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Physicists Still Can_t Agree on What Time Actually Is · Night Science Stories

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  Speaker 1: Good evening, and welcome to The Sleepy Scientist. It's lovely
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• 0:13
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• 0:21
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• 0:47
  Speaker 1: out into the world. Both take about a second, though
• 0:51
  Speaker 1: after tonight you may have some questions about what that
• 0:53
  Speaker 1: actually means. If you're already with us, thank you. Your
• 0:56
  Speaker 1: company on these evenings means a great deal. Now, I'll
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  Speaker 1: do my best not to make tonight's subject so interesting
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  Speaker 1: that it keeps you awake. Although time is perhaps a
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  Speaker 1: slightly dangerous thing to dwell on while lying in bed,
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  Speaker 1: wondering how late it's got. Still, if sleep takes longer
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  Speaker 1: than planned, at least you'll have something more interesting to
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  Speaker 1: blame than the clock beside you. Because for something so
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  Speaker 1: utterly ordinary, time turns out to be remarkably difficult to
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  Speaker 1: explain the moment you ask what it actually is. Physics
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  Speaker 1: can measure it with extraordinary precision. We can detect the
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  Speaker 1: tiny differences that speed and gravity make to a ticking clock.
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  Speaker 1: We can calculate the age of the universe, predict where
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  Speaker 1: the planets will be centuries from now, and build clocks
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  Speaker 1: so accurate they'd barely lose a second over the entire
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  Speaker 1: history of the cosmos. And yet physicists still can't agree
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  Speaker 1: on what the thing they're measuring actually is. Is time
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  Speaker 1: a real part of the universe in its own right?
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  Speaker 1: or simply our way of describing change? Does the past
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  Speaker 1: still exist somewhere, tucked away? Is the future already out there, waiting?
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  Speaker 1: Does time genuinely flow at all, or is that just
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  Speaker 1: something our minds invent to keep us company? And at
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  Speaker 1: the very deepest level of physics, might time turn out
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  Speaker 1: not to be fundamental at all? So shall we begin?
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  Speaker 1: What actually is time, and how can physics measure it
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  Speaker 1: so precisely? while still not agreeing on what it really is.
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  Speaker 1: Time feels like one of the most obvious things in
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  Speaker 1: the world. A clock ticks. A calendar turns over. Morning
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  Speaker 1: becomes afternoon. Summer becomes autumn. Children grow older. Stars burn
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  Speaker 1: through their fuel. Unstable atoms decay, and planets continue along
• 2:47
  Speaker 1: their orbits. Everything around us seems to provide evidence that
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  Speaker 1: time is there. steadily accompanying change, and in practical terms,
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  Speaker 1: physics handles time extremely well. We can measure a fraction
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  Speaker 1: of a second with astonishing precision. We can predict when
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  Speaker 1: an eclipse will occur, calculate how long light takes to
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  Speaker 1: cross the distance between Earth and the Moon, determine the
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  Speaker 1: ages of ancient rocks, and compare clocks separated by thousands
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  Speaker 1: of kilometres. Modern atomic clocks are so stable that tiny
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  Speaker 1: differences caused by altitude and motion can be detected directly.
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  Speaker 1: So it might seem that physicists must have a very
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  Speaker 1: clear answer to the simple question, what is time? They
• 3:32
  Speaker 1: do not. That does not mean physics has somehow misplaced
• 3:35
  Speaker 1: an important piece of equipment. The problem is subtler. Physicists
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  Speaker 1: agree remarkably well about how to measure time and how
• 3:43
  Speaker 1: temporal measurements behave. The disagreement begins when we ask what
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  Speaker 1: those measurements are telling us about reality itself. There are
• 3:52
  Speaker 1: several questions hiding inside the ordinary word time, and everyday
• 3:56
  Speaker 1: conversation tends to bundle them together. One question concerns order.
• 4:02
  Speaker 1: Which event happened first, and which happened later? Another concerns duration.
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  Speaker 1: How much time passed between two events? Then there is
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  Speaker 1: the question of directions. Why can we remember yesterday but
• 4:15
  Speaker 1: not tomorrow? Why does a hot cup of tea cool
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  Speaker 1: down while a cold cup does not spontaneously become hot
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  Speaker 1: by borrowing heat from the room? There is also the
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  Speaker 1: idea of the present. Is there one universal moment called
• 4:29
  Speaker 1: now shared by the entire universe? And perhaps strangest of all,
• 4:34
  Speaker 1: there is the feeling that time flows. We experience a
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  Speaker 1: moving present, with the future becoming the present and the
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  Speaker 1: present becoming the past. But is that flow a physical
• 4:46
  Speaker 1: feature of the universe, or a feature of how conscious
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  Speaker 1: creatures experience change? These questions sound similar, but physics does
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  Speaker 1: not necessarily treat them as the same problem. A useful
• 4:58
  Speaker 1: distinction is between operational time and ontological time. Operational time
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  Speaker 1: is the practical side. It asks what procedure should be
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  Speaker 1: followed to measure an interval. Put a clock beside one event.
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  Speaker 1: Compare its reading with the reading at another event, or
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  Speaker 1: synchronise clocks according to a clearly defined method and compare
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  Speaker 1: what they record. Physics is exceptionally good at this. Ontological
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  Speaker 1: time asks a different question. What, if anything, exists beneath
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  Speaker 1: those measurements? Are clocks responding to a fundamental feature of
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  Speaker 1: reality called time? Is time simply a way of describing
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  Speaker 1: relationships among changing things? Is it part of space-time geometry?
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  Speaker 1: Or could it emerge from something deeper that does not
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  Speaker 1: look temporal at all? That is where agreement begins to loosen.
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  Speaker 1: Isaac Newton imagined time as an independent background. In Newtonian physics,
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  Speaker 1: time exists in its own right and advances uniformly whether
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  Speaker 1: anything interesting happens or not. The entire universe can, in principle,
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  Speaker 1: share one universal time. Albert Einstein changed that picture dramatically.
• 6:11
  Speaker 1: In relativity, the amount of time measured between events can
• 6:15
  Speaker 1: depend on motion and gravity. Two clocks that begin together,
• 6:19
  Speaker 1: follow different paths, and later reunite do not always have
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  Speaker 1: to agree on how much time has elapsed. This is
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  Speaker 1: not because one of them is faulty. The difference can
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  Speaker 1: be a genuine physical consequence of the paths they followed
• 6:32
  Speaker 1: through spacetime. That alone makes time rather less straightforward than
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  Speaker 1: an invisible cosmic metronome ticking identically everywhere. Quantum mechanics introduces
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  Speaker 1: another difficulty. In its usual formulation, time often appears as
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  Speaker 1: an external parameter. A quantum state changes with time, but
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  Speaker 1: time itself is not normally treated in exactly the same
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  Speaker 1: way as properties such as position or momentum. The theory
• 6:59
  Speaker 1: tells us how quantum systems evolve as time passes without
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  Speaker 1: necessarily explaining what time fundamentally is. Then quantum gravity complicates
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  Speaker 1: things further. Any theory that attempts to combine quantum mechanics
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  Speaker 1: with Einstein's theory of gravitation must somehow deal with spacetime
• 7:17
  Speaker 1: itself becoming part of the quantum problem. In some approaches,
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  Speaker 1: the familiar time variable becomes difficult to identify at the
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  Speaker 1: most fundamental level. This has led some physicists to investigate
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  Speaker 1: whether time might be emergent rather than basic. None of
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  Speaker 1: this means that physicists disagree about whether clocks work. They do.
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  Speaker 1: Nor does it mean that relativity, quantum mechanics or astronomy
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  Speaker 1: are unable to make precise predictions involving time. Quite the opposite.
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  Speaker 1: These are among the most successful theories ever constructed. The
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  Speaker 1: uncertainty lies in what those successes imply about the underlying
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  Speaker 1: nature of reality. There is also an important difference between
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  Speaker 1: physical time and our personal experience of it. A tedious
• 8:01
  Speaker 1: meeting may feel much longer than an enjoyable afternoon, but
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  Speaker 1: a reliable clock does not become sympathetic and speed up
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  Speaker 1: to help. Human beings experience duration psychologically, and our perception
• 8:14
  Speaker 1: of time can change with attention, memory, emotion and age.
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  Speaker 1: The physical equations do not contain a variable describing the
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  Speaker 1: sensation that the present moment is moving forwards. They contain
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  Speaker 1: measurable quantities and relationships. This distinction matters because the feeling
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  Speaker 1: of passage is extraordinarily persuasive. It is difficult to imagine
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  Speaker 1: time without imagining something flowing from future to past through
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  Speaker 1: a moving now. Yet the existence of that experience does
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  Speaker 1: not by itself establish that the universe contains an objectively
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  Speaker 1: flowing present. Physics therefore leaves us with several notions that
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  Speaker 1: may belong together, or may not. There is duration, measured
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  Speaker 1: by clocks. There is causal order, describing which events can
• 9:03
  Speaker 1: influence which others. There is the thermodynamic direction associated with
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  Speaker 1: increasing entropy. There is the geometry of spacetime. There is
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  Speaker 1: the psychological experience of passage. And there is the deeper
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  Speaker 1: question of whether time is fundamental at all. Calling all
• 9:19
  Speaker 1: of these things time is convenient, but convenience can hide
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  Speaker 1: important differences. So the real scientific disagreement is not whether
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  Speaker 1: events occur at different clock readings. That part is straightforward
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  Speaker 1: to measure. The deeper disagreement concerns what structure beneath those
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  Speaker 1: readings deserves to be called time in the first place.
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  Speaker 1: And that is what makes such an ordinary word one
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  Speaker 1: of the most persistent questions in modern physics. For a
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  Speaker 1: long time, the most natural way to imagine time was
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  Speaker 1: also the simplest. There was one time for everything. A
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  Speaker 1: clock in London, a falling stone in Italy, a planet
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  Speaker 1: moving around the sun, and a distance star shining somewhere
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  Speaker 1: in the night sky could all, in principle, be placed
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  Speaker 1: against the same invisible background. Events might happen in different places,
• 10:10
  Speaker 1: but there would still be one correct answer to the
• 10:12
  Speaker 1: question of when they happened. That picture became deeply embedded
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  Speaker 1: in classical physics through the work of Isaac Newton. In 1687,
• 10:22
  Speaker 1: Newton published his great work on mechanics and gravitation, commonly
• 10:27
  Speaker 1: known by its shortened Latin title, The Principia. In it,
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  Speaker 1: he made an important distinction between the time measured by
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  Speaker 1: ordinary clocks and what he called absolute, true and mathematical time.
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  Speaker 1: Newton's idea was that true time progressed uniformly by its
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  Speaker 1: own nature. It did not depend on whether planets were moving,
• 10:47
  Speaker 1: clocks were ticking, pendulums were swinging, or human beings were
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  Speaker 1: around to count anything at all. Even in a completely
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  Speaker 1: empty universe, Newtonian time would still pass. This is an
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  Speaker 1: extraordinarily intuitive picture. Imagine a perfect clock somewhere outside everything else,
• 11:05
  Speaker 1: steadily keeping track of the entire universe. Every event could
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  Speaker 1: be assigned a place on that universal timeline. If two
• 11:13
  Speaker 1: distant stars exploded at the same moment according to this
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  Speaker 1: ideal cosmic clock, then they simply happened simultaneously. No observer
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  Speaker 1: was required to make that simultaneity real. Time was already there.
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  Speaker 1: This is what is meant by an absolute conception of time.
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  Speaker 1: Time acts rather like an invisible stage on which physical
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  Speaker 1: events occur. The actors may move around, collide, appear and disappear,
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  Speaker 1: but the stage itself does not depend on them. In
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  Speaker 1: philosophical language, this is closely connected to a substantial view
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  Speaker 1: of time. The basic idea is that time has some
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  Speaker 1: form of existence independently of the events occurring within it.
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  Speaker 1: Temporal positions and intervals are not merely ways of comparing
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  Speaker 1: physical changes. They belong to the structure of reality itself.
• 12:04
  Speaker 1: But even in Newton's own era, this was not the
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  Speaker 1: only possible way to think. Gottfried Wilhelm Leibniz took a
• 12:11
  Speaker 1: very different position. Leibniz was one of the great mathematicians
• 12:16
  Speaker 1: and philosophers of the late 17th and early 18th centuries,
• 12:20
  Speaker 1: and he resisted the idea that space and time needed
• 12:23
  Speaker 1: to exist as invisible containers surrounding the physical world. For Leibniz,
• 12:28
  Speaker 1: time could instead be understood relationally. On this view, saying
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  Speaker 1: that one event happened before another does not require both
• 12:35
  Speaker 1: events to sit inside a separate substance called time. The
• 12:39
  Speaker 1: temporal fact lies in the relationship between the events themselves.
• 12:44
  Speaker 1: One thing changes, then another thing changes. One event precedes another.
• 12:49
  Speaker 1: One process lasts longer than another. Time, in this picture,
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  Speaker 1: is not an independent river carrying everything forward. It is
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  Speaker 1: a way of describing the order and relations among changes.
• 13:02
  Speaker 1: This is known as the relational view of time. The
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  Speaker 1: difference sounds abstract, but the disagreement is quite sharp. Suppose
• 13:10
  Speaker 1: every object in the universe suddenly disappeared. For Newton, time
• 13:15
  Speaker 1: could continue passing in the empty universe. For a strict
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  Speaker 1: relational view, the question becomes much more awkward. If there
• 13:23
  Speaker 1: are no events, no changes, and no physical things standing
• 13:27
  Speaker 1: in temporal relations to one another, what would it mean
• 13:30
  Speaker 1: to say that ten minutes had passed? There would be
• 13:33
  Speaker 1: no clock. More importantly, there might be nothing for that
• 13:37
  Speaker 1: supposed passage to consist of. Newton believed that absolute structures
• 13:42
  Speaker 1: were necessary to make proper sense of motion, particularly accelerated motion.
• 13:48
  Speaker 1: His famous bucket argument was designed mainly around space and rotation,
• 13:53
  Speaker 1: but it reveals the broader structure of his thinking. Imagine
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  Speaker 1: a bucket partly filled with water and suspended by a rope.
• 14:00
  Speaker 1: Twist the rope tightly and release it. At first the
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  Speaker 1: bucket begins to rotate while the water remains relatively still.
• 14:08
  Speaker 1: The surface of the water stays nearly flat. Gradually, friction
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  Speaker 1: between the bucket and the water causes the water to
• 14:15
  Speaker 1: rotate as well. As its rotation increases, the surface of
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  Speaker 1: the water becomes curved, rising slightly at the edges and
• 14:23
  Speaker 1: sinking towards the centre. Eventually, the bucket and water may
• 14:27
  Speaker 1: rotate together. Their motion relative to each other is now small,
• 14:31
  Speaker 1: yet the water still forms a curved surface. Newton argued
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  Speaker 1: that this showed something important. The effect could not be
• 14:39
  Speaker 1: explained simply by the motion of the water relative to
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  Speaker 1: the bucket. The water's rotation appeared to have meaning relative
• 14:46
  Speaker 1: to something deeper. Newton interpreted this as evidence for absolute space.
• 14:51
  Speaker 1: Acceleration and rotation were not merely relationships between nearby objects.
• 14:56
  Speaker 1: They could be defined with respect to an underlying spatial structure.
• 15:01
  Speaker 1: And although the bucket argument concerned rotation and space more
• 15:05
  Speaker 1: directly than time … Newton treated both space and time
• 15:09
  Speaker 1: as parts of an absolute framework within which physical motion
• 15:13
  Speaker 1: could be described. The relational challenge did not disappear. Much later,
• 15:18
  Speaker 1: during the 19th century, the Austrian physicist and philosopher Ernst
• 15:23
  Speaker 1: Mach criticised the idea that motion needed to be defined
• 15:27
  Speaker 1: against an invisible absolute background. Mack argued that the behaviour
• 15:32
  Speaker 1: of an object might instead be understood through its relationships
• 15:35
  Speaker 1: with the matter distributed throughout the universe. In the case
• 15:39
  Speaker 1: of the rotating water, perhaps the important relationship was not
• 15:43
  Speaker 1: simply between the water and the bucket, but between the
• 15:46
  Speaker 1: water and all the distant stars and matter surrounding it.
• 15:50
  Speaker 1: This line of thought later became associated with what is
• 15:53
  Speaker 1: often called Mack's principle, although the phrase covers several related
• 15:58
  Speaker 1: ideas rather than one single precise law. The central theme
• 16:02
  Speaker 1: was relational. Perhaps motion should ultimately be defined through relationships
• 16:07
  Speaker 1: among physical things, rather than against an unseen stage existing
• 16:11
  Speaker 1: independently of them. These arguments might make classical physics sound
• 16:15
  Speaker 1: deeply uncertain. In practice, it was anything but. Newtonian mechanics
• 16:21
  Speaker 1: became one of the most successful scientific frameworks ever developed.
• 16:26
  Speaker 1: For centuries it described falling objects, cannonballs, pendulums, planetary orbits
• 16:32
  Speaker 1: and engineering systems with remarkable accuracy. The reason is that
• 16:37
  Speaker 1: Newton's picture of time works extremely well under ordinary conditions.
• 16:42
  Speaker 1: Relativistic differences in elapsed time become significant when speeds approach
• 16:47
  Speaker 1: the speed of light or when gravitational conditions differ strongly.
• 16:51
  Speaker 1: A person walking through a room is moving far too
• 16:55
  Speaker 1: slowly for relativistic time dilation to matter in any noticeable
• 17:00
  Speaker 1: everyday sense. Even an aircraft travelling hundreds of kilometres per
• 17:04
  Speaker 1: hour is still moving at only a tiny fraction of
• 17:06
  Speaker 1: the speed of light. Likewise, the difference in gravitational time
• 17:10
  Speaker 1: between clocks sitting on different floors of an ordinary building
• 17:13
  Speaker 1: is real, but fantastically small. For almost everything that 17th,
• 17:19
  Speaker 1: 18th and 19th century physicists were trying to calculate, one
• 17:23
  Speaker 1: universal time was an excellent approximation. That practical success is
• 17:28
  Speaker 1: worth remembering. Newton did not create a foolish picture that
• 17:32
  Speaker 1: later physicists simply threw away. He created a framework of
• 17:36
  Speaker 1: extraordinary power whose assumptions become incomplete only when nature is
• 17:41
  Speaker 1: examined under conditions far beyond ordinary experience. And the disagreement
• 17:46
  Speaker 1: between Newton and Leibniz reveals something important. The question of
• 17:50
  Speaker 1: what time actually is did not begin with quantum mechanics.
• 17:54
  Speaker 1: It did not begin with black holes. atomic clocks, or
• 17:58
  Speaker 1: attempts to quantise gravity. Even within classical physics, there are
• 18:02
  Speaker 1: already two very different possibilities. Time might be an independent
• 18:07
  Speaker 1: feature of reality, existing whether anything changes or not. Or
• 18:11
  Speaker 1: time might arise from relationships among physical events, with no
• 18:16
  Speaker 1: need for a separate cosmic clock ticking behind them. Physics
• 18:20
  Speaker 1: had not yet reached relativity, but the basic argument was
• 18:24
  Speaker 1: already there. A clock seems like a very simple object.
• 18:28
  Speaker 1: It gives a number, perhaps accompanied by a ticking sound,
• 18:32
  Speaker 1: and tells us what time it is. But physically, a
• 18:36
  Speaker 1: clock is not detecting an invisible substance drifting past it.
• 18:40
  Speaker 1: There is no known instrument inside a watch that catches
• 18:43
  Speaker 1: tiny pieces of time as they move by. Instead, every
• 18:47
  Speaker 1: clock works by following some physical process that changes in
• 18:50
  Speaker 1: a sufficiently regular and predictable way. In other words, The
• 18:55
  Speaker 1: clock measures one kind of change by comparing it with another.
• 19:00
  Speaker 1: For much of human history, the most obvious repeating processes
• 19:03
  Speaker 1: were astronomical. The rotation of Earth produced the cycle of
• 19:08
  Speaker 1: day and night. The motion of the Moon helped organise months.
• 19:12
  Speaker 1: Earth's orbit around the Sun established the year. These cycles
• 19:16
  Speaker 1: were not perfectly simple, but they were regular enough to
• 19:19
  Speaker 1: become the foundations of early timekeeping. Later came more controlled
• 19:24
  Speaker 1: physical systems. A pendulum clock uses the repeated swing of
• 19:28
  Speaker 1: a pendulum. A mechanical watch uses an oscillating balance wheel.
• 19:33
  Speaker 1: A quartz clock relies on the remarkably regular vibration of
• 19:36
  Speaker 1: a quartz crystal when electrical energy is applied to it.
• 19:40
  Speaker 1: The details differ, but the principle is the same. Find
• 19:44
  Speaker 1: something that changes repeatedly in a reliable way. Count the changes.
• 19:50
  Speaker 1: Use that count to assign a numerical interval between events.
• 19:54
  Speaker 1: A clock, therefore, does not need to know what time is.
• 19:57
  Speaker 1: It only needs something stable enough to act as a reference.
• 20:01
  Speaker 1: This becomes especially interesting when we reach modern atomic clocks.
• 20:07
  Speaker 1: The international definition of the second is based on the
• 20:10
  Speaker 1: behaviour of atoms of caesium-133. More specifically, it is tied
• 20:16
  Speaker 1: to the unperturbed ground-state hyperphene transition of the caesium-133 atom.
• 20:23
  Speaker 1: That phrase sounds rather formidable, but the basic idea is manageable.
• 20:28
  Speaker 1: An atom can occupy particular quantum states. Cesium-133 has two
• 20:34
  Speaker 1: extremely closely spaced energy states associated with the interaction between
• 20:39
  Speaker 1: the magnetic properties of its nucleus and its electrons. When
• 20:43
  Speaker 1: the atom changes between these states, it interacts with electromagnetic
• 20:47
  Speaker 1: radiation of an exceptionally precise frequency. One second is defined
• 20:52
  Speaker 1: using exactly 9,192,631,770 periods of that radiation. That number is
• 21:03
  Speaker 1: not an estimate. It is part of the definition. In effect,
• 21:07
  Speaker 1: physicists replace the slightly irregular turning of Earth with a
• 21:11
  Speaker 1: much more stable quantum process. The modern second is therefore
• 21:16
  Speaker 1: not defined by waiting for the sun to cross the sky.
• 21:19
  Speaker 1: It is defined through a reproducible feature of atomic physics.
• 21:23
  Speaker 1: The International Bureau of Weights and Measures helps maintain the
• 21:27
  Speaker 1: international system of measurement that includes this definition. while organisations
• 21:32
  Speaker 1: such as the National Institute of Standards and Technology in
• 21:35
  Speaker 1: the United States operate some of the world's most precise
• 21:38
  Speaker 1: clocks and contribute to comparisons of time and frequency standards.
