Latest / Boring Science / Physicists Still Can_t Agree on Whether Time Is Real _ Science For Sleep

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Physicists Still Can_t Agree on Whether Time Is Real _ Science For Sleep · Boring Science

• 0:00
  Speaker 1: Time is the one thing every human being is certain of,
• 0:04
  Speaker 1: and the one thing physicists cannot agree on. You wake
• 0:07
  Speaker 1: up because of time. You age because of time. Every
• 0:11
  Speaker 1: plan you have made, every memory you carry, every fear
• 0:15
  Speaker 1: about what comes next, all of it lives inside time.
• 0:19
  Speaker 1: It is so constant, so immediate, so woven into every
• 0:22
  Speaker 1: moment of being alive, that questioning it feels almost absurd.
• 0:27
  Speaker 1: And yet for more than a century, some of the
• 0:29
  Speaker 1: most careful and rigorous thinkers in physics have been building
• 0:33
  Speaker 1: a case that time as you experience it, flowing forward,
• 0:37
  Speaker 1: dividing a real present from a fixed past and an
• 0:40
  Speaker 1: open future, may not correspond to anything fundamental in the
• 0:44
  Speaker 1: universe at all. If you find comfort in these gentle
• 0:47
  Speaker 1: explorations of space and science, I'd be grateful if you'd
• 0:51
  Speaker 1: consider subscribing and giving this video a like. This is
• 1:00
  Speaker 1: not a fringe idea. It is not speculation from the
• 1:04
  Speaker 1: edges of the field. The physicists making this argument are
• 1:07
  Speaker 1: working from the same equations that gave us GPS satellites,
• 1:11
  Speaker 1: nuclear reactors, and the most precisely tested predictions in the
• 1:15
  Speaker 1: history of science. They are reading those equations carefully and
• 1:19
  Speaker 1: reporting what they find. What they find is strange. The
• 1:23
  Speaker 1: laws of physics look essentially the same whether time runs
• 1:27
  Speaker 1: forward or backward. Film a collision between two particles at
• 1:30
  Speaker 1: the quantum level and play it in reverse. The reversed
• 1:34
  Speaker 1: film is physically valid. Nothing in the fundamental equations prefers
• 1:39
  Speaker 1: one direction over the other. The arrow you feel so strongly,
• 1:44
  Speaker 1: the sense that yesterday is behind you and tomorrow is ahead,
• 1:47
  Speaker 1: does not appear to be written into the basic rules
• 1:50
  Speaker 1: of how the universe works. It gets stranger. Write down
• 1:54
  Speaker 1: the equations of physics and look for the special moment
• 1:57
  Speaker 1: called now. You won't find it. There is no variable
• 2:01
  Speaker 1: in any fundamental equation that marks the present as distinct
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  Speaker 1: from any other moment. In Einstein's framework, past, present, and
• 2:10
  Speaker 1: future are coordinates in a four-dimensional structure related to each
• 2:14
  Speaker 1: other the way north and south are related in space.
• 2:18
  Speaker 1: The idea that right now is uniquely real That this
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  Speaker 1: moment exists in a way yesterday no longer does and
• 2:24
  Speaker 1: tomorrow does not yet, finds no support in the mathematics.
• 2:28
  Speaker 1: Then there is the deepest problem of all. When physicists
• 2:32
  Speaker 1: try to combine quantum mechanics with general relativity into a
• 2:36
  Speaker 1: single unified theory, time does something even more unsettling. It disappears.
• 2:43
  Speaker 1: The equation John Wheeler and Bryce DeWitt derived in the
• 2:47
  Speaker 1: 1960s What should have been the master equation governing the
• 2:51
  Speaker 1: quantum universe contains no time variable, none. The universe described
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  Speaker 1: by that equation does not evolve. It simply exists, static,
• 3:02
  Speaker 1: in a superposition of all its possible arrangements. Time in
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  Speaker 1: that picture is not a fundamental feature of reality. It
• 3:10
  Speaker 1: is something that has to be explained, derived, recovered from
• 3:14
  Speaker 1: something deeper that contains no time at all. These are
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  Speaker 1: not three separate puzzles. They are three windows into the
• 3:22
  Speaker 1: same problem. And the problem is this. The most immediate,
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  Speaker 1: undeniable feature of human experience, the sense that time flows,
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  Speaker 1: that now is real, that the future is genuinely open,
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  Speaker 1: appears nowhere in the foundational laws that describe the universe
• 3:40
  Speaker 1: with the greatest precision we have ever achieved. Either those
• 3:44
  Speaker 1: laws are missing something important, or our experience is telling
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  Speaker 1: us something that isn't true about the underlying structure of reality.
• 3:52
  Speaker 1: Physicists have been arguing about this for over a century.
• 3:56
  Speaker 1: The argument has not quieted down. It has intensified. The
• 4:00
  Speaker 1: people on one side include some of the architects of
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  Speaker 1: modern physics, researchers who believe time is an emergent phenomenon,
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  Speaker 1: a large-scale approximation that dissolves when you look closely enough
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  Speaker 1: at the fundamental level. The people on the other side
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  Speaker 1: include equally serious physicists who argue that abandoning time as
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  Speaker 1: a fundamental quantity is a mistake that has sent the
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  Speaker 1: field down a path it cannot find its way back from.
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  Speaker 1: Both sides have genuine technical arguments. Neither side has won.
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  Speaker 1: What makes this debate different from most arguments in physics
• 4:34
  Speaker 1: is the nature of the disagreement itself. It is not
• 4:37
  Speaker 1: about the value of a constant, or which competing model
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  Speaker 1: fits the data slightly better. It is about whether something
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  Speaker 1: you cannot escape, something you are inside of right now,
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  Speaker 1: reading these words, is real in any fundamental sense. The
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  Speaker 1: disagreement is not about details at the edges of the theory.
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  Speaker 1: It is about whether the most obvious feature of your
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  Speaker 1: existence is real. To understand what physicists are actually arguing about,
• 5:03
  Speaker 1: you have to be clear about what is being put
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  Speaker 1: on trial. The word time carries several distinct meanings at once.
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  Speaker 1: And physicists arguing about whether time is real are not
• 5:14
  Speaker 1: always attacking the same one. Some are questioning whether the
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  Speaker 1: felt sense of moments passing corresponds to anything in the
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  Speaker 1: physical world. Others are questioning whether there is any objective
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  Speaker 1: fact about which moment counts as now. Others still are
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  Speaker 1: asking whether time is a fundamental ingredient of the universe
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  Speaker 1: or something that emerges from something deeper. These are different questions.
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  Speaker 1: Start with the most familiar version. You wake up in
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  Speaker 1: the morning and time has passed. The night is gone.
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  Speaker 1: What felt like nothing was, from the outside, eight hours
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  Speaker 1: of clocks ticking and planets turning. This felt sense of duration,
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  Speaker 1: of moments arriving and receding, is one of the most
• 6:00
  Speaker 1: immediate things about being conscious. Neuroscientists are fairly confident this
• 6:05
  Speaker 1: experience is constructed by the brain. Your perception of the
• 6:09
  Speaker 1: present moment is not an instantaneous snapshot. It is a
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  Speaker 1: window roughly a few hundred milliseconds wide, assembled from sensory
• 6:18
  Speaker 1: signals that arrive at different speeds and get stitched together
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  Speaker 1: into something coherent. The now you feel is already a
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  Speaker 1: kind of reconstruction. That part is not very controversial. The
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  Speaker 1: brain builds an experience of flowing time. Nearly Eve, Rion
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  Speaker 1: in the debate, accepts this. What is controversial comes next.
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  Speaker 1: The question is whether that constructed experience is tracking something
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  Speaker 1: real in the physical world, or whether the brain is
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  Speaker 1: generating a vivid fiction with no external counterpart. Two physicists
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  Speaker 1: can agree that the feeling of time passing is a
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  Speaker 1: product of neural processing, and still disagree sharply about whether
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  Speaker 1: there is anything in the structure of the universe that
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  Speaker 1: actually flows, actually passes, actually moves from past to future.
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  Speaker 1: Then there is the directionality problem. Time does not just pass,
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  Speaker 1: it passes in one direction. You remember yesterday, you do
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  Speaker 1: not remember tomorrow. Eggs break and stay broken. Heat moves
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  Speaker 1: from hot objects into cold ones and not the other way.
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  Speaker 1: This arrow, pointing always from past to future, feels as
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  Speaker 1: fundamental as anything in experience. But the laws of physics,
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  Speaker 1: written down in their basic form, do not contain it.
• 7:38
  Speaker 1: Run the equations forward or backward and the mathematics works
• 7:42
  Speaker 1: either way. The arrow of time you feel in every
• 7:45
  Speaker 1: moment of your life has no obvious home in the
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  Speaker 1: equations that are supposed to describe every moment of your life.
• 7:52
  Speaker 1: That gap is one of the central puzzles physicists are
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  Speaker 1: trying to close. Separate from the arrow is the question
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  Speaker 1: of the present. Even granting that time has a direction,
• 8:02
  Speaker 1: why should any particular moment count as special? Right now
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  Speaker 1: feels different from yesterday and tomorrow in a way that
• 8:09
  Speaker 1: goes beyond mere memory or anticipation. There is a sense
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  Speaker 1: in which only now is actually happening. This idea has
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  Speaker 1: a name in philosophy, presentism. On the presentist view, the
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  Speaker 1: present moment is the only thing that genuinely exists. The
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  Speaker 1: past is gone, not just inaccessible, but truly no longer real.
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  Speaker 1: The future has not arrived, not just uncertain, but genuinely
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  Speaker 1: not yet in existence. Reality is this instant and nothing else.
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  Speaker 1: The opposing view is called eternalism. On this picture, all
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  Speaker 1: moments are equally real. Your birth, this moment, your death.
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  Speaker 1: They all exist the way cities in different places all exist,
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  Speaker 1: even though you are only standing in one of them.
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  Speaker 1: The past has not vanished. The future has not failed
• 9:00
  Speaker 1: to arrive. They are simply located at different coordinates. The
• 9:04
  Speaker 1: words past and future describe relations between events, not differences
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  Speaker 1: in their reality. Many physicists lean toward this view because
• 9:12
  Speaker 1: it fits naturally with the mathematics of Einstein's theories, which
• 9:17
  Speaker 1: treat time as one dimension among four and assign no
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  Speaker 1: special status to any particular moment along it. There is
• 9:24
  Speaker 1: also a middle position. The growing block theory says the
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  Speaker 1: past and present are real, but the future is not yet.
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  Speaker 1: Each moment that occurs becomes fixed, joining the permanent structure
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  Speaker 1: of what has happened, while what lies ahead remains genuinely open.
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  Speaker 1: This preserves the intuition that the future is not yet real,
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  Speaker 1: without collapsing into a pure presentism that struggles with what
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  Speaker 1: physics tells us. It also has its own serious problems,
• 9:52
  Speaker 1: which surface the moment you press on what the present
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  Speaker 1: actually means in a universe where different observers, moving at
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  Speaker 1: different speeds, disagree about what is happening right now. Each
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  Speaker 1: of these positions, presentism, eternalism, the growing block, has been
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  Speaker 1: developed carefully, defended rigorously, and attacked from multiple directions. None
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  Speaker 1: of them is obviously correct. None of them is obviously wrong.
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  Speaker 1: The problems are genuine on all sides, and this is
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  Speaker 1: not a situation where more thinking will obviously help, because
• 10:27
  Speaker 1: the difficulties are not just logical, they connect to unresolved
• 10:31
  Speaker 1: questions in fundamental physics about the nature of space-time, the
• 10:35
  Speaker 1: relationship between quantum mechanics and gravity, and what the basic
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  Speaker 1: equations of the universe actually imply about the nature of reality.
• 10:44
  Speaker 1: For most of human history, these questions belonged entirely to philosophy.
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  Speaker 1: Aristotle wrote about time. Augustine wondered whether time could exist
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  Speaker 1: without a mind to perceive it. Kant argued that time
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  Speaker 1: was not a feature of the world but a structure
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  Speaker 1: that the mind imposed on experience. These are serious contributions,
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  Speaker 1: and the questions they raised have never fully been answered,
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  Speaker 1: but they were argued in words, through intuition and logic,
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  Speaker 1: without any mathematical machinery precise enough to generate testable predictions.
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  Speaker 1: Before Einstein, Whether time was real was mostly a question
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  Speaker 1: for philosophers. After him, it became a question you could
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  Speaker 1: write equations about. In 1905, Einstein didn't just change physics.
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  Speaker 1: He changed the geometry of reality. The idea he introduced
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  Speaker 1: was not complicated to state, but it was almost impossible
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  Speaker 1: to absorb. Two observers moving at different speeds relative to
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  Speaker 1: each other will disagree about whether two events happened at
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  Speaker 1: the same time. Not because one of them made a mistake,
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  Speaker 1: not because their instruments were faulty, but because simultaneity is
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  Speaker 1: not a fixed fact about the universe. It is something
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  Speaker 1: that depends on the observer. Two lightning strikes that appear
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  Speaker 1: to happen at exactly the same moment to a person
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  Speaker 1: standing still on a platform will appear to happen at
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  Speaker 1: different moments to a person on a train moving past
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  Speaker 1: that platform at high speed. Both observations are correct. Both
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  Speaker 1: are measuring what time actually does in their reference frame.
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  Speaker 1: There is no higher truth behind them that settles the matter.
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  Speaker 1: This is not a perceptual illusion. That distinction matters enormously.
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  Speaker 1: We can explain away a lot of things by pointing
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  Speaker 1: to the limits of human perception, the imprecision of instruments,
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  Speaker 1: the tricks of perspective. Einstein was not pointing to any
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  Speaker 1: of those. He was describing a structural fact about the universe.
• 12:41
  Speaker 1: The order of events in some cases genuinely depends on
• 12:44
  Speaker 1: how fast you are moving. At everyday speeds the difference
• 12:48
  Speaker 1: is too small to detect, but it is there, and
• 12:51
  Speaker 1: at high enough speeds it becomes dramatic. Two events that
• 12:55
  Speaker 1: appear simultaneous to one observer can appear to happen in
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  Speaker 1: opposite orders to two observers moving in different directions. Neither
• 13:02
  Speaker 1: observer is wrong. The universe does not resolve the disagreement
• 13:06
  Speaker 1: between them. Hermann Minkowski saw what this implied and pushed
• 13:10
  Speaker 1: it further. In 1908, three years after Einstein's paper, Minkowski
• 13:16
  Speaker 1: gave a lecture that opened with one of the most
• 13:19
  Speaker 1: striking sentences in the history of physics. Henceforth, space by
• 13:23
  Speaker 1: itself and time by itself are doomed to fade away
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  Speaker 1: into mere shadows, he said. And only a kind of
• 13:30
  Speaker 1: union of the two will preserve an independent reality. What
• 13:34
  Speaker 1: Minkowski was pointing to was this. Once you accept that
• 13:38
  Speaker 1: different observers measure time and space differently depending on their motion,
• 13:42
  Speaker 1: You have to stop treating space and time as two
• 13:45
  Speaker 1: separate things and start treating them as two aspects of
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  Speaker 1: a single four-dimensional structure. What one observer calls distance, another
• 13:54
  Speaker 1: observer moving at high speed calls a mixture of distance
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  Speaker 1: and time. What one observer calls time, another calls a
• 14:01
  Speaker 1: mixture of time and distance. Neither description is fundamental. The
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  Speaker 1: fundamental object is space-time, the four-dimensional whole Think about what
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  Speaker 1: this means for the concept of now. If you are
• 14:15
  Speaker 1: sitting still and your friend is flying past you on
• 14:18
  Speaker 1: a spacecraft at a significant fraction of the speed of light,
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  Speaker 1: your slices of simultaneity, the set of all events you
• 14:25
  Speaker 1: consider to be happening right now, are tilted relative to
• 14:29
  Speaker 1: your friend's slices. Your now and your friend's now cut
• 14:33
  Speaker 1: through the four-dimensional block at different angles. Events that are
• 14:37
  Speaker 1: happening simultaneously for you and not happening simultaneously for your friend.
• 14:43
  Speaker 1: As the spacecraft moves faster, the difference grows larger. This
• 14:47
  Speaker 1: is not a small technical complication. It cuts straight at
• 14:51
  Speaker 1: the idea that there is one universal present moment shared
• 14:54
  Speaker 1: by everyone. The experimental evidence for all of this is
• 14:58
  Speaker 1: overwhelming and direct. Take two atomic clocks, the most precise
• 15:03
  Speaker 1: timekeeping instruments ever built. Place one on the ground, put
• 15:07
  Speaker 1: the other on a jet flying around the world. When
• 15:10
  Speaker 1: the jet lands and the clocks are brought together, they
• 15:13
  Speaker 1: no longer agree. The clock that flew has ticked slightly
• 15:16
  Speaker 1: fewer seconds than the clock that stayed on the ground.
• 15:20
  Speaker 1: The difference is small, a matter of nanoseconds, but it
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  Speaker 1: is real, reproducible, and exactly what the theory predicts. Time
• 15:28
  Speaker 1: ran at different rates for two clocks that started together
• 15:31
  Speaker 1: and ended together, but took different paths through spacetime. This
• 15:36
  Speaker 1: is not an abstract theoretical result. GPS satellites orbit Earth
• 15:41
  Speaker 1: at high speeds, roughly 14,000 kilometers per hour. At those speeds,
• 15:46
  Speaker 1: relativistic effects cause onboard clocks to run slightly slower compared
• 15:51
  Speaker 1: to clocks on the ground. But the satellites are also
• 15:54
  Speaker 1: higher up, farther from Earth's gravitational pull, which causes their
• 15:59
  Speaker 1: clocks to run slightly faster. The two effects don't cancel perfectly.
• 16:04
  Speaker 1: Engineers have to correct for both. Without those corrections, the
• 16:08
  Speaker 1: accumulated error would grow at roughly seven kilometers per day.
• 16:13
  Speaker 1: Every navigation system on Earth, every phone, every aircraft instrument,
• 16:18
  Speaker 1: every shipping vessel, depends on physicists having correctly accounted for
• 16:22
  Speaker 1: the fact that time does not run at the same
• 16:25
  Speaker 1: rate everywhere. This is one of those places where the
• 16:28
  Speaker 1: physics stops being philosophy and starts costing money if you
• 16:32
  Speaker 1: get it wrong. Time dilation is not a thought experiment.
• 16:37
  Speaker 1: It is a calibration problem with a dollar figure attached.
• 16:40
  Speaker 1: What comes out of the Minkowski picture is something stark.
