps no interest in
1:43:49
Speaker 1: chemistry at all. But a small fraction could have combinations
1:43:53
Speaker 1: of parameters that permit complexity. Observers would obviously appear only
1:43:58
Speaker 1: within that fraction. A second route towards a multiverse-like picture
1:44:02
Speaker 1: comes from string theory. String theory appears to allow an
1:44:05
Speaker 1: enormous number of possible vacuum states. Different ways of arranging
1:44:09
Speaker 1: the extra dimensions and fields can lead to different low-energy physics.
1:44:13
Speaker 1: This set of possible solutions is often called the string landscape.
1:44:17
Speaker 1: Leonard Susskind has been one of the prominent physicists arguing
1:44:20
Speaker 1: that the string landscape, combined with ideas from maternal inflation,
1:44:24
Speaker 1: could make anthropic selection scientifically relevant. In that picture, different
1:44:28
Speaker 1: regions of the wider cosmos could settle into different vacuum states.
1:44:32
Speaker 1: Different vacua could produce different particle masses, interaction strengths or
1:44:36
Speaker 1: cosmological constants. Our observed constants would then represent one local
1:44:41
Speaker 1: outcome among many possibilities. This is a dramatic proposal. It
1:44:45
Speaker 1: is also extremely difficult to test. That difficulty is central.
1:44:50
Speaker 1: If other universes are causally disconnected from ours, we cannot
1:44:53
Speaker 1: simply point a telescope towards one. No light may ever
1:44:56
Speaker 1: reach us from them. No spacecraft could travel there. No
1:45:00
Speaker 1: direct message could arrive announcing that their fine structure constant
1:45:03
Speaker 1: is slightly different. A physical theory therefore needs more than
1:45:07
Speaker 1: the statement that inaccessible universes exist. To become scientifically useful,
1:45:12
Speaker 1: the wider framework must ideally generate consequences that can be
1:45:16
Speaker 1: compared with observations made here. Researchers have considered whether collisions
1:45:21
Speaker 1: between different inflationary regions might leave traces in the cosmic
1:45:24
Speaker 1: microwave background. They have examined whether inflationary models make distinctive
1:45:28
Speaker 1: predictions for primordial gravitational waves or other cosmological observables. So
1:45:33
Speaker 1: far there is no accepted observation establishing the existence of
1:45:37
Speaker 1: other universes. There is also a difficult mathematical problem even
1:45:41
Speaker 1: if an enormous multiverse does exist. It is known as
1:45:44
Speaker 1: the measure problem. Suppose eternal inflation produces an endlessly growing
1:45:49
Speaker 1: collection of regions. Some kinds of region may occur infinitely
1:45:52
Speaker 1: many times. Others may also occur infinitely many times. How
1:45:56
Speaker 1: do you then calculate the probability that a randomly selected
1:45:59
Speaker 1: observer experiences one type rather than another? Infinity divided by
1:46:04
Speaker 1: infinity does not automatically give a meaningful answer. Different methods
1:46:08
Speaker 1: of regulating or counting the regions can produce different probabilities.
1:46:13
Speaker 1: Without a well-defined measure, predictions based on multiverse statistics can
1:46:17
Speaker 1: become ambiguous. This creates a deeper philosophical issue as well.
1:46:21
Speaker 1: Imagine that anthropic selection successfully explains why observers measure a
1:46:25
Speaker 1: particular cosmological constant. It still might not explain why the
1:46:30
Speaker 1: larger multiverse exists. Why those underlying laws? Why that inflationary mechanism?
1:46:36
Speaker 1: Why that landscape of possible states? Selection effects can explain
1:46:40
Speaker 1: why an observer should expect to find themselves in a
1:46:42
Speaker 1: habitable part of an ensemble. They do not necessarily explain
1:46:46
Speaker 1: the origin of the ensemble itself. The same is true
1:46:49
Speaker 1: for necessity. Even if a future theory shows that only
1:46:52
Speaker 1: one mathematically consistent set of constants is possible, someone can
1:46:57
Speaker 1: still ask why that particular deeper theory governs reality. And
1:47:01
Speaker 1: brute fact simply stops the chain by saying that, eventually,
1:47:05
Speaker 1: explanation may have to end somewhere. So the apparent fine-tuning
1:47:09
Speaker 1: of nature opens several doors. The constants may be coincidences.
1:47:13
Speaker 1: They may be consequences of deeper laws. Observers may be
1:47:17
Speaker 1: seeing a biased sample from a much larger range of possibilities,
1:47:20
Speaker 1: There may be an enormous multiverse in which many combinations
1:47:24
Speaker 1: are realised, or the entire way of posing the question
1:47:27
Speaker 1: may change once a more fundamental theory is discovered. At present,
1:47:31
Speaker 1: no scientific consensus has established one of these possibilities as
1:47:34
Speaker 1: the final answer. They are not equivalent explanations, and they
1:47:39
Speaker 1: do not all possess the same degree of testability. But
1:47:43
Speaker 1: each is an attempt to deal with the same quiet mystery.
1:47:46
Speaker 1: The laws of physics contain certain numbers. Those numbers allow
1:47:50
Speaker 1: a universe with stars, chemistry and complex structure, and we
1:47:54
Speaker 1: still do not know whether that is because the numbers
1:47:56
Speaker 1: had to be this way, happen to be this way,
1:47:59
Speaker 1: or are merely the values seen from one small corner
1:48:01
Speaker 1: of a much larger reality. Time feels as though it
1:48:05
Speaker 1: has a direction. That statement seems so obvious that it
1:48:09
Speaker 1: is easy to forget how strange it is. A glass
1:48:11
Speaker 1: can fall from a table, strike the floor and shatter
1:48:14
Speaker 1: into pieces, but we never see the fragments leap upwards.
1:48:18
Speaker 1: assemble themselves into a perfect glass and settle gently back
1:48:21
Speaker 1: onto the table. Smoke spreads through a room, it does
1:48:24
Speaker 1: not normally gather itself from the air, compress into a
1:48:27
Speaker 1: narrow stream and return neatly into a flame. An egg
1:48:30
Speaker 1: can be broken and scrambled, a scrambled egg does not
1:48:33
Speaker 1: spontaneously reorganise itself into a yolk, a white and an
1:48:37
Speaker 1: intact shell. The distinction between past and future seems built
1:48:41
Speaker 1: into ordinary experience. We remember yesterday. We do not remember tomorrow.
1:48:46
Speaker 1: We can leave records of events that have happened, but
1:48:49
Speaker 1: not ordinarily of events that have not happened yet. Causes
1:48:53
Speaker 1: seem to precede effects. Ageing moves in one direction. There
1:48:57
Speaker 1: is an unmistakable arrow, and yet many of the fundamental
1:49:01
Speaker 1: laws of physics are far less concerned about which direction
1:49:04
Speaker 1: time is running. Isaac Newton's laws provide a simple example.