• 21:44
  Speaker 1: In Britain, an important part of this history came from
• 21:47
  Speaker 1: the National Physical Laboratory. During the 1950s, physicist Louis Essen
• 21:52
  Speaker 1: worked with Jack Parry to develop the first cesium atomic
• 21:55
  Speaker 1: clock sufficiently accurate and stable to become a practical time standard.
• 22:00
  Speaker 1: The achievement helped demonstrate that atomic transitions could provide something
• 22:04
  Speaker 1: far better than astronomical rotation for defining precise intervals, and
• 22:09
  Speaker 1: atomic clocks have improved enormously since then. A cesium clock
• 22:14
  Speaker 1: can be imagined as a device that carefully adjusts electromagnetic
• 22:18
  Speaker 1: radiation until it matches the natural resonance of cesium atoms.
• 22:23
  Speaker 1: When the frequency is correct, the atoms respond in a
• 22:26
  Speaker 1: characteristic way. The clock then uses that frequency as an
• 22:30
  Speaker 1: extraordinarily stable reference. It is a little like tuning an instrument,
• 22:35
  Speaker 1: except the instrument is a collection of atoms and the
• 22:37
  Speaker 1: note is so precisely defined by quantum mechanics that it
• 22:41
  Speaker 1: can be reproduced in laboratories separated by continents. Modern optical
• 22:46
  Speaker 1: atomic clocks push the idea even further. Instead of using
• 22:50
  Speaker 1: microwave frequency transitions such as those in cesium, Optical clocks
• 22:54
  Speaker 1: use transitions associated with visible or near-visible electromagnetic frequencies. These
• 23:00
  Speaker 1: oscillate vastly more rapidly. Because there are so many more
• 23:04
  Speaker 1: cycles available to count within each second, optical clocks can
• 23:08
  Speaker 1: compare frequencies with extraordinary precision. Laboratories working with atoms such
• 23:14
  Speaker 1: as strontium, ytterbium and aluminium ions have produced clocks capable
• 23:19
  Speaker 1: of detecting differences in elapsed time so small The changes
• 23:23
  Speaker 1: in gravitational potential across modest height differences can become measurable.
• 23:28
  Speaker 1: A clock placed slightly higher in Earth's gravitational field can
• 23:32
  Speaker 1: tick at a slightly different rate from one below it.
• 23:35
  Speaker 1: The difference is tiny, but modern clocks can detect it.
• 23:39
  Speaker 1: That fact reveals something deeper. Improving a clock does not
• 23:44
  Speaker 1: merely improve our ability to organise appointments. It allows us
• 23:48
  Speaker 1: to test the structure of spacetime itself. According to relativity,
• 23:53
  Speaker 1: a clock measures proper time along its path through spacetime.
• 23:57
  Speaker 1: Two clocks that take different paths, move at different speeds,
• 24:01
  Speaker 1: or experience different gravitational conditions can accumulate different amounts of
• 24:06
  Speaker 1: proper time. A sufficiently accurate clock can reveal that difference directly.
• 24:11
  Speaker 1: Yet there is an important limit to what increasing precision
• 24:14
  Speaker 1: can tell us. Suppose physicists build a clock a thousand
• 24:18
  Speaker 1: times more accurate than anything available today. then suppose they
• 24:23
  Speaker 1: improve it again, and again. Eventually they might compare intervals
• 24:28
  Speaker 1: with almost unimaginable precision. But none of that by itself
• 24:32
  Speaker 1: tells us whether time is fundamental. A perfect clock would
• 24:36
  Speaker 1: tell us how much proper time had elapsed along a
• 24:39
  Speaker 1: particular path. It would not automatically tell us whether time
• 24:43
  Speaker 1: exists independently of physical change, whether it emerges from deeper relationships,
• 24:49
  Speaker 1: or whether it appears only as an effective concept in
• 24:52
  Speaker 1: certain physical regimes. Measurement and interpretation are not the same thing.
• 24:58
  Speaker 1: This brings us back to a surprisingly subtle point. A
• 25:01
  Speaker 1: clock defines an interval by counting regular physical changes, but
• 25:05
  Speaker 1: we usually describe those changes by saying that they occur
• 25:08
  Speaker 1: through time. That can sound slightly circular. We use change
• 25:13
  Speaker 1: to measure time, while using time to describe change. Physics
• 25:17
  Speaker 1: avoids practical confusion by defining precise operational procedures. Choose a
• 25:23
  Speaker 1: reliable process. Count its repetitions. Compare one physical system against another.
• 25:29
  Speaker 1: The procedure works beautifully. The deeper question remains. When two
• 25:34
  Speaker 1: events are separated by a particular number of atomic oscillations,
• 25:39
  Speaker 1: what exactly is that interval telling us about reality? Perhaps
• 25:43
  Speaker 1: time is something fundamental that clocks reveal. Perhaps clocks are
• 25:47
  Speaker 1: simply comparing one change with another. Perhaps the concept of
• 25:52
  Speaker 1: time emerges only when physical relationships become organised in a
• 25:56
  Speaker 1: certain way. The clock itself does not answer. It simply
• 26:01
  Speaker 1: keeps counting. For more than two centuries, Newton's picture of
• 26:06
  Speaker 1: time worked so well that there seemed little reason to
• 26:09
  Speaker 1: doubt it. There might be disagreements about what time ultimately was,
• 26:14
  Speaker 1: But for practical physics, the assumption seemed safe. Somewhere beneath
• 26:18
  Speaker 1: all the motion and change in the universe, there was
• 26:21
  Speaker 1: one common temporal rhythm. Different clocks might be inaccurate, but
• 26:25
  Speaker 1: in principle, there was a single correct time that every
• 26:29
  Speaker 1: perfect clock ought to reveal. Then, in 1905, Albert Einstein
• 26:34
  Speaker 1: removed that universal clock. His special theory of relativity began
• 26:39
  Speaker 1: with two principles that sound fairly modest. The first... was
• 26:43
  Speaker 1: that the laws of physics should take the same form
• 26:46
  Speaker 1: for all inertial observers. An inertial observer is simply someone
• 26:50
  Speaker 1: moving at constant velocity without acceleration. There should be no
• 26:55
  Speaker 1: special state of uniform motion that counts as truly stationary.
• 26:59
  Speaker 1: The second principle concerned light. Einstein proposed that every inertial
• 27:04
  Speaker 1: observer measuring light travelling through a vacuum would obtain the
• 27:08
  Speaker 1: same vacuum speed of light, regardless of the motion of
• 27:11
  Speaker 1: the source or the observer. That speed is roughly 300,000
• 27:16
  Speaker 1: kilometers per second. This creates an immediate problem for ordinary intuition.
• 27:22
  Speaker 1: Suppose you're standing beside a railway line and a train
• 27:25
  Speaker 1: passes you. If someone inside the train throws a ball forwards,
• 27:29
  Speaker 1: you would normally combine the speed of the train with
• 27:31
  Speaker 1: the speed of the ball to determine how quickly the
• 27:34
  Speaker 1: ball is moving relative to you. That is how everyday
• 27:38
  Speaker 1: velocities behave. Light refuses to cooperate. If someone aboard an
• 27:43
  Speaker 1: extremely fast spacecraft shines a beam forwards, an observer on
• 27:47
  Speaker 1: the spacecraft measures the same vacuum speed of light as
• 27:51
  Speaker 1: an observer watching the spacecraft fly past. The observers do
• 27:55
  Speaker 1: not reconcile their measurements by giving light different speeds. Instead,
• 28:00
  Speaker 1: they disagree about space and time. This is the extraordinary
• 28:04
  Speaker 1: move at the heart of special relativity. Distance and duration
• 28:09
  Speaker 1: are no longer fixed quantities that everyone must share. One
• 28:13
  Speaker 1: of the clearest consequences is the relativity of simultaneity. Imagine
• 28:18
  Speaker 1: a very long train moving rapidly along a straight track.
• 28:22
  Speaker 1: Two flashes of light occur, one near the front of
• 28:24
  Speaker 1: the train and another near the rear. An observer standing
• 28:29
  Speaker 1: exactly midway between those locations on the ground can arrange
• 28:33
  Speaker 1: for the two flashes to reach them together. After taking
• 28:37
  Speaker 1: the travel time of the light into account, that observer
• 28:40
  Speaker 1: concludes that the flashes occurred simultaneously. Now imagine another observer
• 28:45
  Speaker 1: sitting at the midpoint of the moving train. While the
• 28:49
  Speaker 1: light is travelling, this observer is moving towards one flash
• 28:52
  Speaker 1: and away from the other. Since both observers must still
• 28:56
  Speaker 1: measure light travelling at the same vacuum speed, the observer
• 28:59
  Speaker 1: on the train cannot explain the different arrival pattern by
• 29:03
  Speaker 1: saying one light beam simply travelled faster. Instead, The observer
• 29:08
  Speaker 1: concludes that the two flashes did not occur at the
• 29:11
  Speaker 1: same time. This is not a trick caused by slow communication.
• 29:16
  Speaker 1: It is tempting to imagine that the disagreement disappears once
• 29:19
  Speaker 1: everyone corrects. For how long the light took to reach them,
• 29:23
  Speaker 1: it does not. After those corrections are made, observers moving
• 29:28
  Speaker 1: relative to each other can still disagree about whether spatially
• 29:31
  Speaker 1: separated events were simultaneous. there is simply no universal definition
• 29:36
  Speaker 1: of distant simultaneity that all inertial observers share. That is
• 29:41
  Speaker 1: a profound departure from Newton. If there is no observer-independent
• 29:46
  Speaker 1: agreement about which distant events are happening at the same moment,
• 29:50
  Speaker 1: then there is no obvious way to slice the entire
• 29:52
  Speaker 1: universe into a unique sequence of universal nows. The mathematics
• 29:58
  Speaker 1: connecting these different measurements had important roots in the work
• 30:02
  Speaker 1: of the Dutch physicist Hendrik Lorentz. The Lorentz transformations describe
• 30:07
  Speaker 1: how measurements of position and time made by one inertial
• 30:11
  Speaker 1: observer are related to measurements made by another observer moving
• 30:16
  Speaker 1: at constant velocity relative to the first. They mix space
• 30:20
  Speaker 1: and time together. A distance measured by one observer can
• 30:24
  Speaker 1: be related partly to a distance and partly to a
• 30:27
  Speaker 1: time measured by another. Similarly, a temporal interval in one
• 30:32
  Speaker 1: reference frame cannot always be considered separately from spatial separation.
• 30:37
  Speaker 1: One famous consequence is time dilation. Imagine two identical clocks.
• 30:43
  Speaker 1: One remains with you, while the other moves past at
• 30:46
  Speaker 1: high speed. According to special relativity, the moving clock accumulates
• 30:51
  Speaker 1: less elapsed time between appropriately compared events, than clocks at
• 30:55
  Speaker 1: rest in your inertial frame. This is not because the
• 30:59
  Speaker 1: moving clock has become damaged, its gears have not slowed
• 31:02
  Speaker 1: because of friction, and its atoms are not somehow becoming lazy.
• 31:07
  Speaker 1: Every suitable physical process associated with that moving system shows
• 31:11
  Speaker 1: the same relativistic behaviour. A useful way to see why
• 31:16
  Speaker 1: involves an imaginary light clock. Picture two mirrors facing each other,
• 31:21
  Speaker 1: one above the other, with a pulse of light bouncing
• 31:24
  Speaker 1: between them. Each journey from one mirror to the other
• 31:27
  Speaker 1: acts like a tick. For someone travelling alongside the clock,
• 31:31
  Speaker 1: the light simply moves vertically up and down. Now imagine
• 31:35
  Speaker 1: watching the same clock move rapidly sideways. During the time
• 31:40
  Speaker 1: in which the light travels between the mirrors, the entire
• 31:43
  Speaker 1: apparatus moves sideways as well. From your perspective, the light
• 31:47
  Speaker 1: therefore follows a diagonal path. That diagonal path is longer
• 31:52
  Speaker 1: than the vertical path measured by someone travelling with the clock.
• 31:56
  Speaker 1: Yet both observers must measure the same vacuum speed of light.
• 32:00
  Speaker 1: If the light travels farther without travelling faster, more time
• 32:04
  Speaker 1: must separate those ticks in your frame. Time dilation is
• 32:08
  Speaker 1: therefore woven into the geometry required to preserve the invariant
• 32:11
  Speaker 1: speed of light. At ordinary speeds, the effect is tiny.
• 32:16
  Speaker 1: At speeds approaching the vacuum speed of light, it becomes dramatic.
• 32:21
  Speaker 1: This sometimes produces an apparent puzzle. If you see a
• 32:24
  Speaker 1: rapidly moving clock running slowly, someone travelling with that clock
• 32:28
  Speaker 1: can regard you as the one who is moving. So
• 32:31
  Speaker 1: should they not say that your clock is running slowly? Yes.
• 32:36
  Speaker 1: For observers remaining in uniform relative motion, that reciprocity is
• 32:40
  Speaker 1: perfectly consistent. The apparent contradiction disappears because the observers do
• 32:46
  Speaker 1: not share the same judgements about simultaneity. They disagree about
• 32:50
  Speaker 1: which distant clock readings should be compared at the same moment.
• 32:55
  Speaker 1: Things become more interesting if the observers meet twice. This
• 32:58
  Speaker 1: is the basis of the travelling twin scenario. Imagine identical
• 33:03
  Speaker 1: twins starting together on Earth. One remains at home while
• 33:07
  Speaker 1: the other travels through space at very high speed, changes
• 33:10
  Speaker 1: direction and eventually returns. When they reunite, they can place
• 33:14
  Speaker 1: their clocks side by side again. The travelling twin may
• 33:18
  Speaker 1: have experienced less elapsed time. There is no disagreement left
• 33:22
  Speaker 1: to hide behind distant observations. Both clocks are together and
• 33:26
  Speaker 1: can be compared directly. The reason this does not violate
• 33:29
  Speaker 1: the apparent symmetry is that the twins have not followed
• 33:33
  Speaker 1: equivalent histories. The stay-at-home twin and the travelling twin have
• 33:37
  Speaker 1: taken different paths through space-time. The traveller must change inertial
• 33:42
  Speaker 1: frames during the journey While the twin remaining on Earth
• 33:45
  Speaker 1: follows a different spacetime trajectory, different paths can contain different
• 33:50
  Speaker 1: amounts of proper time. That phrase, proper time, is especially important.
• 33:56
  Speaker 1: It refers to the elapsed time recorded by a clock
• 33:59
  Speaker 1: travelling along a particular path through spacetime. Special relativity therefore
• 34:04
  Speaker 1: does not say that time becomes meaningless, or that anything
• 34:08
  Speaker 1: can happen in any order. Its predictions are exact. Observers
• 34:13
  Speaker 1: may disagree about durations, distances and simultaneity, but the disagreements
• 34:18
  Speaker 1: are connected by precise mathematical relationships. There is structure beneath
• 34:23
  Speaker 1: the disagreement. What disappeared was not physics. What disappeared was
• 34:28
  Speaker 1: the idea that the universe possesses one universal clock that
• 34:32
  Speaker 1: everyone is secretly measuring imperfectly. Newton had allowed time to
• 34:37
  Speaker 1: march forward independently of the observer. Einstein showed that measured
• 34:42
  Speaker 1: temporal intervals are inseparable from motion. Two perfectly functioning clocks
• 34:47
  Speaker 1: can follow different paths and genuinely record different amounts of
• 34:50
  Speaker 1: elapsed time. Two equally valid observers can disagree about which
• 34:55
  Speaker 1: distant events happen simultaneously. Neither needs to be wrong. The
• 35:00
  Speaker 1: laws connecting their descriptions remain consistent, precise, and experimentally testable.
• 35:06
  Speaker 1: But the comforting idea of a single cosmic present advancing
• 35:11
  Speaker 1: at the same rate everywhere, can no longer be maintained.
• 35:15
  Speaker 1: Time had not disappeared. It had become personal to the
• 35:19
  Speaker 1: path through spacetime. Once relativity had removed the idea of
• 35:23
  Speaker 1: one universal clock, a deeper question appeared. If different observers
• 35:28
  Speaker 1: could disagree about distances and durations but still describe the
• 35:32
  Speaker 1: same physical reality, perhaps space and time were not truly
• 35:36
  Speaker 1: separate things in the first place. That idea was developed
• 35:40
  Speaker 1: with remarkable clarity by the mathematician Hermann Minkowski. In 1908,
• 35:45
  Speaker 1: only a few years after Albert Einstein introduced special relativity,
• 35:50
  Speaker 1: Minkowski presented a geometric reformulation of the theory, in which
• 35:54
  Speaker 1: space and time became parts of a single four-dimensional structure, spacetime.
• 36:00
  Speaker 1: The name sounds grand, but the basic idea is surprisingly simple.
• 36:05
  Speaker 1: To describe where something happens, three spatial coordinates are not enough.
• 36:09
  Speaker 1: You also need to know when it happens. A cup
• 36:12
  Speaker 1: sitting on a table at noon is not the same
• 36:15
  Speaker 1: physical event as the same cup sitting in the same
• 36:17
  Speaker 1: place at midnight. The spatial location may be identical. The
• 36:21
  Speaker 1: temporal coordinate is different. In relativity, an event is therefore
• 36:26
  Speaker 1: something specified by both where and when. A flash of light.
• 36:30
  Speaker 1: Two particles colliding. A clock striking midnight. a spacecraft firing
• 36:35
  Speaker 1: its engine. Each of these can be assigned a position
• 36:39
  Speaker 1: in space and a position in time. Together, those coordinates
• 36:42
  Speaker 1: locate the event in spacetime. This changes the way motion
• 36:46
  Speaker 1: can be imagined. In ordinary thinking, an object exists in
• 36:50
  Speaker 1: space and then moves through time. In spacetime, the complete
• 36:55
  Speaker 1: history of that object can instead be represented by a
• 36:58
  Speaker 1: path called a worldline. Imagine marking your position every moment
• 37:03
  Speaker 1: from birth onwards. Each point represents an event. As the
• 37:07
  Speaker 1: moments accumulate, the points form a continuous path through spacetime.
• 37:12
  Speaker 1: That path is your worldline. A stationary object still has
• 37:16
  Speaker 1: a worldline because it continues from one moment to another,
• 37:20
  Speaker 1: even if its spatial position does not change. A moving
• 37:24
  Speaker 1: object has a worldline tilted differently through spacetime because its
• 37:27
  Speaker 1: spatial coordinates change as well. Minkowski's reformulation made the strange
• 37:33
  Speaker 1: results of special relativity look less like a collection of
• 37:37
  Speaker 1: separate effects, and more like parts of one underlying geometry.
• 37:41
  Speaker 1: Time dilation, length contraction, and the relativity of simultaneity are
• 37:47
  Speaker 1: not independent tricks being played by nature. They are connected
• 37:51
  Speaker 1: consequences of the geometry of spacetime. One of the most
• 37:55
  Speaker 1: important quantities in that geometry is the spacetime interval. Different
• 37:59
  Speaker 1: observers in relative motion may disagree about how much spatial
• 38:03
  Speaker 1: distance separates two events. They may also disagree about how
• 38:07
  Speaker 1: much time separates them, but when those spatial and temporal
• 38:10
  Speaker 1: separations are combined in the particular way required by special relativity,
• 38:16
  Speaker 1: all inertial observers agree on the resulting space-time interval. That
• 38:20
  Speaker 1: invariant quantity plays a role rather like distance in ordinary geometry.
• 38:25
  Speaker 1: Two people can use different coordinate systems to describe the
• 38:28
  Speaker 1: same pair of points on a sheet of paper, but
• 38:31
  Speaker 1: the geometric separation between the points does not depend on
• 38:35
  Speaker 1: the coordinate system. Space-time works similarly, though the geometry is
• 38:40
  Speaker 1: not ordinary Euclidean geometry. Space and time enter the interval differently.
• 38:46
  Speaker 1: That difference is crucial. It is what allows different observers
• 38:50
  Speaker 1: to disagree about spatial and temporal components while still agreeing
• 38:54
  Speaker 1: on the deeper spacetime structure. For events connected by the
• 38:57
  Speaker 1: path of a massive object, the spacetime interval is closely
• 39:01
  Speaker 1: related to proper time. Proper time is what an ideal
• 39:05
  Speaker 1: clock travelling along that worldline would actually record. This gives
• 39:09
  Speaker 1: relativity a wonderfully concrete answer to one important question. How
• 39:14
  Speaker 1: much time did a particular observer experience between two meetings?
• 39:19
  Speaker 1: Follow the observer's worldline. The proper time along that path
• 39:22
  Speaker 1: is the answer. Different worldlines between the same meetings can
• 39:27
  Speaker 1: contain different amounts of proper time. This is why two
• 39:31
  Speaker 1: clocks that separate and later reunite can display different elapsed
• 39:35
  Speaker 1: times without either clock being defective. They follow different paths
• 39:40
  Speaker 1: through spacetime. The geometry itself allows those paths to contain
• 39:44
  Speaker 1: different temporal lengths. Spacetime also determines which events can influence
• 39:49
  Speaker 1: which other events. This is described using light cones. Imagine
• 39:54
  Speaker 1: an event occurring at one point in spacetime. Perhaps a
• 39:57
  Speaker 1: lamp is switched on. Light leaves that event and spreads
• 40:01
  Speaker 1: outward at the invariant vacuum speed of light. If we
• 40:05
  Speaker 1: draw space horizontally and time vertically, the possible paths of
• 40:09
  Speaker 1: those light rays form the edges of a cone. Events
• 40:12
  Speaker 1: lying inside the future light cone can, in principle, influenced
• 40:16
  Speaker 1: by the original event. A slower signal could reach them.
• 40:20
  Speaker 1: A particle moving below the speed of light could reach them.
• 40:24
  Speaker 1: A light signal could reach the boundary itself. Events outside
• 40:28
  Speaker 1: that cone are different. They are too far away in space,
• 40:32
  Speaker 1: relative to their separation in time, for any signal travelling
• 40:36
  Speaker 1: at or below the vacuum speed of light to connect them.
• 40:39
  Speaker 1: These events are said to be space-like separated, For such events,
• 40:43
  Speaker 1: different observers can even disagree about which occurred first. One
• 40:47
  Speaker 1: observer may say event 1 happened before event 2. Another
• 40:51
  Speaker 1: may say event 2 happened before event 1. A third
• 40:55
  Speaker 1: may regard them as simultaneous. That sounds alarming until one
• 40:58
  Speaker 1: important fact is remembered. If the events are space-like separated,
• 41:03
  Speaker 1: neither can causally influence the other. No signal travelling within
• 41:07
  Speaker 1: the limits of relativity can get from one to the
• 41:09
  Speaker 1: other in time. so their order cannot be used to
• 41:12
  Speaker 1: create a contradiction in cause and effect. By contrast, if
• 41:16
  Speaker 1: one event can causally influence another, their temporal order remains
• 41:20
  Speaker 1: fixed for all ordinary inertial observers. A cause does not
• 41:24
  Speaker 1: become its own consequence simply because someone changes a reference frame.