• 16:44
  Speaker 1: The four-dimensional space-time block contains all events, past, present and future,
• 16:51
  Speaker 1: as coordinates. An event at what you call five years
• 16:54
  Speaker 1: ago and an event at what you call five years
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  Speaker 1: from now are both simply points in the structure. Neither
• 17:01
  Speaker 1: one is more or less there than the other. Minkowski's
• 17:04
  Speaker 1: universe does not have a flowing present. It does not
• 17:07
  Speaker 1: have a preferred now. It has a four-dimensional geometry in
• 17:11
  Speaker 1: which every moment occupies its position the way a point
• 17:14
  Speaker 1: in space occupies its location. The sun is not more
• 17:18
  Speaker 1: real than the Andromeda galaxy because the sun is closer,
• 17:21
  Speaker 1: and by the same logic in the block picture, yesterday
• 17:25
  Speaker 1: is not more real than tomorrow because it is closer
• 17:28
  Speaker 1: to now. This is where the physics becomes genuinely disorienting.
• 17:33
  Speaker 1: because it appears to contradict something every person takes for
• 17:36
  Speaker 1: granted every second of their lives. The present moment feels
• 17:40
  Speaker 1: absolutely real. The past feels gone. The future feels unwritten.
• 17:45
  Speaker 1: But the mathematics of special relativity offers no support for
• 17:49
  Speaker 1: any of that. It doesn't provide a mechanism for a
• 17:52
  Speaker 1: moving present. It doesn't distinguish between the reality of past
• 17:56
  Speaker 1: events and future events. It simply describes a four-dimensional structure
• 18:01
  Speaker 1: and says that time is one of the axes. Einstein
• 18:05
  Speaker 1: himself was not unaware of what this implied. He lived
• 18:08
  Speaker 1: with it for 50 years. When his closest friend died
• 18:12
  Speaker 1: in 1955, Einstein wrote to the family that the distinction
• 18:16
  Speaker 1: between past, present, and future is, in his words, only
• 18:21
  Speaker 1: a stubbornly persistent illusion. He died himself a month later.
• 18:26
  Speaker 1: He had not softened his position. The universe Minkowski and
• 18:29
  Speaker 1: Einstein described is one where a person sitting at a
• 18:32
  Speaker 1: cafe in Paris in 1921 occupies a location in spacetime
• 18:38
  Speaker 1: just as real as a person sitting in a cafe
• 18:40
  Speaker 1: right now. The distance between those two events is measured
• 18:44
  Speaker 1: along the time axis rather than a spatial axis, but
• 18:47
  Speaker 1: the structure that contains them both is equally real at
• 18:50
  Speaker 1: every point. Distance in space does not make a location
• 18:54
  Speaker 1: less real. Time in spacetime should not either. If there
• 18:58
  Speaker 1: is no universal now, then the past isn't something that
• 19:02
  Speaker 1: has vanished and the future isn't something that hasn't happened yet.
• 19:05
  Speaker 1: They are simply regions of space-time you haven't visited. The
• 19:09
  Speaker 1: picture that falls naturally out of Einstein's work is one
• 19:12
  Speaker 1: that most people find deeply uncomfortable. The universe, according to
• 19:17
  Speaker 1: this reading, is not a stage on which events unfold
• 19:20
  Speaker 1: over time. It is a four-dimensional object, already complete, in
• 19:25
  Speaker 1: which every event that has ever happened or will ever
• 19:28
  Speaker 1: happen simply exists. Your birth, the extinction of the dinosaurs,
• 19:33
  Speaker 1: your death, all of it present, all of it equally real,
• 19:38
  Speaker 1: spread across the geometry of space-time, the way cities are
• 19:42
  Speaker 1: spread across a map. This is what physicists call the
• 19:46
  Speaker 1: block universe, and it follows almost directly from the mathematics
• 19:50
  Speaker 1: that Einstein handed the world in 1905. The logic runs
• 19:55
  Speaker 1: like this. Special relativity showed that there is no universal present.
• 20:01
  Speaker 1: Two observers moving relative to each other will disagree about
• 20:04
  Speaker 1: which events are simultaneous. Not because one of them is mistaken,
• 20:10
  Speaker 1: because simultaneity itself is not an objective feature of the universe.
• 20:15
  Speaker 1: It depends on how fast you are moving. When Hermann
• 20:18
  Speaker 1: Minkowski reformulated Einstein's work in 1908, he described space and
• 20:24
  Speaker 1: time not as separate things, but as a single four-dimensional structure, space-time.
• 20:31
  Speaker 1: What any individual observer measures as time or space is
• 20:35
  Speaker 1: a kind of shadow, a projection of the underlying four-dimensional
• 20:39
  Speaker 1: geometry onto their particular path through it. The geometry itself
• 20:43
  Speaker 1: is fixed. The geometry itself does not flow. It simply is.
• 20:48
  Speaker 1: This is where the block universe enters. If there is
• 20:52
  Speaker 1: no universal present, no moment that all observers agree is
• 20:56
  Speaker 1: happening right now, then there is no principal basis for
• 21:00
  Speaker 1: calling any moment more real than any other. The present
• 21:03
  Speaker 1: is special to you. in the same way New York
• 21:06
  Speaker 1: is special to someone standing in New York. New York
• 21:10
  Speaker 1: is not more real than London. It is simply where
• 21:13
  Speaker 1: you happen to be. Eternalists say the same thing about time.
• 21:18
  Speaker 1: The moment you are living through right now is not
• 21:21
  Speaker 1: more real than last Tuesday or next March. It is
• 21:25
  Speaker 1: simply where your consciousness happens to be located in the
• 21:28
  Speaker 1: four-dimensional structure. Past and future are directions in space-time. the
• 21:33
  Speaker 1: same way north and south are directions in space. Einstein
• 21:37
  Speaker 1: himself believed this. When his closest friend Michel Besso died
• 21:42
  Speaker 1: in March of 1955, Einstein wrote a letter to Besso's family. Michel,
• 21:48
  Speaker 1: he wrote, had preceded him a little by parting from
• 21:52
  Speaker 1: this strange world. This means nothing. For us believing physicists,
• 21:57
  Speaker 1: he wrote, the distinction between past, present and future is
• 22:01
  Speaker 1: only a stubbornly persistent illusion. Einstein died one month after
• 22:06
  Speaker 1: writing those words. He was not speaking loosely. He was
• 22:10
  Speaker 1: not offering comfort and stretching the truth to do it.
• 22:13
  Speaker 1: He meant the statement in the strictest physical sense. Besso
• 22:17
  Speaker 1: was not gone. He was located at different space-time coordinates.
• 22:22
  Speaker 1: Think about what this actually implies. The dinosaurs are not
• 22:25
  Speaker 1: extinct in the sense of having vanished into nothingness. They exist,
• 22:30
  Speaker 1: fully and completely at their coordinates in the four-dimensional block.
• 22:34
  Speaker 1: The moment you were born exists right now, located 60
• 22:39
  Speaker 1: or 70 or 80 years behind you in the temporal direction.
• 22:44
  Speaker 1: That moment has not disappeared. It is sitting at its
• 22:47
  Speaker 1: space-time address, unchanged, permanent, as real as anything you are
• 22:53
  Speaker 1: experiencing at this instant. And the moment of your death,
• 22:57
  Speaker 1: whenever it falls, already exists somewhere ahead of you in
• 23:00
  Speaker 1: the block. It has not failed to happen yet. It
• 23:04
  Speaker 1: is simply located at coordinates you have not reached. In
• 23:08
  Speaker 1: the block universe, nothing is ever truly lost. Nothing ever
• 23:13
  Speaker 1: truly arrives. Events simply are at their positions in four-dimensional space-time.
• 23:19
  Speaker 1: The sense that time flows, the deep and immediate feeling
• 23:23
  Speaker 1: that moments are passing, that the present is slipping into
• 23:27
  Speaker 1: the past and the future is becoming now, is, on
• 23:30
  Speaker 1: this view, a feature of how consciousness moves through the block.
• 23:34
  Speaker 1: Not a feature of the block itself. The block does
• 23:37
  Speaker 1: not move. The block does not change. What changes is
• 23:41
  Speaker 1: your position within it. Something like the way a reader's
• 23:44
  Speaker 1: eye moves across a page. The page does not change
• 23:47
  Speaker 1: as the eye crosses it. The story was always there
• 23:51
  Speaker 1: in full. The experience of reading it unfolds for the reader,
• 23:55
  Speaker 1: not for the page. This is where most people push back.
• 23:59
  Speaker 1: The future doesn't feel as real as the present. There
• 24:02
  Speaker 1: is something obviously open about tomorrow that yesterday no longer has.
• 24:07
  Speaker 1: Tomorrow feels like it could go different ways. Yesterday is locked.
• 24:11
  Speaker 1: How can they be equally real? Defenders of the block
• 24:14
  Speaker 1: universe have a direct answer to this. The openness of
• 24:18
  Speaker 1: the future is a feature of your epistemic position inside
• 24:21
  Speaker 1: the block, not a feature of the block itself. You
• 24:25
  Speaker 1: remember the past because your brain contains records of it.
• 24:28
  Speaker 1: You do not remember the future because your brain does
• 24:31
  Speaker 1: not yet have access to the information stored there. The
• 24:34
  Speaker 1: asymmetry is in your information, not in the ontology of
• 24:38
  Speaker 1: the universe. The future is closed. You simply haven't read
• 24:42
  Speaker 1: that part of the page yet. Feelings, eternalists point out,
• 24:46
  Speaker 1: Amat physics, the sensation that tomorrow is uncertain carries no
• 24:51
  Speaker 1: weight against the mathematical structure of general relativity, which assigns
• 24:55
  Speaker 1: no special status to any moment whatsoever. Some physicists find
• 25:00
  Speaker 1: this answer clean. The equations don't privilege the present. Nothing
• 25:04
  Speaker 1: in the geometry of space-time marks any moment as now.
• 25:08
  Speaker 1: The block universe is what you get when you take
• 25:11
  Speaker 1: the theory seriously and follow it without flinching. Others find
• 25:15
  Speaker 1: it deeply unsatisfying, not because it feels wrong, though it
• 25:19
  Speaker 1: does feel wrong, but because it seems to leave something out.
• 25:22
  Speaker 1: General relativity is not a complete theory of everything. It
• 25:26
  Speaker 1: breaks down at the quantum scale. It cannot currently be
• 25:30
  Speaker 1: merged with quantum mechanics in any fully consistent way, and
• 25:33
  Speaker 1: so the possibility remains that the timeless four-dimensional block is
• 25:37
  Speaker 1: a feature of an incomplete description of the universe. That
• 25:41
  Speaker 1: a more fundamental theory, one that successfully unifies gravity and
• 25:45
  Speaker 1: quantum mechanics, might restore something like a real present, might
• 25:49
  Speaker 1: give time back its teeth. That question is not resolved.
• 25:54
  Speaker 1: It is not close to resolved. Either the equations are
• 25:57
  Speaker 1: right and our sense of a flowing present is the
• 26:00
  Speaker 1: universe's most successful lie, or the equations are incomplete and
• 26:04
  Speaker 1: something important about time has been left out of the model.
• 26:08
  Speaker 1: In 1915, Einstein went further. Special relativity had already done
• 26:13
  Speaker 1: something remarkable. It had dissolved the idea of a universal
• 26:17
  Speaker 1: present moment, showing that simultaneity is a matter of perspective
• 26:21
  Speaker 1: rather than fact. But special relativity was still a theory
• 26:25
  Speaker 1: of flat space-time, a universe without gravity, without mass pulling
• 26:29
  Speaker 1: at the fabric of things. General relativity changed that entirely.
• 26:34
  Speaker 1: Mass and energy, Einstein showed, do not simply sit inside
• 26:38
  Speaker 1: space and time. They warp them. The clearest way to
• 26:42
  Speaker 1: see this is through clocks. A clock sitting on the
• 26:45
  Speaker 1: surface of the Earth where gravity is strong. Ticks slightly
• 26:49
  Speaker 1: slower than a clock floating in empty space far from
• 26:52
  Speaker 1: any massive object. This is gravitational time dilation. Not slower
• 26:57
  Speaker 1: in any loose or metaphorical sense. Measurably, precisely, quantifiably slower.
• 27:03
  Speaker 1: The GPS satellites orbiting Earth sit in weaker gravity than
• 27:06
  Speaker 1: the receivers in your pocket. and as a result their
• 27:09
  Speaker 1: onboard clocks run faster by roughly 45 microseconds per day.
• 27:13
  Speaker 1: That is not a large number. But it compounds. Without
• 27:17
  Speaker 1: corrections built into the system, GPS position errors would accumulate
• 27:21
  Speaker 1: at a rate of several kilometers per day. Every navigation
• 27:25
  Speaker 1: system that works on this planet works partly because engineers
• 27:28
  Speaker 1: accounted for general relativity. Time run. S at different rates
• 27:33
  Speaker 1: depending on where you are in a gravitational field. That
• 27:36
  Speaker 1: is not a theoretical curiosity. It is an engineering requirement.
• 27:42
  Speaker 1: Go higher up a mountain and a clock ticks fractionally
• 27:45
  Speaker 1: faster than one at sea level. The difference is tiny,
• 27:48
  Speaker 1: but it is real. And modern atomic clocks are sensitive
• 27:51
  Speaker 1: enough to detect it over a vertical distance of just
• 27:54
  Speaker 1: a few centimetres. Raise a clock by 30 centimetres and
• 27:59
  Speaker 1: it ticks at a measurably different rate than a clock
• 28:01
  Speaker 1: sitting on the floor beneath it. Time is not uniform.
• 28:06
  Speaker 1: It is not steady. It bends. Near a black hole,
• 28:10
  Speaker 1: this bending becomes extreme in a way that strains ordinary language.
• 28:15
  Speaker 1: A black hole is a region of space-time where gravity
• 28:18
  Speaker 1: is so intense that the curvature becomes at the event
• 28:22
  Speaker 1: horizon essentially infinite from the perspective of the outside universe.
• 28:28
  Speaker 1: For an observer watching from far away, A clock falling
• 28:32
  Speaker 1: toward a black hole appears to slow as it approaches
• 28:35
  Speaker 1: the horizon. At the horizon itself, from the outside observer's perspective,
• 28:41
  Speaker 1: time appears to stop. The infalling clock does not appear
• 28:45
  Speaker 1: to cross the boundary. It appears to freeze there, suspended,
• 28:49
  Speaker 1: dimming and redshifting, as light struggles to climb out of
• 28:53
  Speaker 1: the gravitational well. The person falling in experiences something completely different,
• 28:59
  Speaker 1: from their own perspective, time flows normally they cross the
• 29:03
  Speaker 1: event horizon without any dramatic sensation at that specific moment
• 29:07
  Speaker 1: they continue inward to them the universe outside appears distorted
• 29:11
  Speaker 1: and compressed but they are not frozen they are moving
• 29:15
  Speaker 1: so which is true is the person inside or frozen
• 29:18
  Speaker 1: at the edge this is not a question with a
• 29:21
  Speaker 1: clean answer both descriptions are physically valid within their respective
• 29:25
  Speaker 1: frames there is no third perspective no privileged outside view
• 29:30
  Speaker 1: from which you can say definitively what is happening. The
• 29:33
  Speaker 1: situation is not paradoxical in the sense of being self-contradictory,
• 29:38
  Speaker 1: but it is deeply strange. Time near a black hole
• 29:41
  Speaker 1: is not one thing, it is different things to different observers,
• 29:45
  Speaker 1: and general relativity offers no mechanism for choosing between them.
• 29:50
  Speaker 1: This brings up the deeper implication of general relativity, the
• 29:54
  Speaker 1: one that goes beyond GPS corrections and black hole horizons.
• 29:58
  Speaker 1: In special relativity, Space-time was still a kind of fixed stage.
• 30:03
  Speaker 1: The actors moved through it, they experienced time differently depending
• 30:06
  Speaker 1: on their speed. But the stage itself was static. General
• 30:10
  Speaker 1: relativity abolished the stage. Space-time in Einstein's 1915 theory is
• 30:16
  Speaker 1: not a container for the universe. It is the universe.
• 30:19
  Speaker 1: It is a dynamical entity that curves and bends and
• 30:23
  Speaker 1: ripples in response to every piece of matter and energy
• 30:26
  Speaker 1: moving through it. When two black holes collide hundreds of
• 30:30
  Speaker 1: millions of light years away, the distortion propagates outward as
• 30:33
  Speaker 1: gravitational waves, ripples in the fabric of space-time itself, detected
• 30:38
  Speaker 1: on Earth as distortions smaller than a proton. The fabric
• 30:42
  Speaker 1: is real and it moves. Time, in this picture, is
• 30:46
  Speaker 1: not running through the universe. Time is part of the fabric.
• 30:50
  Speaker 1: It is one of the four dimensions of the space-time manifold.
• 30:54
  Speaker 1: woven together with the three spatial dimensions in a structure
• 30:57
  Speaker 1: that responds to mass and energy. You do not have
• 31:00
  Speaker 1: a universe sitting inside a temporal flow. You have a
• 31:04
  Speaker 1: four-dimensional object, complete, and what we experience as the passage
• 31:08
  Speaker 1: of time is something that happens inside that object, not
• 31:12
  Speaker 1: something the object moves through. This is the picture general
• 31:15
  Speaker 1: relativity gives, and it creates a collision that has not
• 31:19
  Speaker 1: been resolved. Quantum mechanics was built on different foundations, The
• 31:24
  Speaker 1: Schrodinger equation, which governs how quantum systems evolve, treats time
• 31:29
  Speaker 1: as a fixed external parameter, a background clock that ticks
• 31:33
  Speaker 1: uniformly and provides the independent variable against which everything else
• 31:38
  Speaker 1: is measured. Time in quantum mechanics is not something the
• 31:42
  Speaker 1: theory explains. It is something the theory assumes, built into
• 31:46
  Speaker 1: the scaffolding from the beginning. General relativity says there is
• 31:50
  Speaker 1: no such fixed background clock, Time is part of the
• 31:54
  Speaker 1: dynamical system, not an external setting for it. When physicists
• 31:59
  Speaker 1: try to combine these two frameworks, as they must, because
• 32:03
  Speaker 1: both are needed to describe reality at its most extreme,
• 32:06
  Speaker 1: the conflict between their treatments of time is not a
• 32:09
  Speaker 1: technical obstacle that can be papered over. It is a
• 32:13
  Speaker 1: foundational incompatibility. One theory requires a fixed temporal backdrop. The
• 32:19
  Speaker 1: other says that backdrop does not exist. Both theories are
• 32:22
  Speaker 1: extraordinarily well confirmed within their respective domains. Both cannot be
• 32:27
  Speaker 1: complete as they stand. Quantum mechanics needs a fixed time
• 32:31
  Speaker 1: to do its calculations. And general relativity says no such
• 32:35
  Speaker 1: thing exists. And that collision, unresolved for a century, is
• 32:40
  Speaker 1: where the deepest questions about time actually live. Even setting
• 32:44
  Speaker 1: aside the block universe debate, time has a second problem
• 32:48
  Speaker 1: that is arguably even stranger. The block universe argument is
• 32:52
  Speaker 1: about whether all moments are equally real. That is one
• 32:55
  Speaker 1: kind of puzzle. This is a different kind. Even if
• 32:59
  Speaker 1: you accept, completely and without reservation, that past and future
• 33:04
  Speaker 1: exist on equal footing inside a four-dimensional space-time block, you
• 33:08
  Speaker 1: still have to explain something that feels blindingly obvious. Time
• 33:12
  Speaker 1: has a direction. Yesterday is different from tomorrow, in a
• 33:16
  Speaker 1: way that North and South are not different from each other.