1:49:09
Speaker 1: Imagine recording two planets orbiting one another. If the film
1:49:12
Speaker 1: were played backwards, their velocities would reverse, but the reversed
1:49:16
Speaker 1: motion could still satisfy the same underlying equations. Nothing in
1:49:20
Speaker 1: Newtonian mechanics automatically labels one version as the physically permitted
1:49:24
Speaker 1: future and the other as impossible. The equations describe relationships
1:49:29
Speaker 1: between positions, velocities, accelerations and forces, reverse the appropriate motions,
1:49:35
Speaker 1: and another mathematically valid history can often be obtained. A
1:49:39
Speaker 1: similar property appears in much of electromagnetism. If the relevant
1:49:42
Speaker 1: quantities are transformed correctly, the equations can describe processes running
1:49:47
Speaker 1: in either temporal direction. General relativity also does not contain
1:49:51
Speaker 1: an obvious built-in instruction saying that spacetime must evolve towards
1:49:56
Speaker 1: what we call the future rather than the past. And
1:49:59
Speaker 1: at the microscopic level, many of the interactions used in
1:50:02
Speaker 1: statistical mechanics are reversible. Two molecules collide. Their later motions
1:50:07
Speaker 1: follow from their earlier ones, reverse all the relevant velocities
1:50:11
Speaker 1: with perfect precision, and the equations can lead them back
1:50:14
Speaker 1: towards the earlier configuration. There is an important qualification. Nature
1:50:19
Speaker 1: is not perfectly symmetric under simple time reversal in every interaction.
1:50:24
Speaker 1: The weak nuclear interaction violates certain symmetries. Experiments have shown
1:50:29
Speaker 1: charge parity violation, meaning that some weak processes do not
1:50:33
Speaker 1: behave identically. when particles are replaced by antiparticles and spatial
1:50:37
Speaker 1: coordinates are reflected. Under the extremely well-tested principle known as charge,
1:50:42
Speaker 1: parity and time reversal symmetry, this implies a corresponding violation
1:50:46
Speaker 1: involving time reversal. Direct evidence of time reversal violation has
1:50:50
Speaker 1: also been observed in particle systems. So microscopic physics is
1:50:54
Speaker 1: not absolutely indifferent to the direction of time. But these
1:50:58
Speaker 1: effects are tiny and highly specific. They do not explain
1:51:01
Speaker 1: why coffee cools rather than spontaneously heating itself from the
1:51:05
Speaker 1: surrounding room. They do not explain why perfume spreads through
1:51:08
Speaker 1: the air and does not gather itself back into the bottle.
1:51:12
Speaker 1: The enormous everyday distinction between past and future seems to
1:51:15
Speaker 1: come from somewhere else. That somewhere is thermodynamics. The central
1:51:19
Speaker 1: quantity is entropy. The second law of thermodynamics says that
1:51:23
Speaker 1: the entropy of an isolated macroscopic system tends to increase,
1:51:27
Speaker 1: or at least not decrease. This is often simplified by
1:51:30
Speaker 1: saying that disorder increases. That can be useful as a
1:51:33
Speaker 1: rough image but it can also be misleading. Entropy has
1:51:37
Speaker 1: a more precise statistical meaning. During the 19th century Ludwig
1:51:42
Speaker 1: Boltzmann helped connect thermodynamics with the microscopic motion of atoms
1:51:46
Speaker 1: and molecules. A macroscopic state can correspond to an enormous
1:51:50
Speaker 1: number of possible microscopic arrangements. Consider a box filled with gas.
1:51:56
Speaker 1: If every gas molecule happens to occupy the left half
1:51:58
Speaker 1: of the box That is a possible state. But it
1:52:01
Speaker 1: is an extraordinarily special one. There are vastly more ways
1:52:05
Speaker 1: for the molecules to be spread throughout the entire box
1:52:08
Speaker 1: than for every molecule to remain confined to one side.
1:52:11
Speaker 1: The evenly spread gas therefore corresponds to a much larger
1:52:14
Speaker 1: number of microscopic configurations. It has higher entropy. If a
1:52:18
Speaker 1: partition is removed, the molecules begin colliding and moving through
1:52:22
Speaker 1: the available space. Before long, the gas spreads throughout the box.
1:52:26
Speaker 1: Nothing mysterious is pushing it towards. disorder. There are simply
1:52:30
Speaker 1: overwhelmingly more microscopic arrangements corresponding to the spread out state
1:52:34
Speaker 1: than to the highly concentrated one. The system moves from
1:52:38
Speaker 1: an unusual low entropy macrostate towards a vastly more probable
1:52:42
Speaker 1: high entropy macrostate. This statistical tendency gives us a powerful
1:52:46
Speaker 1: explanation for many irreversible looking processes. Heat flows from a
1:52:50
Speaker 1: hot object into a colder environment because there are enormously
1:52:54
Speaker 1: more microscopic arrangements in which the energy is distributed than
1:52:57
Speaker 1: arrangements in which it remains concentrated. A drop of ink
1:53:01
Speaker 1: disperses through water because mixed configurations vastly outnumber configurations where
1:53:06
Speaker 1: every ink molecule remains gathered in one small region. A
1:53:10
Speaker 1: broken object does not spontaneously reassemble because the microscopic arrangements
1:53:15
Speaker 1: corresponding to a shattered object are incomparably more numerous than
1:53:19
Speaker 1: those corresponding to one perfectly reconstructed glass sitting on a table.
1:53:23
Speaker 1: None of this makes the reverse process strictly forbidden by
1:53:26
Speaker 1: the microscopic laws. It makes it fantastically improbable. For everyday
1:53:30
Speaker 1: objects containing unimaginably large numbers of particles, fantastically improbable effectively
1:53:36
Speaker 1: becomes never. But this creates another puzzle. During the 19th century,
1:53:41
Speaker 1: the physicist Josef Loschmidt pointed out a difficulty now known
1:53:44
Speaker 1: as Loschmidt's reversibility objection. Suppose microscopic physical laws are reversible.
1:53:50
Speaker 1: Imagine a system evolving from low entropy to high entropy. Now,
1:53:54
Speaker 1: at some instant, reverse the velocity of every particle with
1:53:57
Speaker 1: perfect precision. The same microscopic laws should then carry the
1:54:01
Speaker 1: system back towards its earlier low entropy state. Entropy would decrease.
1:54:07
Speaker 1: So how can reversible microscopic laws produce an apparently irreversible
1:54:11
Speaker 1: second law? Boltzmann's statistical picture provides much of the answer.