• 41:28
  Speaker 1: Light cones therefore give spacetime a causal structure. They separate
• 41:33
  Speaker 1: what can influence what from what cannot. and this geometric
• 41:37
  Speaker 1: picture creates another challenge for the everyday idea of a
• 41:40
  Speaker 1: universal present. In Newtonian physics, it is natural to imagine
• 41:45
  Speaker 1: the universe divided into slices. One slice contains everything happening now.
• 41:51
  Speaker 1: Another contains everything that happened a moment ago. Another contains
• 41:56
  Speaker 1: everything that will happen a moment later. Relativity does not
• 41:59
  Speaker 1: provide one unique slicing of spacetime that every observer must accept.
• 42:04
  Speaker 1: Observers moving relative to one another can define different sets
• 42:08
  Speaker 1: of distant events as simultaneous. There is no experimentally privileged
• 42:13
  Speaker 1: universal surface of now built into special relativity. This fact
• 42:17
  Speaker 1: has encouraged a particular philosophical interpretation often called eternalism, or
• 42:23
  Speaker 1: more informally, the block universe view. According to this interpretation
• 42:27
  Speaker 1: … Past, present and future events can all be regarded
• 42:31
  Speaker 1: as parts of one four-dimensional space-time structure. Your childhood, this
• 42:37
  Speaker 1: moment and events in your future would occupy different locations
• 42:41
  Speaker 1: in space-time in much the same way that London and
• 42:44
  Speaker 1: Tokyo occupy different locations in space. The difference is temporal
• 42:49
  Speaker 1: rather than spatial. From this perspective, the universe does not
• 42:54
  Speaker 1: need to be imagined as a three-dimensional world continually updating itself.
• 42:59
  Speaker 1: It can be described as a four-dimensional whole. This interpretation
• 43:03
  Speaker 1: fits naturally with the mathematics of relativity. But it is
• 43:07
  Speaker 1: important not to confuse that with experimental proof that eternalism
• 43:12
  Speaker 1: is the only possible interpretation of reality. Relativity establishes observable
• 43:17
  Speaker 1: facts about clock readings, causal structure, simultaneity and space-time intervals.
• 43:25
  Speaker 1: The philosophical conclusion that all moments exist equally, is a
• 43:29
  Speaker 1: further interpretation of that structure. Some physicists find it compelling.
• 43:34
  Speaker 1: Others are more cautious about treating the geometry as a
• 43:37
  Speaker 1: complete statement about what exists. Either way, Minkowski's spacetime changed
• 43:43
  Speaker 1: the language of the problem. The old question had been
• 43:46
  Speaker 1: how objects move through space while one universal time passes everywhere.
• 43:51
  Speaker 1: Relativity suggested a different picture. Objects trace world lines through spacetime,
• 43:58
  Speaker 1: Clocks record proper time along those world lines. Observers divide
• 44:02
  Speaker 1: space and time differently, yet agree on invariant spacetime relationships.
• 44:07
  Speaker 1: Causal influence is organised by light cones rather than by
• 44:11
  Speaker 1: a universal cosmic present. And time is no longer an
• 44:14
  Speaker 1: independent background ticking at the same rate for everything. It
• 44:18
  Speaker 1: has become one part of the geometry describing how events
• 44:21
  Speaker 1: are connected. Special relativity had already removed the idea that
• 44:26
  Speaker 1: every observer must share one universal time, but Albert Einstein
• 44:30
  Speaker 1: was not finished with time. In 1915, he completed the
• 44:35
  Speaker 1: mature form of his general theory of relativity, and gravity
• 44:39
  Speaker 1: changed from being a force acting across space into something
• 44:43
  Speaker 1: far more geometric. In Isaac Newton's picture, space and time
• 44:49
  Speaker 1: formed a fixed background. Objects moved through that background, and
• 44:54
  Speaker 1: gravity pulled masses towards one another. General relativity replaced that arrangement.
• 45:00
  Speaker 1: Matter and energy influence the geometry of spacetime itself, and
• 45:04
  Speaker 1: objects move through that curved geometry. This change has an
• 45:08
  Speaker 1: immediate consequence for clocks. If spacetime is curved, then the
• 45:12
  Speaker 1: amount of proper time accumulated by a clock can depend
• 45:15
  Speaker 1: not only on how fast it moves, but also on
• 45:18
  Speaker 1: where it is within a gravitational field. Gravity can change
• 45:22
  Speaker 1: how much time passes. One of Einstein's key ideas was
• 45:26
  Speaker 1: the equivalence principle. Imagine standing inside a sealed lift with
• 45:31
  Speaker 1: no windows. If the lift is sitting on the surface
• 45:34
  Speaker 1: of a planet, you feel your weight pulling you towards
• 45:37
  Speaker 1: the floor. But suppose instead that the lift is far
• 45:41
  Speaker 1: from any planet or star and is accelerating upwards through
• 45:44
  Speaker 1: empty space. You would still feel pressed against the floor.
• 45:49
  Speaker 1: Within a sufficiently small region, the effects of uniform acceleration
• 45:54
  Speaker 1: can closely resemble those of a gravitational field. Einstein realised
• 45:59
  Speaker 1: that this connection between gravity and acceleration had profound consequences.
• 46:04
  Speaker 1: Imagine a beam of light crossing an accelerating lift. While
• 46:08
  Speaker 1: the light travels from one side to the other, the
• 46:11
  Speaker 1: lift moves upwards. To someone inside, the path of the
• 46:14
  Speaker 1: light appears bent. If acceleration and gravity are locally equivalent,
• 46:19
  Speaker 1: then gravity should influence the apparent path of light as well.
• 46:23
  Speaker 1: That insight eventually became part of a much larger idea.
• 46:27
  Speaker 1: Gravity is associated with curved spacetime, and because time is
• 46:31
  Speaker 1: woven into spacetime, gravity affects clocks. Consider two identical clocks
• 46:37
  Speaker 1: held at different heights above Earth. The clock lower down
• 46:41
  Speaker 1: is deeper within Earth's gravitational potential. The higher clock is
• 46:45
  Speaker 1: farther away from Earth's centre. and sits in a slightly
• 46:48
  Speaker 1: different gravitational environment. When those clocks are compared carefully, the
• 46:53
  Speaker 1: lower clock accumulates slightly less proper time. It runs more
• 46:57
  Speaker 1: slowly relative to the higher one. This effect is known
• 47:00
  Speaker 1: as gravitational time dilation. The wording can occasionally make it
• 47:04
  Speaker 1: sound as though gravity reaches inside a clock and interferes
• 47:08
  Speaker 1: with its mechanism. That is not what is happening. A
• 47:11
  Speaker 1: mechanical clock, an atomic clock, a chemical reaction and any
• 47:16
  Speaker 1: other reliable physical process all reflect the same difference. It
• 47:21
  Speaker 1: is not the clock being damaged by gravity. It is
• 47:24
  Speaker 1: a difference in the amount of proper time associated with
• 47:27
  Speaker 1: different paths through curved spacetime. Near Earth, the effect is
• 47:31
  Speaker 1: very small, but it is real. A clock on the
• 47:35
  Speaker 1: ground and a clock high on a mountain do not
• 47:37
  Speaker 1: accumulate exactly the same amount of time. Move into a
• 47:40
  Speaker 1: much stronger gravitational field, such as the region near a
• 47:44
  Speaker 1: neutron star or black hole, and the difference can become
• 47:47
  Speaker 1: far more dramatic. The same physics also appears in the
• 47:51
  Speaker 1: behaviour of light. Suppose light is emitted from a lower
• 47:54
  Speaker 1: point in a gravitational field and received higher up. The
• 47:58
  Speaker 1: receiver can measure the light at a lower frequency than
• 48:01
  Speaker 1: the emitter would measure locally. This is gravitational redshift. The
• 48:07
  Speaker 1: light is shifted towards lower frequency. and therefore towards the
• 48:11
  Speaker 1: red end of the visible spectrum if the radiation happens
• 48:14
  Speaker 1: to lie within visible wavelengths. The reverse situation produces gravitational
• 48:19
  Speaker 1: blue shift. These changes are closely connected to differences in
• 48:23
  Speaker 1: clock rates. An atom emitting radiation at a particular frequency
• 48:28
  Speaker 1: can be thought of as providing a very precise rhythm.
• 48:32
  Speaker 1: If identical atoms at different gravitational potentials produce signals that
• 48:36
  Speaker 1: are compared, the difference in measured frequency reflects the fact
• 48:40
  Speaker 1: that proper time is accumulating differently at those locations. This
• 48:45
  Speaker 1: was tested directly in one of the classic experiments of
• 48:48
  Speaker 1: 20th century relativity. Around 1959 and 1960, physicists Robert Pound
• 48:55
  Speaker 1: and Glenn Rebker performed an experiment at Harvard University using
• 49:00
  Speaker 1: gamma radiation. Their experiment took place over a vertical distance
• 49:04
  Speaker 1: of only about 22 metres inside a tower. that is
• 49:08
  Speaker 1: hardly the sort of distance normally associated with dramatic effects
• 49:12
  Speaker 1: of general relativity. Yet Earth's gravitational field changes slightly even
• 49:17
  Speaker 1: across those 22 metres. Pound and Rep used the Musbauer effect,
• 49:22
  Speaker 1: which allows extremely precise measurements of gamma-ray energies to detect
• 49:26
  Speaker 1: the tiny gravitational frequency shift predicted by general relativity. The
• 49:31
  Speaker 1: effect was minute, but it was measurable. That result helped
• 49:35
  Speaker 1: confirm that gravitational redshift was not merely an elegant consequence
• 49:39
  Speaker 1: of the theory on paper. It occurs here on Earth.
• 49:43
  Speaker 1: Clocks themselves soon became precise enough to provide another route.
• 49:48
  Speaker 1: In 1971, Joseph Huffle and Richard Keating carried highly accurate
• 49:54
  Speaker 1: cesium atomic clocks aboard commercial aircraft flying around the world.
• 49:59
  Speaker 1: Some journeys travelled eastward and others westward. When the airborne
• 50:03
  Speaker 1: clocks returned, their readings were compared with atomic clocks that
• 50:07
  Speaker 1: had remained on the ground. Two relativistic effects had contributed.
• 50:13
  Speaker 1: The aircraft were moving relative to Earth, so special relativistic
• 50:17
  Speaker 1: time dilation mattered, but they were also flying at altitude,
• 50:21
  Speaker 1: higher in Earth's gravitational field, so gravitational time dilation mattered
• 50:26
  Speaker 1: as well. The two effects can act in different directions.
• 50:31
  Speaker 1: Motion tends to reduce accumulated proper time relative to suitable
• 50:35
  Speaker 1: reference clocks, while being higher above Earth can increase it
• 50:39
  Speaker 1: relative to clocks lower down. Once both effects were included,
• 50:44
  Speaker 1: the observed clock differences agreed with relativity within the experimental
• 50:48
  Speaker 1: precision available at the time. Today, clock technology has become
• 50:52
  Speaker 1: vastly more sensitive. Modern optical atomic clocks can compare frequencies
• 50:58
  Speaker 1: so precisely that gravitational time differences can be detected across
• 51:02
  Speaker 1: surprisingly small changes in height. Experiments have measured relativistic clock
• 51:08
  Speaker 1: shifts over height differences of well under a metre. That
• 51:11
  Speaker 1: means the top of a laboratory bench can, in principle,
• 51:15
  Speaker 1: accumulate time at a measurably different rate from the floor
• 51:18
  Speaker 1: beneath it. The difference is astonishingly small, but modern clocks
• 51:22
  Speaker 1: are astonishingly good. This creates the slightly peculiar situation in
• 51:27
  Speaker 1: which your head is ageing, by a tiny relativistic amount,
• 51:31
  Speaker 1: faster than your feet while you stand upright. The effect
• 51:34
  Speaker 1: is far too small to help with birthdays. But it
• 51:38
  Speaker 1: is physically real. General relativity therefore turns time into something
• 51:43
  Speaker 1: connected directly to gravitational geometry. There is no single rate
• 51:48
  Speaker 1: at which all clocks throughout the universe must tick. A
• 51:51
  Speaker 1: clock near Earth, one aboard a satellite, one near the Sun,
• 51:56
  Speaker 1: and one close to a neutron star can all accumulate
• 51:59
  Speaker 1: different amounts of proper time. And those differences are not
• 52:02
  Speaker 1: caused by poor engineering. They belong to the structure of spacetime.
• 52:07
  Speaker 1: This is the conceptual departure from Newton that matters most.
• 52:12
  Speaker 1: In Newtonian physics, the stage remains fixed while matter moves
• 52:15
  Speaker 1: across it. In general relativity, the stage participates. Matter and
• 52:21
  Speaker 1: energy influence spacetime geometry. Space-time geometry influences how matter moves,
• 52:28
  Speaker 1: and that same geometry determines the temporal intervals that clocks
• 52:31
  Speaker 1: record along their paths. Space and time are no longer
• 52:35
  Speaker 1: passive containers, they are part of the dynamics. This means
• 52:39
  Speaker 1: that when physicists ask how much time passes, the answer
• 52:42
  Speaker 1: can depend on where the clock is, how it moves,
• 52:46
  Speaker 1: and what gravitational geometry surrounds it. Time is no longer
• 52:50
  Speaker 1: something the universe simply carries around at one uniform rate.
• 52:54
  Speaker 1: it is woven into the shape of spacetime itself. Once
• 52:58
  Speaker 1: relativity is taken seriously, the question how much time has
• 53:02
  Speaker 1: passed becomes slightly incomplete. Passed for whom, and along which path?
• 53:07
  Speaker 1: That sounds like an unnecessary complication, until we remember what
• 53:11
  Speaker 1: a clock actually measures. A physical clock records proper time
• 53:15
  Speaker 1: along its own journey through spacetime. If two clocks begin together,
• 53:20
  Speaker 1: travel along different world lines and later meet again, They
• 53:23
  Speaker 1: can show different elapsed times without either of them being wrong.
• 53:28
  Speaker 1: Each clock has measured the time along its own path.
• 53:31
  Speaker 1: This makes proper time one of the most physically direct
• 53:34
  Speaker 1: notions of time in relativity. If you carry a clock
• 53:38
  Speaker 1: with you, the reading it accumulates is your proper time.
• 53:41
  Speaker 1: But physicists often need something else as well. They need
• 53:44
  Speaker 1: coordinate time. Coordinate time is a way of assigning time
• 53:48
  Speaker 1: labels to events across a region of spacetime. It allows
• 53:52
  Speaker 1: physicists to organise calculations, describe motion and compare events occurring
• 53:57
  Speaker 1: at widely separated places. The important point is that a
• 54:01
  Speaker 1: coordinate system is partly a choice. It is a useful
• 54:05
  Speaker 1: mathematical framework rather than necessarily a universal clock built into nature.
• 54:10
  Speaker 1: Think of a map of Earth. We can choose longitude
• 54:13
  Speaker 1: so that zero degrees passes through Greenwich. but the planet
• 54:16
  Speaker 1: does not contain a glowing line telling us that Greenwich
• 54:19
  Speaker 1: was cosmically selected as the centre of longitude. The coordinate
• 54:24
  Speaker 1: is useful. It is not a physical object. Time coordinates
• 54:28
  Speaker 1: and relativity can work in a similar way. Different coordinate
• 54:32
  Speaker 1: systems may assign different numerical labels to the same event
• 54:36
  Speaker 1: while agreeing on the physical relationships that can actually be measured.
• 54:40
  Speaker 1: The coordinates may change. The observable physics does not. This
• 54:45
  Speaker 1: distinction becomes especially important in general relativity, where spacetime can
• 54:50
  Speaker 1: be curved. There may be many mathematically valid ways to
• 54:53
  Speaker 1: label the same region. One coordinate system might make a
• 54:57
  Speaker 1: particular calculation simple. Another might make the geometry easier to visualise.
• 55:03
  Speaker 1: A third might be chosen because it behaves conveniently near
• 55:06
  Speaker 1: a planet, a star or a black hole. None of
• 55:09
  Speaker 1: this means reality changes when the coordinates change. it means
• 55:13
  Speaker 1: that the labels are not the thing being labelled. Proper time,
• 55:17
  Speaker 1: by contrast, is tied directly to a world line. If
• 55:21
  Speaker 1: an ideal clock travels from one event to another, the
• 55:24
  Speaker 1: amount it records is not merely a convenient choice of coordinates,
• 55:27
  Speaker 1: it is a physical quantity. And because different paths through
• 55:31
  Speaker 1: space-time can contain different amounts of proper time, two observers
• 55:35
  Speaker 1: can genuinely age by different amounts between meetings. Modern technology
• 55:40
  Speaker 1: has to account for this. Global navigation satellite systems provide
• 55:45
  Speaker 1: one of the clearest everyday examples. Satellites used for navigation
• 55:49
  Speaker 1: carry extremely accurate atomic clocks. Receivers on Earth determine their
• 55:54
  Speaker 1: position by comparing the arrival times of signals transmitted from
• 55:58
  Speaker 1: several satellites. Because light travels at roughly 300,000 kilometres per
• 56:04
  Speaker 1: second in vacuum, even a timing error of a tiny
• 56:07
  Speaker 1: fraction of a second can translate into a large position error.
• 56:11
  Speaker 1: so the clocks need to be extremely well understood. But
• 56:15
  Speaker 1: satellite clocks do not tick at exactly the same rate
• 56:18
  Speaker 1: as clocks on Earth. There are two important relativistic effects. First,
• 56:25
  Speaker 1: the satellites are moving rapidly relative to observers on Earth.
• 56:28
  Speaker 1: Special relativity therefore predicts a velocity-related time dilation that tends
• 56:33
  Speaker 1: to make their clocks accumulate slightly less proper time. the
• 56:38
  Speaker 1: satellites orbit far above Earth's surface, where Earth's gravitational field
• 56:43
  Speaker 1: is weaker. General relativity predicts that clocks higher in Earth's
• 56:48
  Speaker 1: gravitational potential accumulate slightly more proper time than comparable clocks
• 56:53
  Speaker 1: lower down. For the satellites used by the Global Positioning System,
• 56:58
  Speaker 1: the gravitational effect is larger than the velocity effect. The
• 57:02
  Speaker 1: combined difference amounts to roughly 38 microseconds per day. Compared
• 57:07
  Speaker 1: with suitable clocks on Earth, 38 millionths of a second
• 57:11
  Speaker 1: does not sound like much, but if it were ignored,
• 57:15
  Speaker 1: positioning errors would accumulate rapidly, reaching kilometres after only a
• 57:19
  Speaker 1: relatively short period. Relativity is therefore not an optional philosophical
• 57:25
  Speaker 1: decoration added to satellite navigation. It is built into the
• 57:29
  Speaker 1: timing calculations that allow the system to work accurately. And
• 57:34
  Speaker 1: yet even here, Physicists do not need to declare one
• 57:37
  Speaker 1: universal time for the entire universe. They use well-defined coordinate
• 57:42
  Speaker 1: times for the navigation system and relate them carefully to
• 57:46
  Speaker 1: the proper times measured by clocks moving along different paths.
• 57:51
  Speaker 1: Different meanings of time can coexist because they are doing
• 57:54
  Speaker 1: different jobs. This becomes even more interesting on cosmological scales.
• 58:00
  Speaker 1: When physicists describe the expanding universe, They often use a
• 58:04
  Speaker 1: family of models known as the Friedman, Lemaitre, Robertson and
• 58:09
  Speaker 1: Walker models. These models assume that on sufficiently large scales
• 58:14
  Speaker 1: the universe is approximately homogeneous and isotropic. Homogeneous means that
• 58:19
  Speaker 1: matter is distributed roughly evenly when viewed over enormous distances.
• 58:24
  Speaker 1: Isotropic means that, on those scales, the universe looks broadly
• 58:28
  Speaker 1: similar in every direction. Within such a description a particularly
• 58:33
  Speaker 1: useful time coordinate appears. It is called cosmic time. Imagine
• 58:38
  Speaker 1: observers who are carried along with the average expansion of
• 58:41
  Speaker 1: the universe. They are not racing through the cosmos relative
• 58:45
  Speaker 1: to that large-scale expansion. Instead, they move with what cosmologists
• 58:50
  Speaker 1: call the Hubble flow. For these idealised observers, cosmic time
• 58:54
  Speaker 1: provides a shared way of describing the age and evolution
• 58:57
  Speaker 1: of the universe. Using the standard cosmological model, and a
• 59:01
  Speaker 1: range of astronomical observations, physicists infer that the universe is
• 59:06
  Speaker 1: roughly 13.8 billion years old. This can sound suspiciously like
• 59:12
  Speaker 1: Newton's universal time returning through the back door. But it
• 59:16
  Speaker 1: is not the same thing. Cosmic time is useful because
• 59:20
  Speaker 1: the actual universe has a large-scale distribution of matter and
• 59:23
  Speaker 1: radiation that provides a natural cosmological reference. It is not
• 59:28
  Speaker 1: a fundamental law saying that every conceivable observer everywhere must
• 59:33
  Speaker 1: agree on one absolute temporal rhythm. Observers moving rapidly relative
• 59:39
  Speaker 1: to the cosmic expansion can still measure different proper times.
• 59:43
  Speaker 1: So can observers experiencing different gravitational conditions. Cosmic time is
• 59:48
  Speaker 1: a particularly convenient description for a particular family of observers
• 59:53
  Speaker 1: in a particular cosmological model. It is not Newtonian absolute
• 59:57
  Speaker 1: time reborn. The cosmic microwave background provides a beautiful physical
• 1:00:02
  Speaker 1: example of this preferred cosmological frame. The cosmic microwave background
• 1:00:08
  Speaker 1: is ancient radiation left from a time when the universe
• 1:00:12
  Speaker 1: had cooled enough for light to travel freely over large distances. Today,
• 1:00:17
  Speaker 1: it fills space almost uniformly. An observer moving relative to
• 1:00:23
  Speaker 1: this radiation sees a slight temperature difference across the sky.
• 1:00:27
  Speaker 1: The cosmic microwave background appears a little warmer in the
• 1:00:30
  Speaker 1: direction of motion and a little cooler in the opposite direction.
• 1:00:35
  Speaker 1: By measuring this pattern, known as a dipole, astronomers can
• 1:00:39
  Speaker 1: determine motion relative to the frame in which the cosmic
• 1:00:42
  Speaker 1: microwave background looks most nearly uniform in all directions. That
• 1:00:47
  Speaker 1: frame is extraordinarily useful. but it does not violate special relativity.