• 33:19
  Speaker 1: You remember Tuesday, but not next Thursday. A coffee cup
• 33:23
  Speaker 1: shatters on a kitchen floor, and the pieces do not
• 33:26
  Speaker 1: fly back up. Smoke pours from a chimney and spreads
• 33:29
  Speaker 1: across the sky and never pulls itself back into a column.
• 33:33
  Speaker 1: A block of ice dropped into a warm glass of
• 33:35
  Speaker 1: water melts, and the water never spontaneously refreezes, collecting its co,
• 33:41
  Speaker 1: owled back into a cube. This directional quality runs through
• 33:45
  Speaker 1: every experience you have ever had, It feels like bedrock.
• 33:50
  Speaker 1: It feels like the most obvious thing in the universe.
• 33:54
  Speaker 1: Here is the problem. The laws of physics do not
• 33:57
  Speaker 1: have a direction. Newton's equations of motion are time-symmetric. Run
• 34:02
  Speaker 1: them forward and they describe a planet orbiting a star.
• 34:06
  Speaker 1: Run them backward and they describe the same planet orbiting
• 34:09
  Speaker 1: the same star, perfectly legally, nothing violated. Maxwell's equations, which
• 34:15
  Speaker 1: govern electricity and magnetism and light, are time symmetric. The
• 34:20
  Speaker 1: Schrodinger equation, which governs how quantum systems evolve, is time symmetric.
• 34:26
  Speaker 1: Einstein's field equations in general relativity are time symmetric. Every
• 34:31
  Speaker 1: fundamental law that physicists have written down to describe how
• 34:35
  Speaker 1: the universe works operates identically whether the variable called time
• 34:39
  Speaker 1: increases or decreases. There is no arrow built into any
• 34:43
  Speaker 1: of them. Forward and backward are, at the level of
• 34:46
  Speaker 1: fundamental law, the same. To feel how strange this is,
• 34:51
  Speaker 1: try a simple thought experiment. Imagine filming two billiard balls
• 34:56
  Speaker 1: rolling toward each other, colliding and bouncing apart. Now play
• 35:00
  Speaker 1: that film backward. What you see is two billiard balls
• 35:04
  Speaker 1: rolling toward each other, colliding and bouncing apart. The reversed
• 35:09
  Speaker 1: film looks completely normal. Nothing flags it as impossible. Every
• 35:14
  Speaker 1: detail of the backward film is consistent with Newton's laws.
• 35:18
  Speaker 1: Now imagine filming a raw egg dropped from a countertop.
• 35:22
  Speaker 1: The egg falls, hits the floor, and the shell cracks apart,
• 35:27
  Speaker 1: the yolk and white spreading across the tile. Play that backward.
• 35:31
  Speaker 1: What you see is so jarring it almost looks like
• 35:34
  Speaker 1: a special effect. Broken shell fragments slide together from across
• 35:38
  Speaker 1: the floor. Yolk and albumen gather themselves up. A perfect
• 35:43
  Speaker 1: egg rises smoothly from the tile and into the air.
• 35:46
  Speaker 1: Every intuition you have screams that this cannot happen. But
• 35:51
  Speaker 1: here is what physics says. There is no law in
• 35:54
  Speaker 1: the equation that forbids it. The reversed film does not
• 35:57
  Speaker 1: violate Newton, Maxwell, Schrodinger or Einstein. Every microscopic interaction happening
• 36:04
  Speaker 1: in that film, every molecule bouncing off every other molecule
• 36:08
  Speaker 1: is bounced, jars, bowing at. Every microscopic interaction happening in
• 36:14
  Speaker 1: that film Every molecule bouncing off every other molecule is
• 36:20
  Speaker 1: individually reversible. The egg reassembling itself is, at the level
• 36:25
  Speaker 1: of fundamental physics, a legal move. And yet it never happens.
• 36:31
  Speaker 1: Not once in the history of the observable universe has
• 36:34
  Speaker 1: anyone watched a broken egg reconstitute itself from fragments. This
• 36:39
  Speaker 1: gap between what the laws permit at the microscopic level
• 36:43
  Speaker 1: And what actually happens at the macroscopic level is the
• 36:46
  Speaker 1: arrow of time paradox. Ludwig Boltzmann was the first to
• 36:51
  Speaker 1: formalize it seriously in the 1870s, and it cost him
• 36:55
  Speaker 1: years of his intellectual life. He understood the problem with
• 36:59
  Speaker 1: piercing clarity and spent enormous effort trying to explain how
• 37:03
  Speaker 1: a world of reversible microscopic interactions could produce the relentlessly
• 37:08
  Speaker 1: one directional macroscopic world we inhabit. He was mocked by
• 37:12
  Speaker 1: some contemporaries who doubted atoms existed at all. He died
• 37:17
  Speaker 1: without seeing the problem resolved. The problem is still not resolved.
• 37:22
  Speaker 1: The asymmetry runs deeper than eggs and coffee cups. Consider memory.
• 37:28
  Speaker 1: You carry a record of the past in your brain,
• 37:31
  Speaker 1: but you have no analogous record of the future. This
• 37:34
  Speaker 1: feels obvious, but ask why. Your brain is a physical system.
• 37:39
  Speaker 1: Its neurons are made of atoms. Those atoms obey the
• 37:43
  Speaker 1: time-symmetric laws we just described. Why should a collection of
• 37:47
  Speaker 1: time-symmetric parts produce something that so firmly points in one direction?
• 37:52
  Speaker 1: Information flows from past to present in a way it
• 37:56
  Speaker 1: does not flow from future to present, and the laws
• 37:59
  Speaker 1: of physics offer no straightforward account of why. Consider causation.
• 38:04
  Speaker 1: You feel certain that the past caused the present, not
• 38:08
  Speaker 1: the other way around. Causes precede effects. But in the
• 38:12
  Speaker 1: time-symmetric fundamental equations, the relationship between earlier and later states
• 38:18
  Speaker 1: is perfectly mutual. The present state of the universe determines
• 38:22
  Speaker 1: the future state, just as surely as the past state
• 38:25
  Speaker 1: determined the present. There is no fundamental asymmetry between influencing
• 38:30
  Speaker 1: forward and influencing backward. The arrow of causation you experience
• 38:36
  Speaker 1: so viscerally has no obvious home in the underlying physics.
• 38:40
  Speaker 1: At the microscopic level, this is precise and specific. Take
• 38:44
  Speaker 1: any single collision between two particles, an electron bouncing off
• 38:48
  Speaker 1: a proton, two gas molecules rebounding from each other inside
• 38:52
  Speaker 1: a container. Record every detail of the collision and reverse
• 38:56
  Speaker 1: the velocities of every particle involved. The reversed collision is
• 39:00
  Speaker 1: physically valid. Every microscopic interaction in the universe is reversible
• 39:05
  Speaker 1: in this sense. None of them prefer a direction. Yet
• 39:08
  Speaker 1: pile enough of these reversible interactions together, trillions upon trillions
• 39:13
  Speaker 1: of molecules bouncing around inside a gas or a liquid
• 39:16
  Speaker 1: or a solid, and something emerges at the larger scale
• 39:19
  Speaker 1: that is not reversible at all. The irreversibility is not
• 39:23
  Speaker 1: there in the parts. It appears when you zoom out
• 39:26
  Speaker 1: to the whole. How? That is the question. The standard
• 39:29
  Speaker 1: answer physicists reach for is entropy. Entropy loosely is a
• 39:34
  Speaker 1: measure of how many ways a system can be arranged
• 39:37
  Speaker 1: while still looking the same from the outside. A gas
• 39:41
  Speaker 1: with all its molecules clustered in one corner of a
• 39:43
  Speaker 1: room has low entropy. A gas with its molecules spread
• 39:47
  Speaker 1: evenly throughout the room has high entropy. there are vastly
• 39:51
  Speaker 1: more ways to have the gas spread evenly than clustered
• 39:53
  Speaker 1: in a corner. So if the molecules are moving around randomly,
• 39:57
  Speaker 1: bouncing off each other in the walls, the system will
• 40:00
  Speaker 1: almost certainly be found in a high entropy state rather
• 40:03
  Speaker 1: than a low entropy one. Not because the laws push
• 40:06
  Speaker 1: it that way, but because high entropy states vastly outnumber
• 40:10
  Speaker 1: low entropy ones. The second law of thermodynamics captures this.
• 40:14
  Speaker 1: Entropy in an isolated system tends to increase over time.
• 40:19
  Speaker 1: disorder accumulates. Order does not spontaneously reappear, and that direction,
• 40:25
  Speaker 1: from lower entropy to higher entropy, is the direction we
• 40:29
  Speaker 1: call the future. This sounds like a resolution. It sounds
• 40:33
  Speaker 1: like an explanation for why time runs one way and
• 40:36
  Speaker 1: not the other. It is, in many respects, a genuine
• 40:40
  Speaker 1: and important insight. It explains why your coffee goes cold
• 40:44
  Speaker 1: and not hot. why a drop of ink spreads through
• 40:47
  Speaker 1: water and never pulls back together, why the past feels
• 40:51
  Speaker 1: fixed and the future feels open. The entropy answer is
• 40:54
  Speaker 1: not wrong. The standard answer, entropy, solves the surface of
• 40:58
  Speaker 1: the problem and opens a much deeper one underneath. The
• 41:02
  Speaker 1: entropy explanation for time's direction works as long as you
• 41:05
  Speaker 1: don't push on it too hard. The second law of
• 41:08
  Speaker 1: thermodynamics is one of the most reliable statements in all
• 41:12
  Speaker 1: of science. In any isolated system, entropy tends to increase.
• 41:17
  Speaker 1: Entropy is roughly a measure of disorder, of how many
• 41:20
  Speaker 1: ways the parts of a system can be rearranged while
• 41:23
  Speaker 1: still looking the same from the outside. A tidy room
• 41:27
  Speaker 1: has low entropy. A scattered one has high entropy. An
• 41:31
  Speaker 1: ice cube dropped into warm water has lower entropy than
• 41:34
  Speaker 1: the lukewarm mixture it becomes. The direction time runs. Forward
• 41:39
  Speaker 1: is the direction entropy runs. This is why you remember
• 41:42
  Speaker 1: yesterday but not tomorrow. The past is the low entropy direction.
• 41:47
  Speaker 1: The future is the high entropy one. It sounds clean.
• 41:51
  Speaker 1: It sounds complete. Then you ask the next question and
• 41:55
  Speaker 1: everything gets complicated. If entropy increases over time, then the
• 42:00
  Speaker 1: further back you go, the lower the entropy must have been.
• 42:05
  Speaker 1: Follow that logic all the way back and you arrive
• 42:08
  Speaker 1: at the Big Bang. The universe at its birth was
• 42:12
  Speaker 1: in a state of astonishingly low entropy, not just a
• 42:16
  Speaker 1: little lower than today, inconceivably, absurdly, almost impossibly lower. Everything
• 42:24
  Speaker 1: we know about the arrow of time, every egg that
• 42:27
  Speaker 1: has ever broken, every drop of ink that has ever
• 42:30
  Speaker 1: spread through water, every fire that has ever burned down
• 42:34
  Speaker 1: to ash, every single one of those irreversible processes depends
• 42:39
  Speaker 1: entirely on that one starting fact. The universe began in
• 42:45
  Speaker 1: a very special state. Physicists call this the past hypothesis.
• 42:50
  Speaker 1: And it is less an explanation than a placeholder for one.
• 42:54
  Speaker 1: Here is why it matters so much. The laws of
• 42:57
  Speaker 1: physics are time symmetric. Run any microscopic interaction backward, and
• 43:02
  Speaker 1: the reversed version is equally valid. A molecule bouncing off
• 43:06
  Speaker 1: another molecule looks the same in either direction. The equations
• 43:10
  Speaker 1: don't care which way time points. So the laws themselves
• 43:14
  Speaker 1: cannot be the source of the arrow of time. That
• 43:16
  Speaker 1: leaves the initial conditions. The asymmetry we observe everywhere around us,
• 43:22
  Speaker 1: the irreversibility, the decay, the one-way flow, is not built
• 43:27
  Speaker 1: into the rules. It is built into where things started.
• 43:30
  Speaker 1: The universe isn't asymmetric because its laws demand it. The
• 43:35
  Speaker 1: universe is asymmetric because it began in an extraordinary configuration.
• 43:41
  Speaker 1: Roger Penrose has pushed this further than almost anyone. He
• 43:45
  Speaker 1: calculated the probability of the universe beginning with the low
• 43:48
  Speaker 1: entropy we observe. The number he arrived at is staggering,
• 43:52
  Speaker 1: roughly 1 in 10 to the power of 10 to
• 43:54
  Speaker 1: the power of 123. Writing that number out fully would
• 43:59
  Speaker 1: require more digits than there are atoms in the observable universe.
• 44:03
  Speaker 1: It is not a small probability. It is a probability
• 44:06
  Speaker 1: so remote that calling it small is almost misleading. Pemrus
• 44:10
  Speaker 1: did not put this number forward to celebrate the miraculous.
• 44:14
  Speaker 1: He put it forward to emphasize that the past hypothesis
• 44:17
  Speaker 1: is not an explanation. It is a fact in desperate
• 44:20
  Speaker 1: need of one. Saying the universe started with low entropy
• 44:24
  Speaker 1: is like explaining why you won the lottery by noting
• 44:27
  Speaker 1: that someone had to win. True, but not illuminating. Some
• 44:33
  Speaker 1: physicists have tried to make the past hypothesis less mysterious
• 44:36
  Speaker 1: by reaching for cosmological frameworks. Sean Carroll, a theoretical physicist
• 44:42
  Speaker 1: at Johns Hopkins, has worked extensively on this problem. His
• 44:46
  Speaker 1: approach accepts the past hypothesis as a real feature of
• 44:49
  Speaker 1: the universe and asks whether a larger picture of cosmology
• 44:53
  Speaker 1: can make the low entropy beginning less surprising. One proposal
• 44:57
  Speaker 1: involves eternal inflation and a multiverse, in which our observable
• 45:02
  Speaker 1: universe is one of many, with our particular branch having
• 45:05
  Speaker 1: started in a low entropy state by chance. Whether this
• 45:09
  Speaker 1: resolves the mystery, or just multiplies it, is genuinely debated.
• 45:14
  Speaker 1: A multiverse where every possible initial condition is realized somewhere
• 45:19
  Speaker 1: doesn't explain why we find ourselves in a low entropy branch,
• 45:23
  Speaker 1: it simply asserts that such branches exist. Ludwig Boltzmann, gave
• 45:29
  Speaker 1: us much of the mathematical foundation of statistical mechanics in
• 45:33
  Speaker 1: the 1870s, was tormented by exactly this problem. He understood
• 45:38
  Speaker 1: that entropy increase was statistical rather than absolute. A system
• 45:42
  Speaker 1: overwhelmingly tends toward higher entropy because there are so many
• 45:46
  Speaker 1: more high entropy states than low entropy ones. But he
• 45:50
  Speaker 1: also understood the implication. If entropy increase is statistical, then
• 45:55
  Speaker 1: a sufficiently large or sufficiently old universe should spend almost
• 45:59
  Speaker 1: all its time near maximum entropy, with occasional random fluctuations downward.
• 46:04
  Speaker 1: The low entropy universe we observe is not what you
• 46:07
  Speaker 1: would predict from pure statistical reasoning applied to an eternal cosmos.
• 46:12
  Speaker 1: Boltzmann spent years trying to resolve this, and never managed
• 46:16
  Speaker 1: to his own satisfaction. The problem outlived him. Lee Smolin
• 46:20
  Speaker 1: takes a different position. He argues that appealing to special
• 46:24
  Speaker 1: initial conditions is a sign that something is fundamentally wrong
• 46:28
  Speaker 1: with how the question is being framed. A truly satisfying
• 46:32
  Speaker 1: theory of physics in Smolin's view should explain the arrow
• 46:37
  Speaker 1: of time from the inside, from the laws themselves, rather
• 46:41
  Speaker 1: than importing it through boundary conditions that are simply assumed
• 46:45
  Speaker 1: without justification. If the only way to explain why time
• 46:49
  Speaker 1: runs forward is to posit that the universe happened to
• 46:52
  Speaker 1: start in one particular extraordinary configuration, then physics has not
• 46:57
  Speaker 1: explained the arrow of time at all. It has pushed
• 47:01
  Speaker 1: the explanation outside the theory. This is the edge where
• 47:05
  Speaker 1: the entropy story breaks down. The second law is real.
• 47:09
  Speaker 1: Entropy increases. The arrow of time points in the direction
• 47:13
  Speaker 1: of increasing disorder, and that tells you something genuine about
• 47:17
  Speaker 1: why the past feels different from the future. But underneath
• 47:21
  Speaker 1: it sits a question that the second law cannot answer
• 47:24
  Speaker 1: on its own. Why was the entropy low to begin with?
• 47:27
  Speaker 1: The arrow doesn't come from the law. It comes from
• 47:31
  Speaker 1: the starting point. And nobody has a satisfying account of
• 47:35
  Speaker 1: why the starting point was what it was. A theory
• 47:38
  Speaker 1: that explains time's direction by pointing to special initial conditions
• 47:42
  Speaker 1: hasn't explained anything. It has just pushed the mystery one
• 47:46
  Speaker 1: step further back. Quantum mechanics has its own time problem.
• 47:50
  Speaker 1: and it runs even deeper than anything in relativity. Start
• 47:53
  Speaker 1: with what the theory actually says. In quantum mechanics, the
• 47:57
  Speaker 1: fundamental object describing any physical system is something called the
• 48:01
  Speaker 1: wave function. It is not a list of where things are.
• 48:05
  Speaker 1: It is a list of all the places things could be,
• 48:08
  Speaker 1: weighted by probability. Before you measure a quantum system, it
• 48:12
  Speaker 1: is genuinely in multiple states at once. This is not
• 48:16
  Speaker 1: a statement about ignorance. It is not the case that
• 48:19
  Speaker 1: the electron is somewhere definite and we simply haven't looked yet.