1:54:16
Speaker 1: The second law is not an absolute statement that entropy
1:54:18
Speaker 1: can never decrease. under any imaginable circumstances, it is overwhelmingly probabilistic.
1:54:25
Speaker 1: A high entropy macrostate corresponds to vastly more microscopic configurations
1:54:30
Speaker 1: than a low entropy one. Almost every microscopic evolution available
1:54:34
Speaker 1: to an ordinary macroscopic system therefore leads towards or remains
1:54:38
Speaker 1: within states of high entropy. But Loschmidt's objection reveals something deeper.
1:54:43
Speaker 1: If the equations themselves work in both temporal directions, then
1:54:47
Speaker 1: entropy increase cannot by itself explain why we identify one
1:54:51
Speaker 1: direction as the future. A film of molecules evolving from
1:54:54
Speaker 1: a rare low entropy state towards equilibrium looks ordinary. Run
1:54:58
Speaker 1: the film backwards and the equations may still permit the motion.
1:55:02
Speaker 1: What looks extraordinary is the starting condition. The reversed film
1:55:06
Speaker 1: begins with molecules arranged in an incredibly precise high entropy
1:55:10
Speaker 1: looking configuration whose velocities are coordinated so perfectly that they
1:55:15
Speaker 1: all conspire to produce a lower entropy state. That is
1:55:19
Speaker 1: possible mathematically. It is just extraordinarily special. And this moves
1:55:24
Speaker 1: the mystery to the beginning. The universe today contains an
1:55:27
Speaker 1: enormous thermodynamic arrow because its earlier states had lower entropy.
1:55:33
Speaker 1: But why? Why did the universe begin in such an
1:55:36
Speaker 1: unusual condition? The early cosmos was hot and dense, yet
1:55:40
Speaker 1: from the perspective of gravity it appears to have been
1:55:43
Speaker 1: remarkably smooth and special. there was room for entropy to
1:55:47
Speaker 1: increase enormously, as matter later formed stars, galaxies and black holes.
1:55:53
Speaker 1: If the early universe had already begun in a generic
1:55:55
Speaker 1: high entropy state, the thermodynamic arrow familiar to us might
1:56:00
Speaker 1: never have existed in the same way. So entropy explains
1:56:04
Speaker 1: a great deal about why the future looks different from
1:56:06
Speaker 1: the past. It explains why heat spreads, why structures decay,
1:56:11
Speaker 1: why records accumulate, why irreversible processes surround us. but it
1:56:16
Speaker 1: leaves one question sitting underneath all of them. Why was
1:56:19
Speaker 1: entropy lower in the past? The arrow of time may
1:56:22
Speaker 1: therefore not originate in the local laws of physics at all.
1:56:25
Speaker 1: It may arise from the boundary conditions of the universe.
1:56:28
Speaker 1: The reason a glass falls and breaks but does not
1:56:31
Speaker 1: rebuild itself may ultimately trace back, not to anything unusual
1:56:35
Speaker 1: about the glass, but to the extraordinarily special state in
1:56:38
Speaker 1: which the entire universe began. If the arrow of time
1:56:42
Speaker 1: depends on entropy increasing … then the most important question
1:56:45
Speaker 1: is not simply why entropy rises. It is why entropy
1:56:49
Speaker 1: was ever low enough to rise in the first place.
1:56:51
Speaker 1: That takes us back towards the beginning of the universe.
1:56:54
Speaker 1: The early universe was hot. It was dense. It was
1:56:58
Speaker 1: filled with radiation and matter at temperatures vastly higher than
1:57:01
Speaker 1: anything found naturally on Earth today. At first glance, that
1:57:05
Speaker 1: sounds like an extremely disordered state. In ordinary life, hot
1:57:09
Speaker 1: things often feel more chaotic than cold things. Molecules move rapidly.
1:57:14
Speaker 1: Energy is spread widely. Everything appears thoroughly mixed. So it
1:57:18
Speaker 1: may seem natural to assume that the young universe must
1:57:21
Speaker 1: have possessed very high entropy. But gravity changes the story.
1:57:26
Speaker 1: A hot gas inside a box tends towards greater uniformity.
1:57:29
Speaker 1: A gravitating system often does the opposite. Matter attracts matter.
1:57:34
Speaker 1: Slight concentrations grow. Gas gathers into clouds. Clouds collapse into stars.
1:57:40
Speaker 1: Stars collect into galaxies. Galaxies form clusters. and under sufficiently
1:57:45
Speaker 1: extreme conditions, matter can collapse into black holes. For a
1:57:49
Speaker 1: system dominated by gravity, a perfectly smooth distribution can therefore
1:57:53
Speaker 1: represent a highly special state rather than the most generic one.
1:57:57
Speaker 1: The early universe appears to have been astonishingly smooth. Measurements
1:58:01
Speaker 1: of the cosmic microwave background show tiny variations in density
1:58:05
Speaker 1: and temperature, but these variations were extremely small. Across vast
1:58:09
Speaker 1: regions the young cosmos was remarkably uniform. Matter had not
1:58:13
Speaker 1: yet collapsed into stars and galaxies. Black holes did not
1:58:17
Speaker 1: dominate the cosmic entropy budget. Large-scale gravitational structures were still
1:58:21
Speaker 1: waiting to form. This means that, from the perspective of gravity,
1:58:25
Speaker 1: the early universe was in a highly ordered and unusually
1:58:28
Speaker 1: low entropy condition. That is one of the deepest puzzles
1:58:32
Speaker 1: in cosmology. The universe began hot and dense, yet gravitationally
1:58:36
Speaker 1: it appears to have started in a very special state.
1:58:39
Speaker 1: Roger Penrose has emphasised this problem for decades. Penrose has
1:58:43
Speaker 1: argued that the low entropy of the early universe is
1:58:46
Speaker 1: closely connected to the behaviour of gravitational degrees of freedom.
1:58:51
Speaker 1: In general relativity, spacetime curvature can be separated mathematically into
1:58:55
Speaker 1: different components. One particularly important part is described by what
1:59:00
Speaker 1: is called the Whale curvature. Roughly speaking, Whale curvature is
1:59:04
Speaker 1: associated with tidal gravitational effects and the sort of free
1:59:08
Speaker 1: gravitational structure found around objects such as black holes and
1:59:12
Speaker 1: gravitational waves. In the early universe, the large-scale geometry appears
1:59:16
Speaker 1: to have been exceptionally smooth. Penrose proposed what is known
1:59:20
Speaker 1: as the Whale curvature hypothesis. The basic idea is that
1:59:23
Speaker 1: the initial state of the universe was constrained so that
1:59:26
Speaker 1: the whale curvature was extremely small. That would provide a
1:59:30
Speaker 1: way of characterising the special gravitational order of the beginning.