• 1:00:52
  Speaker 1: Special relativity says that there is no privileged inertial frame
• 1:00:57
  Speaker 1: built into the fundamental laws of physics. It does not
• 1:01:00
  Speaker 1: say that the contents of the universe cannot define convenient
• 1:01:04
  Speaker 1: reference frames. A room full of air gives you a
• 1:01:07
  Speaker 1: useful frame in which the air is approximately at rest.
• 1:01:11
  Speaker 1: The existence of that air does not make one inertial
• 1:01:14
  Speaker 1: frame fundamental to all physics. In much the same way,
• 1:01:18
  Speaker 1: The large-scale distribution of cosmic matter and radiation provides a
• 1:01:23
  Speaker 1: natural cosmological reference without creating an absolute time that overrides relativity.
• 1:01:30
  Speaker 1: Modern physics is therefore perfectly comfortable using several notions of
• 1:01:34
  Speaker 1: time at once. Proper time tells us what a particular
• 1:01:38
  Speaker 1: clock experiences along a particular world line. Coordinate time provides
• 1:01:42
  Speaker 1: a convenient set of labels for organising events within a
• 1:01:45
  Speaker 1: chosen description. Cosmic time gives cosmologists a useful way to
• 1:01:50
  Speaker 1: describe the large-scale history of an expanding universe. None of
• 1:01:54
  Speaker 1: these needs to replace all the others. They answer different questions,
• 1:01:58
  Speaker 1: and that is perhaps one of relativity's most important lessons.
• 1:02:03
  Speaker 1: There is no single clock somewhere in the cosmos against
• 1:02:06
  Speaker 1: which every other clock is secretly being judged. There are
• 1:02:10
  Speaker 1: physical clocks following world lines. There are coordinate systems chosen
• 1:02:14
  Speaker 1: to organise events. There are cosmological reference frames produced by
• 1:02:19
  Speaker 1: the actual contents of our universe. Time remains precise. It
• 1:02:23
  Speaker 1: remains measurable. But it no longer needs to be one
• 1:02:27
  Speaker 1: universal thing, ticking identically for everything that exists. There is
• 1:02:32
  Speaker 1: another feature of time that seems so obvious that it
• 1:02:35
  Speaker 1: is easy to overlook. The past and future do not
• 1:02:38
  Speaker 1: feel interchangeable. We remember breakfast, but not tomorrow's dinner. Smoke
• 1:02:44
  Speaker 1: spreads from a candle after it is blown out. but
• 1:02:47
  Speaker 1: never gathers itself neatly from across a room and dives
• 1:02:50
  Speaker 1: back into the wick. A dropped glass may shatter into fragments,
• 1:02:54
  Speaker 1: yet the fragments do not leap from the floor, assemble
• 1:02:57
  Speaker 1: themselves in mid-air and land gently in your hand. The
• 1:03:02
  Speaker 1: world appears to have a direction, and yet many of
• 1:03:05
  Speaker 1: the fundamental equations used in physics do not contain such
• 1:03:08
  Speaker 1: an obvious preference. If the motions of idealised particles are
• 1:03:13
  Speaker 1: described using certain classical laws, The equations can often be
• 1:03:17
  Speaker 1: run forwards or backwards in time and still describe physically
• 1:03:21
  Speaker 1: allowed motion. The same broad idea appears in several areas
• 1:03:25
  Speaker 1: of microscopic physics. This creates a puzzle. If the underlying
• 1:03:30
  Speaker 1: laws are so often compatible with both temporal directions, why
• 1:03:33
  Speaker 1: does ordinary life look so strongly one way? A large
• 1:03:37
  Speaker 1: part of the answer comes from thermodynamics. During the 19th century, physicists,
• 1:03:43
  Speaker 1: including Rudolf Clausius, developed the laws governing heat, work and energy.
• 1:03:49
  Speaker 1: Clusius introduced the concept of entropy as a way of
• 1:03:51
  Speaker 1: describing an important feature of thermodynamic processes. Later, Ludwig Boltzmann
• 1:03:58
  Speaker 1: connected entropy to the microscopic behaviour of matter through statistical mechanics.
• 1:04:03
  Speaker 1: That connection changed the problem. Instead of treating heat only
• 1:04:07
  Speaker 1: as a smooth macroscopic quantity, Statistical mechanics considers the enormous
• 1:04:13
  Speaker 1: numbers of atoms and molecules moving within a physical system.
• 1:04:17
  Speaker 1: Imagine a sealed box filled with gas. From a distance,
• 1:04:21
  Speaker 1: the gas might look perfectly uniform. We could describe its pressure,
• 1:04:25
  Speaker 1: temperature and volume without knowing where every molecule is. That
• 1:04:29
  Speaker 1: large-scale description is called a macrostate. But underneath that simple
• 1:04:34
  Speaker 1: description lies an astonishing number of possible microscopic arrangements. every
• 1:04:39
  Speaker 1: molecule could occupy a slightly different position and possess a
• 1:04:43
  Speaker 1: different velocity. A detailed specification of those microscopic conditions is
• 1:04:48
  Speaker 1: called a microstate. Many different microstates can correspond to the
• 1:04:53
  Speaker 1: same macrostate. That fact is central to entropy. Suppose all
• 1:04:58
  Speaker 1: the gas molecules somehow begin crowded into one small corner
• 1:05:01
  Speaker 1: of the box. That is a possible physical arrangement. But
• 1:05:05
  Speaker 1: there are relatively few microscopic configurations corresponding to such an
• 1:05:09
  Speaker 1: unusually concentrated state. Now, release them. The molecules bounce around,
• 1:05:15
  Speaker 1: collide with one another and spread throughout the box. There
• 1:05:19
  Speaker 1: are vastly more possible microscopic arrangements in which the gas
• 1:05:22
  Speaker 1: is distributed broadly through the available volume. So the spread-out
• 1:05:26
  Speaker 1: state is overwhelmingly more probable. This is the statistical heart
• 1:05:31
  Speaker 1: of the second law of thermodynamics. For an isolated macroscopic system,
• 1:05:36
  Speaker 1: evolution towards higher entropy is overwhelmingly likely. It is not
• 1:05:41
  Speaker 1: usually helpful to imagine entropy as an invisible force pushing
• 1:05:45
  Speaker 1: events into the future. Nothing has to reach into the
• 1:05:48
  Speaker 1: gas and instruct the molecules to spread out. They simply
• 1:05:51
  Speaker 1: move according to ordinary microscopic dynamics. The reason the gas
• 1:05:56
  Speaker 1: appears to move from ordered to disordered arrangements is that
• 1:05:59
  Speaker 1: there are enormously more microscopic ways for it to be
• 1:06:02
  Speaker 1: spread out than confined to one corner. Probability does the
• 1:06:08
  Speaker 1: heavy lifting. The same principle appears in familiar examples. Place
• 1:06:13
  Speaker 1: a warm mug in a cooler room and thermal energy
• 1:06:15
  Speaker 1: spreads into the surroundings. The mug cools. In principle, the
• 1:06:19
  Speaker 1: microscopic motions of the molecules are not individually labelled with
• 1:06:23
  Speaker 1: arrows saying they must carry energy away from the mug.
• 1:06:27
  Speaker 1: But there are overwhelmingly more microscopic arrangements in which energy
• 1:06:31
  Speaker 1: has become distributed between mug and room than arrangements in
• 1:06:34
  Speaker 1: which the surrounding air spontaneously transfers just the right energy
• 1:06:38
  Speaker 1: back into the mug. So cooling is what we observe.
• 1:06:42
  Speaker 1: Spray perfume into one corner of a room and the
• 1:06:45
  Speaker 1: molecules disperse. They collide with air molecules and wander into
• 1:06:49
  Speaker 1: an enormous number of possible positions. There is nothing in
• 1:06:53
  Speaker 1: principle preventing a fantastically precise sequence of molecular motions from
• 1:06:58
  Speaker 1: bringing every perfume molecule back into one tiny region. But
• 1:07:02
  Speaker 1: the number of microstates corresponding to a dispersed perfume cloud
• 1:07:06
  Speaker 1: is so enormously larger that spontaneous reassembly is effectively never seen.
• 1:07:10
  Speaker 1: A shattered glass gives the same lesson in a more
• 1:07:14
  Speaker 1: dramatic form. The broken pieces, vibrations in the floor, Sound
• 1:07:19
  Speaker 1: waves in the air and thermal energy produced during the
• 1:07:21
  Speaker 1: impact together form a vastly more probable range of microscopic
• 1:07:26
  Speaker 1: configurations than the highly organised state of one intact glass
• 1:07:30
  Speaker 1: sitting quietly on a table. The reverse is not forbidden
• 1:07:34
  Speaker 1: in the same simple sense as travelling faster than light
• 1:07:37
  Speaker 1: is forbidden for an ordinary massive object. It is instead
• 1:07:41
  Speaker 1: fantastically improbable. This statistical tendency towards higher entropy produces what
• 1:07:47
  Speaker 1: physicists call the thermodynamic arrow of time. It gives us
• 1:07:51
  Speaker 1: a physical distinction between two temporal directions. In one direction,
• 1:07:56
  Speaker 1: heat spreads, gases mix, structures decay, and entropy tends to rise.
• 1:08:01
  Speaker 1: In the opposite direction, those same processes would appear to
• 1:08:04
  Speaker 1: run backwards. But this still does not mean that entropy
• 1:08:08
  Speaker 1: explains a literal flow of time. The second law does
• 1:08:11
  Speaker 1: not tell us that time itself is a substance moving forwards.
• 1:08:15
  Speaker 1: It tells us that physical systems exhibit a strong asymmetry
• 1:08:19
  Speaker 1: when arranged in certain low entropy conditions and allowed to evolve.
• 1:08:24
  Speaker 1: That distinction matters. There are also other arrows of time.
• 1:08:28
  Speaker 1: The psychological arrow refers to the fact that we remember
• 1:08:31
  Speaker 1: the past but not the future. Memories are physical records,
• 1:08:36
  Speaker 1: created through processes in brains and other systems. and those
• 1:08:39
  Speaker 1: processes themselves involve thermodynamic irreversibility. The radiation arrow concerns the
• 1:08:46
  Speaker 1: familiar observation that electromagnetic radiation is usually seen spreading outward
• 1:08:52
  Speaker 1: from sources rather than arriving from the future in precisely
• 1:08:56
  Speaker 1: coordinated waves that converge onto them. There is also a
• 1:08:59
  Speaker 1: cosmological arrow associated with the large-scale history of the universe
• 1:09:04
  Speaker 1: and its expansion. These arrows may be related, but they
• 1:09:08
  Speaker 1: are not automatically identical. The thermodynamic arrow is especially powerful
• 1:09:13
  Speaker 1: because it appears everywhere in ordinary macroscopic physics. It explains
• 1:09:18
  Speaker 1: why ageing has a direction, Why stars consume fuel rather
• 1:09:23
  Speaker 1: than unburn it. Why information is easy to erase irreversibly
• 1:09:28
  Speaker 1: but difficult to reconstruct perfectly once dispersed. Why yesterday leaves
• 1:09:33
  Speaker 1: records while tomorrow does not arrive with photographs already attached.
• 1:09:38
  Speaker 1: But statistical mechanics also creates a deeper problem. If high
• 1:09:42
  Speaker 1: entropy states are overwhelmingly more probable … then why was
• 1:09:46
  Speaker 1: the early universe in a condition that allowed entropy to
• 1:09:48
  Speaker 1: increase so dramatically in the first place? The universe we
• 1:09:52
  Speaker 1: observe today contains stars, galaxies, black holes, planets and enormous
• 1:09:59
  Speaker 1: amounts of structure. All of that history depends on the
• 1:10:02
  Speaker 1: universe beginning in a very unusual state. A low entropy
• 1:10:07
  Speaker 1: past gives physical systems room to evolve towards higher entropy.
• 1:10:12
  Speaker 1: Without that special starting condition, the familiar thermodynamic arrow would
• 1:10:16
  Speaker 1: be much harder to explain. So entropy explains a great
• 1:10:20
  Speaker 1: deal about why time appears directional, but it also pushes
• 1:10:23
  Speaker 1: the mystery backwards. The equations can describe how low entropy
• 1:10:27
  Speaker 1: states evolve into more probable high entropy ones. What they
• 1:10:32
  Speaker 1: do not automatically explain is why the universe began in
• 1:10:35
  Speaker 1: such an extraordinarily special, low entropy condition at all. The
• 1:10:40
  Speaker 1: arrow of time is therefore not simply written into every
• 1:10:43
  Speaker 1: microscopic equation. It may depend profoundly on the history of
• 1:10:47
  Speaker 1: the universe itself. And once again, the ordinary feeling that
• 1:10:51
  Speaker 1: time obviously flows forwards turns out to contain a much
• 1:10:55
  Speaker 1: deeper physical question. Entropy gives physics a remarkably good explanation
• 1:11:00
  Speaker 1: for why so many processes around us have a preferred direction.
• 1:11:04
  Speaker 1: Heat spreads from hot objects into cooler surroundings. Gases mix,
• 1:11:09
  Speaker 1: stars burn fuel. Fragile objects break far more easily than
• 1:11:13
  Speaker 1: they reconstruct themselves. But there is a catch hiding inside
• 1:11:17
  Speaker 1: that explanation. Saying that entropy tends to increase does not,
• 1:11:22
  Speaker 1: by itself, explain why the universe has an arrow of time.
• 1:11:27
  Speaker 1: For entropy to increase, there must first be somewhere for
• 1:11:29
  Speaker 1: it to increase from. A system already sitting in complete
• 1:11:33
  Speaker 1: thermodynamic equilibrium has no obvious large-scale gradient left to exploit.
• 1:11:39
  Speaker 1: There is no hot region next to a cold one,
• 1:11:42
  Speaker 1: no compressed gas waiting to expand, and no strongly ordered
• 1:11:46
  Speaker 1: configuration ready to become more probable. The familiar arrow therefore
• 1:11:50
  Speaker 1: depends on an earlier condition of lower entropy. Push that
• 1:11:54
  Speaker 1: reasoning far enough backwards and it becomes a question about
• 1:11:58
  Speaker 1: the universe itself. Why was the early universe in such
• 1:12:01
  Speaker 1: a special state? One way of expressing the problem is
• 1:12:05
  Speaker 1: through what has become known as the past hypothesis. The
• 1:12:09
  Speaker 1: past hypothesis proposes that the universe began, or at least
• 1:12:13
  Speaker 1: existed extremely early in its history, in an extraordinarily low
• 1:12:17
  Speaker 1: entropy macrostate. It does not merely say that yesterday had
• 1:12:21
  Speaker 1: slightly less entropy than today. It places a special low
• 1:12:25
  Speaker 1: entropy condition near the beginning of the observable history of
• 1:12:28
  Speaker 1: the universe. Once that condition is supplied, statistical mechanics can
• 1:12:33
  Speaker 1: do much of the remaining work, There are vastly more
• 1:12:36
  Speaker 1: high entropy microscopic arrangements available than low entropy ones, so
• 1:12:41
  Speaker 1: systems beginning away from equilibrium will overwhelmingly tend to evolve
• 1:12:45
  Speaker 1: towards the larger region of possibilities. That gives one temporal
• 1:12:49
  Speaker 1: direction a distinctive statistical character. We call the direction towards
• 1:12:54
  Speaker 1: the lower entropy boundary the past. We call the direction
• 1:12:58
  Speaker 1: in which entropy generally rises the future. But this immediately
• 1:13:03
  Speaker 1: raises another question. Why was there such a boundary condition
• 1:13:06
  Speaker 1: in the first place? Ludwig Boltzmann spent much of his
• 1:13:10
  Speaker 1: scientific life wrestling with closely related difficulties. His statistical interpretation
• 1:13:16
  Speaker 1: of thermodynamics explained irreversible looking processes through the probable behaviour
• 1:13:21
  Speaker 1: of enormous numbers of particles. Yet the microscopic laws he
• 1:13:26
  Speaker 1: was working with created an uncomfortable complication. In many classical systems,
• 1:13:31
  Speaker 1: if a particular microscopic sequence of motions is physically permitted,
• 1:13:36
  Speaker 1: then a suitably reversed sequence can also satisfy the equations.
• 1:13:41
  Speaker 1: This point was pressed sharply by the physicist Josef Loschmidt.
• 1:13:45
  Speaker 1: Loschmidt's reversibility objection asks us to imagine a gas evolving
• 1:13:50
  Speaker 1: from a relatively ordered condition towards equilibrium. Its molecules collide,
• 1:13:56
  Speaker 1: scatter and spread. until the gas reaches a more probable macrostate. Now,
• 1:14:01
  Speaker 1: at some instant, imagine reversing every particle velocity with impossible precision.
• 1:14:08
  Speaker 1: Every molecule heads back along the reverse of its previous motion.
• 1:14:12
  Speaker 1: Under suitable classical mechanics, the system would retrace its microscopic history.
• 1:14:18
  Speaker 1: Instead of entropy increasing, it would decrease. The dispersed gas
• 1:14:22
  Speaker 1: could return towards its earlier concentrated state. The point was
• 1:14:26
  Speaker 1: not that anyone could realistically perform such a reversal. The
• 1:14:30
  Speaker 1: point was conceptual. If the microscopic equations permit both histories,
• 1:14:36
  Speaker 1: then the equations alone cannot explain why we overwhelmingly observe
• 1:14:40
  Speaker 1: one direction rather than the other. Something else is needed.
• 1:14:44
  Speaker 1: The state from which the system begins matters. For the universe,
• 1:14:49
  Speaker 1: that means the boundary conditions matter. Boltzmann explored several ideas
• 1:14:53
  Speaker 1: concerning how low entropy conditions might arise. One possibility involves fluctuations.
• 1:15:00
  Speaker 1: Imagine an enormously old system remaining in thermodynamic equilibrium for
• 1:15:04
  Speaker 1: immense periods. Random microscopic motion can occasionally produce fluctuations away
• 1:15:10
  Speaker 1: from perfect equilibrium. Small fluctuations are much more likely than
• 1:15:14
  Speaker 1: large ones, but given enough time, extraordinarily unlikely fluctuations can occur. Perhaps,
• 1:15:22
  Speaker 1: Boltzmann considered, a sufficiently large low-entropy fluctuation could create a
• 1:15:27
  Speaker 1: region resembling the ordered universe we observe. This is ingenious.
• 1:15:31
  Speaker 1: It also produces a disturbing problem. If an equilibrium cosmos
• 1:15:36
  Speaker 1: generates order through random fluctuations, then a small fluctuation is
• 1:15:40
  Speaker 1: fantastically more probable than an enormous one. Producing an entire
• 1:15:44
  Speaker 1: observable universe containing billions of galaxies, a long cosmological history, planets,
• 1:15:51
  Speaker 1: laboratories and astronomers requires an enormous departure from equilibrium. A
• 1:15:56
  Speaker 1: much smaller fluctuation capable of producing only the immediate circumstances
• 1:16:01
  Speaker 1: of one observer would generally be less improbable. Taken too literally,
• 1:16:07
  Speaker 1: this style of reasoning creates bizarre questions about why we
• 1:16:10
  Speaker 1: observe such a vast and consistent cosmos at all. Why
• 1:16:14
  Speaker 1: do we see ancient starlight, distant galaxies, geological records and
• 1:16:19
  Speaker 1: a cosmic history extending billions of years if a far
• 1:16:23
  Speaker 1: smaller fluctuation could have produced a much more limited observer
• 1:16:27
  Speaker 1: with apparently convincing memories. Modern discussions sometimes express the extreme
• 1:16:32
  Speaker 1: version of this puzzle through the idea of a Boltzmann brain,
• 1:16:36
  Speaker 1: a hypothetical self-aware fluctuation appearing briefly in an equilibrium environment.
• 1:16:42
  Speaker 1: It is not proposed as a realistic origin for human beings.
• 1:16:46
  Speaker 1: It is used to expose a problem with cosmological models
• 1:16:49
  Speaker 1: in which random fluctuations would predict that isolated observers should
• 1:16:53
  Speaker 1: be more common than observers inhabiting a vast orderly universe
• 1:16:57
  Speaker 1: like ours. The early universe itself makes the entropy question
• 1:17:00
  Speaker 1: more subtle, because gravity behaves differently from an ordinary gas
• 1:17:04
  Speaker 1: in a box. Observations of the cosmic microwave background show
• 1:17:09
  Speaker 1: that the young universe was remarkably smooth on large scales.
• 1:17:14
  Speaker 1: Its temperature differed across the sky only by roughly a
• 1:17:17
  Speaker 1: few parts in 100,000, once major foreground and motion effects
• 1:17:22
  Speaker 1: were accounted for. At first glance, a smooth distribution might
• 1:17:27
  Speaker 1: sound like a high entropy state. For an ordinary gas
• 1:17:31
  Speaker 1: without significant self-gravity, spreading uniformly through a container is indeed
• 1:17:36
  Speaker 1: associated with equilibrium. Gravity changes the story. A smooth gravitational
• 1:17:42
  Speaker 1: system can evolve into clumps. Slightly denser regions attract more matter.
• 1:17:48
  Speaker 1: Over immense periods, matter forms galaxies, stars and compact objects.
• 1:17:53
  Speaker 1: Black holes in particular carry enormous entropy. This means that
• 1:17:57
  Speaker 1: the smooth, early universe can be understood as having had
• 1:18:00
  Speaker 1: extremely low gravitational entropy despite being hot and close to
• 1:18:06
  Speaker 1: thermal equilibrium in several other respects. Roger Penrose has placed
• 1:18:10
  Speaker 1: particular emphasis on this feature. He has argued that the
• 1:18:14
  Speaker 1: extraordinarily special nature of the early gravitational field is central
• 1:18:19
  Speaker 1: to understanding the arrow of time. Penrose proposed what is
• 1:18:23
  Speaker 1: known as the whale curvature hypothesis. The whale tensor describes
• 1:18:28
  Speaker 1: aspects of spacetime curvature associated with tidal gravitational fields and
• 1:18:33
  Speaker 1: gravitational radiation. Very roughly, Penrose suggested that the initial cosmological
• 1:18:39
  Speaker 1: state should possess tightly constrained whale curvature. corresponding to an
• 1:18:44
  Speaker 1: unusually smooth gravitational configuration, while later gravitational clumping allows increasingly
• 1:18:50
  Speaker 1: complicated structures to develop. It is an attempt to identify
• 1:18:55
  Speaker 1: something precise about the special gravitational character of the beginning.
• 1:19:00
  Speaker 1: It is not a universally accepted final solution. Cosmology nevertheless
• 1:19:05
  Speaker 1: gives strong evidence for the broad observational picture behind the puzzle.