• 48:23
  Speaker 1: The theory says the electron occupies multiple possibilities simultaneously, and
• 48:28
  Speaker 1: experiments confirm this in ways that leave no comfortable alternative.
• 48:33
  Speaker 1: The double slit experiment makes this concrete. Fire a single
• 48:37
  Speaker 1: electron at a barrier with two narrow gaps in it.
• 48:40
  Speaker 1: On the other side, a detector screen records where the
• 48:43
  Speaker 1: electron lands. Do this once and the electron hits at
• 48:47
  Speaker 1: a single point. Do it thousands of times and the
• 48:50
  Speaker 1: pattern that builds up is not what you would get
• 48:53
  Speaker 1: if the electron were going through one slit or the other.
• 48:56
  Speaker 1: It is an interference pattern. The kind of pattern you
• 48:59
  Speaker 1: get when a wave passes through both slits at once
• 49:02
  Speaker 1: and the two halves interfere with each other. A single
• 49:06
  Speaker 1: electron acting like a wave passing through both gaps simultaneously
• 49:11
  Speaker 1: interfering with itself. Block one slit and the interference vanishes.
• 49:16
  Speaker 1: The electron behaves like a particle again, Open both slits
• 49:20
  Speaker 1: and the wave behavior returns. The electron somehow knows both
• 49:25
  Speaker 1: slits are available. This is strange. It becomes stranger when
• 49:30
  Speaker 1: you ask what happens at the moment of measurement. When
• 49:33
  Speaker 1: you observe a quantum system, when a detector fires, when
• 49:37
  Speaker 1: a measurement is made, the wave function collapses, all the
• 49:41
  Speaker 1: probability spread across multiple possible states concentrates suddenly into one
• 49:46
  Speaker 1: definite outcome. Before measurement, the electron was in a superposition
• 49:51
  Speaker 1: of positions. After measurement, it is here, at this specific point,
• 49:56
  Speaker 1: with certainty. The before and after are separated by the
• 50:00
  Speaker 1: moment of observation. This is where time enters quantum mechanics.
• 50:04
  Speaker 1: In a way that nobody has ever fully explained, The
• 50:08
  Speaker 1: wave function evolves according to the Schrodinger equation between measurements.
• 50:13
  Speaker 1: That equation is deterministic. Given the wave function now, it
• 50:17
  Speaker 1: tells you exactly what the wave function will be at
• 50:19
  Speaker 1: any future time. It is also completely time symmetric. Run
• 50:24
  Speaker 1: it forward or backward, the physics is the same. There
• 50:27
  Speaker 1: is nothing in the Schrodinger equation that distinguishes past from future,
• 50:31
  Speaker 1: and nothing in it that singles out any special moment.
• 50:34
  Speaker 1: The equation describes smooth, continuous, reversible evolution. That is all
• 50:39
  Speaker 1: it does. The collapse is none of those things. It
• 50:42
  Speaker 1: is abrupt. It is irreversible. It picks out one outcome
• 50:47
  Speaker 1: from many. And it is not described by the Schrodinger
• 50:50
  Speaker 1: equation at all. The two processes, smooth wavefunction evolution and
• 50:54
  Speaker 1: sudden collapse, are governed by entirely different rules. The theory
• 50:59
  Speaker 1: uses both, but never explains how or why the switch
• 51:02
  Speaker 1: between them happens. This is the measurement problem, and physicists
• 51:06
  Speaker 1: have been sitting with it since the 1920s. The measurement
• 51:10
  Speaker 1: problem is sometimes described as a technical puzzle waiting for
• 51:14
  Speaker 1: a cleaner formulation. It is more serious than that. It
• 51:18
  Speaker 1: is a question about when and why time stops being symmetric.
• 51:22
  Speaker 1: Before measurement, the quantum description of reality is fully time-reversible.
• 51:27
  Speaker 1: After measurement, something irreversible has occurred. A definite outcome exists
• 51:33
  Speaker 1: where many possibilities existed before. That transition is a genuine
• 51:37
  Speaker 1: asymmetry in time, injected into a theory whose fundamental equation
• 51:42
  Speaker 1: has no such asymmetry. Where does it come from? The
• 51:45
  Speaker 1: theory doesn't say. The various interpretations of quantum mechanics are
• 51:50
  Speaker 1: largely different ways of handling this. Each one answers the
• 51:54
  Speaker 1: measurement problem differently, and each one carries with it a
• 51:57
  Speaker 1: different account of time. In the Copenhagen interpretation, the collapse
• 52:02
  Speaker 1: is simply a feature of what happens when a quantum
• 52:05
  Speaker 1: system interacts with a classical measuring device. The wave function
• 52:09
  Speaker 1: is not a description of physical reality. It is a
• 52:12
  Speaker 1: tool for calculating probabilities. When a measurement is made, the
• 52:17
  Speaker 1: wave function updates to reflect the new information. Time enters
• 52:21
  Speaker 1: through the measurement process itself. There is a before, an after,
• 52:26
  Speaker 1: and an irreversible event in between. This sounds reasonable until
• 52:30
  Speaker 1: you ask what counts as a measurement, what distinguishes a
• 52:34
  Speaker 1: quantum system from a classical measuring device, and why the
• 52:38
  Speaker 1: boundary between them exists. No satisfying answer has been produced.
• 52:43
  Speaker 1: The many worlds interpretation takes a different path. In this picture,
• 52:48
  Speaker 1: the wave function never collapses. The Schrodinger equation always holds.
• 52:53
  Speaker 1: When a measurement is made, the universe branches, Every possible
• 52:58
  Speaker 1: outcome occurs in separate branches that can no longer interfere
• 53:02
  Speaker 1: with each other. There is no collapse, no irreversibility, no
• 53:06
  Speaker 1: preferred direction of time. All branches exist simultaneously in the
• 53:11
  Speaker 1: larger wavefunction of the universe. Time is symmetric. What looks
• 53:15
  Speaker 1: like an irreversible measurement from inside one branch is actually
• 53:19
  Speaker 1: a branching of the universal wavefunction, that is, at the
• 53:22
  Speaker 1: deepest level, perfectly reversible. Pilot wave theory, developed by David
• 53:28
  Speaker 1: Bohm in the 1950s, restores a definite particle trajectory at
• 53:32
  Speaker 1: all times, guided by a real wave that evolves according
• 53:35
  Speaker 1: to the Schrodinger equation. The particle always has a definite position.
• 53:41
  Speaker 1: Measurement reveals that position rather than creating it. This removes
• 53:45
  Speaker 1: the mystery of collapse but reintroduces it elsewhere. The wave
• 53:49
  Speaker 1: function still evolves in a high-dimensional configuration space in ways
• 53:53
  Speaker 1: that are difficult to interpret physically. Relational quantum mechanics developed
• 53:57
  Speaker 1: by Carlo Rovelli in the 1990s says that quantum states
• 54:01
  Speaker 1: are not absolute descriptions of systems, but descriptions relative to
• 54:06
  Speaker 1: other systems. There is no observer-independent wavefunction for the universe.
• 54:11
  Speaker 1: Each system has a quantum state relative to each other
• 54:14
  Speaker 1: system it interacts with. Time, in this view, is relational
• 54:18
  Speaker 1: in the same way. Events happen relative to physical systems,
• 54:22
  Speaker 1: not against a fixed background clock. This resolves some puzzles
• 54:26
  Speaker 1: and creates others. What all of these interpretations share is
• 54:30
  Speaker 1: that none of them fully dissolves the problem. Copenhagen leaves
• 54:34
  Speaker 1: the boundary between quantum and classical unexplained. Many worlds struggles
• 54:39
  Speaker 1: to explain why we experience only one branch and why
• 54:42
  Speaker 1: probabilities have the values they do. Pilot wave is difficult
• 54:47
  Speaker 1: to reconcile with special relativity. Relational quantum mechanics is consistent
• 54:52
  Speaker 1: but resists many attempts to make it precise. And every
• 54:57
  Speaker 1: single one of them makes a different implicit assumption about
• 55:00
  Speaker 1: what time is. Whether collapse is real, whether time is
• 55:04
  Speaker 1: symmetric at the fundamental level, whether the wave function is
• 55:08
  Speaker 1: a feature of reality or a bookkeeping device. Pick an
• 55:12
  Speaker 1: interpretation and you've made a bet about the nature of time.
• 55:16
  Speaker 1: And every available bet has serious problems. In the 1960s,
• 55:21
  Speaker 1: John Wheeler and Bryce DeWitt sat down to write the
• 55:24
  Speaker 1: most ambitious equation in physics, the Schrodinger equation for the
• 55:28
  Speaker 1: entire universe. The goal was straightforward, at least in principle.
• 55:33
  Speaker 1: Quantum mechanics had a master equation. The Schrodinger equation described
• 55:37
  Speaker 1: how any quantum system evolved through time. Apply it to everything,
• 55:42
  Speaker 1: all matter, all energy, all of space-time, and you would
• 55:45
  Speaker 1: have the fundamental quantum description of reality itself. Wheeler and
• 55:50
  Speaker 1: DeWitt sat down to do exactly that. What came out?
• 55:54
  Speaker 1: Stopped them cold. The time variable was gone. Not approximately gone.
• 55:59
  Speaker 1: Not hidden in some corner of the equation where it
• 56:02
  Speaker 1: might be recovered later. Gone entirely. The Wheeler-DeWitt equation, which
• 56:08
  Speaker 1: was supposed to be the quantum heartbeat of the universe,
• 56:11
  Speaker 1: had no time parameter at all. The equation described the
• 56:15
  Speaker 1: quantum state of everything. and said nothing about how that
• 56:19
  Speaker 1: state changed, because in this equation it didn't. The universe,
• 56:24
  Speaker 1: according to the math, was static, not evolving, not flowing,
• 56:30
  Speaker 1: simply existing frozen and complete in a superposition of all
• 56:34
  Speaker 1: its possible configurations simultaneously. This was not what anyone expected.
• 56:40
  Speaker 1: It was not a sign that something had gone wrong
• 56:42
  Speaker 1: with the calculation. It was a sign that something had
• 56:45
  Speaker 1: gone wrong with the assumptions that every physicist had been
• 56:47
  Speaker 1: carrying into the room. The result was not an error,
• 56:51
  Speaker 1: it was a consequence. Here is the problem at the
• 56:54
  Speaker 1: root of it. In ordinary quantum mechanics, time is treated
• 56:58
  Speaker 1: as a background parameter. It is not part of the
• 57:01
  Speaker 1: quantum system being described. It is the clock running outside
• 57:05
  Speaker 1: the system, telling the wave function when to evolve. Schrodinger's
• 57:10
  Speaker 1: equation takes time as a given. Time is the stage.
• 57:14
  Speaker 1: The quantum system is what performs on it. This works
• 57:17
  Speaker 1: perfectly well for electrons and atoms and even large molecules.
• 57:21
  Speaker 1: But when you try to write a quantum equation for
• 57:24
  Speaker 1: the entire universe, there is no outside. There is no
• 57:28
  Speaker 1: clock that isn't part of the system. Everything is inside
• 57:31
  Speaker 1: the equation. Time cannot be a background parameter when there
• 57:36
  Speaker 1: is no background. General relativity makes this worse. In Einstein's theory,
• 57:41
  Speaker 1: Time is not a fixed stage at all. It is
• 57:45
  Speaker 1: part of the dynamical fabric of space-time, something that curves
• 57:49
  Speaker 1: and warps in response to mass and energy. Time is
• 57:53
  Speaker 1: a variable in general relativity, not a constant. So when
• 57:57
  Speaker 1: you combine general relativity with quantum mechanics and ask how
• 58:02
  Speaker 1: the quantum state of the universe evolves, you are asking
• 58:05
  Speaker 1: a question that the mathematics cannot process cleanly. Time is
• 58:10
  Speaker 1: both inside the system because general relativity puts it there,
• 58:14
  Speaker 1: and supposed to be outside the system, because quantum mechanics
• 58:19
  Speaker 1: needs an external clock. The two frameworks pull in opposite directions.
• 58:24
  Speaker 1: When Wheeler and DeWitt pushed through the algebra to find
• 58:27
  Speaker 1: a resolution, what resolved was time itself. It cancelled. The
• 58:32
  Speaker 1: equation that remained described a universe with no temporal evolution whatsoever.
• 58:39
  Speaker 1: Physicists call this the problem of time, It is one
• 58:42
  Speaker 1: of the central unsolved puzzles in all of theoretical physics.
• 58:46
  Speaker 1: Not a footnote in the literature, but a foundational crisis
• 58:49
  Speaker 1: sitting underneath decades of work on quantum gravity. What the
• 58:53
  Speaker 1: Wheeler-DeWitt equation implies, if you take it seriously, is stark.
• 58:57
  Speaker 1: Time is not fundamental. The deepest description of the universe
• 59:01
  Speaker 1: available to physics contains no time. Whatever time is, the
• 59:05
  Speaker 1: thing you feel passing right now, the thing that orders
• 59:08
  Speaker 1: your memories, the thing that sets cause from effect, it
• 59:12
  Speaker 1: is not written into the bedrock equations. It must come
• 59:15
  Speaker 1: from somewhere else. It must emerge. The analogy that helps
• 59:18
  Speaker 1: here is temperature. Individual molecules don't have a temperature. Temperature
• 59:24
  Speaker 1: is a property that appears only when you have enormous
• 59:27
  Speaker 1: numbers of molecules interacting. and it describes the average behavior
• 59:31
  Speaker 1: of their motion. Ask what the temperature of a single
• 59:35
  Speaker 1: molecule is and the question doesn't make sense. Ask what
• 59:39
  Speaker 1: the temperature of a gas is and you get a precise, measurable,
• 59:42
  Speaker 1: physically real answer. Temperature is emergent. It is real at
• 59:47
  Speaker 1: the scale where it makes sense and absent at the
• 59:50
  Speaker 1: scale where it doesn't. Time, the Wheeler-DeWitt equation suggests, may
• 59:55
  Speaker 1: work the same way. At the most fundamental level, the
• 59:59
  Speaker 1: quantum gravitational level, there is no time. There are configurations,
• 1:00:05
  Speaker 1: there are quantum states, there are correlations between parts of
• 1:00:09
  Speaker 1: the system. But there is no clock ticking through it all,
• 1:00:13
  Speaker 1: no parameter measuring how things evolve, because nothing evolves. Then,
• 1:00:19
  Speaker 1: at larger scales, something that looks and behaves like time appears.
• 1:00:24
  Speaker 1: The flow you experience, the arrow pointing from past to future,
• 1:00:29
  Speaker 1: the sense that now is distinct from then. These may
• 1:00:32
  Speaker 1: be properties of large, complex, thermodynamically rich systems, the way
• 1:00:38
  Speaker 1: temperature is a property of large collections of molecules, real
• 1:00:42
  Speaker 1: at that scale, absent at the deepest one. This is
• 1:00:46
  Speaker 1: not a comfortable position. It means that the most immediate
• 1:00:50
  Speaker 1: and certain feature of your experience, that time is passing,
• 1:00:55
  Speaker 1: that things are happening, that the present moment is distinct
• 1:00:58
  Speaker 1: and real, has no counterpart in the fundamental description of
• 1:01:02
  Speaker 1: the universe. The universe at its core is not happening.
• 1:01:06
  Speaker 1: It is not unfolding. It simply is, all at once,
• 1:01:10
  Speaker 1: in a timeless quantum state. Some physicists treat the Wheeler-DeWitt
• 1:01:15
  Speaker 1: equation as a mathematical curiosity, a symptom of an incomplete
• 1:01:19
  Speaker 1: theory that will eventually be fixed when better tools arrive.
• 1:01:23
  Speaker 1: Others treat it as a genuine revelation about the nature
• 1:01:26
  Speaker 1: of reality. the clearest signal the mathematics has sent about
• 1:01:30
  Speaker 1: what the universe is actually like. That disagreement is not resolved.
• 1:01:34
  Speaker 1: It is, if anything, sharper now than when the equation
• 1:01:37
  Speaker 1: was first written down. What is resolved is this. You
• 1:01:41
  Speaker 1: cannot combine quantum mechanics with general relativity without confronting the
• 1:01:46
  Speaker 1: problem of time directly. A generation of physicists, Ravelli, Barber, Smolin,
• 1:01:52
  Speaker 1: built their careers on different answers to the question that
• 1:01:55
  Speaker 1: Wheeler and DeWitt's equation forced into the open. The equation
• 1:01:59
  Speaker 1: gave no time. The question it left behind was what
• 1:02:02
  Speaker 1: to do with that fact. The implication is not subtle.
• 1:02:06
  Speaker 1: If time doesn't appear in the fundamental equation of the universe,
• 1:02:10
  Speaker 1: then time, in some very real sense, is not fundamental.
• 1:02:14
  Speaker 1: Carlo Rovelli does not think time is fundamental, and he
• 1:02:18
  Speaker 1: has spent decades building the mathematical machinery to back that up.
• 1:02:22
  Speaker 1: He is not working from the fringes. Ravelli is one
• 1:02:26
  Speaker 1: of the founders of loop quantum gravity, which sits alongside
• 1:02:30
  Speaker 1: string theory as one of the two most developed attempts
• 1:02:32
  Speaker 1: to build a quantum theory of gravity. When he says
• 1:02:36
  Speaker 1: time doesn't exist at the fundamental level, he is not speculating.
• 1:02:40
  Speaker 1: He is drawing a conclusion from a framework that thousands
• 1:02:43
  Speaker 1: of physicists take seriously, one that has been tested against
• 1:02:47
  Speaker 1: the mathematics of general relativity and found to be consistent.
• 1:02:52
  Speaker 1: That distinction matters. There is a difference between a philosopher
• 1:02:56
  Speaker 1: suggesting time might be an illusion and a theoretical physicist
• 1:03:00
  Speaker 1: who helped construct one of the leading candidates for a
• 1:03:03
  Speaker 1: final theory of physics saying the same thing. Revelli is
• 1:03:07
  Speaker 1: the second kind. The starting point is loop quantum gravity's
• 1:03:11
  Speaker 1: picture of space itself. In ordinary physics, space is a
• 1:03:15
  Speaker 1: smooth continuous fabric, the stage on which everything else happens.
• 1:03:20
  Speaker 1: Loop quantum gravity says The picture breaks down at the
• 1:03:23
  Speaker 1: smallest conceivable scales. At the Planck length, roughly 10 to
• 1:03:28
  Speaker 1: the power of negative 35 meters, space is not smooth
• 1:03:32
  Speaker 1: at all. It is granular. It comes in discrete chunks
• 1:03:36
  Speaker 1: of area and volume, the way matter comes in atoms.