1:59:34
Speaker 1: As the universe evolved, structure formation could then increase gravitational complexity.
1:59:39
Speaker 1: Stars formed, galaxies assembled, black holes appeared. The gravitational contribution
1:59:45
Speaker 1: to entropy could rise enormously. Penrose's proposal is not an
1:59:49
Speaker 1: established final theory of the beginning, but it captures the
1:59:52
Speaker 1: central difficulty. The early universe was not simply one random
1:59:56
Speaker 1: state among all imaginable states. It seems to have been
1:59:59
Speaker 1: extraordinarily special. There is another way physicists and philosophers describe
2:00:04
Speaker 1: the same basic idea. It is called the past hypothesis.
2:00:08
Speaker 1: The past hypothesis says that the universe began in a
2:00:11
Speaker 1: very low entropy macrostate. From that initial condition, the ordinary
2:00:15
Speaker 1: thermodynamic arrow follows naturally. Entropy then increases towards what we
2:00:19
Speaker 1: call the future. Heat spreads, structures decay, records accumulate, irreversible
2:00:25
Speaker 1: looking processes emerge. The past hypothesis is powerful because it
2:00:29
Speaker 1: explains why the second law of thermodynamics works so reliably.
2:00:33
Speaker 1: If the universe starts in an exceptionally low entropy state,
2:00:37
Speaker 1: then almost every ordinary evolution away from that state leads
2:00:40
Speaker 1: towards greater entropy. But there is an obvious weakness. The
2:00:44
Speaker 1: past hypothesis tells us what the initial condition was. It
2:00:47
Speaker 1: does not tell us why the universe had that condition.
2:00:50
Speaker 1: Saying that the universe began with very low entropy is
2:00:52
Speaker 1: not the same as explaining what physical principle made it
2:00:55
Speaker 1: begin that way. It is a boundary condition, and boundary
2:00:58
Speaker 1: conditions can be mysterious. To see why, consider how cosmic
2:01:03
Speaker 1: structure evolved. The early universe began almost smooth. Tiny density
2:01:07
Speaker 1: variations were present. Gravity gradually amplified them. Regions containing slightly
2:01:12
Speaker 1: more matter attracted even more matter. Gas collapsed into the
2:01:16
Speaker 1: first stars. Stars gathered into galaxies. Galaxies merged and grew.
2:01:21
Speaker 1: Dense stellar remnants formed. Black holes became increasingly important. From
2:01:26
Speaker 1: an everyday perspective, this may look like order emerging from disorder.
2:01:30
Speaker 1: Beautiful galaxies appear where once there was nearly uniform plasma.
2:01:34
Speaker 1: But thermodynamically, the total entropy is still increasing. Gravity allows
2:01:38
Speaker 1: matter to form clumps while opening an enormous number of
2:01:41
Speaker 1: new microscopic possibilities. Black holes, in particular, carry extraordinary amounts
2:01:46
Speaker 1: of entropy. A single supermassive black hole can contain vastly
2:01:50
Speaker 1: more entropy than all the ordinary matter and radiation surrounding it.
2:01:54
Speaker 1: So the growth of cosmic structure is not a reversal
2:01:57
Speaker 1: of the second law. It is one of the ways
2:01:59
Speaker 1: the universe moves towards higher entropy. The strange part is
2:02:03
Speaker 1: that such evolution was possible only because the initial gravitational
2:02:06
Speaker 1: state was so special. And this low entropy beginning may
2:02:10
Speaker 1: be connected to something very intimate. Memory. A memory is
2:02:14
Speaker 1: a physical state. Patterns of neural activity change. Connections between
2:02:18
Speaker 1: neurons are modified. Information becomes encoded in matter. Photographs are
2:02:23
Speaker 1: physical arrangements of molecules. Fossils are physical structures. Tree rings,
2:02:28
Speaker 1: footprints and scratches on stone are all records. Each preserves
2:02:31
Speaker 1: correlations with earlier events. Records do not appear from nowhere.
2:02:35
Speaker 1: They form through physical processes that themselves obey thermodynamics. A
2:02:40
Speaker 1: photograph exists because light interacted with a detector or photosensitive material.
2:02:45
Speaker 1: A fossil exists because an organism left a structure that
2:02:47
Speaker 1: survived through later geological processes. A memory exists because a
2:02:51
Speaker 1: brain changed in response to earlier events. These records point
2:02:55
Speaker 1: towards the past because the universe possesses a thermodynamic arrow.
2:02:59
Speaker 1: Processes that create stable records typically involve increases in entropy elsewhere.
2:03:05
Speaker 1: The low entropy pass provides the conditions that allow records
2:03:07
Speaker 1: to accumulate. This helps explain a familiar asymmetry. We possess
2:03:11
Speaker 1: evidence of yesterday. We have photographs, memories, documents, light that
2:03:16
Speaker 1: has travelled from distant stars, fossils from ancient life. But
2:03:20
Speaker 1: we do not ordinarily possess equivalent records of tomorrow. There
2:03:23
Speaker 1: are no photographs sitting on a desk that were produced
2:03:25
Speaker 1: by events which have not happened yet. Our brains do
2:03:29
Speaker 1: not contain reliable memories of next Tuesday. The physical world
2:03:32
Speaker 1: is filled with traces of the past and not of
2:03:34
Speaker 1: the future. That distinction may ultimately reflect the same thermodynamic
2:03:38
Speaker 1: asymmetry that causes heat to flow and smoke to disperse.
2:03:43
Speaker 1: This raises the question of the psychological arrow of time.
2:03:46
Speaker 1: Human experience feels directed. The past seems fixed. The future
2:03:51
Speaker 1: feels open. We remember one and anticipate the other. Part
2:03:55
Speaker 1: of this may be explained by the way brains physically
2:03:57
Speaker 1: create and store records in a universe whose entropy is increasing,
2:04:01
Speaker 1: but it would be too strong to claim that thermodynamics
2:04:03
Speaker 1: has already explained every aspect of our subjective experience of time.
2:04:07
Speaker 1: The relationship between physical time, memory and conscious experience remains
2:04:11
Speaker 1: a deep subject of its own. Still, the connection is striking.
2:04:15
Speaker 1: The reason you, remember childhood but not old age in advance,
2:04:18
Speaker 1: may ultimately be tied to the thermodynamic history of the
2:04:21
Speaker 1: entire cosmos. This reveals an important distinction. Physics can explain
2:04:26
Speaker 1: a great deal about how the arrow of time works
2:04:28
Speaker 1: once a low entropy beginning is assumed. Statistical mechanics tells
2:04:32
Speaker 1: us why systems overwhelmingly move towards more probable macroscopic states.