• 1:19:09
  Speaker 1: The universe was once much hotter and denser than it
• 1:19:13
  Speaker 1: is today. The cosmic microwave background reveals an early cosmos
• 1:19:17
  Speaker 1: that was extraordinarily uniform on large scales, with tiny density
• 1:19:22
  Speaker 1: variations that later provided the seeds for galaxies and larger structures.
• 1:19:28
  Speaker 1: As cosmic expansion continued, gravity amplified some of those variations.
• 1:19:34
  Speaker 1: Stars formed, galaxies assembled, black holes appeared, the range of
• 1:19:39
  Speaker 1: accessible high-entropy configurations grew enormously. What observations do not currently
• 1:19:45
  Speaker 1: tell us is why the universe possessed the relevant special
• 1:19:49
  Speaker 1: initial condition. Perhaps a deeper cosmological theory will explain it.
• 1:19:55
  Speaker 1: Perhaps quantum gravity will alter how the question should be asked.
• 1:19:59
  Speaker 1: Perhaps the low-entropy boundary must ultimately be treated as a
• 1:20:03
  Speaker 1: fundamental fact about our universe. There is no accepted answer yet.
• 1:20:08
  Speaker 1: and this matters directly to the nature of time. The
• 1:20:11
  Speaker 1: arrow separating past from future may not be explained by
• 1:20:14
  Speaker 1: a tiny local mechanism hidden inside a clock. It may
• 1:20:18
  Speaker 1: depend on the boundary condition of the entire cosmos. The
• 1:20:21
  Speaker 1: reason a warm drink cools, why records point towards yesterday
• 1:20:25
  Speaker 1: rather than tomorrow, and why broken things do not ordinarily
• 1:20:29
  Speaker 1: reconstruct themselves, may ultimately trace back to the extraordinary condition
• 1:20:34
  Speaker 1: of the universe billions of years ago. Entropy tells us
• 1:20:38
  Speaker 1: why systems evolve overwhelmingly in one direction once a low
• 1:20:42
  Speaker 1: entropy past is given. The deeper puzzle is why that
• 1:20:47
  Speaker 1: past was there to begin with. To understand why time
• 1:20:51
  Speaker 1: seems to possess a direction, physics may therefore have to
• 1:20:54
  Speaker 1: understand not only the laws governing the universe, but why
• 1:20:58
  Speaker 1: the universe began in the particular state that it did.
• 1:21:02
  Speaker 1: If you filmed an ordinary morning and played the recording backwards,
• 1:21:05
  Speaker 1: something would feel wrong almost immediately. Steam would gather itself
• 1:21:09
  Speaker 1: from the air and dive back into a cup of tea.
• 1:21:12
  Speaker 1: Crumbs would leap from the floor and reassemble into toast.
• 1:21:16
  Speaker 1: A smashed plate might lift itself from the kitchen tiles,
• 1:21:20
  Speaker 1: join perfectly along every crack and settle gently onto the table.
• 1:21:25
  Speaker 1: Nothing about the reversed film would be difficult to recognise.
• 1:21:29
  Speaker 1: It would simply look impossible. And yet, At the microscopic level,
• 1:21:33
  Speaker 1: physics is often much less concerned about which direction the
• 1:21:37
  Speaker 1: film is running. This is where time reversal symmetry enters
• 1:21:41
  Speaker 1: the story. A law has time reversal symmetry, roughly speaking,
• 1:21:46
  Speaker 1: if a physically allowed process can be transformed into a
• 1:21:48
  Speaker 1: corresponding process running in the opposite temporal direction while still
• 1:21:53
  Speaker 1: satisfying the same underlying law. That definition needs a little care.
• 1:22:00
  Speaker 1: Reversing time does not simply mean replacing every moment with
• 1:22:03
  Speaker 1: the same scene played backwards while leaving every physical quantity untouched.
• 1:22:09
  Speaker 1: Some quantities must also change. Momentum is one example. If
• 1:22:14
  Speaker 1: a ball is travelling from left to right and the
• 1:22:16
  Speaker 1: motion is reversed, its momentum must now point from right
• 1:22:20
  Speaker 1: to left. Angular momentum changes direction as well. Magnetic fields
• 1:22:26
  Speaker 1: require appropriate transformation under time reversal too. Once these changes
• 1:22:32
  Speaker 1: are handled correctly, many familiar laws show a striking symmetry
• 1:22:36
  Speaker 1: between the two temporal directions. Newtonian mechanics provides a simple example.
• 1:22:42
  Speaker 1: Imagine two billiard balls colliding on a perfectly smooth table.
• 1:22:47
  Speaker 1: One ball approaches, they strike one another and they move apart.
• 1:22:52
  Speaker 1: If every velocity is reversed with perfect accuracy, The balls
• 1:22:56
  Speaker 1: retrace their paths and return to their original configuration. Newton's
• 1:23:01
  Speaker 1: equations do not object. They describe the reverse process just
• 1:23:05
  Speaker 1: as happily as the forward one. Classical electromagnetism also possesses
• 1:23:11
  Speaker 1: a form of time-reversal symmetry when electric and magnetic quantities
• 1:23:15
  Speaker 1: are transformed appropriately. And the ordinary Schrödinger equation of non-relativistic
• 1:23:20
  Speaker 1: quantum mechanics can also display time-reversal symmetry for many systems,
• 1:23:26
  Speaker 1: provided the quantum state is transformed in the correct way.
• 1:23:30
  Speaker 1: This creates an important distinction. Fundamental equations can be time-reversal
• 1:23:35
  Speaker 1: symmetric even though the macroscopic world overwhelmingly is not. A
• 1:23:40
  Speaker 1: scrambled egg does not naturally unscramble, but that is not
• 1:23:43
  Speaker 1: because every microscopic interaction inside the egg contains a tiny
• 1:23:48
  Speaker 1: instruction saying forwards only. The difference comes largely from statistics
• 1:23:53
  Speaker 1: and boundary conditions. There are enormously more microscopic arrangements corresponding
• 1:23:58
  Speaker 1: to a scrambled egg than to a perfectly organised raw one.
• 1:24:03
  Speaker 1: The reverse process is not generally forbidden by the microscopic laws.
• 1:24:07
  Speaker 1: It is fantastically improbable. So when physicists say that many
• 1:24:11
  Speaker 1: laws are time-reversal symmetric, they are not suggesting that broken
• 1:24:15
  Speaker 1: glasses routinely reassemble when nobody's watching. They are describing a
• 1:24:19
  Speaker 1: mathematical property of the underlying dynamics. For a long time,
• 1:24:24
  Speaker 1: it was tempting to wonder whether all fundamental physics might
• 1:24:27
  Speaker 1: treat the two temporal directions in essentially the same way.
• 1:24:31
  Speaker 1: Then the weak nuclear interaction complicated the picture. The weak
• 1:24:35
  Speaker 1: interaction is one of the fundamental interactions of nature. It
• 1:24:40
  Speaker 1: plays a crucial role in processes such as radioactive beta
• 1:24:44
  Speaker 1: decay and in the nuclear reactions that help power stars.
• 1:24:48
  Speaker 1: During the 20th century, experiments revealed that the weak interaction
• 1:24:53
  Speaker 1: does not respect every symmetry that physicists had once expected
• 1:24:58
  Speaker 1: nature to obey. One of these is parity. Parity concerns
• 1:25:02
  Speaker 1: what happens when a physical process is reflected, as though
• 1:25:06
  Speaker 1: viewed in a mirror. For many familiar interactions, the reflected
• 1:25:10
  Speaker 1: version behaves according to the same laws. The weak interaction
• 1:25:15
  Speaker 1: does not always do this. Physicists then examined a broader
• 1:25:19
  Speaker 1: combined symmetry involving charge conjugation and parity. Charge conjugation means
• 1:25:25
  Speaker 1: replacing particles with their corresponding antiparticles. Parity performs the mirror reflection.
• 1:25:32
  Speaker 1: For a while, there was hope that although the weak
• 1:25:35
  Speaker 1: interaction violated parity by itself, the combined operation of charge
• 1:25:40
  Speaker 1: conjugation and parity might still be an exact symmetry. That
• 1:25:45
  Speaker 1: hope failed. in 1964. James Cronin and Val Fitch, working
• 1:25:52
  Speaker 1: with neutral kaons, discovered that some weak interaction processes violate
• 1:25:57
  Speaker 1: the combined symmetry of charge conjugation and parity. Neutral kaons
• 1:26:02
  Speaker 1: are unstable particles that can transform and decay in ways
• 1:26:05
  Speaker 1: sensitive to the weak interaction. Cronin and Fitch found that
• 1:26:09
  Speaker 1: the behaviour of these particles was not perfectly symmetric under
• 1:26:13
  Speaker 1: the combined replacement of particles with antiparticles and spatial reflection.
• 1:26:19
  Speaker 1: The result was profound. It meant that nature contained a
• 1:26:22
  Speaker 1: small but genuine microscopic asymmetry that could not be removed
• 1:26:27
  Speaker 1: simply by combining those two transformations. This matters for time
• 1:26:32
  Speaker 1: because of a deeper result known as the charge, parity
• 1:26:35
  Speaker 1: and time reversal theorem. Within ordinary relativistic quantum field theories
• 1:26:41
  Speaker 1: that satisfy several important assumptions, including locality and Lorentz invariance,
• 1:26:47
  Speaker 1: the combined transformation of charge conjugation, parity and time reversal
• 1:26:52
  Speaker 1: is expected to remain an exact symmetry. This creates a
• 1:26:57
  Speaker 1: powerful connection. If the combination of charge conjugation and parity
• 1:27:02
  Speaker 1: is violated, While the full combination of charge conjugation, parity
• 1:27:07
  Speaker 1: and time reversal is preserved, then time reversal symmetry cannot
• 1:27:11
  Speaker 1: remain perfectly intact in the corresponding processes. Later experiments found
• 1:27:17
  Speaker 1: direct evidence of time reversal violation. Studies of neutral meson systems,
• 1:27:22
  Speaker 1: including neutral kaons and neutral b-mesons, allowed physicists to compare
• 1:27:27
  Speaker 1: transitions that are related specifically by reversing the direction of time.
• 1:27:32
  Speaker 1: Experiments such as those performed by the Barbar Collaboration at
• 1:27:36
  Speaker 1: the Stanford Linear Accelerator Center reported direct observation of time
• 1:27:41
  Speaker 1: reversal violation in neutral B mesons. This was not a
• 1:27:45
  Speaker 1: thermodynamic effect. It was not the result of billions of
• 1:27:49
  Speaker 1: particles drifting towards a more probable macrostate. It was a
• 1:27:53
  Speaker 1: microscopic asymmetry within fundamental particle physics. Nature, at this level,
• 1:27:58
  Speaker 1: really does distinguish between certain time-reversed processes. That sounds at
• 1:28:04
  Speaker 1: first like the missing explanation for the arrow of time.
• 1:28:07
  Speaker 1: Perhaps eggs scramble rather than unscramble, because the fundamental laws
• 1:28:12
  Speaker 1: themselves possess a preferred temporal direction. Unfortunately, the connection is
• 1:28:18
  Speaker 1: not so simple. The microscopic time-reversal violation seen in weak
• 1:28:23
  Speaker 1: interactions is extremely specific. It appears in particular particle processors.
• 1:28:29
  Speaker 1: it does not automatically produce the enormous everyday asymmetry associated
• 1:28:35
  Speaker 1: with thermodynamics. A room does not fill with perfume in
• 1:28:39
  Speaker 1: one direction. rather than the other because neutral kaons happen
• 1:28:43
  Speaker 1: to violate time-reversal symmetry. The statistical explanation involving entropy remains essential.
• 1:28:51
  Speaker 1: Even if weak interaction and time-reversal violation were absent, a
• 1:28:56
  Speaker 1: low entropy glass could still shatter into a vastly more
• 1:28:59
  Speaker 1: probable collection of fragments, vibrations and heat. And even with
• 1:29:05
  Speaker 1: time reversal violation present, the fundamental question remains of why
• 1:29:10
  Speaker 1: the universe began in a condition from which entropy could
• 1:29:13
  Speaker 1: rise so dramatically. Microscopic and macroscopic arrows therefore have to
• 1:29:18
  Speaker 1: be kept separate. At the microscopic level, some fundamental interactions
• 1:29:24
  Speaker 1: genuinely violate time reversal symmetry. At the macroscopic level, the
• 1:29:29
  Speaker 1: overwhelming directionality of cooling, mixing, ageing and decay is still
• 1:29:34
  Speaker 1: explained primarily through statistical mechanics and the special low entropy
• 1:29:39
  Speaker 1: condition of the past. The two facts may ultimately belong
• 1:29:43
  Speaker 1: within one deeper picture of the universe, but physics has
• 1:29:47
  Speaker 1: not established a simple chain in which one explains the other.
• 1:29:50
  Speaker 1: That leaves us with an unexpectedly subtle answer. Many fundamental
• 1:29:55
  Speaker 1: laws do not care very much which way time points.
• 1:29:58
  Speaker 1: Some do. And the dramatic arrow of time we experience
• 1:30:02
  Speaker 1: every day still appears to depend less on a universal
• 1:30:05
  Speaker 1: command hidden in every equation, and more on probability, entropy,
• 1:30:10
  Speaker 1: and the remarkable condition from which the universe began. Quantum
• 1:30:15
  Speaker 1: mechanics introduces another difficulty into the story of time. And
• 1:30:19
  Speaker 1: this time, the problem does not begin with clocks moving
• 1:30:22
  Speaker 1: quickly or sitting in strong gravitational fields. It begins inside
• 1:30:28
  Speaker 1: the mathematical structure of the theory itself. In the 1920s, physicists,
• 1:30:33
  Speaker 1: including Erwin Schrödinger, developed a new way of describing microscopic
• 1:30:37
  Speaker 1: systems such as atoms, electrons and particles of light. At
• 1:30:42
  Speaker 1: the centre of Schrödinger' s formulation is the quantum state. This
• 1:30:46
  Speaker 1: state contains the information needed to calculate the probabilities of
• 1:30:51
  Speaker 1: different measurement outcomes, and the Schrodinger equation tells us how
• 1:30:55
  Speaker 1: that quantum state changes, but there is something unusual hiding
• 1:30:59
  Speaker 1: in the equation. Time appears as an external parameter. The
• 1:31:04
  Speaker 1: state evolves with time, but time itself is not usually
• 1:31:08
  Speaker 1: treated as a quantum observable in the same way as
• 1:31:11
  Speaker 1: quantities such as position or momentum. That distinction matters. In
• 1:31:17
  Speaker 1: ordinary quantum mechanics, position is represented by a mathematical object
• 1:31:22
  Speaker 1: called an operator. Momentum is represented by an operator as well,
• 1:31:27
  Speaker 1: so are angular momentum, spin and energy. These operators act
• 1:31:31
  Speaker 1: on the quantum state and allow physicists to calculate possible
• 1:31:36
  Speaker 1: measurement results and their probabilities. Time normally has a different job.
• 1:31:41
  Speaker 1: Instead of being represented by an operator telling us the
• 1:31:44
  Speaker 1: possible values of time in the same way that a
• 1:31:47
  Speaker 1: position operator gives possible positions, time usually labels the evolution
• 1:31:52
  Speaker 1: of the system. It tells us when the quantum state
• 1:31:54
  Speaker 1: is being considered. Very loosely, the theory says something like this.
• 1:31:59
  Speaker 1: Here is the quantum state now. Here is how that
• 1:32:02
  Speaker 1: state changes as the time parameter changes. But the time
• 1:32:06
  Speaker 1: parameter itself usually sits outside the quantum system being described
• 1:32:11
  Speaker 1: It behaves more like part of the background against which
• 1:32:14
  Speaker 1: quantum events unfold. This is conceptually striking because relativity has
• 1:32:19
  Speaker 1: already taught us to be suspicious of fixed backgrounds. General
• 1:32:23
  Speaker 1: relativity says that space-time is dynamical. Its geometry responds to
• 1:32:28
  Speaker 1: matter and energy. Quantum mechanics, meanwhile, usually begins by assuming
• 1:32:33
  Speaker 1: that some temporal structure has already been provided so that
• 1:32:37
  Speaker 1: the quantum state knows how to evolve. The difference is
• 1:32:41
  Speaker 1: easy to ignore when studying an atom in a laboratory.
• 1:32:44
  Speaker 1: It becomes much harder to ignore when trying to describe
• 1:32:47
  Speaker 1: the entire universe. The unusual role of time in quantum
• 1:32:52
  Speaker 1: mechanics was recognised early. Wolfgang Pauli is often associated with
• 1:32:57
  Speaker 1: an important argument concerning what would happen if time were
• 1:33:00
  Speaker 1: represented by a universal self-adjoint operator in exactly the right
• 1:33:06
  Speaker 1: canonical relationship with energy. In quantum mechanics, position and momentum
• 1:33:11
  Speaker 1: have a special mathematical relationship. Roughly speaking, this relationship helps
• 1:33:17
  Speaker 1: produce the familiar position-momentum uncertainty principle. One might therefore wonder
• 1:33:23
  Speaker 1: whether time could be represented by a similar operator canonically
• 1:33:27
  Speaker 1: paired with energy. Pauli pointed out a serious difficulty. If
• 1:33:32
  Speaker 1: such a universal time operator had the required canonical relationship
• 1:33:36
  Speaker 1: with the Hamiltonian, the operator representing energy, then under the
• 1:33:41
  Speaker 1: usual assumptions it would imply an energy spectrum extending without
• 1:33:45
  Speaker 1: bound in both directions. That would mean no lowest possible energy.
• 1:33:51
  Speaker 1: But many real quantum systems do have a lowest energy state. Atoms,
• 1:33:55
  Speaker 1: for example, possess ground states. So the simple idea of
• 1:34:00
  Speaker 1: treating time exactly like position runs into trouble. This is
• 1:34:04
  Speaker 1: sometimes summarised too strongly by saying that quantum mechanics does
• 1:34:08
  Speaker 1: not allow time observables. That is not correct. Physicists can
• 1:34:13
  Speaker 1: and do study quantities such as arrival times, dwell times
• 1:34:17
  Speaker 1: and characteristic decay times. Various operational and mathematical constructions can
• 1:34:22
  Speaker 1: describe temporal measurements. The important point is narrower. There is
• 1:34:27
  Speaker 1: no universal time operator in ordinary quantum mechanics that behaves
• 1:34:31
  Speaker 1: in exactly the same way as position. while remaining canonically
• 1:34:35
  Speaker 1: conjugate to a Hamiltonian with a lower bounded energy spectrum.
• 1:34:40
  Speaker 1: That subtlety also matters when discussing the energy-time uncertainty relationship.
• 1:34:45
  Speaker 1: The position momentum uncertainty principle involves two operators whose measurement
• 1:34:51
  Speaker 1: distributions cannot both be made arbitrarily sharp in the same
• 1:34:54
  Speaker 1: quantum state. The energy-time case is different. Time is not
• 1:34:59
  Speaker 1: generally represented by an operator in the same standard way.
• 1:35:03
  Speaker 1: So energy-time uncertainty should not simply be imagined as meaning
• 1:35:07
  Speaker 1: that the universe cannot simultaneously know the energy and the time. Instead,
• 1:35:13
  Speaker 1: several related forms exist. One common version connects the spread
• 1:35:18
  Speaker 1: in energy of a quantum state with the characteristic timescale
• 1:35:22
  Speaker 1: over which that state can change appreciably. A state with
• 1:35:26
  Speaker 1: a very narrow range of energies may evolve slowly into
• 1:35:30
  Speaker 1: a distinguishably different state. A state containing a broader spread
• 1:35:34
  Speaker 1: of energies can change more rapidly. Another familiar context appears
• 1:35:39
  Speaker 1: in unstable quantum systems. An excited atomic state or unstable
• 1:35:44
  Speaker 1: particle may have a characteristic lifetime. A very short-lived state
• 1:35:49
  Speaker 1: is often associated with a broader spread in measured energy.
• 1:35:54
  Speaker 1: These relationships are enormously useful, but they do not place
• 1:35:57
  Speaker 1: time and energy on precisely the same footing as position
• 1:36:01
  Speaker 1: and momentum. Time remains deeply involved in quantum predictions nonetheless.
• 1:36:07
  Speaker 1: Consider radioactive decay. Quantum mechanics can calculate the probability that
• 1:36:13
  Speaker 1: an unstable system will remain undecayed after a certain interval.
• 1:36:17
  Speaker 1: The theory does not usually predict the exact moment when
• 1:36:21
  Speaker 1: one particular, unstable nucleus will decay. Instead, it predicts a
• 1:36:27
  Speaker 1: probability distribution over time. Half-life is therefore a temporal concept
• 1:36:32
  Speaker 1: built directly into the statistical behaviour. Or consider quantum oscillations.
• 1:36:38
  Speaker 1: Some quantum systems can evolve back and forth between distinguishable states.
• 1:36:43
  Speaker 1: Neutrinos provide a striking example. As they travel, quantum interference
• 1:36:48
  Speaker 1: between different mass states can change the probability of detecting
• 1:36:52
  Speaker 1: one neutrino flavour rather than another. The amount of evolution
• 1:36:56
  Speaker 1: depends on distance, energy. and the associated propagation time. Quantum
• 1:37:01
  Speaker 1: interference more broadly is also impossible to describe without keeping
• 1:37:05
  Speaker 1: track of relative phases that evolve over time. A superposition
• 1:37:10
  Speaker 1: of energy states does not simply sit unchanged in every respect.
• 1:37:14
  Speaker 1: Its components acquire different phases as the time parameter advances.
• 1:37:19
  Speaker 1: Those changing phase relationships can alter measurable interference patterns. So
• 1:37:24
  Speaker 1: quantum mechanics uses time constantly. It could hardly function without it.