• 1:03:40
  Speaker 1: These chunks are described mathematically by structures called spin networks,
• 1:03:45
  Speaker 1: webs of nodes and links whose combinatorial properties determine the
• 1:03:49
  Speaker 1: geometry of space. There is no background stage. Space itself
• 1:03:54
  Speaker 1: emerges from the relationships between the nodes of the network.
• 1:03:58
  Speaker 1: Remove the network and there is no space underneath it.
• 1:04:02
  Speaker 1: Space is the network. This is what physicists mean when
• 1:04:06
  Speaker 1: they call loop quantum gravity background independent. The theory does
• 1:04:11
  Speaker 1: not assume a pre-existing space-time and then put physics into it.
• 1:04:15
  Speaker 1: It builds space-time from something more primitive. Time in this
• 1:04:19
  Speaker 1: picture is in the same position as space. It does
• 1:04:22
  Speaker 1: not exist as a background parameter ticking away behind the scenes.
• 1:04:26
  Speaker 1: What we call time is a description of how events
• 1:04:29
  Speaker 1: relate to each other, not a container those events sit inside.
• 1:04:33
  Speaker 1: This is the relational view of time, and Rovelli pushes
• 1:04:37
  Speaker 1: it harder than almost anyone. There is no universal clock
• 1:04:40
  Speaker 1: running through the universe. There is no fact about what
• 1:04:44
  Speaker 1: time it is everywhere simultaneously. Events don't happen at a
• 1:04:48
  Speaker 1: time the way objects sit at a location. They happen
• 1:04:51
  Speaker 1: relative to other events, the way spatial positions are only
• 1:04:55
  Speaker 1: meaningful relative to other objects. Strip away all the physical
• 1:04:59
  Speaker 1: systems and there is nothing left called time. This sounds abstract.
• 1:05:04
  Speaker 1: The thermal time hypothesis is where it becomes concrete. In
• 1:05:08
  Speaker 1: any large physical system with many interacting components, thermodynamics picks
• 1:05:13
  Speaker 1: out a preferred direction. Entropy increases in one direction and
• 1:05:17
  Speaker 1: not the other, and that asymmetry defines what we call
• 1:05:20
  Speaker 1: the future. Rovelli and Alain Conis proposed that this thermodynamic
• 1:05:25
  Speaker 1: direction is where the experience of flowing time actually comes from.
• 1:05:29
  Speaker 1: It is not that the universe has a built-in arrow
• 1:05:32
  Speaker 1: of time at the fundamental level. It is that any
• 1:05:35
  Speaker 1: observer embedded inside a thermodynamically complex system will experience a
• 1:05:39
  Speaker 1: direction of time. Because the correlations between past states and
• 1:05:43
  Speaker 1: future states are asymmetric, the flow is real in the
• 1:05:47
  Speaker 1: same sense that temperature is real. A single molecule does
• 1:05:50
  Speaker 1: not have a tempi-bracher. Temperature is a property that emerges
• 1:05:55
  Speaker 1: when you have many molecules and you average over their behavior.
• 1:05:59
  Speaker 1: The experience of time flowing forward is a property that
• 1:06:02
  Speaker 1: emerges when you have a large, thermodynamically complex system and
• 1:06:07
  Speaker 1: you are part of it. Real, but not fundamental. Ravelli
• 1:06:11
  Speaker 1: laid this out in full in his book The Order
• 1:06:14
  Speaker 1: of Time, which became an unexpected bestseller, which says something
• 1:06:19
  Speaker 1: about how urgently people want to understand this. His argument
• 1:06:23
  Speaker 1: there is not just technical, it is almost archaeological, peeling
• 1:06:28
  Speaker 1: back the layers of what we call time to show
• 1:06:31
  Speaker 1: that none of them are basic. The distinction between past
• 1:06:34
  Speaker 1: and future comes from entropy. Without a thermodynamic arrow, past
• 1:06:39
  Speaker 1: and future are the same direction. The sense of the present,
• 1:06:43
  Speaker 1: the specialness of now, comes from our particular perspective inside
• 1:06:48
  Speaker 1: the world. We interact with only a small part of
• 1:06:51
  Speaker 1: the universe at any given moment, and that limited interaction
• 1:06:55
  Speaker 1: is what creates the impression of a local now, the
• 1:06:58
  Speaker 1: impression of flow, the sense that time is moving rather
• 1:07:02
  Speaker 1: than that we are moving through tunar. J-time comes from
• 1:07:06
  Speaker 1: the way the brain encodes memory. and builds a continuous
• 1:07:10
  Speaker 1: narrative out of discrete neural events. Each of these features
• 1:07:14
  Speaker 1: has an explanation. Not one of them requires time to
• 1:07:18
  Speaker 1: be fundamental. What remains when you remove all of them?
• 1:07:22
  Speaker 1: Ravelli's answer is a network of events and relations. The
• 1:07:26
  Speaker 1: fundamental description of the universe is not a movie, not
• 1:07:30
  Speaker 1: a sequence of moments unfolding one after another. It is
• 1:07:34
  Speaker 1: something closer to a vast web, in which each node
• 1:07:38
  Speaker 1: is an event and each connection describes a physical relation
• 1:07:42
  Speaker 1: between events. The web has no preferred direction. It has
• 1:07:46
  Speaker 1: no global ordering. It does not happen in time because
• 1:07:50
  Speaker 1: time is one of the things that emerges from it
• 1:07:52
  Speaker 1: at larger scales, not one of the things that was
• 1:07:55
  Speaker 1: there to begin with. The Wheeler-DeWitt equation, which came up
• 1:07:59
  Speaker 1: before this, gives Rovelli's position strong mathematical support. The fact
• 1:08:04
  Speaker 1: that time drops out of the fundamental quantum equation for
• 1:08:07
  Speaker 1: the universe is not, in his reading, an artifact to
• 1:08:11
  Speaker 1: be fixed. It is a signal. The mathematics is telling
• 1:08:15
  Speaker 1: you something. When the only candidate for the fundamental quantum
• 1:08:19
  Speaker 1: description of the universe has no time variable, the reasonable
• 1:08:23
  Speaker 1: conclusion is that time is not in the foundations. This
• 1:08:27
  Speaker 1: is not a proof, but it is serious evidence. The
• 1:08:31
  Speaker 1: objection almost everyone raises is that this seems to contradict
• 1:08:35
  Speaker 1: something immediate and undeniable. You are experiencing time right now.
• 1:08:40
  Speaker 1: The clock on the wall is moving. None of this
• 1:08:43
  Speaker 1: feels like an emergent approximation. Rovelli's response is patient and precise.
• 1:08:48
  Speaker 1: Emergent phenomena are not fake. Heat is emergent, and it
• 1:08:52
  Speaker 1: is real enough to burn you. The solidity of a
• 1:08:55
  Speaker 1: table is emergent from the electromagnetic interactions between atoms, and
• 1:08:59
  Speaker 1: it is real enough to stop your fist. The experience
• 1:09:02
  Speaker 1: of time is emergent from the relational structure of events
• 1:09:06
  Speaker 1: in a thermodynamically asymmetric world, and it is real enough
• 1:09:09
  Speaker 1: to organize every moment of a human life. The word
• 1:09:13
  Speaker 1: emergent does not mean illusory. It means that the description
• 1:09:17
  Speaker 1: at one level of the physics is a large-scale consequence
• 1:09:20
  Speaker 1: of a different description at a more fundamental level. For Ravelli,
• 1:09:25
  Speaker 1: time is not less real for being emergent. A rainbow
• 1:09:28
  Speaker 1: is emergent too, and nobody thinks rainbows aren't real. Julian
• 1:09:33
  Speaker 1: Barber arrived at the timelessness of the universe by a
• 1:09:36
  Speaker 1: completely different road, and he did it mostly alone. Ravelli
• 1:09:41
  Speaker 1: built his case from quantum gravity, from the collapse of
• 1:09:44
  Speaker 1: time in loop quantum structures, from the failure of a
• 1:09:47
  Speaker 1: universal parameter to survive contact with the Planck scale. Barber's
• 1:09:52
  Speaker 1: starting point was older and stranger. He went back to
• 1:09:55
  Speaker 1: classical mechanics, Newton, Hamilton, the equations physicists had been using
• 1:10:00
  Speaker 1: for three centuries and asked whether time was actually in
• 1:10:03
  Speaker 1: them in the way everyone assumed. What he found, after
• 1:10:07
  Speaker 1: years of working largely outside universities, without the institutional scaffolding
• 1:10:13
  Speaker 1: that most theoretical physicists rely on, was that it wasn't. Time,
• 1:10:18
  Speaker 1: he argued, is not a container in which the universe
• 1:10:21
  Speaker 1: sits and changes. It is a name we give to
• 1:10:24
  Speaker 1: the sequence of configurations we observe. Strip the name away
• 1:10:28
  Speaker 1: and what remains is change. Not change in time, just change.
• 1:10:32
  Speaker 1: This distinction sounds small. It is not. Newton believed in
• 1:10:36
  Speaker 1: absolute time, a river flowing uniformly through the universe, indifferent
• 1:10:42
  Speaker 1: to matter, indifferent to observers, ticking at the same rate
• 1:10:45
  Speaker 1: everywhere whether anything happened or not. Barber thinks this is
• 1:10:49
  Speaker 1: wrong in the deepest possible way. If you removed every
• 1:10:53
  Speaker 1: particle from the universe, every field, every trace of energy,
• 1:10:57
  Speaker 1: Newton's time would still be passing. Barber says that is
• 1:11:01
  Speaker 1: not a feature of reality. It is a philosophical mistake.
• 1:11:05
  Speaker 1: Time without change is not time. It is a ghost
• 1:11:09
  Speaker 1: that does no work and leaves no trace. His central
• 1:11:13
  Speaker 1: concept is what he calls Platonia. The name comes from Plato,
• 1:11:18
  Speaker 1: and the idea is this. Imagine the space of all
• 1:11:21
  Speaker 1: possible configurations of the universe, every arrangement of every particle,
• 1:11:26
  Speaker 1: every possible shape of every field, laid out like an
• 1:11:30
  Speaker 1: enormous landscape of distinct snapshots. Each snapshot is complete. It
• 1:11:35
  Speaker 1: contains everything there is to know about the state of
• 1:11:38
  Speaker 1: the universe at that instant. Now here is what Barber insists.
• 1:11:42
  Speaker 1: These snapshots do not sit inside time. They simply exist.
• 1:11:47
  Speaker 1: All of them. The universe is not a film being
• 1:11:50
  Speaker 1: projected frame by frame through a projector. It is a
• 1:11:53
  Speaker 1: stack of frames, each one real, none of them moving anywhere.
• 1:11:58
  Speaker 1: What we call time is the path we trace through
• 1:12:00
  Speaker 1: that stack. This requires a moment to absorb. Change is real.
• 1:12:06
  Speaker 1: Barber is emphatic about this. Two configurations that differ from
• 1:12:10
  Speaker 1: each other are genuinely different. The difference is real. What
• 1:12:14
  Speaker 1: is not real is the independent dimension that supposedly carries
• 1:12:18
  Speaker 1: you from one configuration to the next. In Barber's picture,
• 1:12:22
  Speaker 1: there is no river. There are only riverbanks at different positions.
• 1:12:27
  Speaker 1: And what we call time is the relation between them.
• 1:12:31
  Speaker 1: His technical framework for this is called shape dynamics. The
• 1:12:34
  Speaker 1: core idea is that the only physically meaningful quantities in
• 1:12:38
  Speaker 1: the universe are shapes. not absolute positions, not absolute sizes,
• 1:12:43
  Speaker 1: not durations, but the ratios and angles that describe how
• 1:12:47
  Speaker 1: things are arranged relative to each other. The distance between
• 1:12:51
  Speaker 1: two particles means nothing by itself. What matters is the
• 1:12:55
  Speaker 1: ratio of that distance to other distances. You cannot point
• 1:12:59
  Speaker 1: to a location in absolute space. You can only describe
• 1:13:02
  Speaker 1: where something is in relation to everything else. The same
• 1:13:06
  Speaker 1: logic applies to time. There is no absolute moment, there
• 1:13:10
  Speaker 1: is only this configuration, and what it is like compared
• 1:13:14
  Speaker 1: to another configuration. Shape dynamics is not purely philosophical. Barber
• 1:13:20
  Speaker 1: and collaborators have shown that it reproduces general relativity in
• 1:13:24
  Speaker 1: certain regimes, that the same physics you get from Einstein's
• 1:13:28
  Speaker 1: equations falls out of the shape-based framework under specific conditions.
• 1:13:33
  Speaker 1: The background independence of general relativity The fact that the
• 1:13:37
  Speaker 1: laws of physics make no reference to any fixed coordinate
• 1:13:41
  Speaker 1: system maps naturally onto shape dynamics in a way that
• 1:13:44
  Speaker 1: suggests something is right about the underlying idea. What looks
• 1:13:49
  Speaker 1: like temporal evolution, when translated into this language, becomes a
• 1:13:54
  Speaker 1: path through the space of shapes. The path is real.
• 1:13:57
  Speaker 1: The background time through which the path supposedly moves is not.
• 1:14:02
  Speaker 1: Here is where the picture gets genuinely strange. If all
• 1:14:06
  Speaker 1: configurations simply exist, if Plutonia is the full structure of
• 1:14:11
  Speaker 1: reality and nothing is moving through it, then why does
• 1:14:14
  Speaker 1: time feel like it flows? Why do you remember yesterday
• 1:14:18
  Speaker 1: and not tomorrow? Why does the universe appear to have
• 1:14:21
  Speaker 1: a direction? Barber's answer involves memory, and it is one
• 1:14:26
  Speaker 1: of the most striking ideas in the whole debate. Each
• 1:14:29
  Speaker 1: configuration of the universe that constitutes a present moment contains
• 1:14:34
  Speaker 1: records of configurations that came before it. Your brain holds memories.
• 1:14:39
  Speaker 1: Fossils hold shapes that match living organisms that existed earlier.
• 1:14:43
  Speaker 1: Photographs carry light patterns from past arrangements of matter. The
• 1:14:48
  Speaker 1: craters on the Moon record impacts. The light reaching a
• 1:14:51
  Speaker 1: telescope left its source thousands of years ago. Every present
• 1:14:55
  Speaker 1: configuration is saturated with information about previous configurations. Not because
• 1:15:01
  Speaker 1: time carried those configurations away into the past, but because
• 1:15:05
  Speaker 1: the present configuration was built from them and retains their imprint.
• 1:15:09
  Speaker 1: This is why you feel like you've come from somewhere.
• 1:15:12
  Speaker 1: Not because a river called time has carried you here.
• 1:15:16
  Speaker 1: Because you, right now, in this precise configuration of neurons
• 1:15:20
  Speaker 1: and memories and physical state, contain traces of what came before.
• 1:15:25
  Speaker 1: The past is not behind you in some temporal hallway
• 1:15:27
  Speaker 1: you can no longer enter. The past is inside you,
• 1:15:31
  Speaker 1: encoded in the structure of the present. The sense of
• 1:15:34
  Speaker 1: temporal flow, on this view, is the experience of a
• 1:15:37
  Speaker 1: configuration that is full of records. The feeling of moving
• 1:15:40
  Speaker 1: forward through time is the feeling of being a complex
• 1:15:43
  Speaker 1: system whose current state is deeply shaped by prior states.
• 1:15:47
  Speaker 1: A system that contains its own history. Whether this is
• 1:15:51
  Speaker 1: a complete explanation of time's arrow, or an elegant re-description
• 1:15:55
  Speaker 1: of the same mystery wearing different clothes is a question
• 1:15:59
  Speaker 1: Barber's critics press hard. But the framework is coherent and
• 1:16:03
  Speaker 1: mathematically serious, and it arrives at timelessness by a route
• 1:16:07
  Speaker 1: that has nothing to do with quantum mechanics, nothing to
• 1:16:11
  Speaker 1: do with spin networks or Planck scale discreteness. It comes
• 1:16:15
  Speaker 1: from asking what Newton actually needed time for and finding
• 1:16:19
  Speaker 1: that the answer is less than he thought. The past
• 1:16:23
  Speaker 1: doesn't recede because time carries it away. The past is
• 1:16:26
  Speaker 1: visible in the present because the present carries its records.
• 1:16:31
  Speaker 1: In 1983, two physicists did something almost no one noticed
• 1:16:36
  Speaker 1: at the time. They derived the flow of time from
• 1:16:39
  Speaker 1: a completely timeless quantum equation. Don Page and William Wuters
• 1:16:43
  Speaker 1: were working on one of the strangest consequences of the
• 1:16:46
  Speaker 1: Wheeler-DeWitt equation. That equation, as the previous section established, describes
• 1:16:52
  Speaker 1: the quantum state of the entire universe without any time
• 1:16:56
  Speaker 1: parameter at all. The universe, according to it, simply sits there, static, frozen, unchanging.
• 1:17:03
  Speaker 1: Page and Wuters took that conclusion seriously and then asked
• 1:17:07
  Speaker 1: a different question. Not whether time could be put back
• 1:17:10
  Speaker 1: into the equation from outside, but whether time could somehow
• 1:17:14
  Speaker 1: arise from within it. Their proposal was this. Take the
• 1:17:18
  Speaker 1: quantum state of the universe, timeless, static, satisfying the wheeler-dewitt
• 1:17:23
  Speaker 1: equation now divide the universe into two parts one part
• 1:17:28
  Speaker 1: is a clock system the other part is everything else
• 1:17:32
  Speaker 1: all the matter all the fields all the physical processes
• 1:17:36
  Speaker 1: that make up the rest of reality these two parts
• 1:17:39
  Speaker 1: are quantum entangled And entanglement, in quantum mechanics, means the
• 1:17:44
  Speaker 1: state of one part is correlated with the state of
• 1:17:47
  Speaker 1: the other, in a way that cannot be reduced to
• 1:17:50
  Speaker 1: the properties of either part alone. When you look at
• 1:17:53
  Speaker 1: the clock, you find a specific reading. When the clock
• 1:17:56
  Speaker 1: reads a certain value, the rest of the universe is
• 1:18:00
  Speaker 1: in a specific corresponding configuration. Different clock readings correlate with
• 1:18:05
  Speaker 1: different states of everything else. From outside the system, nothing
• 1:18:09
  Speaker 1: is changing. The global quantum state is static. There is
• 1:18:13
  Speaker 1: no evolution, no flow, no before or after. But there
• 1:18:17
  Speaker 1: is no observer outside the universe. Every observer who actually
• 1:18:21
  Speaker 1: exists is inside the system, part of the entangled whole.