2:04:37
Speaker 1: Thermodynamics describes the consequences. Cosmology shows how structure forms. Information
2:04:43
Speaker 1: theory helps explain why records persist, but none of that
2:04:46
Speaker 1: yet explains why the initial state was so special. Perhaps
2:04:50
Speaker 1: a deeper cosmological theory will. Maybe quantum gravity will reveal
2:04:54
Speaker 1: that the beginning could only have occurred in a restricted
2:04:57
Speaker 1: set of states. Perhaps what looks like an arbitrary boundary
2:05:00
Speaker 1: condition will eventually emerge from a deeper law. There are
2:05:03
Speaker 1: proposals involving inflation, quantum cosmology, bouncing universes and other models
2:05:09
Speaker 1: of the earliest cosmos. None has yet provided a universally
2:05:12
Speaker 1: accepted explanation for the low initial entropy. So the puzzle remains.
2:05:17
Speaker 1: The direction in which a cup cools. The direction in
2:05:20
Speaker 1: which a star burns its fuel. The direction in which
2:05:23
Speaker 1: a person ages. The direction in which memories accumulate. All
2:05:27
Speaker 1: of these may trace back to one extraordinary fact. The
2:05:31
Speaker 1: universe began in a state that was far more ordered
2:05:33
Speaker 1: than it had any obvious reason to be. Every ordinary
2:05:36
Speaker 1: moment of time may therefore carry a faint imprint of
2:05:39
Speaker 1: the beginnings. The arrow we experience from one second to
2:05:42
Speaker 1: the next may have been set by conditions established billions
2:05:45
Speaker 1: of years before the first stars existed. And the reason
2:05:48
Speaker 1: those conditions were there at all remains one of the
2:05:50
Speaker 1: quietest and most profound mysteries in physics. There is one
2:05:54
Speaker 1: mystery that does not fit comfortably beside the others. Dark
2:05:57
Speaker 1: matter is clearly a problem for physics. Quantum gravity is
2:06:01
Speaker 1: clearly a problem for physics. The cosmological constant is clearly
2:06:05
Speaker 1: a problem for physics. Consciousness is more difficult to place.
2:06:09
Speaker 1: It may be a problem for physics. It may belong
2:06:11
Speaker 1: more naturally to neuroscience, cognitive science or philosophy. Or it
2:06:16
Speaker 1: may eventually require several of those fields working together. The
2:06:20
Speaker 1: difficulty begins with something that is not especially controversial. Human
2:06:24
Speaker 1: conscious experience is closely tied to the physical brain. Change
2:06:28
Speaker 1: the brain and experience can change with it. Anesthetic drugs
2:06:32
Speaker 1: can remove conscious awareness. Sleep alters it. Damage to particular
2:06:37
Speaker 1: brain regions can change perception, memory, language or personality. Electrical
2:06:43
Speaker 1: stimulation can sometimes produce sensations, movements or fragments of experience.
2:06:48
Speaker 1: Psychoactive drugs can dramatically alter perception, mood and the structure
2:06:53
Speaker 1: of thought. These observations provide strong evidence that consciousness depends
2:06:57
Speaker 1: in some important way on the physical state of the
2:07:00
Speaker 1: nervous system. Neurosciences made considerable progress in studying that relationship.
2:07:05
Speaker 1: Researchers can compare brain activity during wakefulness and unconsciousness. They
2:07:10
Speaker 1: can identify neural systems involved in vision, memory, attention and decision-making.
2:07:15
Speaker 1: They can examine which patterns of activity correlate with a person,
2:07:19
Speaker 1: reporting that they saw an image, heard a sound or
2:07:22
Speaker 1: experienced a particular sensation. These are sometimes called neural correlates
2:07:26
Speaker 1: of consciousness. They are scientifically useful because they connect measurable
2:07:31
Speaker 1: brain activity with reported experience. but identifying a correlate is
2:07:35
Speaker 1: not necessarily the same as explaining consciousness itself. Suppose a
2:07:39
Speaker 1: particular pattern of neural activity occurs whenever someone sees the
2:07:43
Speaker 1: colour red. That would be an important discovery. It might
2:07:47
Speaker 1: tell us which brain regions are active, how information flows
2:07:50
Speaker 1: between them, and what signals are required for the person
2:07:53
Speaker 1: to report seeing red. But another question remains. Why should
2:07:58
Speaker 1: that physical activity feel like anything at all? Why is
2:08:01
Speaker 1: there a subjective experience of redness rather than merely electrical
2:08:05
Speaker 1: and chemical processing taking place in darkness? The philosopher David
2:08:09
Speaker 1: Chalmers gave this problem a particularly memorable name. He called
2:08:13
Speaker 1: it the hard problem of consciousness. The hard problem is
2:08:16
Speaker 1: not simply the challenge of explaining how the brain recognises objects,
2:08:20
Speaker 1: stores memories or controls behaviour? Those are difficult scientific problems,
2:08:25
Speaker 1: but they can in principle be approached through mechanisms. The
2:08:29
Speaker 1: harder question is why physical processing should be accompanied by
2:08:32
Speaker 1: first-person experience. Why should information processing have an inner side?
2:08:37
Speaker 1: Why should pain hurt? Why should music sound like something?
2:08:42
Speaker 1: Why should light of different wavelengths ultimately be accompanied by
2:08:45
Speaker 1: the experience of colour? This is sometimes described as an
2:08:49
Speaker 1: explanatory gap Physics can describe electrical potentials across neuronal membranes.
2:08:54
Speaker 1: Chemistry can describe neurotransmitters crossing synapses. Neuroscience can describe networks
2:09:00
Speaker 1: processing sensory information. Computer models can describe transformations of information.
2:09:06
Speaker 1: All of those descriptions are objective. They can, at least
2:09:10
Speaker 1: in principle, be observed from outside. Experience is different. The
2:09:14
Speaker 1: fact that pain feels painful is available most directly to
2:09:18
Speaker 1: the individual having the experience. That first-person quality is what
2:09:22
Speaker 1: makes consciousness unusually difficult to fit into ordinary physical explanation.