• 1:37:30
  Speaker 1: The strange part is that time is usually supplied to
• 1:37:33
  Speaker 1: the theory rather than treated as another ordinary quantum degree
• 1:37:37
  Speaker 1: of freedom belonging to the system. For laboratory physics, this
• 1:37:41
  Speaker 1: arrangement works extremely well. A clock on the wall or
• 1:37:45
  Speaker 1: an electronic timing system can be treated approximately as external
• 1:37:49
  Speaker 1: to the atom or particle being studied. The laboratory has
• 1:37:53
  Speaker 1: a sufficiently well-defined temporal background. The experiment begins at one
• 1:37:58
  Speaker 1: time and ends at another. Nothing seems especially troubling. But
• 1:38:04
  Speaker 1: now compare this with general relativity. General relativity says that
• 1:38:08
  Speaker 1: the geometry of spacetime is itself a physical field. It
• 1:38:12
  Speaker 1: can curve, it can change. Its temporal structure depends on matter,
• 1:38:18
  Speaker 1: energy and gravitational conditions. Quantum mechanics usually assume some background
• 1:38:24
  Speaker 1: time against which evolution occurs. General relativity says that the
• 1:38:28
  Speaker 1: very thing supplying temporal structure is dynamical that tension becomes
• 1:38:33
  Speaker 1: unavoidable when gravity itself must be quantized a quantum theory
• 1:38:38
  Speaker 1: of gravity cannot simply place quantum matter inside one perfectly
• 1:38:42
  Speaker 1: classical space-time and declare the problem solved In regimes where
• 1:38:47
  Speaker 1: spacetime geometry itself must obey quantum rules, the clock and
• 1:38:51
  Speaker 1: the background can no longer remain completely separate from the
• 1:38:55
  Speaker 1: quantum system. The object telling quantum states how to evolve
• 1:38:59
  Speaker 1: becomes part of what must itself be described quantum mechanically,
• 1:39:03
  Speaker 1: and that is where a seemingly technical difference in the
• 1:39:05
  Speaker 1: Schrodinger equation turns into one of the deepest problems in
• 1:39:09
  Speaker 1: fundamental physics. Quantum mechanics needs time. General relativity makes time
• 1:39:14
  Speaker 1: part of dynamical spacetime. A complete theory of quantum gravity
• 1:39:18
  Speaker 1: must somehow reconcile the two. It must explain how quantum
• 1:39:22
  Speaker 1: evolution works when the temporal structure that normally organises that
• 1:39:26
  Speaker 1: evolution is no longer fixed in advance. Once again, physics
• 1:39:31
  Speaker 1: can describe an enormous amount about how systems change. The
• 1:39:35
  Speaker 1: harder question is what time must be for those descriptions
• 1:39:38
  Speaker 1: to fit together. If time is not supplied from outside
• 1:39:41
  Speaker 1: a quantum system, perhaps it can be built from relationships
• 1:39:44
  Speaker 1: inside the system instead. That possibility sounds strange at first,
• 1:39:50
  Speaker 1: because ordinary physics encourages us to imagine time as something
• 1:39:53
  Speaker 1: already present. We set up an experiment. We start a clock.
• 1:39:58
  Speaker 1: We watch a system evolve. The clock appears to sit
• 1:40:02
  Speaker 1: outside the experiment, calmly keeping track of events while the
• 1:40:05
  Speaker 1: interesting physics happen somewhere else. But a real clock is
• 1:40:09
  Speaker 1: never completely outside nature. It is made from atoms, fields
• 1:40:13
  Speaker 1: and matter. It has energy. It interacts with its surroundings.
• 1:40:18
  Speaker 1: And if quantum mechanics applies universally, then the clock itself
• 1:40:22
  Speaker 1: must ultimately be a quantum system too. That suggests a
• 1:40:26
  Speaker 1: different way of thinking. Instead of asking how a system
• 1:40:30
  Speaker 1: changes with respect to an external time that exists independently
• 1:40:33
  Speaker 1: of everything else, we can ask how one physical system
• 1:40:37
  Speaker 1: changes relative to another. One system can act as a clock.
• 1:40:41
  Speaker 1: The other can be the system we want to study.
• 1:40:44
  Speaker 1: the idea becomes particularly interesting in quantum theory. In 1983,
• 1:40:49
  Speaker 1: physicists Don Page and William Wooters developed a relational approach
• 1:40:53
  Speaker 1: to time that explored how an effective sense of evolution
• 1:40:57
  Speaker 1: could emerge from correlations between different parts of a quantum system.
• 1:41:02
  Speaker 1: The central idea is easier to understand if we begin
• 1:41:05
  Speaker 1: with something simple. Imagine two physical systems. One changes in
• 1:41:09
  Speaker 1: a regular way and acts as a clock. The other
• 1:41:12
  Speaker 1: has some state that also changes. Normally, we might say
• 1:41:16
  Speaker 1: that the second system is in one condition at one
• 1:41:18
  Speaker 1: o'clock and another condition at two o'clock. But notice what
• 1:41:22
  Speaker 1: that statement really uses. It compares the state of one
• 1:41:25
  Speaker 1: system with the reading of another. So instead of saying
• 1:41:29
  Speaker 1: that the second system changes with respect to some invisible
• 1:41:32
  Speaker 1: external time, we can phrase the same information relationally. When
• 1:41:36
  Speaker 1: the clock system has one reading, the other system has
• 1:41:40
  Speaker 1: one state. When the clock system has another reading, the
• 1:41:43
  Speaker 1: other system has changed. Time, in this description, is encoded
• 1:41:47
  Speaker 1: in correlations. That is the essential intuition behind relational time.
• 1:41:53
  Speaker 1: The Page and Wouter's approach takes this idea into quantum mechanics.
• 1:41:57
  Speaker 1: A combined quantum system can, under the right conditions, be
• 1:42:00
  Speaker 1: described in a way that is globally stationary. At first
• 1:42:03
  Speaker 1: this sounds disastrous. If the full quantum state is stationary,
• 1:42:07
  Speaker 1: where has change gone? But the answer is that change
• 1:42:11
  Speaker 1: can still appear internally. One part of the system can
• 1:42:14
  Speaker 1: act as a clock. The state of another part can
• 1:42:17
  Speaker 1: then be described conditionally, relative to what the clock part
• 1:42:20
  Speaker 1: is showing. From the perspective of the complete system, the
• 1:42:25
  Speaker 1: overall state need not evolve in the familiar external time sense,
• 1:42:29
  Speaker 1: yet observers inside the system can still recover an effective
• 1:42:32
  Speaker 1: sequence of changes by comparing one subsystem with another. The
• 1:42:37
  Speaker 1: whole can be stationary, while the relationships within it contain
• 1:42:40
  Speaker 1: something that behaves like time. That sounds almost paradoxical, but
• 1:42:44
  Speaker 1: a simple analogy helps. Imagine a large book containing every
• 1:42:49
  Speaker 1: frame of a film. The book itself does not need
• 1:42:52
  Speaker 1: to move. All the pages already exist. Yet if one
• 1:42:57
  Speaker 1: part of the page tells you which frame you're looking at,
• 1:43:00
  Speaker 1: and another part shows the scene associated with that frame,
• 1:43:03
  Speaker 1: then an ordered sequence can still be reconstructed from the
• 1:43:06
  Speaker 1: relationships between them. The analogy is imperfect. especially because quantum
• 1:43:12
  Speaker 1: correlations are subtler than pages in a book, but it
• 1:43:15
  Speaker 1: captures the broad idea. Temporal order can emerge from correlations
• 1:43:19
  Speaker 1: without requiring an external clock sitting beyond the entire system.
• 1:43:24
  Speaker 1: For this to work physically, the clock cannot be an
• 1:43:26
  Speaker 1: abstract label. It must itself be a real quantum object.
• 1:43:30
  Speaker 1: And real quantum clocks are not perfect. A quantum clock
• 1:43:34
  Speaker 1: is subject to uncertainty. Its state can spread. It can
• 1:43:38
  Speaker 1: interact with other systems. It has limited energy. Its components
• 1:43:42
  Speaker 1: cannot be prepared and measured with infinite precision. Any practical
• 1:43:47
  Speaker 1: clock therefore has finite accuracy. This is not simply an
• 1:43:50
  Speaker 1: engineering inconvenience. At a deep enough level, it reflects the
• 1:43:54
  Speaker 1: fact that measuring time requires physical resources. A clock must
• 1:43:59
  Speaker 1: change from one distinguishable state to another. If nothing about
• 1:44:03
  Speaker 1: the clock changed, it could not mark the difference between
• 1:44:06
  Speaker 1: one moment and another. This connects to an important result
• 1:44:09
  Speaker 1: associated with the Soviet physicists Leonid Mandelstam and Igor Tam.
• 1:44:15
  Speaker 1: The Mandelstam-Tam energy-time uncertainty relation links the spread in energy
• 1:44:20
  Speaker 1: of a quantum state to the characteristic timescale, over which
• 1:44:24
  Speaker 1: that state can change appreciably. In broad terms, a state
• 1:44:28
  Speaker 1: with a very narrow energy distribution cannot evolve into a
• 1:44:32
  Speaker 1: clearly distinguishable state arbitrarily quickly. A larger spread in energy
• 1:44:37
  Speaker 1: allows faster distinguishable evolution. This does not mean that time
• 1:44:41
  Speaker 1: itself becomes fuzzy in exactly the same way that position
• 1:44:44
  Speaker 1: can be uncertain. The relation is subtler than that, but
• 1:44:49
  Speaker 1: it does reveal a fundamental link between temporal resolution and
• 1:44:52
  Speaker 1: the physical state of the clock. To mark very fine intervals,
• 1:44:57
  Speaker 1: a clock must contain physical dynamics capable of changing on
• 1:45:00
  Speaker 1: those intervals. That makes timekeeping an active physical process rather
• 1:45:05
  Speaker 1: than a passive reading of an invisible background. There are
• 1:45:08
  Speaker 1: also limits imposed by interactions. A clock that is completely
• 1:45:12
  Speaker 1: isolated may be difficult to read. A clock that interacts
• 1:45:16
  Speaker 1: strongly enough to be read may be disturbed. A quantum
• 1:45:20
  Speaker 1: clock cannot simply stand outside the laws of quantum mechanics
• 1:45:24
  Speaker 1: and announce the time with perfect authority. It participates in
• 1:45:28
  Speaker 1: the same physics as everything else. This idea fits surprisingly
• 1:45:32
  Speaker 1: well with one of the broader lessons of relativity. Albert
• 1:45:36
  Speaker 1: Einstein's theories already taught physicists to take physical clocks seriously.
• 1:45:41
  Speaker 1: Relativity does not begin by assuming one universal cosmic time
• 1:45:45
  Speaker 1: and then asking how imperfect clocks respond to it. Instead,
• 1:45:49
  Speaker 1: an ideal clock travelling along a world line measures proper
• 1:45:52
  Speaker 1: time associated with that path through spacetime. The clock is
• 1:45:56
  Speaker 1: part of the physical situation. Quantum theory pushes that operational
• 1:46:00
  Speaker 1: idea further. If clocks are physical systems, then at a
• 1:46:04
  Speaker 1: fundamental level their own quantum nature cannot always be ignored.
• 1:46:09
  Speaker 1: One object can provide a temporal reference for another, but
• 1:46:12
  Speaker 1: that reference is never completely detached from the universe it
• 1:46:15
  Speaker 1: is helping us describe. This does not prove that time
• 1:46:19
  Speaker 1: is unreal. That distinction is essential. A successful relational description
• 1:46:24
  Speaker 1: shows that some temporal information can be encoded through correlations
• 1:46:28
  Speaker 1: between physical systems. It does not by itself establish that
• 1:46:32
  Speaker 1: time has no fundamental existence, nor does it prove that
• 1:46:36
  Speaker 1: every appearance of temporal evolution must ultimately be explained relationally.
• 1:46:41
  Speaker 1: It offers one possible way of understanding how time-like behaviour
• 1:46:45
  Speaker 1: can emerge inside a quantum framework without assuming that an
• 1:46:49
  Speaker 1: external master clock must always be present. The idea becomes
• 1:46:53
  Speaker 1: especially important when thinking about the universe as a whole.
• 1:46:57
  Speaker 1: In an ordinary laboratory we can always point to a
• 1:46:59
  Speaker 1: clock outside the specific quantum system under study. The atom
• 1:47:03
  Speaker 1: evolves while the laboratory clock runs. The photon is detected
• 1:47:07
  Speaker 1: at a particular time according to the electronics on the bench,
• 1:47:11
  Speaker 1: but if the entire universe is treated as one quantum system,
• 1:47:14
  Speaker 1: what external clock could possibly be used? There is nowhere
• 1:47:18
  Speaker 1: outside the universe to place it. No second universe needs
• 1:47:22
  Speaker 1: to sit nearby holding a stopwatch. Any clock we can
• 1:47:25
  Speaker 1: actually refer to must be part of the system itself.
• 1:47:29
  Speaker 1: That makes relational time more than an interesting philosophical option.
• 1:47:33
  Speaker 1: It becomes a practical clue for how quantum cosmology might
• 1:47:36
  Speaker 1: describe change. Instead of asking how the universe evolves with
• 1:47:40
  Speaker 1: respect to an external time, one might ask how one
• 1:47:43
  Speaker 1: part of the universe changes relative to another. One degree
• 1:47:47
  Speaker 1: of freedom becomes the clock. Another becomes the system being measured.
• 1:47:52
  Speaker 1: Temporal order appears through their correlations. The resulting picture is
• 1:47:56
  Speaker 1: very different from Newton's universal time flowing independently of everything.
• 1:48:01
  Speaker 1: It is also different from simply treating time as an
• 1:48:03
  Speaker 1: unexamined parameter. Here, time is tied to physical relationships. A
• 1:48:09
  Speaker 1: clock does not merely reveal a background. It helps define
• 1:48:12
  Speaker 1: the reference against which change is described, and if that
• 1:48:16
  Speaker 1: idea is correct at a deep level, then time may
• 1:48:19
  Speaker 1: not always need to be something imposed on the universe
• 1:48:21
  Speaker 1: from outside. It may be something physical systems create for
• 1:48:25
  Speaker 1: one another. General relativity and quantum mechanics are both extraordinarily successful. Unfortunately,
• 1:48:32
  Speaker 1: they do not treat time in the same way. Quantum
• 1:48:35
  Speaker 1: mechanics usually describes a system evolving against a time parameter
• 1:48:39
  Speaker 1: that has already been supplied. General relativity does something very different.
• 1:48:44
  Speaker 1: In general relativity, space-time itself is dynamical. Its geometry is
• 1:48:49
  Speaker 1: not merely a fixed background. Matter and energy influence that geometry,
• 1:48:54
  Speaker 1: and the geometry determines how clocks tick, how light travels
• 1:48:58
  Speaker 1: and how objects move. That difference becomes impossible to ignore
• 1:49:02
  Speaker 1: when gravity itself must be described quantum mechanically. A complete
• 1:49:06
  Speaker 1: theory of quantum gravity would have to describe situations in
• 1:49:10
  Speaker 1: which spacetime geometry is not perfectly classical. Instead, the geometry
• 1:49:15
  Speaker 1: itself would somehow possess quantum behaviour. That means the very
• 1:49:19
  Speaker 1: structure used to define distances, durations and causal relationships would
• 1:49:24
  Speaker 1: become part of the quantum system. And once that happens,
• 1:49:28
  Speaker 1: the usual idea of evolution with respect to an external
• 1:49:31
  Speaker 1: time becomes much harder to maintain. One of the most
• 1:49:35
  Speaker 1: important attempts to confront this problem arose from the canonical
• 1:49:39
  Speaker 1: approach to quantum gravity. John Archibald Wheeler and Bryce DeWitt
• 1:49:43
  Speaker 1: played central roles in developing this framework during the 20th century.
• 1:49:48
  Speaker 1: The resulting equation became known as the Wheeler-DeWitt equation. Its
• 1:49:52
  Speaker 1: purpose was ambitious. Instead of quantising matter inside a fixed spacetime,
• 1:49:58
  Speaker 1: the idea was to apply quantum principles to the gravitational
• 1:50:01
  Speaker 1: field itself. In a simplified quantum cosmological setting, the wavefunction
• 1:50:08
  Speaker 1: is no longer merely a wavefunction for one particle moving
• 1:50:11
  Speaker 1: through a background. It can describe possible configurations of geometry
• 1:50:15
  Speaker 1: and matter together. That is already a major conceptual shift.
• 1:50:20
  Speaker 1: But the strangest feature appears when one asks the most
• 1:50:23
  Speaker 1: familiar quantum question. How does the state change with time?
• 1:50:28
  Speaker 1: In the ordinary Schrödinger equation, the quantum state evolves with
• 1:50:32
  Speaker 1: respect to an external time parameter. The equation explicitly connects
• 1:50:36
  Speaker 1: changes in the state to that time variable. The Wheeler-DeWitt
• 1:50:40
  Speaker 1: equation does not contain time in that familiar form. At
• 1:50:44
  Speaker 1: first glance, it can look as though time has simply vanished.
• 1:50:47
  Speaker 1: This is sometimes described by saying that the equation for
• 1:50:51
  Speaker 1: the universe is timeless. That phrase is memorable. but it
• 1:50:54
  Speaker 1: can also be misleading. The missing external time variable is
• 1:50:59
  Speaker 1: not a random omission. It arises from the structure of
• 1:51:03
  Speaker 1: general relativity itself. General relativity can be reformulated in what
• 1:51:07
  Speaker 1: is called Hamiltonian form. In that formulation, the theory contains constraints.
• 1:51:14
  Speaker 1: One of the most important is the Hamiltonian constraint. Very roughly,
• 1:51:19
  Speaker 1: Instead of having an ordinary Hamiltonian that generates evolution relative
• 1:51:22
  Speaker 1: to an external universal time, general relativity is invariant under
• 1:51:27
  Speaker 1: different choices of how spacetime is sliced into space and time.
• 1:51:32
  Speaker 1: The theory does not identify one preferred external time parameter
• 1:51:36
  Speaker 1: that all observers must use. When the corresponding Hamiltonian constraint
• 1:51:40
  Speaker 1: is carried into the quantum theory, the result has the
• 1:51:43
  Speaker 1: form associated with the Wheeler-DeWitt equations. So the apparent disappearance
• 1:51:49
  Speaker 1: of time reflects something deep. The classical theory being quantized
• 1:51:53
  Speaker 1: already treats temporal description differently from ordinary mechanics. This is
• 1:51:58
  Speaker 1: the heart of what physicists call the problem of time.
• 1:52:03
  Speaker 1: Despite the singular name, it is not really one problem.
• 1:52:07
  Speaker 1: It is a family of related problems. How should physical
• 1:52:10
  Speaker 1: change be described if there is no external time parameter?
• 1:52:15
  Speaker 1: Which quantities count as genuine observables when general relativity allows
• 1:52:20
  Speaker 1: many different coordinate descriptions of the same physical situation? How
• 1:52:24
  Speaker 1: should one define temporal ordering? How does the familiar notion
• 1:52:28
  Speaker 1: of evolution emerge? And how can a theory without an
• 1:52:32
  Speaker 1: obvious fundamental time variable reproduce the ordinary experience of clocks
• 1:52:36
  Speaker 1: ticking and systems changing? These are technical questions, not merely
• 1:52:42
  Speaker 1: philosophical ones. The difficulty appears because quantum mechanics and general
• 1:52:47
  Speaker 1: relativity begin from very different starting points. In ordinary quantum mechanics,
• 1:52:52
  Speaker 1: time is generally supplied from outside the quantum system. The
• 1:52:57
  Speaker 1: quantum state then evolves with respect to it. In general relativity,
• 1:53:02
  Speaker 1: the geometry that determines physical clock rates belongs to the
• 1:53:05
  Speaker 1: system itself. There is no need for a hidden universal
• 1:53:09
  Speaker 1: clock beyond spacetime. So if the gravitational field becomes quantum mechanical,
• 1:53:14
  Speaker 1: the thing normally used to organise quantum evolution becomes quantum
• 1:53:18
  Speaker 1: mechanical too. The background has joined the experiment. That makes
• 1:53:23
  Speaker 1: the usual separation between clock and system much harder to sustain.
• 1:53:28
  Speaker 1: Suppose the geometry of spacetime exists in a quantum superposition
• 1:53:32
  Speaker 1: of different configurations. Then different possible geometries may correspond to
• 1:53:37
  Speaker 1: different relationships among clocks, distances, and causal structures. What would
• 1:53:43
  Speaker 1: it mean to say that the entire state evolves at
• 1:53:46
  Speaker 1: one particular external time? Which time? Defined by which geometry?
• 1:53:52
  Speaker 1: These questions become especially sharp in quantum cosmology. If the
• 1:53:56
  Speaker 1: whole universe is being treated as a quantum system, there
• 1:54:00
  Speaker 1: is no obvious external observer and no master clock placed
• 1:54:04
  Speaker 1: outside the cosmos. Every possible clock is part of the
• 1:54:07
  Speaker 1: system under description. This helps explain why relational approaches to
• 1:54:12
  Speaker 1: time become attractive. One physical degree of freedom might be
• 1:54:16
  Speaker 1: used as a clock relative to another. Instead of asking
• 1:54:20
  Speaker 1: how the universe changes with respect to an external time,
• 1:54:24
  Speaker 1: one can ask how one part changes when another part
• 1:54:27
  Speaker 1: takes on a particular value. In that sense, time might
• 1:54:30
  Speaker 1: reappear internally through correlations. But that is only one strategy.
• 1:54:36
  Speaker 1: Different approaches to quantum gravity treat the problem in different ways.
• 1:54:40
  Speaker 1: Some search for an internal clock variable. Some try to
• 1:54:44
  Speaker 1: recover an approximate notion of time only in certain regimes.
• 1:54:48
  Speaker 1: Others reformulate the theory so that fundamental statements are relational
• 1:54:52
  Speaker 1: from the beginning. The disagreement is not merely about terminology.
• 1:54:57
  Speaker 1: It concerns how the mathematics should describe change when spacetime
• 1:55:01
  Speaker 1: itself is quantum. It is also important not to over-interpret
• 1:55:05
  Speaker 1: the Wheeler-DeWitt equation. The absence of an ordinary external time
• 1:55:10
  Speaker 1: variable does not mean that experiments have shown the universe
• 1:55:13
  Speaker 1: to be literally frozen. Nothing about the equation proves that
• 1:55:18
  Speaker 1: change is an illusion. People age, stars evolve, particles decay,
• 1:55:24
  Speaker 1: clocks accumulate readings. These are physical facts that any successful
• 1:55:29
  Speaker 1: deeper theory must reproduce. The challenge is to explain how
• 1:55:33
  Speaker 1: such time-dependent behaviour emerges from a framework in which time
• 1:55:37
  Speaker 1: does not appear in the usual fundamental form. There are
• 1:55:42
  Speaker 1: several ways this can happen in simplified models. A distinction
• 1:55:46
  Speaker 1: can sometimes be made between slow and fast degrees of freedom,
• 1:55:50
  Speaker 1: allowing an approximate time parameter to emerge for part of
• 1:55:53
  Speaker 1: the system. One variable can function as a relational clock.
• 1:55:57
  Speaker 1: Semiclassical approximations can recover equations resembling ordinary quantum evolution against
• 1:56:03
  Speaker 1: an effective background spacetime. In suitable limits, familiar temporal behaviour
• 1:56:08
  Speaker 1: can return. That is an important clue. A concept does
• 1:56:12
  Speaker 1: not have to appear in the most fundamental equation in
• 1:56:15
  Speaker 1: exactly the same form in which it appears at larger scales.