• 1:18:25
  Speaker 1: And from the inside, from the perspective of an observer
• 1:18:28
  Speaker 1: who is part of the rest of the universe looking
• 1:18:30
  Speaker 1: at the clock, something different appears. The clock has a
• 1:18:33
  Speaker 1: definite reading. That reading corresponds to a definite state of
• 1:18:37
  Speaker 1: the surrounding world. As the entanglement structure of the system
• 1:18:41
  Speaker 1: varies across configurations, the clock reads differently, and the surrounding
• 1:18:46
  Speaker 1: world looks different. The observer experiences this as change. as
• 1:18:50
  Speaker 1: time passing. This is not a vague metaphor. Page and
• 1:18:54
  Speaker 1: Wuters worked through the mathematics carefully, and what they found
• 1:18:58
  Speaker 1: was remarkable. Starting from a quantum state with no time
• 1:19:02
  Speaker 1: evolution at all, they were able to derive the time-dependent
• 1:19:06
  Speaker 1: Schrodinger equation, the standard equations that describes how quantum systems
• 1:19:12
  Speaker 1: change over time. That equation was not put in by hand.
• 1:19:16
  Speaker 1: It emerged from the correlations between entangled subsystems of a
• 1:19:20
  Speaker 1: timeless whole. Time, in their framework, is what entanglement looks
• 1:19:25
  Speaker 1: like from the inside. For roughly three decades, almost nobody cared.
• 1:19:31
  Speaker 1: The paper was acknowledged, noted occasionally in discussions of quantum gravity,
• 1:19:36
  Speaker 1: and largely set aside. it seemed too abstract, too remote
• 1:19:41
  Speaker 1: from anything testable, too philosophically strange to be taken as
• 1:19:45
  Speaker 1: a serious physical proposal. That changed gradually, as the problem
• 1:19:50
  Speaker 1: of time in quantum gravity became harder to ignore, and
• 1:19:54
  Speaker 1: as experimental tools for manipulating small quantum systems became more precise.
• 1:20:00
  Speaker 1: In 2013, a group of physicists at the Istituto Nazionale
• 1:20:05
  Speaker 1: di Ricerca Metrologica in Turin ran an experiment that tested
• 1:20:10
  Speaker 1: a version of the Page-Witters mechanism in a small optical system.
• 1:20:14
  Speaker 1: They prepared pairs of entangled photons, used one photon as
• 1:20:18
  Speaker 1: a clock and the other as the system being observed,
• 1:20:21
  Speaker 1: and measured whether the correlations between them were consistent with
• 1:20:24
  Speaker 1: the mechanism's predictions. They were. The experiment did not prove
• 1:20:29
  Speaker 1: that time is generated by entanglement at a cosmic scale.
• 1:20:33
  Speaker 1: It was far too small for that. but it demonstrated
• 1:20:37
  Speaker 1: that the mathematical structure Page and Waters described was not
• 1:20:41
  Speaker 1: merely formal. It produced predictions that could be confirmed in
• 1:20:45
  Speaker 1: the laboratory. The implications, if the mechanism scales up to
• 1:20:49
  Speaker 1: the universe as a whole, are significant. Time is not
• 1:20:53
  Speaker 1: a dimension the universe is embedded in. It is not
• 1:20:56
  Speaker 1: a background clock ticking away independently of everything else. It
• 1:21:00
  Speaker 1: is a relationship. a set of correlations between entangled parts
• 1:21:05
  Speaker 1: of a quantum system. Different subsystems of the universe serve
• 1:21:09
  Speaker 1: as clocks for other subsystems. The experience of duration, of
• 1:21:13
  Speaker 1: one moment following another, is the experience of being a
• 1:21:17
  Speaker 1: physical system correlated with other physical systems in a particular
• 1:21:21
  Speaker 1: structured way. Take away the entanglement and the correlations disappear.
• 1:21:26
  Speaker 1: Take away the correlations and time disappears with them. This
• 1:21:31
  Speaker 1: reframes the question of time's reality in a specific and
• 1:21:34
  Speaker 1: interesting way. The Wheeler-DeWitt equation says the universe as a
• 1:21:39
  Speaker 1: whole has no time. The Page-Wooters mechanism says that is
• 1:21:43
  Speaker 1: consistent with time being perfectly real for observers inside the universe.
• 1:21:49
  Speaker 1: There is no contradiction. The global state is timeless. The
• 1:21:54
  Speaker 1: relational structure is temporal. Both statements are true simultaneously. because
• 1:21:59
  Speaker 1: they are statements about different things, the whole versus the parts,
• 1:22:04
  Speaker 1: the view from outside versus the view from within. There
• 1:22:08
  Speaker 1: is something almost vertiginous about this. The sensation you have
• 1:22:12
  Speaker 1: right now of being located in a present moment, of
• 1:22:15
  Speaker 1: the past being behind you and the future ahead, would be,
• 1:22:19
  Speaker 1: on this account, a consequence of the fact that you
• 1:22:22
  Speaker 1: are a quantum system entangled with other quantum systems. The
• 1:22:26
  Speaker 1: clock on the wall, The decay of atoms in your body,
• 1:22:30
  Speaker 1: the firing of neurons in your brain, all of these
• 1:22:34
  Speaker 1: are entangled with each other in ways that produce consistent,
• 1:22:37
  Speaker 1: correlated readings. That web of correlations is time. Not a
• 1:22:42
  Speaker 1: container those processes happen inside, the correlations themselves. Julian Barber
• 1:22:48
  Speaker 1: would say something similar. Change is real, but time as
• 1:22:52
  Speaker 1: an independent backdrop is not. The pagewater's picture adds a
• 1:22:55
  Speaker 1: quantum mechanical mechanism for how that change gets structured, how
• 1:22:59
  Speaker 1: it becomes ordered, how it comes to feel like a progression,
• 1:23:03
  Speaker 1: rather than a jumble of disconnected states. The entanglement does
• 1:23:07
  Speaker 1: the work that time was supposed to do, and it
• 1:23:10
  Speaker 1: does it without requiring time to exist as a fundamental
• 1:23:13
  Speaker 1: element of the theory. If time is relational, if it
• 1:23:17
  Speaker 1: lives in the correlation between entangled quantum systems rather than
• 1:23:20
  Speaker 1: in a background dimension, Then asking whether time is real
• 1:23:24
  Speaker 1: is like asking whether a conversation is real. It doesn't
• 1:23:28
  Speaker 1: exist without two participants, but between them, it is entirely genuine.
• 1:23:33
  Speaker 1: Lee Smolin helped build loop quantum gravity alongside Rovelli, looked
• 1:23:38
  Speaker 1: at the same equations, and arrived at the exact opposite conclusion.
• 1:23:42
  Speaker 1: That is worth pausing on. These are not two physicists
• 1:23:46
  Speaker 1: working in different traditions, talking past each other across a
• 1:23:50
  Speaker 1: disciplinary divide. Smolin and Ravelli were collaborators. They came up
• 1:23:55
  Speaker 1: through the same research program, wrestled with the same mathematical objects,
• 1:24:00
  Speaker 1: and read the same results. And where Ravelli looked at
• 1:24:03
  Speaker 1: the Wheeler-DeWitt equation and saw confirmation that time is not fundamental,
• 1:24:08
  Speaker 1: Smolin looked at the same equation and saw a warning
• 1:24:11
  Speaker 1: sign that something had gone seriously wrong. His argument is
• 1:24:15
  Speaker 1: not that the math is incorrect. It is that the
• 1:24:18
  Speaker 1: math is incomplete. and that the conclusions physicists have drawn
• 1:24:22
  Speaker 1: from it represent one of the most consequential category errors
• 1:24:26
  Speaker 1: in the history of the field. The book where Smolin
• 1:24:29
  Speaker 1: laid this out most forcefully is called Time Reborn, published
• 1:24:33
  Speaker 1: in 2013. The central claim is stark. If the laws
• 1:24:38
  Speaker 1: of physics are genuinely timeless, if they exist outside of time,
• 1:24:42
  Speaker 1: fixed and eternal, governing the universe from some mathematical beyond,
• 1:24:47
  Speaker 1: then they cannot be explained. You cannot ask why they
• 1:24:50
  Speaker 1: are what they are. There is no process by which
• 1:24:53
  Speaker 1: they came to be. No earlier state of affairs selected them.
• 1:24:57
  Speaker 1: They simply are, without cause, without history, without any account
• 1:25:02
  Speaker 1: of why these laws rather than some other set entirely.
• 1:25:05
  Speaker 1: For Smolin, this is not a deep truth about physics.
• 1:25:09
  Speaker 1: It is a dead end. A timeless law is an
• 1:25:12
  Speaker 1: unexplained law. and unexplained laws are a signal that you
• 1:25:16
  Speaker 1: haven't actually understood what you're looking at yet. Flip it
• 1:25:20
  Speaker 1: around and the argument becomes clearer. If time is real, genuinely,
• 1:25:25
  Speaker 1: fundamentally real, then the laws of physics have a history.
• 1:25:29
  Speaker 1: They can change. They can evolve. And if they can evolve,
• 1:25:34
  Speaker 1: then in principle you can ask why they evolved into
• 1:25:37
  Speaker 1: the particular form we observe. And in principle you can
• 1:25:40
  Speaker 1: construct an explanation. the question becomes tractable. The moment you
• 1:25:45
  Speaker 1: remove time from the picture, you remove the possibility of
• 1:25:49
  Speaker 1: that explanation entirely. Smolin is direct about where this leaves
• 1:25:53
  Speaker 1: the block universe picture. He does not think the block
• 1:25:57
  Speaker 1: universe is obviously wrong as a description of what the
• 1:25:59
  Speaker 1: equations say. He thinks the equations are incomplete and that
• 1:26:03
  Speaker 1: taking them as a final account of reality is the mistake.
• 1:26:07
  Speaker 1: The time symmetry of current physical laws is a feature
• 1:26:10
  Speaker 1: of our current incomplete theories. When physicists finally write down
• 1:26:14
  Speaker 1: a successful theory of quantum gravity, one that actually works
• 1:26:18
  Speaker 1: rather than producing infinities and contradictions, Smolin predicts that time
• 1:26:23
  Speaker 1: will come back, not as an approximation, not as an
• 1:26:26
  Speaker 1: emergent convenience, as something fundamental, built into the structure of
• 1:26:31
  Speaker 1: the theory at the deepest level. This is not a
• 1:26:34
  Speaker 1: vague hope. Smolin has spent decades trying to build the
• 1:26:38
  Speaker 1: technical case for it. His argument connects to a broader
• 1:26:42
  Speaker 1: proposal about how the laws of physics themselves should be understood.
• 1:26:46
  Speaker 1: The laws we observe appear to be, in a specific
• 1:26:49
  Speaker 1: technical sense, finely tuned. The constants of nature sit in
• 1:26:54
  Speaker 1: a narrow range that permits complex chemistry, stable atoms, long-lived stars,
• 1:27:00
  Speaker 1: and ultimately the kind of slow-patient complexity that produces life.
• 1:27:04
  Speaker 1: This has bothered physicists for a long time. The standard
• 1:27:08
  Speaker 1: response invokes the multiverse. There are many universes with different constants.
• 1:27:13
  Speaker 1: We happen to live in one, where the constants permit
• 1:27:16
  Speaker 1: us to exist and there is nothing more to explain.
• 1:27:19
  Speaker 1: Smolin finds this unsatisfying. It explains the observation by making
• 1:27:23
  Speaker 1: the explanation impossibly to check. His alternative is cosmological natural selection.
• 1:27:31
  Speaker 1: The idea runs like this. When a black hole forms,
• 1:27:35
  Speaker 1: It may give rise to a new universe with slightly
• 1:27:38
  Speaker 1: different physical constants, a kind of cosmic reproduction. Universes that
• 1:27:43
  Speaker 1: produce many black holes leave more descendants than universes that
• 1:27:46
  Speaker 1: produce few. The laws of physics we observe may have
• 1:27:50
  Speaker 1: been selected over many generations of universes because they happen
• 1:27:54
  Speaker 1: to produce a large number of black holes. The constants
• 1:27:58
  Speaker 1: are what they are not by chance, and not by
• 1:28:00
  Speaker 1: anthropic selections. but because they have been shaped by a
• 1:28:04
  Speaker 1: process that works like evolution. What matters here is the
• 1:28:08
  Speaker 1: implication for time. Cosmological natural selection only makes sense in
• 1:28:13
  Speaker 1: a universe where time is real. Evolution requires succession. It
• 1:28:18
  Speaker 1: requires that earlier states give rise to later ones, that
• 1:28:22
  Speaker 1: history has genuine content, that the past actually happened and
• 1:28:26
  Speaker 1: the future has not yet been determined. A timeless block
• 1:28:30
  Speaker 1: universe cannot evolve in any meaningful sense. If all moments
• 1:28:34
  Speaker 1: are equally real and equally fixed, there is no room
• 1:28:38
  Speaker 1: for a process that selects some configurations over others across generations.
• 1:28:43
  Speaker 1: The entire framework collapses without a real direction of time
• 1:28:47
  Speaker 1: running through it. Smolin's critics point out that cosmological natural
• 1:28:51
  Speaker 1: selection has its own serious problems. The mechanism of reproduction
• 1:28:56
  Speaker 1: through black holes is speculative. The constants that maximize black
• 1:29:00
  Speaker 1: hole production do not obviously match the constants we observe,
• 1:29:03
  Speaker 1: and the theory is difficult to test in any direct way.
• 1:29:07
  Speaker 1: Smolin acknowledges the objections. He does not present cosmological natural
• 1:29:11
  Speaker 1: selection as settled science. He presents it as the kind
• 1:29:15
  Speaker 1: of explanation that becomes possible once you treat time as real,
• 1:29:18
  Speaker 1: and impossible the moment you treat it as emergent or illusory.
• 1:29:22
  Speaker 1: The most striking part of Smolin's position is what he
• 1:29:25
  Speaker 1: does with the subjective experience of time. Every other approach
• 1:29:30
  Speaker 1: in this debate treats the felt sense of the present
• 1:29:32
  Speaker 1: moment as something that needs to be explained away, a
• 1:29:35
  Speaker 1: product of neural processing, an artifact of thermodynamics, a macroscopic
• 1:29:41
  Speaker 1: approximation that dissolves at the fundamental level. Smolin refuses this move.
• 1:29:47
  Speaker 1: He argues that the present moment, the raw experience of
• 1:29:50
  Speaker 1: right now, is not a bug in our cognitive machinery
• 1:29:54
  Speaker 1: or a confusion to be resolved. It is data. It
• 1:29:57
  Speaker 1: is the most immediate and universal observation any conscious entity
• 1:30:02
  Speaker 1: can report. A physical theory that cannot account for it,
• 1:30:06
  Speaker 1: or that dismisses it as an illusion, is not more
• 1:30:09
  Speaker 1: rigorous than a theory that takes it seriously. It is
• 1:30:12
  Speaker 1: less complete. For Smolin, the sense that right now is
• 1:30:16
  Speaker 1: real is not a brain bug to be debugged. it
• 1:30:19
  Speaker 1: is the most important datum physics has been ignoring. The
• 1:30:23
  Speaker 1: physicists working on quantum gravity are not the only ones
• 1:30:27
  Speaker 1: trying to figure out whether time is real. Philosophers have
• 1:30:30
  Speaker 1: been running a parallel argument for decades, and the two
• 1:30:34
  Speaker 1: communities are only beginning to talk to each other. While
• 1:30:37
  Speaker 1: Smolin and Rovelli debate equations, philosophers have been building arguments
• 1:30:42
  Speaker 1: from logic, language, and the structure of reality itself. the
• 1:30:47
  Speaker 1: conclusions they've reached are unsettling in their own right. And
• 1:30:51
  Speaker 1: what makes this philosophical debate particularly important is that it
• 1:30:55
  Speaker 1: doesn't depend on any particular interpretation of quantum mechanics. It
• 1:31:00
  Speaker 1: cuts deeper than that. The question of whether the past
• 1:31:03
  Speaker 1: still exists, whether the future already exists, and whether only
• 1:31:08
  Speaker 1: right now is real, turns out to be extraordinarily difficult
• 1:31:12
  Speaker 1: to answer even without a single equation in sight. Start
• 1:31:16
  Speaker 1: with the position that most closely matches common sense. Presentism
• 1:31:21
  Speaker 1: holds that only the present moment is real. The past
• 1:31:25
  Speaker 1: is genuinely gone, not stored somewhere, not accessible in any
• 1:31:29
  Speaker 1: physical sense, simply absent from reality. The future is genuinely
• 1:31:34
  Speaker 1: not yet there. This is the view most people hold
• 1:31:38
  Speaker 1: without knowing it has a name. When you say that
• 1:31:40
  Speaker 1: Julius Caesar crossed the Rubicon, The presentist reading is that
• 1:31:44
  Speaker 1: this sentence describes something that no longer exists. The crossing happened.
• 1:31:49
  Speaker 1: It influenced the world. But the event itself has vanished completely,
• 1:31:54
  Speaker 1: leaving only its effects behind. This matches how most people
• 1:31:58
  Speaker 1: intuitively understand causation, identity and change. Things happen and then cease.
• 1:32:05
  Speaker 1: New things come into being. Reality is an ever-moving knife
• 1:32:09
  Speaker 1: edge between a go-nay-past, and an unformed future. The problem
• 1:32:14
  Speaker 1: is that special relativity makes presentism almost impossible to maintain.
• 1:32:19
  Speaker 1: If there is no absolute simultaneity, if two observers moving
• 1:32:24
  Speaker 1: at different velocities genuinely disagree about which events happen at
• 1:32:27
  Speaker 1: the same time, then the question of what counts as
• 1:32:30
  Speaker 1: the present becomes observer-dependent. Consider two events happening far apart,
• 1:32:36
  Speaker 1: a supernova in one galaxy and a birth in another.
• 1:32:40
  Speaker 1: One observer might find these simultaneous. A second observer, moving
• 1:32:44
  Speaker 1: rapidly relative to the first, might find the supernova happened
• 1:32:48
  Speaker 1: before the birth. A third, moving the other way, might
• 1:32:52
  Speaker 1: find the birth came first. None of these observers is wrong.
• 1:32:57
  Speaker 1: None is measuring carelessly. The disagreement is written into the
• 1:33:01
  Speaker 1: structure of space-time. So if only the present exists, which
• 1:33:05
  Speaker 1: present is the of? That gets to count as real?
• 1:33:10
  Speaker 1: If the answer is that each observer has their own
• 1:33:12
  Speaker 1: real present, then reality is fractured into as many simultaneous
• 1:33:17
  Speaker 1: presents as there are observers moving at different speeds. That
• 1:33:21
  Speaker 1: is an uncomfortable place to end up. Eternalism takes the
• 1:33:25
  Speaker 1: opposite position and avoids the relativity problem entirely by conceding it.