2:09:27
Speaker 1: One widely held position is physicalism. Physicalism holds, broadly speaking,
2:09:33
Speaker 1: that consciousness ultimately depends entirely on physical processes. On this view,
2:09:39
Speaker 1: there is no additional non-physical ingredient required. A sufficiently complete
2:09:43
Speaker 1: understanding of the brain may eventually explain conscious experience in
2:09:47
Speaker 1: the same broad way that molecular physics explains temperature. or
2:09:51
Speaker 1: fluid dynamics explains waves. Temperature does not appear as a
2:09:55
Speaker 1: fundamental property of an isolated elementary particle. It emerges statistically
2:10:00
Speaker 1: from the behaviour of enormous numbers of particles. Likewise, some
2:10:04
Speaker 1: researchers suggest that consciousness may be an emergent property of
2:10:08
Speaker 1: sufficiently complex physical systems. But the analogy has limits. Once
2:10:13
Speaker 1: the microscopic behaviour of particles is understood, it is reasonably
2:10:17
Speaker 1: clear how quantities such as pressure and temperature can be
2:10:20
Speaker 1: constructed from it. Consciousness appears harder because the thing needing
2:10:25
Speaker 1: explanation is not merely a pattern of behaviour, it is
2:10:29
Speaker 1: the existence of experience itself. For that reason, alternative philosophical
2:10:34
Speaker 1: positions remain active. Property dualism suggests that conscious properties may
2:10:40
Speaker 1: be real features of the world that are not reducible
2:10:42
Speaker 1: in a straightforward way to the physical properties currently described
2:10:46
Speaker 1: by science. Panpsychism goes further in another direction, proposing that
2:10:51
Speaker 1: extremely basic forms of experiential property may be fundamental and widespread,
2:10:56
Speaker 1: with complex consciousness emerging from their organisation. Neither of these
2:11:01
Speaker 1: positions is established science. They are philosophical attempts to address
2:11:05
Speaker 1: a problem that has not yet yielded to a single
2:11:08
Speaker 1: accepted framework. Meanwhile, neuroscience continues to build testable theories about
2:11:14
Speaker 1: the organisation associated with conscious processing. One influential proposal is
2:11:20
Speaker 1: integrated information theory, associated particularly with Giulio Tononi. It attempts
2:11:26
Speaker 1: to connect consciousness with the way information is integrated within
2:11:30
Speaker 1: a system. The broad idea is that consciousness depends not
2:11:34
Speaker 1: merely on having information, but on that information being organised
2:11:38
Speaker 1: in a deeply interconnected way that cannot be decomposed into
2:11:41
Speaker 1: independent parts without losing something essential about the whole. Another
2:11:46
Speaker 1: major proposal is global workspace theory, introduced by Bernard Baez
2:11:50
Speaker 1: and later developed in neuroscientific form by researchers including Stanislas Deheene.
2:11:56
Speaker 1: In this family of models, information becomes conscious when it
2:12:00
Speaker 1: is made globally available across a wide network of brain systems.
2:12:04
Speaker 1: A visual signal may initially be processed locally. If it
2:12:08
Speaker 1: gains access to a global workspace, it can influence memory, decision-making, verbal,
2:12:14
Speaker 1: report and other systems across the brain. These theories are
2:12:17
Speaker 1: scientifically valuable because they generate predictions about brain activity and behaviour.
2:12:22
Speaker 1: They can be compared with experiments involving perception, masking, attention,
2:12:28
Speaker 1: anaesthesia and altered states of awareness. but neither should be
2:12:32
Speaker 1: treated as though it has solved every aspect of consciousness.
2:12:36
Speaker 1: A theory may explain when information becomes globally accessible. Another
2:12:40
Speaker 1: may quantify certain patterns of integration. The further question of
2:12:45
Speaker 1: why those processes should be accompanied by subjective experience may
2:12:49
Speaker 1: still remain. And this is where physics sometimes re-enters the
2:12:53
Speaker 1: conversation in a less careful way. Quantum mechanics is frequently
2:12:57
Speaker 1: invoked in popular discussions of consciousness. Because quantum theory is
2:13:01
Speaker 1: strange and consciousness is mysterious, the two are often placed
2:13:04
Speaker 1: beside one another as though one mystery naturally explains the other.
2:13:10
Speaker 1: There is no compelling scientific evidence that consciousness is required
2:13:13
Speaker 1: for ordinary quantum measurement. A detector can register a particle
2:13:17
Speaker 1: without a human being watching it. A photographic plate can
2:13:21
Speaker 1: store information. An atom can interact with its environment. Quantum
2:13:25
Speaker 1: mechanics does not require a conscious mind to sit nearby
2:13:29
Speaker 1: and persuade reality to choose an outcome. There are speculative
2:13:32
Speaker 1: theories proposing roles for quantum effects in the brain, and
2:13:35
Speaker 1: quantum phenomena certainly occur in biological matter because all matter
2:13:39
Speaker 1: is quantum mechanical at a fundamental level. But that is
2:13:42
Speaker 1: very different from demonstrating that specifically, quantum processes are necessary
2:13:47
Speaker 1: to explain consciousness. No such conclusion is presently established. Perhaps
2:13:52
Speaker 1: that expectation places too much responsibility on fundamental physics in
2:13:56
Speaker 1: the first place. Physics describes matter, energy, fields, space-time and
2:14:03
Speaker 1: the interactions between them. It may turn out that consciousness
2:14:07
Speaker 1: requires additional levels of explanation. Biology uses concepts that do
2:14:11
Speaker 1: not appear directly in particle physics. Evolution, ecosystems and natural
2:14:17
Speaker 1: selection are not fundamental forces. yet they are essential for
2:14:21
Speaker 1: understanding living systems. Likewise, concepts from computation, information processing, cognitive
2:14:29
Speaker 1: science and neuroscience may prove indispensable for understanding conscious minds.
2:14:35
Speaker 1: Asking a fundamental physicist to explain subjective experience using only
2:14:38
Speaker 1: equations for elementary particles might therefore be like asking a
2:14:42
Speaker 1: chemist to explain a novel using the periodic table. The
2:14:46
Speaker 1: chemistry is necessary. It is not obviously the most useful
2:14:49
Speaker 1: level of description, But the opposite possibility remains open. Perhaps
2:14:54
Speaker 1: there is no fundamental gap at all. Perhaps neuroscience is
2:14:57
Speaker 1: still young enough that our present confusion should not surprise us.
2:15:01
Speaker 1: A future theory of brain organisation may reveal why certain
2:15:04
Speaker 1: physical processes generate conscious experience with such clarity that the
2:15:08
Speaker 1: present mystery begins to seem less mysterious. Science has crossed
2:15:13
Speaker 1: conceptual gaps before. Life once appeared to require a special
2:15:17
Speaker 1: vital force. Today, living systems are understood through chemistry, molecular biology,
2:15:23
Speaker 1: evolution and physics without invoking a separate substance of life.
2:15:28
Speaker 1: Consciousness might eventually follow a similar path, or it might not.