• 1:56:19
  Speaker 1: Temperature provides a familiar example. Individual atoms do not each
• 1:56:23
  Speaker 1: possess the temperature of a gas in the same sense
• 1:56:26
  Speaker 1: that the gas as a whole does. Temperature emerges from
• 1:56:29
  Speaker 1: collective behaviour. Time might conceivably behave in a similarly layered
• 1:56:34
  Speaker 1: way in quantum gravity. That remains a possibility rather than
• 1:56:39
  Speaker 1: an established fact. The technical challenge is that any proposed
• 1:56:43
  Speaker 1: fundamental theory must eventually reproduce the extraordinarily successful temporal structure
• 1:56:49
  Speaker 1: Already tested by relativity and quantum mechanics, it must recover
• 1:56:54
  Speaker 1: proper time along classical world lines. It must explain ordinary
• 1:56:58
  Speaker 1: quantum evolution, where a background space-time is a good approximation.
• 1:57:03
  Speaker 1: It must preserve causal relationships in the regimes where relativity
• 1:57:07
  Speaker 1: has been tested. And it must do all of this
• 1:57:10
  Speaker 1: while providing a consistent description of quantum space-time itself. This
• 1:57:15
  Speaker 1: is where the disagreement over time becomes unavoidable. Physicists are
• 1:57:20
  Speaker 1: not merely choosing different words for the same settled picture.
• 1:57:23
  Speaker 1: They're confronting genuinely different mathematical possibilities for how change, duration
• 1:57:29
  Speaker 1: and causal structure arise at the deepest level. A successful
• 1:57:34
  Speaker 1: theory of quantum gravity cannot simply assume that time works
• 1:57:38
  Speaker 1: exactly as it does in ordinary laboratory quantum mechanics. Nor
• 1:57:43
  Speaker 1: can it ignore the temporal predictions that established physics gets right. Somehow,
• 1:57:49
  Speaker 1: It has to explain why a world full of clocks, histories,
• 1:57:53
  Speaker 1: decay processes and evolving systems emerges from whatever more fundamental
• 1:57:58
  Speaker 1: structure lies underneath. And that is why the problem of
• 1:58:02
  Speaker 1: time remains one of the clearest places where our best
• 1:58:05
  Speaker 1: theories reveal that they are not yet one complete theory
• 1:58:08
  Speaker 1: of reality. There is a possibility more radical than saying
• 1:58:13
  Speaker 1: that time behaves differently in different circumstances. Perhaps time is
• 1:58:17
  Speaker 1: not fundamental at all. That does not mean clocks are imaginary,
• 1:58:21
  Speaker 1: that ageing is unreal, or that yesterday somehow failed to happen.
• 1:58:26
  Speaker 1: The idea is more precise. Some physicists have explored whether
• 1:58:30
  Speaker 1: the temporal behaviour familiar to us could emerge from deeper
• 1:58:33
  Speaker 1: relationships among physical systems, even if the most fundamental description
• 1:58:38
  Speaker 1: of reality contains no universal time variable, resembling the one
• 1:58:43
  Speaker 1: used in ordinary mechanics. Physics already contains examples of this
• 1:58:48
  Speaker 1: kind of emergence. Temperature is real. A thermometer measures something
• 1:58:52
  Speaker 1: physically meaningful, and temperature determines how systems exchange thermal energy.
• 1:58:58
  Speaker 1: Yet an individual atom does not possess temperature in quite
• 1:59:01
  Speaker 1: the same sense as a cup of water does. Temperature
• 1:59:05
  Speaker 1: becomes useful when enormous numbers of microscopic degrees of freedom
• 1:59:09
  Speaker 1: are described collectively. Pressure has a similar character. A gas
• 1:59:14
  Speaker 1: exerts pressure on the walls of its container, even though
• 1:59:17
  Speaker 1: the microscopic description consists of particles colliding with those walls.
• 1:59:23
  Speaker 1: The pressure is not imaginary because it emerges from deeper physics.
• 1:59:27
  Speaker 1: It is simply not fundamental in the same way as
• 1:59:29
  Speaker 1: the microscopic ingredients from which it arises. Time might conceivably
• 1:59:34
  Speaker 1: work in an analogous fashion. One physicist who has developed
• 1:59:38
  Speaker 1: this possibility extensively is Carlo Rovelli. Rovelli has argued for
• 1:59:42
  Speaker 1: a strongly relational understanding of physics. Instead of assuming that
• 1:59:47
  Speaker 1: everything in the universe changes against one external master clock,
• 1:59:51
  Speaker 1: physical statements can be expressed through relationships among observable quantities.
• 1:59:56
  Speaker 1: A pendulum changes relative to the rotation of Earth. The
• 2:00:00
  Speaker 1: position of a planet changes relative to the configuration of
• 2:00:03
  Speaker 1: other bodies. A quantum system can change relative to another
• 2:00:07
  Speaker 1: physical variable being used as a clock. The important information
• 2:00:11
  Speaker 1: lies in correlations. This is closely connected to general relativity.
• 2:00:17
  Speaker 1: Albert Einstein's theory already removed the Newtonian idea of one
• 2:00:21
  Speaker 1: universal time flowing identically everywhere. Different observers can accumulate different
• 2:00:26
  Speaker 1: amounts of proper time, and general relativity permits many coordinate
• 2:00:30
  Speaker 1: descriptions of the same spacetime. Rovelli pushes the relational lesson
• 2:00:35
  Speaker 1: further when thinking about quantum gravity. At a fundamental level,
• 2:00:39
  Speaker 1: perhaps there is no need for one preferred variable called
• 2:00:42
  Speaker 1: time against which every other physical quantity evolves. Instead, the
• 2:00:46
  Speaker 1: theory may describe relations among changing quantitas. One variable changes
• 2:00:51
  Speaker 1: when another takes a particular value. One physical observable serves
• 2:00:55
  Speaker 1: as a reference for another. From those relationships, a familiar
• 2:00:59
  Speaker 1: temporal description may emerge in circumstances where one subsystem behaves
• 2:01:04
  Speaker 1: sufficiently well as a clock. This does not mean that
• 2:01:07
  Speaker 1: everyday time suddenly stops working. Quite the opposite. A successful
• 2:01:12
  Speaker 1: emergent theory must explain why ordinary time works so well.
• 2:01:17
  Speaker 1: When spacetime is approximately classical, gravity is not violently quantum.
• 2:01:22
  Speaker 1: and reliable clocks can be constructed, the usual language of seconds,
• 2:01:26
  Speaker 1: durations and evolution should reappear. The claim is simply that
• 2:01:30
  Speaker 1: this temporal description might be an effective one rather than
• 2:01:33
  Speaker 1: the deepest layer of the theory. An especially intriguing version
• 2:01:37
  Speaker 1: of this idea was developed by the mathematician Alan Cohn
• 2:01:41
  Speaker 1: and Carlo Rovelli. It is known as the thermal time hypothesis.
• 2:01:45
  Speaker 1: Ordinarily physicists begin with the notion of time and then
• 2:01:49
  Speaker 1: describe how a physical state changes with respect to it.
• 2:01:53
  Speaker 1: The thermal time hypothesis explores something almost reversed. Perhaps, in
• 2:01:58
  Speaker 1: an appropriate framework, the state of a system itself can
• 2:02:01
  Speaker 1: determine a natural notion of temporal evolution. The statistical condition
• 2:02:06
  Speaker 1: of the system would help identify the flow used to
• 2:02:09
  Speaker 1: describe its dynamics. This is a subtle, mathematical proposal, rather
• 2:02:14
  Speaker 1: than a claim that ordinary warmth creates time. A cold
• 2:02:18
  Speaker 1: system does not run out of seconds, and placing a
• 2:02:21
  Speaker 1: clock in a refrigerator does not threaten the existence of Tuesday.
• 2:02:25
  Speaker 1: The word thermal refers to the statistical structure involved. The
• 2:02:29
  Speaker 1: deeper suggestion is that temporal flow might not need to
• 2:02:32
  Speaker 1: be specified universally before the physical state is known. Instead,
• 2:02:37
  Speaker 1: a useful notion of time could arise from the state
• 2:02:40
  Speaker 1: and the algebra of physical observables. If something like that
• 2:02:44
  Speaker 1: is correct … time would depend more deeply on physical
• 2:02:47
  Speaker 1: conditions than Newton could have imagined. There would not necessarily
• 2:02:51
  Speaker 1: be one temporal parameter waiting behind every possible description of reality.
• 2:02:57
  Speaker 1: The relevant time could arise from the relationships and statistical
• 2:03:00
  Speaker 1: structure of the physical situation itself. Julian Barber has defended
• 2:03:04
  Speaker 1: an even more radical picture. Barber has explored the possibility
• 2:03:09
  Speaker 1: that time does not exist fundamentally as an independent entity
• 2:03:13
  Speaker 1: at all. In his approach, what fundamentally exists can be
• 2:03:17
  Speaker 1: described in terms of configurations. A configuration is, roughly speaking,
• 2:03:22
  Speaker 1: a possible arrangement of the universe. One arrangement contains matter
• 2:03:26
  Speaker 1: and geometry in one pattern. Another contains them in a
• 2:03:29
  Speaker 1: different pattern. Rather than imagining those configurations as snapshots being
• 2:03:34
  Speaker 1: carried along by flowing time, Barber asks whether the complete
• 2:03:38
  Speaker 1: physical description can be constructed directly from the relationships among
• 2:03:43
  Speaker 1: the configurations. The familiar impression of history could then arise
• 2:03:47
  Speaker 1: because some configurations contain records of others. A configuration might
• 2:03:51
  Speaker 1: contain fossils, memories, photographs, geological structures or traces of earlier
• 2:03:57
  Speaker 1: physical processes. Those records create the appearance of a past.
• 2:04:02
  Speaker 1: They allow one configuration to contain information suggesting a sequence
• 2:04:06
  Speaker 1: of other configurations, even if fundamental reality is not literally
• 2:04:10
  Speaker 1: being pushed from one snapshot to another by an invisible
• 2:04:13
  Speaker 1: flow of time. This is sometimes described as a timeless picture.
• 2:04:18
  Speaker 1: But it is important to handle the word timeless carefully.
• 2:04:22
  Speaker 1: It does not mean that nothing happens in ordinary experience.
• 2:04:26
  Speaker 1: Nor does it require denying the enormous body of experimental
• 2:04:29
  Speaker 1: evidence involving clocks, motion, decay, ageing and causality. It means
• 2:04:35
  Speaker 1: that the deepest mathematical description might not contain time as
• 2:04:39
  Speaker 1: an independently flowing ingredient. The patterns we interpret as change
• 2:04:43
  Speaker 1: in history would have to emerge from relationships within that description.
• 2:04:48
  Speaker 1: Rovelli's relational view, the thermal time hypothesis of Alan Connors
• 2:04:53
  Speaker 1: and Carlo Rovelli, and Barber's timeless approach are therefore not
• 2:04:57
  Speaker 1: three different names for exactly the same proposal. They should
• 2:05:01
  Speaker 1: not be blended together. Relational approaches emphasise the physical meaning
• 2:05:06
  Speaker 1: of quantities changing with respect to one another. The thermal
• 2:05:10
  Speaker 1: time hypothesis explores whether a temporal flow can arise from
• 2:05:13
  Speaker 1: a system's statistical state. Barber's programme goes further by questioning
• 2:05:19
  Speaker 1: whether time should appear fundamentally at all and placing configurations
• 2:05:23
  Speaker 1: and their relationships at the centre of the description. Their
• 2:05:27
  Speaker 1: motivations overlap in places. Their claims are not identical. This
• 2:05:32
  Speaker 1: distinction is useful because phrases such as time is an
• 2:05:36
  Speaker 1: illusion can make the science sound far more settled than
• 2:05:39
  Speaker 1: it actually is. Emergence does not mean illusion. A rainbow
• 2:05:44
  Speaker 1: is produced by the interaction of light with water droplets
• 2:05:47
  Speaker 1: but that does not make the rainbow visually unreal. A
• 2:05:51
  Speaker 1: sound wave is collective motion involving enormous numbers of particles
• 2:05:56
  Speaker 1: yet the sound can still shake a window. Temperature emerges
• 2:06:01
  Speaker 1: from microscopic behaviour but touching a hot stove remains an
• 2:06:04
  Speaker 1: impressively convincing demonstration emergent quantities can matter. So if time
• 2:06:10
  Speaker 1: were eventually shown to be emergent, clocks would not become
• 2:06:14
  Speaker 1: fraudulent devices. The seconds they record could remain perfectly meaningful
• 2:06:19
  Speaker 1: within the physical regime in which temporal behaviour appears. The
• 2:06:23
  Speaker 1: real change would be conceptual. We would no longer regard
• 2:06:26
  Speaker 1: time as one of the irreducible ingredients from which all
• 2:06:29
  Speaker 1: other physical descriptions are built. Instead, It would join the
• 2:06:32
  Speaker 1: long list of things that become meaningful only when deeper
• 2:06:35
  Speaker 1: ingredients are organised in the appropriate way. At present, however,
• 2:06:40
  Speaker 1: physics has not established that this is what nature does.
• 2:06:44
  Speaker 1: No accepted experiment has demonstrated that time disappears at some
• 2:06:48
  Speaker 1: fundamental scale. There is no experimentally confirmed theory of quantum
• 2:06:53
  Speaker 1: gravity showing that temporal structure definitely emerges from something timeless.
• 2:06:58
  Speaker 1: Relational and emergent time approaches are motivated by genuine problems
• 2:07:02
  Speaker 1: in combining quantum theory with general relativity. But motivation is
• 2:07:07
  Speaker 1: not the same as experimental confirmation. It remains possible that
• 2:07:11
  Speaker 1: a deeper theory will preserve some fundamental notion of time.
• 2:07:15
  Speaker 1: It might preserve causal order while allowing familiar duration to emerge.
• 2:07:20
  Speaker 1: It might contain a temporal ingredient unlike anything in present theories.
• 2:07:24
  Speaker 1: Or it might ultimately show that asking whether time is
• 2:07:27
  Speaker 1: fundamental is too crude. because different aspects of what we
• 2:07:30
  Speaker 1: currently call time arise at different levels of description. That
• 2:07:34
  Speaker 1: uncertainty is precisely why the question remains open. Time could
• 2:07:39
  Speaker 1: be part of the universe's deepest architecture, or seconds, durations
• 2:07:43
  Speaker 1: and temporal flow could resemble temperature, appearing only when more
• 2:07:47
  Speaker 1: fundamental relationships are viewed on the right scale. Both possibilities
• 2:07:51
  Speaker 1: must still reproduce the same familiar world. Clocks tick. Stars evolve.
• 2:07:57
  Speaker 1: Atoms decay. People remember the past and anticipate the future.
• 2:08:03
  Speaker 1: The unresolved question is whether all of that happens inside
• 2:08:06
  Speaker 1: a fundamental thing called time, or whether time is what
• 2:08:10
  Speaker 1: those relationships begin to look like once the universe becomes
• 2:08:13
  Speaker 1: complicated enough, once physicists begin asking what time looks like
• 2:08:17
  Speaker 1: in a deeper theory of quantum gravity. the answers start
• 2:08:21
  Speaker 1: to diverge quite sharply. That is because there is still
• 2:08:24
  Speaker 1: no single experimentally confirmed theory describing gravity and quantum mechanics
• 2:08:30
  Speaker 1: as one complete framework. Several major approaches exist. They often
• 2:08:35
  Speaker 1: agree that ordinary spacetime probably cannot remain untouched all the
• 2:08:39
  Speaker 1: way down to the deeper scales. But they do not
• 2:08:42
  Speaker 1: necessarily agree on which parts of spacetime are fundamental, which
• 2:08:46
  Speaker 1: parts are emergent. or what roll time should ultimately play.
• 2:08:51
  Speaker 1: Loop quantum gravity provides one prominent example. The theory is
• 2:08:56
  Speaker 1: associated with physicists including Abhay Ashtekar, Carlo Rovelli, and Lee Smolin.
• 2:09:02
  Speaker 1: Its central goal is to describe geometry itself using quantum principles.
• 2:09:07
  Speaker 1: In ordinary general relativity, spacetime geometry is smooth. Distances, areas,
• 2:09:12
  Speaker 1: and volumes are described continuously. Loop quantum gravity instead leads
• 2:09:17
  Speaker 1: to quantum states of geometry, in which certain geometric quantities
• 2:09:22
  Speaker 1: such as area and volume possess discrete spectra. This is
• 2:09:26
  Speaker 1: sometimes described loosely by saying that space comes in tiny chunks.
• 2:09:31
  Speaker 1: That phrase can be useful as a first intuition, but
• 2:09:33
  Speaker 1: it needs care. The theory does not simply turn spacetime
• 2:09:37
  Speaker 1: into a grid made from miniature cubes. Its quantum geometry
• 2:09:41
  Speaker 1: is described through structures known as spin networks. in which
• 2:09:45
  Speaker 1: mathematical relationships encode discrete geometric properties. When these structures change,
• 2:09:52
  Speaker 1: the resulting histories can be described using related constructions known
• 2:09:56
  Speaker 1: as spin foams. Time becomes difficult here for the same
• 2:10:00
  Speaker 1: reason it becomes difficult in canonical quantum gravity more generally.
• 2:10:04
  Speaker 1: General relativity does not provide one external master clock. The
• 2:10:09
  Speaker 1: geometry is part of the physical system. so loop quantum
• 2:10:13
  Speaker 1: gravity must confront the problem of how dynamics and change
• 2:10:16
  Speaker 1: should be described without quietly restoring Newton's universal time in
• 2:10:20
  Speaker 1: the background. Relational ideas therefore play an important role. Physical
• 2:10:26
  Speaker 1: quantities can be described through how they change relative to
• 2:10:29
  Speaker 1: one another rather than against one preferred external clock. There
• 2:10:34
  Speaker 1: is also a common misconception worth avoiding. Because some geometric
• 2:10:38
  Speaker 1: quantities are quantized, It is often claimed that loop quantum
• 2:10:42
  Speaker 1: gravity proves time advances in tiny indivisible steps. Usually, the
• 2:10:47
  Speaker 1: Planck time is then introduced as though it were the
• 2:10:50
  Speaker 1: duration of the universe's smallest possible tick. The Planck time
• 2:10:54
  Speaker 1: is roughly 5 times 10 to the minus 44 seconds.
• 2:10:58
  Speaker 1: It is an extraordinarily short interval, constructed from fundamental constants.
• 2:11:04
  Speaker 1: But its existence does not mean that nature has been
• 2:11:07
  Speaker 1: shown to update itself once every Planck time. nor does
• 2:11:10
  Speaker 1: loop quantum gravity automatically establish that time is universally discrete.
• 2:11:16
  Speaker 1: Quantist areas and volumes do not by themselves imply a
• 2:11:20
  Speaker 1: cosmic clock advancing frame by frame. That popular picture goes
• 2:11:24
  Speaker 1: well beyond what the theory actually says. A rather different
• 2:11:28
  Speaker 1: possibility appears in causal set theory. The modern causal set
• 2:11:32
  Speaker 1: program was developed through work by physicists including Luca Bombelli,
• 2:11:37
  Speaker 1: Juhan Lee, David Meyer and Raphael Sorkin. Here, the starting
• 2:11:42
  Speaker 1: point is not smooth spacetime. Instead, reality is described through
• 2:11:47
  Speaker 1: a collection of elementary events together with a causal ordering
• 2:11:51
  Speaker 1: that tells us which events can precede and influence which others.
• 2:11:55
  Speaker 1: The structure is discrete. But what matters most is not
• 2:11:59
  Speaker 1: simply discreteness. It is causal order. If one event can
• 2:12:04
  Speaker 1: causally precede another, the relation between them becomes part of
• 2:12:08
  Speaker 1: the fundamental structure. A smooth spacetime, like the one used
• 2:12:12
  Speaker 1: in general relativity, would then have to emerge approximately from
• 2:12:16
  Speaker 1: a sufficiently large causal set. This gives one aspect of
• 2:12:21
  Speaker 1: time a particularly deep role. The distinction between before and after,
• 2:12:26
  Speaker 1: where causal influence is possible, may survive even if familiar
• 2:12:30
  Speaker 1: ideas such as continuous duration do not exist fundamentally in
• 2:12:35
  Speaker 1: their ordinary form. That is an important distinction. Causal order
• 2:12:39
  Speaker 1: is not identical to elapsed time. Knowing that one event
• 2:12:43
  Speaker 1: can occur before another does not automatically tell you that
• 2:12:46
  Speaker 1: exactly three seconds separate them. So a theory can preserve
• 2:12:51
  Speaker 1: temporal ordering while allowing ordinary metric duration to emerge only
• 2:12:55
  Speaker 1: at larger scales. String theory approaches the problem differently again.
• 2:13:00
  Speaker 1: In many formulations of string theory, Calculations begin with strings
• 2:13:05
  Speaker 1: or higher dimensional objects propagating through a spacetime background. That
• 2:13:10
  Speaker 1: background includes temporal structure, so time does not simply disappear
• 2:13:14
  Speaker 1: from the starting point, in the same way suggested by
• 2:13:17
  Speaker 1: some canonical formulations of quantum gravity. String theory also introduces
• 2:13:23
  Speaker 1: additional spatial dimensions in many versions, but time usually retains
• 2:13:28
  Speaker 1: a distinct role. However, Developments within string theory have made
• 2:13:32
  Speaker 1: the picture less straightforward. Dualities can show that two apparently
• 2:13:37
  Speaker 1: different physical descriptions may encode the same underlying physics. This
• 2:13:42
  Speaker 1: raises the possibility that some features of spacetime that look
• 2:13:46
  Speaker 1: fundamental in one description may emerge differently in another. Holographic
• 2:13:51
  Speaker 1: ideas make that possibility even more striking. One of the
• 2:13:55
  Speaker 1: most influential examples is the gauge-gravity duality, introduced by Juan
• 2:14:01
  Speaker 1: Maldacena in the late 1990s. In its best known form,
• 2:14:06
  Speaker 1: the duality relates a gravitational theory and a higher dimensional
• 2:14:10
  Speaker 1: spacetime to a quantum theory without gravity defined on a
• 2:14:14
  Speaker 1: lower dimensional boundary. The two descriptions can encode the same
• 2:14:18
  Speaker 1: physics in very different ways. This has encouraged physicists to
• 2:14:23
  Speaker 1: ask whether spacetime geometry itself might emerge from more basic
• 2:14:27
  Speaker 1: quantum relationships. In some holographic settings, spatial geometry appears to
• 2:14:32
  Speaker 1: be closely connected to patterns of quantum entanglement. That naturally
• 2:14:37
  Speaker 1: raises questions about whether time could also be emergent in
• 2:14:40
  Speaker 1: some broader sense. But this point is often pushed much
• 2:14:44
  Speaker 1: too far in popular explanations. Holography has not proved that
• 2:14:49
  Speaker 1: time is an illusion. It has not demonstrated that our
• 2:14:52
  Speaker 1: experience of temporal passage is fake. and it has not
• 2:14:56
  Speaker 1: established one universally accepted mechanism from which all aspects of
• 2:15:01
  Speaker 1: time emerge. What it has done is provide powerful examples
• 2:15:05
  Speaker 1: in which descriptions containing gravity and spacetime can be equivalent
• 2:15:10
  Speaker 1: to very different descriptions formulated in other variables. That is
• 2:15:14
  Speaker 1: enough to make physicists cautious about declaring every familiar feature
• 2:15:19
  Speaker 1: of spacetime fundamental. Across these different approaches, Several concepts can
• 2:15:25
  Speaker 1: easily be confused. Causal order tells us which events can
• 2:15:29
  Speaker 1: influence which others. Relational change describes how one physical quantity
• 2:15:34
  Speaker 1: varies with respect to another. Proper time is the duration
• 2:15:38
  Speaker 1: measured along a worldline in relativistic spacetime. Emergent time refers
• 2:15:43
  Speaker 1: broadly to a temporal description arising from deeper variables that
• 2:15:48
  Speaker 1: may not themselves look like ordinary time. These ideas overlap.