• 1:33:31
  Speaker 1: All moments in time are equally real. Your birth, this moment,
• 1:33:36
  Speaker 1: your death. All of them exist with equal status in
• 1:33:40
  Speaker 1: the four-dimensional block. The words past and future describe relations
• 1:33:45
  Speaker 1: between events in the block, not differences in their degree
• 1:33:48
  Speaker 1: of reality. Just as New York and Tokyo are both
• 1:33:52
  Speaker 1: real despite being in different places, 1955 and 2025 are
• 1:33:58
  Speaker 1: both real despite being at different temporal coordinates. The relativity
• 1:34:04
  Speaker 1: problem dissolves immediately. there is no single present that needs defending.
• 1:34:09
  Speaker 1: Every observer carves out their own slice of the block,
• 1:34:13
  Speaker 1: and all slices are equally valid cuts through an equally
• 1:34:16
  Speaker 1: real structure. But eternalism creates a different set of problems.
• 1:34:21
  Speaker 1: If the future is already real, if your death exists
• 1:34:25
  Speaker 1: right now as a fixed region of space-time, in what
• 1:34:28
  Speaker 1: sense does anything genuinely happen? Change seems to become an illusion.
• 1:34:34
  Speaker 1: You don't change. Rather, Your earlier temporal stages differ from
• 1:34:38
  Speaker 1: your later ones. The way the left side of a
• 1:34:41
  Speaker 1: stick differs from the right side. Nothing becomes different. Everything
• 1:34:46
  Speaker 1: simply is different at different coordinates. This is a coherent position,
• 1:34:52
  Speaker 1: but it sits badly with ordinary experience and raises serious
• 1:34:55
  Speaker 1: questions about causation and agency. If the event of your
• 1:35:00
  Speaker 1: choosing what to eat tomorrow already exists at a fixed
• 1:35:03
  Speaker 1: location in the block, What does it mean to say
• 1:35:06
  Speaker 1: you are about to choose it? Eternalists have answers. They
• 1:35:10
  Speaker 1: say that causation is real. Later events in the block
• 1:35:13
  Speaker 1: depend on earlier events in a way that is perfectly objective.
• 1:35:18
  Speaker 1: Change is real. It just means temporal variation, rather than
• 1:35:22
  Speaker 1: something coming into being that wasn't there before. Free will
• 1:35:26
  Speaker 1: is harder. Most eternalists who take free will seriously argue
• 1:35:31
  Speaker 1: for compatibilism. The choice exists in the block, but it
• 1:35:34
  Speaker 1: is still genuinely yours because it flows from your own
• 1:35:37
  Speaker 1: mental states rather than external compulsion. Critics find this unsatisfying.
• 1:35:43
  Speaker 1: The debate over free will in a block universe has
• 1:35:46
  Speaker 1: been running for decades with no resolution in sight. The
• 1:35:50
  Speaker 1: growing block tries to find a middle path between these
• 1:35:53
  Speaker 1: two positions. On this view, the past and present are real.
• 1:35:57
  Speaker 1: They exist in the fixed block, but the future does
• 1:36:00
  Speaker 1: not yet exist at all. Time has a genuine direction
• 1:36:04
  Speaker 1: because the block is asymmetric. It grows. New moments come
• 1:36:08
  Speaker 1: into being at the leading edge, become part of the
• 1:36:11
  Speaker 1: permanent past, and the block extends further. This preserves the
• 1:36:15
  Speaker 1: reality of change in a strong sense. New events genuinely
• 1:36:19
  Speaker 1: come into existence rather than being merely revisited coordinates. It
• 1:36:23
  Speaker 1: also preserves the fixity of the past, which matches the
• 1:36:27
  Speaker 1: way causation seems to work. What has happened cannot be undone.
• 1:36:31
  Speaker 1: The difficulty is that the leading edge of the growing block,
• 1:36:34
  Speaker 1: the present moment, faces the same relativity problem as presentism.
• 1:36:39
  Speaker 1: Different observers moving at different velocities disagree about which events
• 1:36:43
  Speaker 1: are simultaneous, which means they disagree about where the leading
• 1:36:46
  Speaker 1: edge is. If the present is the edge of the block,
• 1:36:49
  Speaker 1: and that edge is observer-dependent, then the very boundary between
• 1:36:53
  Speaker 1: real and not yet real becomes a matter of perspective.
• 1:36:56
  Speaker 1: Some philosophers have argued that this can be handled with
• 1:36:59
  Speaker 1: careful analysis of what relativity actually claims. Others think it
• 1:37:03
  Speaker 1: is fatal to the growing bloc, in the same way
• 1:37:06
  Speaker 1: it is fatal to presentism. Three positions, decades of debate,
• 1:37:11
  Speaker 1: and not one of them undefeated, which tells you something
• 1:37:14
  Speaker 1: about how hard the underlying question actually is. Because neither
• 1:37:18
  Speaker 1: presentism nor eternalism fully works, a cottage industry of intermediate
• 1:37:24
  Speaker 1: positions has grown up around the problem. The debate between
• 1:37:27
  Speaker 1: those two camps has been running long enough that philosophers
• 1:37:31
  Speaker 1: have had time to notice where each one breaks down
• 1:37:34
  Speaker 1: and to start building alternatives. Some of those alternatives are
• 1:37:38
  Speaker 1: genuinely creative. None of them are clean. The most developed
• 1:37:43
  Speaker 1: intermediate position is the growing block universe. The philosopher C.D.
• 1:37:48
  Speaker 1: Broad proposed the core idea in the 1920s, and more
• 1:37:52
  Speaker 1: recently Michael Tooley has given it a rigorous formal treatment.
• 1:37:57
  Speaker 1: The picture is this. The universe is a four-dimensional block,
• 1:38:01
  Speaker 1: but not a completed one. New moments keep coming into
• 1:38:05
  Speaker 1: existence and becoming permanently fixed. The past is real and immutable.
• 1:38:10
  Speaker 1: Everything that has happened is now a permanent structure of
• 1:38:13
  Speaker 1: the block. But the future has not yet been added.
• 1:38:17
  Speaker 1: It simply doesn't exist anywhere. The block is growing moment
• 1:38:21
  Speaker 1: by moment in the direction we call forward. This is
• 1:38:25
  Speaker 1: attractive because it seems to honor two things simultaneously. Change
• 1:38:30
  Speaker 1: is genuine. New events really do come to be rather
• 1:38:34
  Speaker 1: than merely being revealed as the universe shuffles through a
• 1:38:37
  Speaker 1: finished structure. And the past is fixed. Nothing that has
• 1:38:42
  Speaker 1: already happened can be undone or altered. The sense that
• 1:38:46
  Speaker 1: the present moment is special The leading edge where reality
• 1:38:49
  Speaker 1: is actively being produced gets a real physical basis. Then
• 1:38:54
  Speaker 1: David Chalmers pointed out something strange. If every past moment
• 1:38:59
  Speaker 1: is equally real and you are located somewhere in that block,
• 1:39:03
  Speaker 1: there is no way from the inside to confirm that
• 1:39:06
  Speaker 1: you are actually at the leading edge. Every moment in
• 1:39:09
  Speaker 1: the entire block would feel exactly like the present. A
• 1:39:13
  Speaker 1: person sitting at a moment that was the present two
• 1:39:16
  Speaker 1: hours ago or two centuries ago would have an experience
• 1:39:19
  Speaker 1: indistinguishable from yours. They would feel the urgency of now,
• 1:39:25
  Speaker 1: remember the past, anticipate the future. From within any moment
• 1:39:30
  Speaker 1: in the block, the subjective experience is identical. So how
• 1:39:34
  Speaker 1: do you know you were at the growing edge rather
• 1:39:37
  Speaker 1: than embedded somewhere in the long fixed interior? You don't.
• 1:39:41
  Speaker 1: You cannot. The growing block doesn't just have a technical
• 1:39:45
  Speaker 1: problem with relativity. It has an epistemic problem that cuts
• 1:39:50
  Speaker 1: at the very feature it was trying to explain. The
• 1:39:53
  Speaker 1: specialness of now was supposed to be the point. The
• 1:39:57
  Speaker 1: theory cannot guarantee that now is actually special. The moving
• 1:40:02
  Speaker 1: spotlight theory takes a different approach to the same underlying intuition.
• 1:40:07
  Speaker 1: It accepts the full static block universe, all moments equally real,
• 1:40:12
  Speaker 1: the whole four-dimensional structure already complete. But it adds one thing,
• 1:40:17
  Speaker 1: an objective present that travels through the block. Think of
• 1:40:21
  Speaker 1: a searchlight sweeping across a fixed landscape. The landscape is
• 1:40:25
  Speaker 1: entirely real everywhere the light isn't shining. But there is
• 1:40:29
  Speaker 1: a fact about which patch is illuminated right now. That
• 1:40:33
  Speaker 1: illuminated patch is the present. Past and future moments exist,
• 1:40:38
  Speaker 1: but the present is the one the spotlight has reached.
• 1:40:41
  Speaker 1: This preserves the block universe's clean relationship with relativity's mathematics,
• 1:40:47
  Speaker 1: and gives the present a genuine physical distinction. The appeal
• 1:40:51
  Speaker 1: is obvious. It seems like a neat surgical fix. Keep
• 1:40:55
  Speaker 1: everything physicists like about the block universe and add back
• 1:40:58
  Speaker 1: the one feature that ordinary experience demands. The problem is
• 1:41:02
  Speaker 1: that the spotlight is doing a lot of work and
• 1:41:05
  Speaker 1: explaining almost nothing. What exactly is it? It isn't a
• 1:41:09
  Speaker 1: physical object, no particle, no field, no known form of energy.
• 1:41:14
  Speaker 1: How fast does it move? One second per second is
• 1:41:17
  Speaker 1: the obvious answer, but one second per second relative to what?
• 1:41:21
  Speaker 1: The block universe has no external time for the spotlight
• 1:41:25
  Speaker 1: to move through. In what medium does it travel? If
• 1:41:28
  Speaker 1: spacetime is the complete structure and the spotlight is moving
• 1:41:31
  Speaker 1: through spacetime, you need a second kind of time to
• 1:41:35
  Speaker 1: describe that motion. and then a third kind to describe
• 1:41:38
  Speaker 1: the motion of the spotlight through the second kind, and
• 1:41:41
  Speaker 1: so on without stopping. The moving spotlight doesn't solve the
• 1:41:44
  Speaker 1: problem of the present. It names the problem and calls
• 1:41:48
  Speaker 1: the name a solution. Philosophers are aware of all of this.
• 1:41:52
  Speaker 1: The positions keep multiplying precisely because each new proposal patches
• 1:41:56
  Speaker 1: one hole and opens another. There are variants of the
• 1:41:59
  Speaker 1: growing block that try to sidestep the charmer's problem by
• 1:42:02
  Speaker 1: adding observer indexing. There are versions of the moving spotlight
• 1:42:06
  Speaker 1: that try to build the second-time structure into the formalism explicitly.
• 1:42:11
  Speaker 1: Each attempt generates a new round of objections, a new literature,
• 1:42:15
  Speaker 1: new refinements. The gap between what physics describes and what
• 1:42:19
  Speaker 1: experience demands is deep enough that no framework spanning both
• 1:42:23
  Speaker 1: sides is held up without serious damage. The fact that
• 1:42:26
  Speaker 1: this many serious proposals exist, each solving some problems and
• 1:42:31
  Speaker 1: creating others, is a reliable sign that the right answer
• 1:42:34
  Speaker 1: hasn't been found yet. The most radical recent proposals don't
• 1:42:38
  Speaker 1: just question whether time is fundamental, they question whether space-time
• 1:42:42
  Speaker 1: itself is fundamental. This is a different kind of claim.
• 1:42:46
  Speaker 1: The arguments from Ravelli and Barbour take the equation seriously
• 1:42:50
  Speaker 1: and conclude that time drops out of the deep description
• 1:42:53
  Speaker 1: of the universe. The arguments from Smolin push back and
• 1:42:56
  Speaker 1: insist time must be restored. But a third line of work,
• 1:43:00
  Speaker 1: Growing out of string theory and black hole physics asks
• 1:43:04
  Speaker 1: a stranger question altogether. Not where time went, or how
• 1:43:08
  Speaker 1: to bring it back, but where space-time came from in
• 1:43:11
  Speaker 1: the first place. The starting point is black hole thermodynamics.
• 1:43:16
  Speaker 1: In the early 1970s, Jacob Bekenstein noticed something odd about
• 1:43:20
  Speaker 1: black holes. The entropy of a black hole, the measure
• 1:43:24
  Speaker 1: of its hidden disorder, is not proportional to its volume.
• 1:43:28
  Speaker 1: It is proportional to the area of its event horizon.
• 1:43:31
  Speaker 1: Stephen Hawking confirmed this and extended it. The amount of
• 1:43:35
  Speaker 1: information a region of space can contain is set by
• 1:43:38
  Speaker 1: the area of its boundary surface, not by the three-dimensional
• 1:43:42
  Speaker 1: space inside it. Think about what that means. If you
• 1:43:46
  Speaker 1: wanted to encode everything happening inside a sphere, you would
• 1:43:49
  Speaker 1: not need storage proportional to the volume of that sphere.
• 1:43:53
  Speaker 1: You would need storage proportional to its surface area, Everything
• 1:43:57
  Speaker 1: in the interior can be described by information written on
• 1:44:00
  Speaker 1: the skin of the region. This became known as the
• 1:44:03
  Speaker 1: holographic principle. And it was deeply strange. The obvious objection
• 1:44:10
  Speaker 1: is immediate. We live in three dimensions. We move through
• 1:44:14
  Speaker 1: three dimensions. We build furniture in three dimensions. How could
• 1:44:18
  Speaker 1: a three-dimensional reality be completely described by a two-dimensional surface?
• 1:44:24
  Speaker 1: That sounds like a trick. But physicists who worked through
• 1:44:28
  Speaker 1: the mathematics found it was not a trick. It was
• 1:44:31
  Speaker 1: pointing at something real about how information and geometry relate
• 1:44:35
  Speaker 1: to each other. The interior is, in a precise technical sense, redundant.
• 1:44:40
  Speaker 1: The boundary contains everything. Juan Maldacena sharpened this into something
• 1:44:45
  Speaker 1: exact in 1997. Working within string theory, he found a
• 1:44:50
  Speaker 1: specific case where the holographic principle becomes a full mathematical equivalence.
• 1:44:55
  Speaker 1: On one side, theory of gravity in a particular curved
• 1:44:58
  Speaker 1: space-time called anti-de Sitter space on the other side a
• 1:45:03
  Speaker 1: quantum field theory with no gravity at all living on
• 1:45:06
  Speaker 1: the boundary of that space-time these two theories are identical
• 1:45:11
  Speaker 1: every calculation you can do in one corresponds exactly to
• 1:45:14
  Speaker 1: a calculation in the other every particle in the bulk
• 1:45:17
  Speaker 1: gravitational theory corresponds to some configuration of the boundary quantum
• 1:45:22
  Speaker 1: field theory this is the ad set CFT correspondence and
• 1:45:26
  Speaker 1: it has been one of the most studied results in
• 1:45:28
  Speaker 1: theoretical physics in the last three decades. What it implies
• 1:45:32
  Speaker 1: for time is striking. The interior space-time, the full four-dimensional
• 1:45:37
  Speaker 1: region where gravity operates, where time is a dimension, is
• 1:45:41
  Speaker 1: encoded in a quantum system on the boundary. That boundary
• 1:45:45
  Speaker 1: theory has no gravity. It does have time in the
• 1:45:48
  Speaker 1: sense of quantum evolution. But the time that appears in
• 1:45:51
  Speaker 1: the interior space-time is not the same as the time
• 1:45:54
  Speaker 1: parameter on the boundary. The geometry of the bulk, including
• 1:45:58
  Speaker 1: the time dimension of the bulk, is a derived structure.
• 1:46:02
  Speaker 1: It is not put in by hand. It emerges from
• 1:46:04
  Speaker 1: the quantum physics of the boundary. If spacetime can emerge
• 1:46:08
  Speaker 1: from a quantum theory that does not itself live in
• 1:46:12
  Speaker 1: that spacetime, then spacetime is not fundamental. It is a consequence.
• 1:46:18
  Speaker 1: Something else is prior, and spacetime is what that something
• 1:46:22
  Speaker 1: else looks like from the inside. Mark van Ramsdonk pushed
• 1:46:26
  Speaker 1: this further around 2010. He found that the geometric structure
• 1:46:30
  Speaker 1: of the bulk spacetime, the way different regions are connected,
• 1:46:34
  Speaker 1: is directly tied to entanglement in the boundary quantum theory.
• 1:46:38
  Speaker 1: Disconnect two regions of the boundary by removing their entanglement
• 1:46:42
  Speaker 1: and the corresponding regions of the bulk geometry pull apart.
• 1:46:46
  Speaker 1: The smooth interior spacetime tears restore the entanglement and the
• 1:46:50
  Speaker 1: geometry reconnects. The connectivity of space is a function of
• 1:46:54
  Speaker 1: the entanglement structure of the quantum theory. Brian Swingle developed
• 1:46:58
  Speaker 1: this further using ideas from quantum information, showing that the
• 1:47:02
  Speaker 1: geometry emerging from the boundary theory has precisely the structure
• 1:47:06
  Speaker 1: you would expect if space were built from entanglement. The
• 1:47:10
  Speaker 1: slogan that physicists started using is short and strange. Space-time
• 1:47:15
  Speaker 1: from entanglement. Entanglement is the primary ingredient. Space is what
• 1:47:20
  Speaker 1: happens when quantum systems are correlated with each other in
• 1:47:23
  Speaker 1: the right way. Time is part of that derived geometric structure.
• 1:47:28
  Speaker 1: This connects to an idea sometimes called ER equals EPR.
• 1:47:32
  Speaker 1: Einstein-Rosen bridges are wormholes, tunnels through spacetime connecting two distant regions.