2:15:33
Speaker 1: That is what makes this mystery different. We do not
2:15:35
Speaker 1: yet know whether subjective experience is an emergent consequence of
2:15:39
Speaker 1: known physical law, whether our physical description is missing something important,
2:15:44
Speaker 1: or whether the demand for physics alone to explain consciousness
2:15:47
Speaker 1: is misunderstands the kind of problem consciousness is. And there
2:15:52
Speaker 1: is something rather appropriate about ending here. The other mysteries
2:15:56
Speaker 1: can be placed far away. Dark matter surrounds distant galaxies.
2:16:01
Speaker 1: Dark energy shapes the expansion of the cosmos. Quantum gravity
2:16:04
Speaker 1: hides near black holes and the earliest moments of the universe.
2:16:09
Speaker 1: Consciousness cannot be placed at a comfortable distance. It is
2:16:12
Speaker 1: present in the act of wondering about all the others.
2:16:15
Speaker 1: The strangest unresolved object. not be something hidden beyond a
2:16:19
Speaker 1: telescope or buried inside an equation. It may be the
2:16:22
Speaker 1: simple fact that the universe contains physical systems capable of
2:16:26
Speaker 1: experiencing anything at all. Physics has an unusual talent for
2:16:30
Speaker 1: making certainty, and ignorance exists side by side. We can
2:16:34
Speaker 1: predict the behaviour of particles with extraordinary precision. We can
2:16:38
Speaker 1: detect gravitational waves produced by black holes colliding more than
2:16:42
Speaker 1: a billion light years away. We can calculate how stars
2:16:46
Speaker 1: burn reconstruct conditions in the young universe, and build machines
2:16:50
Speaker 1: whose operation depends on quantum mechanics behaving exactly as expected.
2:16:55
Speaker 1: And then, having done all of that, we can ask
2:16:58
Speaker 1: what most of the universe is actually made from, and
2:17:01
Speaker 1: the answer is that we do not know. That is
2:17:03
Speaker 1: not an embarrassment. In many ways, recognising precisely what remains
2:17:08
Speaker 1: unexplained is one of the greatest achievements of science. Ignorance
2:17:13
Speaker 1: becomes useful when its boundaries are known, A mystery that
2:17:17
Speaker 1: can be measured, constrained and tested is no longer simply darkness.
2:17:22
Speaker 1: It is a frontier. Dark matter is perhaps the clearest example.
2:17:27
Speaker 1: Something appears to provide additional gravity throughout galaxies and clusters.
2:17:31
Speaker 1: We can infer its distribution from the motions of stars.
2:17:34
Speaker 1: We can map it using gravitational lensing. Its presence helps
2:17:38
Speaker 1: explain how structure formed across cosmic history. It appears to
2:17:43
Speaker 1: make up most of the matter in existence. and yet
2:17:45
Speaker 1: its underlying identity remains unknown. Detectors have searched for hypothetical
2:17:50
Speaker 1: particles deep beneath the ground. Astronomers have tested alternatives. Entire
2:17:56
Speaker 1: classes of candidates have been restricted or excluded. Still, the
2:18:00
Speaker 1: dark matter surrounding galaxies has not introduced itself. Dark energy
2:18:04
Speaker 1: is stranger still. The expansion of the universe is accelerating.
2:18:09
Speaker 1: that conclusion no longer rests on one kind of observation. Supernovae,
2:18:14
Speaker 1: the cosmic microwave background, baryon acoustic oscillations and large-scale cosmic
2:18:19
Speaker 1: structure all contribute to the modern picture. Yet the cause
2:18:23
Speaker 1: remains unresolved. Perhaps dark energy is a cosmological constant. Perhaps
2:18:29
Speaker 1: it is connected with quantum vacuum energy, although that creates
2:18:32
Speaker 1: one of the most severe theoretical problems in physics. Perhaps
2:18:36
Speaker 1: it evolves with cosmic time. Increasingly precise observations are now
2:18:41
Speaker 1: testing that possibility, and the results have been intriguing without
2:18:44
Speaker 1: establishing that dark energy truly changes. The dominant component of
2:18:48
Speaker 1: the present cosmic energy budget is therefore known mainly through
2:18:52
Speaker 1: what it makes the universe do. Then there is ordinary matter.
2:18:56
Speaker 1: The apparently familiar part, even its existence, contains a mystery.
2:19:00
Speaker 1: The hot early universe should have produced matter and anti-matter
2:19:04
Speaker 1: in almost equal quantities. Most of those particles later annihilated
2:19:09
Speaker 1: with their antiparticle partners. But matter possessed some tiny advantage.
2:19:13
Speaker 1: A difference of roughly one surviving matter particle for around
2:19:16
Speaker 1: a billion pairs was enough. That residue became gas. The
2:19:21
Speaker 1: gas formed stars. Stars manufactured heavier elements. Those elements eventually
2:19:26
Speaker 1: became planets, and everything living on at least one of them.
2:19:30
Speaker 1: Physicists know that nature contains some violations of the symmetries
2:19:34
Speaker 1: between matter and antimatter. But the asymmetry provided by the
2:19:38
Speaker 1: standard model appears far too small to explain the cosmic imbalance.
2:19:42
Speaker 1: So every ordinary atom may be quietly carrying evidence that
2:19:46
Speaker 1: something happened in the early universe that known particle physics
2:19:50
Speaker 1: does not yet fully describe. Gravity leads us to another boundary.
2:19:54
Speaker 1: Albert Einstein's general theory of relativity describes spacetime and gravity
2:19:59
Speaker 1: with extraordinary success. Quantum mechanics describes microscopic nature with equally
2:20:05
Speaker 1: extraordinary success. but force both frameworks into the same extreme
2:20:09
Speaker 1: situation and the agreement disappears. Inside black holes and near
2:20:14
Speaker 1: the earliest moments of cosmic history, quantum effects and strong
2:20:18
Speaker 1: spacetime curvature should both matter. A deeper theory is required.
2:20:22
Speaker 1: String theory, loop quantum gravity and other approaches attempt to
2:20:26
Speaker 1: provide it. None has yet received decisive experimental confirmation. We
2:20:31
Speaker 1: therefore possess remarkably successful theories describing both the very large
2:20:35
Speaker 1: and the very small, without knowing how those descriptions become one.
2:20:39
Speaker 1: Quantum mechanics then adds a different kind of uncertainty. There
2:20:43
Speaker 1: is little doubt that the theory works. The argument concerns
2:20:46
Speaker 1: what its success means. A wave function can describe a
2:20:49
Speaker 1: superposition of possible measurement outcomes. An experiment ultimately presents us
2:20:54
Speaker 1: with a definite result. How that transition should be understood
2:20:57
Speaker 1: remains disputed. Perhaps the wave function collapses, perhaps it never
2:21:02
Speaker 1: collapses and all outcomes remain within a larger quantum state.
2:21:05
Speaker 1: Perhaps particles possess definite configurations guided by a quantum wave.