• 1:47:38
  Speaker 1: Einstein-Podolsky-Rosen pairs are entangled quantum particles. The claim, developed by
• 1:47:44
  Speaker 1: Maldacena and Leonard Susskind, is that these two things are
• 1:47:47
  Speaker 1: not merely analogous, They are the same thing described in
• 1:47:51
  Speaker 1: different languages. Entangle two quantum systems and you have created
• 1:47:55
  Speaker 1: a geometric connection between them. The wormhole and the entanglement
• 1:47:59
  Speaker 1: are one object. This picture is not proven. It works
• 1:48:02
  Speaker 1: clearly in the particular mathematical setting of Ad Sifit, which
• 1:48:06
  Speaker 1: uses a kind of curved spacetime that does not precisely
• 1:48:09
  Speaker 1: match the universe we live in. Extending it to cosmology,
• 1:48:13
  Speaker 1: to the actual universe with its positive cosmological constant, remains
• 1:48:17
  Speaker 1: an open technical problem. The programme has serious critics who
• 1:48:21
  Speaker 1: argue that the mathematical equivalence in ADSIFI does not prove
• 1:48:24
  Speaker 1: that space-time is emergent in any philosophically meaningful sense, only
• 1:48:28
  Speaker 1: that the same physics can be described in two different ways.
• 1:48:32
  Speaker 1: But the research programme is serious and active. Physicists are
• 1:48:36
  Speaker 1: building a picture in which quantum information entanglement, correlations between subsystems,
• 1:48:42
  Speaker 1: the flow of quantum data, is the foundational layer of reality.
• 1:48:47
  Speaker 1: Space-time is not the stage on which quantum physics performs.
• 1:48:50
  Speaker 1: Space-time is a structure that quantum physics assembles under the
• 1:48:54
  Speaker 1: right conditions from something that has no spatial or temporal
• 1:48:57
  Speaker 1: character of its own. If space-time is woven from entanglement,
• 1:49:02
  Speaker 1: then time was never a dimension the universe lived in.
• 1:49:05
  Speaker 1: It was always a consequence of the way quantum information
• 1:49:08
  Speaker 1: is organized. Whatever the physics says, you experience time. And
• 1:49:13
  Speaker 1: that experience is so immediate and so constant that it
• 1:49:16
  Speaker 1: seems absurd to dismiss it. You feel the past as
• 1:49:20
  Speaker 1: something gone. You feel the future as something unformed. You
• 1:49:24
  Speaker 1: feel right now as the only moment that is actually here.
• 1:49:28
  Speaker 1: That sense is not occasional or faint. It is the
• 1:49:31
  Speaker 1: background condition of every thought you have ever had. And
• 1:49:35
  Speaker 1: yet the physics described in the preceding sections treats that
• 1:49:38
  Speaker 1: feeling as somewhere between irrelevant and illusory. So the question
• 1:49:43
  Speaker 1: becomes urgent. What exactly is the brain doing when it
• 1:49:46
  Speaker 1: produces an experience this vivid and this universal? The first
• 1:49:51
  Speaker 1: thing to understand is that the brain is not recording
• 1:49:54
  Speaker 1: time the way a camera records light. It is building
• 1:49:57
  Speaker 1: a model. At every moment, your nervous system is taking
• 1:50:02
  Speaker 1: incoming sensory signals, comparing them to predictions it has already
• 1:50:06
  Speaker 1: made about the world, and updating its picture accordingly. The
• 1:50:10
  Speaker 1: present you experience is not a snapshot of reality arriving
• 1:50:14
  Speaker 1: cleanly at your senses. It is a construction assembled from
• 1:50:18
  Speaker 1: signals that travel at different speeds, processed by different neural pathways,
• 1:50:23
  Speaker 1: integrated over a window of several hundred milliseconds. What feels
• 1:50:28
  Speaker 1: like now is actually a brief interval of time stitched
• 1:50:31
  Speaker 1: together and presented to awareness as a single moment. The
• 1:50:36
  Speaker 1: scenes are invisible. but they are there. This matters because
• 1:50:40
  Speaker 1: it means the brain can get the sequence of events wrong.
• 1:50:43
  Speaker 1: Under the right conditions, it does exactly that. A stimulus
• 1:50:47
  Speaker 1: applied to your ankle reaches your brain slightly later than
• 1:50:50
  Speaker 1: a stimulus applied to your hand because the nerve pathway
• 1:50:53
  Speaker 1: is longer, yet you experience them as simultaneous. Researchers have
• 1:50:57
  Speaker 1: designed experiments where two events are reversed in presentation, and
• 1:51:01
  Speaker 1: subjects report experiencing them in the wrong order. The brain
• 1:51:05
  Speaker 1: retroactively adjusts its timeline, deciding after the fact which event
• 1:51:09
  Speaker 1: came first, and the conscious experience reflects that adjustment rather
• 1:51:14
  Speaker 1: than the physical sequence. You are not perceiving time directly,
• 1:51:18
  Speaker 1: you are receiving a narrative the brain has already edited.
• 1:51:22
  Speaker 1: The subjective rate of time is equally unstable. High emotional
• 1:51:27
  Speaker 1: arousal stretches the felt duration of an event. Fear makes
• 1:51:31
  Speaker 1: seconds feel like minutes. Boredom has the same effect. Deep
• 1:51:35
  Speaker 1: absorption in a task compresses time in the other direction
• 1:51:39
  Speaker 1: and hours vanish. Body temperature shifts alter the perceived speed
• 1:51:43
  Speaker 1: of an internal clock. Age changes it systematically over decades.
• 1:51:48
  Speaker 1: None of this is exotic. Everyone has lived it. But
• 1:51:52
  Speaker 1: taken together, these variations tell you something structural about the
• 1:51:56
  Speaker 1: nature of temporal experience. The flow of time that feels
• 1:52:00
  Speaker 1: so objective and so constant is actually a variable output
• 1:52:04
  Speaker 1: of neural processing sensitive to arousal, attention, chemistry, and context.
• 1:52:11
  Speaker 1: Then there is the question of the felt specialness of now.
• 1:52:14
  Speaker 1: Why does this moment feel different from the moment an
• 1:52:17
  Speaker 1: hour ago, which you now only remember, or the moment
• 1:52:21
  Speaker 1: an hour from now which you can only anticipate? Neuroscientists
• 1:52:25
  Speaker 1: describe the brain as fundamentally predictive. It models the immediate
• 1:52:30
  Speaker 1: future continuously, generates expectations, and compares incoming data against those expectations.
• 1:52:38
  Speaker 1: The present moment is the place where prediction meets outcome.
• 1:52:41
  Speaker 1: The sense of nowness may be exactly this, the ongoing
• 1:52:45
  Speaker 1: collision between the brain's model and the world's response. If
• 1:52:49
  Speaker 1: that account is right, the special quality of the present
• 1:52:52
  Speaker 1: is a feature of how the brain operates, not a
• 1:52:55
  Speaker 1: feature of physics. Here is where the argument splits in
• 1:52:58
  Speaker 1: exactly the way the larger debate splits. Ravelli and Barber
• 1:53:02
  Speaker 1: look at this neuroscience and find it entirely consistent with
• 1:53:06
  Speaker 1: their position. If the brain produces the experience of temporal
• 1:53:10
  Speaker 1: flow through its own internal processing, then flow is a
• 1:53:14
  Speaker 1: biological phenomenon. The universe underneath does not need to flow
• 1:53:18
  Speaker 1: for the brain to generate that sensation. A static network
• 1:53:22
  Speaker 1: of relations or a frozen block of space-time can give
• 1:53:25
  Speaker 1: rise to a brain that experiences time as rushing forward.
• 1:53:29
  Speaker 1: because the brain is a physical system that processes differences
• 1:53:33
  Speaker 1: between states and calls the processing now. The experience is
• 1:53:37
  Speaker 1: real as an experience. It has no counterpart in the
• 1:53:41
  Speaker 1: fundamental structure of the universe. Smolin looks at the same
• 1:53:45
  Speaker 1: neuroscience and draws a different conclusion. The brain is a
• 1:53:49
  Speaker 1: physical system embedded in a physical world. When it tracks change,
• 1:53:54
  Speaker 1: anticipates the future, and remembers the past, it is responding
• 1:53:58
  Speaker 1: to something. The question is whether that something is the
• 1:54:01
  Speaker 1: genuine structure of a time-bound universe or merely the local
• 1:54:06
  Speaker 1: statistics of a block. Smolin's view is that the brain's
• 1:54:10
  Speaker 1: sensitivity to time reflects something real about time, the way
• 1:54:14
  Speaker 1: the eye's sensitivity to light reflects something real about photons.
• 1:54:18
  Speaker 1: The fact that the brain constructs its experience does not
• 1:54:22
  Speaker 1: mean the experience is tracking nothing. it may mean the
• 1:54:26
  Speaker 1: brain has evolved to track a feature of reality that
• 1:54:29
  Speaker 1: physics has not yet described adequately. What makes this genuinely
• 1:54:33
  Speaker 1: difficult is that neither interpretation makes a wrong prediction about experience.
• 1:54:39
  Speaker 1: Both the block universe and a fundamentally flowing universe would
• 1:54:43
  Speaker 1: produce exactly the kind of brains we have, constructing exactly
• 1:54:47
  Speaker 1: the kind of temporal experiences we report. The neuroscience doesn't
• 1:54:52
  Speaker 1: break the tie It describes what the brain does without
• 1:54:55
  Speaker 1: settling whether the brain is faithfully representing deep structure or
• 1:54:59
  Speaker 1: creatively constructing something absent from the underlying physics. The one
• 1:55:03
  Speaker 1: piece of evidence everyone agrees on, that time is experienced,
• 1:55:07
  Speaker 1: turns out to be perfectly consistent with both sides of
• 1:55:10
  Speaker 1: the argument, which is part of why the argument never ends.
• 1:55:14
  Speaker 1: After more than a hundred years of this argument, it
• 1:55:16
  Speaker 1: is worth asking an uncomfortable question. Is this actually getting resolved?
• 1:55:21
  Speaker 1: Or are physicists just talking past each other? The honest
• 1:55:25
  Speaker 1: answer is that the disagreement is not shrinking. It may
• 1:55:29
  Speaker 1: be growing. New frameworks keep appearing. Loop quantum gravity, shape dynamics,
• 1:55:36
  Speaker 1: the Page-Wutters mechanism, emergent space-time from entanglement. And each one
• 1:55:41
  Speaker 1: deepens the argument rather than closing it. More physicists are
• 1:55:45
  Speaker 1: engaged with the question of time's fundamental nature than at
• 1:55:49
  Speaker 1: any previous point in history. and they disagree more sharply
• 1:55:53
  Speaker 1: than ever about what the answer is. There are three
• 1:55:56
  Speaker 1: separate reasons the debate refuses to settle. The first is experimental.
• 1:56:01
  Speaker 1: The block universe and presentism make identical predictions for every
• 1:56:06
  Speaker 1: measurement physics can currently perform. You cannot build an apparatus
• 1:56:10
  Speaker 1: that distinguishes between a universe where all moments coexist equally
• 1:56:15
  Speaker 1: and a universe where only the present is real. Both
• 1:56:18
  Speaker 1: pictures produce the same numbers. Similarly, Loop quantum gravity, which
• 1:56:23
  Speaker 1: treats time as emergent and ultimately absent from the foundations,
• 1:56:28
  Speaker 1: and any theory that preserves time as a genuine fundamental element,
• 1:56:32
  Speaker 1: both reproduce general relativity at the scales any existing telescope
• 1:56:36
  Speaker 1: or collider can probe. The disagreement lives at energies of
• 1:56:41
  Speaker 1: around 1019 GV, the Planck scale, where quantum gravitational effects
• 1:56:47
  Speaker 1: become significant. The most powerful particle accelerator ever built reaches
• 1:56:52
  Speaker 1: energies roughly 15 orders of magnitude below that threshold. The
• 1:56:56
  Speaker 1: gap is not a technical problem waiting on better engineering.
• 1:57:00
  Speaker 1: It is staggering. The second reason is that serious physicists
• 1:57:04
  Speaker 1: are looking at the same evidence and weighting it differently.
• 1:57:08
  Speaker 1: Ravelli and Barbour look at the Wheeler-DeWitt equation, the equation
• 1:57:12
  Speaker 1: that should describe the quantum state of the entire universe.
• 1:57:15
  Speaker 1: and they see time vanishing from the mathematics. For them,
• 1:57:19
  Speaker 1: this is decisive. The fundamental equation has no time parameter. Therefore,
• 1:57:25
  Speaker 1: time is not fundamental. The logic is clean. Smolin looks
• 1:57:29
  Speaker 1: at the same situation and sees a different lesson entirely.
• 1:57:33
  Speaker 1: Every experiment ever conducted in physics was performed by a
• 1:57:37
  Speaker 1: specific observer at a specific moment. No measurement has ever
• 1:57:41
  Speaker 1: been made outside of time. The doing of physics is
• 1:57:45
  Speaker 1: irreducibly temporal. You cannot subtract the present moment from the
• 1:57:49
  Speaker 1: act of observation without destroying the observation. Both of these
• 1:57:53
  Speaker 1: arguments are serious. Neither one is obviously wrong. They represent
• 1:57:58
  Speaker 1: a genuine difference in which facts are being treated as primary.
• 1:58:02
  Speaker 1: The third reason is that the question bleeds into philosophy
• 1:58:05
  Speaker 1: in ways that resist purely empirical resolution. What would it
• 1:58:09
  Speaker 1: even mean to confirm that time is real? If you
• 1:58:12
  Speaker 1: run an experiment and find a result, you found it now.
• 1:58:16
  Speaker 1: The finding is itself a temporal event. You cannot use
• 1:58:20
  Speaker 1: temporal evidence to prove that time is fundamental without some circularity.
• 1:58:25
  Speaker 1: And if you try to prove time is not fundamental,
• 1:58:28
  Speaker 1: you face a different version of the same trap. The
• 1:58:31
  Speaker 1: proof happens in time, and the people evaluating it are
• 1:58:35
  Speaker 1: using their experience of temporal sequence to follow the argument.
• 1:58:39
  Speaker 1: The question is not like asking whether the Higgs boson exists.
• 1:58:44
  Speaker 1: It cuts below the level at which experiments give clean verdicts.
• 1:58:48
  Speaker 1: What would actually move the needle? That the experimental frontier,
• 1:58:52
  Speaker 1: Gravitational wave observatories are beginning to probe the Stromfeld regime
• 1:58:57
  Speaker 1: around black hole mergers in ways that may eventually test
• 1:59:01
  Speaker 1: quantum corrections to general relativity. Hawking radiation, the slow thermal
• 1:59:07
  Speaker 1: emission predicted to bleed from black holes, has never been
• 1:59:10
  Speaker 1: directly observed, but future detectors may change that. Hawking radiation
• 1:59:16
  Speaker 1: carries quantum information in a way that connects directly to
• 1:59:19
  Speaker 1: questions about whether information is preserved across the event horizon,
• 1:59:24
  Speaker 1: which connects to whether time evolution is truly unitary, which
• 1:59:28
  Speaker 1: connects to the deepest questions about the nature of time.
• 1:59:32
  Speaker 1: These are not distant abstractions. They are concrete physics that
• 1:59:36
  Speaker 1: observational programs are currently chasing. On the laboratory side, experiments
• 1:59:41
  Speaker 1: testing the Page-Waters mechanism The proposal that time emerges from
• 1:59:46
  Speaker 1: quantum entanglement between subsystems have already been performed in small
• 1:59:51
  Speaker 1: quantum optical systems. The results are consistent with the mechanism.
• 1:59:55
  Speaker 1: Scaling these experiments up using larger and more complex quantum
• 2:00:00
  Speaker 1: systems could sharpen the evidence considerably. If time demonstrably emerges
• 2:00:05
  Speaker 1: from entanglement in a controlled laboratory setting, that is not
• 2:00:09
  Speaker 1: proof that time is absent from the fundamentals of the universe.
• 2:00:13
  Speaker 1: but it is strong circumstantial evidence that emergence is at
• 2:00:16
  Speaker 1: least possible. Cosmological observations of the very early universe offer
• 2:00:21
  Speaker 1: another angle. The cosmic microwave background encodes information about conditions
• 2:00:26
  Speaker 1: when the universe was less than 400,000 years old. Future
• 2:00:30
  Speaker 1: surveys will sharpen that picture dramatically. If quantum gravitational effects
• 2:00:35
  Speaker 1: left imprints on the early universe, signatures in the spectrum
• 2:00:39
  Speaker 1: of primordial fluctuations, then observations of the large-scale structure of
• 2:00:44
  Speaker 1: the cosmos might eventually constrain the theoretical landscape enough to
• 2:00:49
  Speaker 1: rule out whole families of proposals about the nature of time.
• 2:00:54
  Speaker 1: None of this will hand down a verdict quickly, but
• 2:00:57
  Speaker 1: it narrows the territory. What physicists are increasingly clear about
• 2:01:01
  Speaker 1: is that the resolution, when it comes, will require a
• 2:01:05
  Speaker 1: complete theory of quantum gravity, not an approximation, not a limit,
• 2:01:10
  Speaker 1: Not a regime where one theory borrows tools from the other.
• 2:01:14
  Speaker 1: A full theory. And building that theory has been the
• 2:01:17
  Speaker 1: central unsolved problem in fundamental physics for roughly a century.
• 2:01:22
  Speaker 1: Every serious proposal, string theory, loop quantum gravity, causal set theory,
• 2:01:29
  Speaker 1: causal dynamical triangulations, has partial successes and unresolved failures. None
• 2:01:36
  Speaker 1: has made a prediction unique to quantum gravity that was
• 2:01:39
  Speaker 1: then confirmed by experiment. The theories are not yet complete
• 2:01:43
  Speaker 1: enough to be tested in the ways that would matter most.
• 2:01:47
  Speaker 1: Some physicists argue the question itself may be poorly posed.
• 2:01:51
  Speaker 1: Asking whether time is real might be like asking whether
• 2:01:54
  Speaker 1: temperature is real. Temperature is not a fundamental property of
• 2:01:58
  Speaker 1: the universe. It is a statistical description of the collective
• 2:02:02
  Speaker 1: motion of enormous numbers of particles. but it is entirely real.
• 2:02:07
  Speaker 1: It blisters your hand. It governs chemical reactions. It shapes
• 2:02:12
  Speaker 1: weather systems. If time is like temperature, emergent but causally potent,
• 2:02:19
  Speaker 1: absent from the microscopic laws but deeply present at every
• 2:02:23
  Speaker 1: scale where human beings actually live, then the question of
• 2:02:27
  Speaker 1: its fundamental reality is less interesting than the question of
• 2:02:30
  Speaker 1: how it arises and what it is made of. That
• 2:02:34
  Speaker 1: reframing may be the most intellectually honest place to land.
• 2:02:38
  Speaker 1: Not where the time exists, but what kind of thing
• 2:02:41
  Speaker 1: it is and where it comes from. The question of
• 2:02:44
  Speaker 1: whether time is real, and the question of how to
• 2:02:47
  Speaker 1: build a complete theory of quantum gravity, are not two
• 2:02:50
  Speaker 1: separate questions. They are the same question written in different languages,
• 2:02:55
  Speaker 1: and neither one has been answered.