2:21:10
Speaker 1: Perhaps collapse is itself a physical process not yet included
2:21:13
Speaker 1: in standard quantum dynamics. Different interpretations can reproduce much of
2:21:18
Speaker 1: the same experimental evidence, while describing profoundly different realities underneath it.
2:21:24
Speaker 1: Physics knows how to calculate the answer. It is less
2:21:27
Speaker 1: certain what reality is doing while the calculation works. Even
2:21:31
Speaker 1: the numerical constants inside our theories raise questions. Why does
2:21:35
Speaker 1: electromagnetism possess its particular strength? Why do elementary particles have
2:21:40
Speaker 1: the masses they do? Why is the cosmological constant so small?
2:21:44
Speaker 1: Perhaps these values are simply facts. Perhaps a deeper theory
2:21:48
Speaker 1: makes them necessary. Perhaps observers inevitably find themselves only in
2:21:53
Speaker 1: regions where the values permit complex structures. Perhaps there are
2:21:57
Speaker 1: many regions or universes with different effective constants, turning apparent
2:22:01
Speaker 1: fine-tuning into a selection effect. There is no established answer.
2:22:05
Speaker 1: And then there is time. Every moment of ordinary life
2:22:08
Speaker 1: seems to distinguish past from future. We remember one and
2:22:12
Speaker 1: anticipate the other. Heat spreads. Objects break. Stars consume their fuel.
2:22:17
Speaker 1: People grow older. Yet much of fundamental microscopic physics does
2:22:21
Speaker 1: not contain anything resembling this overwhelming everyday arrow. Entropy explains
2:22:27
Speaker 1: why ordinary systems tend towards more probable macroscopic states. but
2:22:31
Speaker 1: that explanation depends on the universe having started in an
2:22:35
Speaker 1: extraordinarily special low entropy condition. Why the early universe possessed
2:22:40
Speaker 1: that condition remains unknown. The direction of time experienced during
2:22:44
Speaker 1: an ordinary quiet evening may therefore trace all the way
2:22:47
Speaker 1: back to an unexplained property of the cosmos near its beginning,
2:22:50
Speaker 1: and finally there is consciousness. That mystery may not even
2:22:54
Speaker 1: belong entirely to physics. We know that conscious experience depends
2:22:58
Speaker 1: intimately on the physical brain. change the brain and experience changes.
2:23:03
Speaker 1: Neuroscience can identify mechanisms involved in perception, memory, attention and wakefulness.
2:23:09
Speaker 1: But whether a complete physical description will eventually explain why
2:23:12
Speaker 1: those processes are accompanied by subjective experience remains unsettled. Perhaps
2:23:17
Speaker 1: consciousness is an emergent property of sufficiently complex physical systems.
2:23:22
Speaker 1: Perhaps new explanatory concepts are required. Perhaps the problem belongs
2:23:26
Speaker 1: partly to neuroscience, computation and philosophy. rather than fundamental physics alone.
2:23:32
Speaker 1: For now, the fact that the universe can be experienced
2:23:35
Speaker 1: from the inside remains at least as strange as many
2:23:37
Speaker 1: of the objects within it. It can be tempting to
2:23:40
Speaker 1: look at this collection of unanswered questions and conclude that
2:23:43
Speaker 1: physics is full of holes. In one sense, it is.
2:23:47
Speaker 1: But those holes are where progress happens. At several points
2:23:51
Speaker 1: in history, physicists have wondered whether the broad foundations of
2:23:54
Speaker 1: the subject were nearly complete. Then, awkward details refuse to disappear.
2:23:59
Speaker 1: Classical mechanics could not explain everything about light and electromagnetism.
2:24:04
Speaker 1: The old physics could not explain atomic stability, blackbody radiation
2:24:08
Speaker 1: or the photoelectric effect. Mercury's orbit contained a small discrepancy.
2:24:14
Speaker 1: Those problems did not merely require tidier calculations. They helped
2:24:18
Speaker 1: lead towards relativity and quantum mechanics. The uncomfortable details became doors.
2:24:23
Speaker 1: There is no guarantee that every modern mystery will produce
2:24:26
Speaker 1: a revolution of comparable scale. Some may eventually have surprisingly
2:24:30
Speaker 1: conventional explanations. A new particle could account for dark matter.
2:24:34
Speaker 1: Improved observations might show that dark energy really is a
2:24:37
Speaker 1: simple cosmological constant. An apparently profound problem may occasionally turn
2:24:42
Speaker 1: out to have been created by asking the question in
2:24:44
Speaker 1: the wrong way. Others may require ideas that currently seem
2:24:48
Speaker 1: almost unimaginable. That uncertainty is the point. Somewhere tonight, an
2:24:54
Speaker 1: instrument may already be collecting data that matters. A detector
2:24:57
Speaker 1: deep underground may record an unusual interaction. A telescope may
2:25:01
Speaker 1: receive ancient photons from an object whose behaviour does not
2:25:04
Speaker 1: quite fit existing theory. A gravitational wave may contain some
2:25:08
Speaker 1: tiny unexpected feature. An experiment may produce a result that looks,
2:25:13
Speaker 1: at first, like an irritating anomaly. Or the important evidence
2:25:17
Speaker 1: may already exist, quietly sitting inside a dataset, waiting for
2:25:21
Speaker 1: someone to notice that one small thing refuses to behave.
2:25:25
Speaker 1: Above us, the sky is full of light that began
2:25:27
Speaker 1: travelling long before there were humans to receive it. Some
2:25:30
Speaker 1: photons arriving tonight left their sources millions or billions of
2:25:33
Speaker 1: years ago. They carry information about galaxies, stars, black holes
2:25:37
Speaker 1: and earlier stages of cosmic history. Among that information may
2:25:41
Speaker 1: be something that eventually changes our understanding of matter, gravity,
2:25:45
Speaker 1: time or the universe itself. Perhaps one of these mysteries
2:25:48
Speaker 1: will be solved soon. Perhaps several will survive for centuries.
2:25:52
Speaker 1: Either outcome would be entirely normal for science. The unanswered
2:25:56
Speaker 1: questions are not evidence that physics has reached its limits.
2:25:59
Speaker 1: They are evidence that it has reached its frontier, and
2:26:02
Speaker 1: somewhere in tonight's sky, or beneath the ground in a
2:26:05
Speaker 1: quiet detector, or hidden inside an equation that someone has
2:26:09
Speaker 1: not yet understood properly, there may already be evidence capable
2:26:12
Speaker 1: of making parts of everything we have just discussed obsolete.
2:26:16
Speaker 1: For a video about the unsolved mysteries of physics, that
2:26:19
Speaker 1: would be the best ending imaginable.