• 2:15:52
  Speaker 1: They are not interchangeable. A theory might regard causal order
• 2:15:56
  Speaker 1: as fundamental while treating metric duration as emergent. Another might
• 2:16:01
  Speaker 1: describe change relationally without claiming that all temporal structure disappears.
• 2:16:07
  Speaker 1: Another might retain a time coordinate in its mathematical formulation
• 2:16:11
  Speaker 1: while proposing that parts of spatial geometry emerge from quantum information.
• 2:16:17
  Speaker 1: That is why simple statements about the final nature of
• 2:16:20
  Speaker 1: time are so risky. Time is discrete sounds decisive, but
• 2:16:25
  Speaker 1: no experiment has established a universal smallest temporal interval. Time
• 2:16:31
  Speaker 1: is an illusion, sounds dramatic, but it hides the fact
• 2:16:35
  Speaker 1: that clocks, causal relationships and proper time differences are experimentally real.
• 2:16:41
  Speaker 1: Time is definitely fundamental sounds equally confident, yet quantum gravity
• 2:16:46
  Speaker 1: has not reached a settled framework in which that conclusion
• 2:16:50
  Speaker 1: can be demonstrated. The disagreement exists because different candidate theories
• 2:16:55
  Speaker 1: assign different jobs to the things we currently bundle together
• 2:16:58
  Speaker 1: under the word time. One may emphasise causal order. Another
• 2:17:03
  Speaker 1: may emphasise relational change. Another may begin with a time
• 2:17:08
  Speaker 1: coordinate and question whether other aspects of spacetime are fundamental.
• 2:17:13
  Speaker 1: Another may try to recover familiar temporal evolution only after
• 2:17:17
  Speaker 1: a deeper description has been established. None has yet earned
• 2:17:21
  Speaker 1: the right to close the argument. Whatever the final theory
• 2:17:24
  Speaker 1: turns out to be, it will have to recover the
• 2:17:27
  Speaker 1: successful physics we already know. It must explain why clocks
• 2:17:31
  Speaker 1: measure proper time, why relativity describes causal structure so accurately,
• 2:17:37
  Speaker 1: why quantum systems evolve with extraordinary precision in laboratory experiments,
• 2:17:42
  Speaker 1: and why ordinary temporal experience emerges so reliably from the
• 2:17:46
  Speaker 1: underlying physics. At present, several researches are still open, and
• 2:17:51
  Speaker 1: that is why modern physics cannot honestly reduce the nature
• 2:17:55
  Speaker 1: of time to one neat sentence. Different theories preserve different
• 2:17:59
  Speaker 1: pieces of it. The difficult part is discovering which pieces
• 2:18:03
  Speaker 1: nature actually keeps. Perhaps the strangest thing about time is
• 2:18:08
  Speaker 1: not that clocks tick at different rates. It is that
• 2:18:11
  Speaker 1: one moment feels special. This one. The present. The past
• 2:18:16
  Speaker 1: seems to be gone. The future seems not to exist yet.
• 2:18:20
  Speaker 1: And the present appears to sit between them, advancing steadily
• 2:18:24
  Speaker 1: as one moment replaces another. That feeling is so immediate
• 2:18:28
  Speaker 1: that it is difficult to imagine reality any other way.
• 2:18:33
  Speaker 1: But physics does not provide a simple, universal object corresponding
• 2:18:38
  Speaker 1: to the present moment. Relativity is the reason. In everyday life,
• 2:18:43
  Speaker 1: and we casually assume that there must be one correct
• 2:18:46
  Speaker 1: answer to the question of what is happening right now
• 2:18:50
  Speaker 1: everywhere in the universe. Somewhere in another city something is
• 2:18:54
  Speaker 1: happening now. Somewhere on Mars something's happening now. Somewhere in
• 2:18:58
  Speaker 1: a distant galaxy surely there must also be an event
• 2:19:01
  Speaker 1: taking place at this exact same cosmic moment. Isaac Newton's
• 2:19:07
  Speaker 1: picture of absolute time fits that intuition beautifully. If one
• 2:19:12
  Speaker 1: universal time flows everywhere, then the entire universe can be
• 2:19:16
  Speaker 1: divided into layers of simultaneity. One layer is the present.
• 2:19:21
  Speaker 1: Earlier layers are the past. Later layers are the future.
• 2:19:25
  Speaker 1: Special relativity removes that simple division. Observers moving relative to
• 2:19:31
  Speaker 1: one another can disagree about whether two spatially separated events
• 2:19:35
  Speaker 1: occur at the same time. Neither observer needs to be mistaken.
• 2:19:40
  Speaker 1: Their clocks can be perfectly accurate. Their calculations can be
• 2:19:43
  Speaker 1: perfectly consistent. The disagreement comes from the structure of spacetime itself.
• 2:19:50
  Speaker 1: Imagine two flashes of light occurring far apart. One observer
• 2:19:55
  Speaker 1: may conclude that they happened simultaneously. Another observer moving relative
• 2:20:00
  Speaker 1: to the first can conclude that one happened before the other.
• 2:20:03
  Speaker 1: A third could reverse that ordering, provided the two events
• 2:20:07
  Speaker 1: are space-like, separated and therefore unable to causally influence one another.
• 2:20:13
  Speaker 1: This means there is no unique observer-independent way to collect
• 2:20:17
  Speaker 1: all distant events into one universal present. Locally, the present
• 2:20:22
  Speaker 1: is easy enough. You are here. Your clock reads what
• 2:20:26
  Speaker 1: it reads. Events happening beside you can be compared directly.
• 2:20:31
  Speaker 1: The difficulty appears when the idea of now is stretched
• 2:20:34
  Speaker 1: across enormous distances. Relativity provides no single cosmic slice that
• 2:20:40
  Speaker 1: all observers must recognise as the true present. That creates
• 2:20:44
  Speaker 1: a fascinating tension between physics and ordinary experience. One philosophical
• 2:20:50
  Speaker 1: response is called presentism. Presentism is the view that only
• 2:20:54
  Speaker 1: the present exists. The past existed, but no longer does.
• 2:20:59
  Speaker 1: The future will exist, but does not yet. This matches
• 2:21:03
  Speaker 1: everyday intuition extremely well. Reality on this view is continually
• 2:21:08
  Speaker 1: being updated. The difficulty is explaining what counts as the
• 2:21:11
  Speaker 1: present across a relativistic universe. If different observers disagree about simultaneity,
• 2:21:18
  Speaker 1: then which observer's present corresponds to what objectively exists? There
• 2:21:23
  Speaker 1: are ways of trying to construct sophisticated presentist views, but
• 2:21:27
  Speaker 1: relativity makes the problem much harder than it was under
• 2:21:31
  Speaker 1: Newtonian physics. Another interpretation is eternalism. Eternalism holds that past,
• 2:21:38
  Speaker 1: present and future events are all equally parts of reality.
• 2:21:42
  Speaker 1: They occupy different positions in spacetime rather than appearing and
• 2:21:46
  Speaker 1: disappearing as time flows. From this perspective, the universe can
• 2:21:51
  Speaker 1: be pictured as a four-dimensional structure. Your childhood is located
• 2:21:56
  Speaker 1: elsewhere in time. Your present experience is located here. Your
• 2:22:01
  Speaker 1: future experiences occupy other temporal locations. The fact that you
• 2:22:06
  Speaker 1: experience only one region of this structure at once does
• 2:22:10
  Speaker 1: not necessarily mean the others fail to exist. This interpretation
• 2:22:15
  Speaker 1: is often associated with the block universe picture. It fits
• 2:22:19
  Speaker 1: naturally with the geometry of relativity because spacetime already treats
• 2:22:24
  Speaker 1: events as elements of one, four-dimensional structure. But relativity itself
• 2:22:30
  Speaker 1: does not simply issue a philosophical decree saying eternalism is true.
• 2:22:35
  Speaker 1: The experiments confirm relativity's predictions about clocks, motion, gravity and simultaneity.
• 2:22:43
  Speaker 1: The claim about what ultimately exists is an interpretation layered
• 2:22:47
  Speaker 1: onto those physical results. A third possibility is the growing
• 2:22:51
  Speaker 1: block view. Here, the past and present are real, while
• 2:22:56
  Speaker 1: the future does not yet exist. Reality grows as new
• 2:23:00
  Speaker 1: events become actual. The past remains part of the block,
• 2:23:04
  Speaker 1: while the edge of the block corresponds to the present.
• 2:23:07
  Speaker 1: This preserves a genuine sense of becoming. But once again,
• 2:23:12
  Speaker 1: relativity makes it difficult to define a universal edge. if
• 2:23:16
  Speaker 1: different observers disagree about which distant events are simultaneous. Presentism,
• 2:23:22
  Speaker 1: eternalism and the growing block view are therefore not three
• 2:23:26
  Speaker 1: experimentally established physical theories competing in the same way that
• 2:23:31
  Speaker 1: two models might compete over the predicted orbit of a planet.
• 2:23:35
  Speaker 1: They are metaphysical interpretations. Physics constrains them. It makes some
• 2:23:40
  Speaker 1: versions easier to formulate and others harder. but it does
• 2:23:44
  Speaker 1: not currently provide an experiment that simply announces which one
• 2:23:47
  Speaker 1: is correct. Some modern physicists have taken strong positions on
• 2:23:52
  Speaker 1: this deeper issue. Lee Smolin has argued that time and
• 2:23:56
  Speaker 1: temporal becoming should be treated as fundamental features of nature.
• 2:24:01
  Speaker 1: In his view, the reality of change should not be
• 2:24:03
  Speaker 1: explained away too quickly by timeless mathematical structures. Carlo Rovelli
• 2:24:09
  Speaker 1: has taken a more strongly relational approach. emphasising that physical
• 2:24:13
  Speaker 1: quantities acquire meaning through their relationships with one another rather
• 2:24:17
  Speaker 1: than through one universal external clock. Julian Barber has gone
• 2:24:21
  Speaker 1: further still, exploring the possibility that time is not fundamental,
• 2:24:26
  Speaker 1: and that reality may ultimately be described through configurations and
• 2:24:30
  Speaker 1: relationships without requiring an independently flowing temporal substance. These are
• 2:24:36
  Speaker 1: not minor disagreements over wording, They represent genuinely different pictures
• 2:24:41
  Speaker 1: of what reality may be. One sees becoming as fundamental.
• 2:24:46
  Speaker 1: Another sees time as relational. Another questions whether time belongs
• 2:24:51
  Speaker 1: in the deepest description at all. The word flow creates
• 2:24:55
  Speaker 1: another difficulty. We often say that time flows. But what
• 2:25:01
  Speaker 1: would the speed of that flow be? One second per
• 2:25:04
  Speaker 1: second sounds natural, but it does not really explain anything.
• 2:25:08
  Speaker 1: A speed requires one quantity to change relative to another.
• 2:25:12
  Speaker 1: Kilometres per hour compares distance with time. But if time
• 2:25:16
  Speaker 1: itself flows, what does it flow relative to? Another time?
• 2:25:20
  Speaker 1: And if that second time also flows, would a third
• 2:25:23
  Speaker 1: time be required to measure it? Standard physics does not
• 2:25:27
  Speaker 1: introduce a separate measurable substance called temporal flow, with its
• 2:25:31
  Speaker 1: own velocity through some deeper background. Clocks measure intervals between events.
• 2:25:37
  Speaker 1: They do not measure a speed at which time itself moves.
• 2:25:41
  Speaker 1: That does not mean change is unreal, change is everywhere.
• 2:25:45
  Speaker 1: Atoms transition between states, stars evolve, living organisms age, planets orbit,
• 2:25:52
  Speaker 1: particles decay, memories form, signals travel, causes produce effects. Those
• 2:25:59
  Speaker 1: processes are undeniable parts of the physical world. What remains
• 2:26:04
  Speaker 1: uncertain is whether they occur because a thing called time
• 2:26:08
  Speaker 1: literally flows, or whether the appearance of flow arises from
• 2:26:12
  Speaker 1: the relationships among changing systems, and from the way conscious
• 2:26:16
  Speaker 1: observers experience records, memory and anticipation. This is where the
• 2:26:22
  Speaker 1: many meanings of time begin to separate. Time can be
• 2:26:25
  Speaker 1: a coordinate used to label events. It can be proper
• 2:26:28
  Speaker 1: time measured along a worldline. It can be causal ordering,
• 2:26:32
  Speaker 1: distinguishing which events can influence which others. It can describe
• 2:26:37
  Speaker 1: relational change between physical systems. It can be associated with
• 2:26:42
  Speaker 1: the thermodynamic arrow produced by increasing entropy. It might emerge
• 2:26:47
  Speaker 1: from deeper quantum relationships, or it might turn out to
• 2:26:51
  Speaker 1: be fundamental after all. Each of these ideas capture something
• 2:26:55
  Speaker 1: real and useful. The difficulty is that they do not
• 2:26:59
  Speaker 1: yet combine into one universally accepted picture. Relativity describes clocks
• 2:27:06
  Speaker 1: and space-time with extraordinary precision. Quantum mechanics describes microscopic evolution
• 2:27:13
  Speaker 1: with equally extraordinary success. Thermodynamics explains the directionality of countless
• 2:27:20
  Speaker 1: macroscopic processes. Candidate theories of quantum gravity explore deeper possibilities
• 2:27:27
  Speaker 1: for how temporal structure might arise. But the theories do
• 2:27:31
  Speaker 1: not yet fit together into one experimentally confirmed account of
• 2:27:35
  Speaker 1: what time ultimately is. That is why physicists consider agree
• 2:27:40
  Speaker 1: on how to calculate so many things involving time while
• 2:27:43
  Speaker 1: still disagreeing about its nature. They can predict how clocks
• 2:27:48
  Speaker 1: behave near Earth, around satellites or close to strong gravitational fields.
• 2:27:53
  Speaker 1: They can calculate decay probabilities and quantum oscillations. They can
• 2:27:58
  Speaker 1: describe the thermodynamic arrow and the geometry of spacetime. And still,
• 2:28:03
  Speaker 1: when asked whether the present moment is fundamentally real, whether
• 2:28:07
  Speaker 1: time flows, or whether time exists independently of change, the
• 2:28:11
  Speaker 1: answer is not settled. The physics works, the clocks work,
• 2:28:16
  Speaker 1: the universe keeps changing. What remains uncertain is what, beneath
• 2:28:21
  Speaker 1: all of that change, deserves to be called time. There
• 2:28:26
  Speaker 1: is something wonderfully strange about looking at an ordinary clock
• 2:28:30
  Speaker 1: after all of this. The numbers still change, the second
• 2:28:34
  Speaker 1: hand still moves, an atomic clock still counts, an exquisitely
• 2:28:39
  Speaker 1: regular physical process. Nothing about the practical act of measuring
• 2:28:44
  Speaker 1: time has become less reliable. In fact, modern physics can
• 2:28:48
  Speaker 1: compare tiny differences in elapsed time with a precision that
• 2:28:53
  Speaker 1: would have seemed almost absurd only a few centuries ago.
• 2:28:57
  Speaker 1: And yet, the deeper question remains. What is the thing
• 2:29:02
  Speaker 1: being measured? That question has followed physics for a very
• 2:29:06
  Speaker 1: long time. Isaac Newton imagined absolute mathematical time, progressing uniformly
• 2:29:12
  Speaker 1: and independently of everything else. In that picture, the universe
• 2:29:17
  Speaker 1: shared one common temporal background– Gottfried Wilhelm Leibniz challenged that idea.
• 2:29:24
  Speaker 1: He argued that time might instead be understood through relationships
• 2:29:27
  Speaker 1: among events and changes. One view treated time as something
• 2:29:32
  Speaker 1: with an existence of its own. The other treated temporal
• 2:29:36
  Speaker 1: facts as relationships between physical things. The disagreement was already
• 2:29:41
  Speaker 1: there before relativity, quantum mechanics or cosmology had arrived. Albert
• 2:29:46
  Speaker 1: Einstein then transformed the discussion. Special relativity removed universal simultaneity.
• 2:29:54
  Speaker 1: Two observers moving relative to one another can disagree about
• 2:29:58
  Speaker 1: whether distant events happened at the same time. Neither has
• 2:30:02
  Speaker 1: to be mistaken. The disagreement is built into the structure
• 2:30:06
  Speaker 1: of spacetime. General relativity went further. Spacetime itself became dynamical.
• 2:30:13
  Speaker 1: Matter and energy influence its geometry. and that geometry influences motion,
• 2:30:18
  Speaker 1: light and clock rates. There is no longer one universal
• 2:30:23
  Speaker 1: cosmic rhythm against which every physical clock can be judged.
• 2:30:27
  Speaker 1: A clock follows a worldline. Along that worldline it records
• 2:30:32
  Speaker 1: proper time. Proper time is one of the clearest operational
• 2:30:36
  Speaker 1: meanings of time that physics possesses. If two clocks meet,
• 2:30:40
  Speaker 1: separate and later reunite, they may have accumulated different amounts
• 2:30:45
  Speaker 1: of proper time. Their readings can be compared directly. The
• 2:30:49
  Speaker 1: difference is real. It can depend on velocity. It can
• 2:30:53
  Speaker 1: depend on gravity. It can be calculated. It can be measured.
• 2:30:57
  Speaker 1: This is not a matter of interpretation. Relativity has been
• 2:31:00
  Speaker 1: tested repeatedly through clocks, particles, satellites and astronomical observations. But
• 2:31:08
  Speaker 1: even proper time does not settle the deeper question of
• 2:31:11
  Speaker 1: what time fundamentally is. It tells us what a clock
• 2:31:14
  Speaker 1: measures along a path through spacetime. It does not tell
• 2:31:18
  Speaker 1: us whether time exists independently of change, whether spacetime is fundamental,
• 2:31:23
  Speaker 1: or whether the temporal structure described by relativity emerges from
• 2:31:27
  Speaker 1: something deeper. The arrow of time adds another layer. Ludwig
• 2:31:31
  Speaker 1: Boltzmann and statistical mechanics help explain why the macroscopic world
• 2:31:36
  Speaker 1: looks so strongly directed. Heat spreads, gases mix, objects break.
• 2:31:42
  Speaker 1: Entropy overwhelmingly tends to increase when systems begin away from equilibrium.
• 2:31:48
  Speaker 1: That statistical asymmetry explains much of the ordinary distinction between
• 2:31:52
  Speaker 1: past and future, but it does not close the problem.
• 2:31:56
  Speaker 1: For entropy to increase, the universe had to begin in
• 2:32:00
  Speaker 1: an extraordinarily special low-entropy condition. Why it did so remains unsettled.
• 2:32:07
  Speaker 1: The familiar arrow of time may therefore reach all the
• 2:32:10
  Speaker 1: way back to the initial condition of the cosmos. Quantum
• 2:32:14
  Speaker 1: mechanics changes the role of time again. In the ordinary
• 2:32:18
  Speaker 1: Schrodinger formulation, the quantum state evolves with respect to a
• 2:32:22
  Speaker 1: time parameter. But that time is usually supplied externally. It
• 2:32:27
  Speaker 1: is not represented in precisely the same way as observables
• 2:32:30
  Speaker 1: such as position or momentum. The theory is extraordinarily successful.
• 2:32:35
  Speaker 1: It predicts interference, decay, atomic behaviour and countless other phenomena
• 2:32:41
  Speaker 1: with astonishing accuracy. Yet it usually assumes the temporal background
• 2:32:46
  Speaker 1: against which evolution takes place. That becomes uncomfortable when gravity enters.
• 2:32:52
  Speaker 1: General relativity says spacetime is dynamical. Quantum mechanics usually wants
• 2:32:57
  Speaker 1: a temporal parameter already available. Trying to combine the two
• 2:33:02
  Speaker 1: forces the question to the mathematics itself. John Archibald Wheeler
• 2:33:06
  Speaker 1: and Bryce DeWitt encountered one of the most striking versions
• 2:33:09
  Speaker 1: of this problem in work on quantum gravity and quantum cosmology.
• 2:33:14
  Speaker 1: In the Wheeler-DeWitt framework, the familiar external time parameter does
• 2:33:18
  Speaker 1: not appear in the same way that it does in
• 2:33:20
  Speaker 1: the ordinary time-dependent Schrödinger equation. That does not prove that
• 2:33:25
  Speaker 1: nothing changes. It does not show that the universe is frozen.
• 2:33:29
  Speaker 1: But it does force physicists to ask, how ordinary temporal
• 2:33:33
  Speaker 1: evolution could emerge from a deeper theory in which the
• 2:33:36
  Speaker 1: usual time variable is absent. From there, several possibilities remain open.
• 2:33:42
  Speaker 1: Time might be fundamental. It might be relational, defined through
• 2:33:47
  Speaker 1: changes between physical systems. It might emerge from quantum correlations.
• 2:33:53
  Speaker 1: It might be associated with the statistical state of a system.
• 2:33:57
  Speaker 1: It might be encoded most deeply through causal order. Some
• 2:34:01
  Speaker 1: familiar aspects of time might be fundamental while others are emergent.
• 2:34:06
  Speaker 1: The word time may even be covering several distinct physical
• 2:34:09
  Speaker 1: ideas that only look like one thing in ordinary experience.
• 2:34:13
  Speaker 1: What physics does know is substantial. Clocks can accumulate different
• 2:34:18
  Speaker 1: amounts of proper time. Motion affects clock readings. Gravity affects
• 2:34:23
  Speaker 1: clock readings. There is no universal simultaneity shared by all
• 2:34:27
  Speaker 1: inertial observers. Entropy supplies a powerful explanation for the macroscopic
• 2:34:33
  Speaker 1: arrow separating past from future. Quantum mechanics makes extraordinarily accurate
• 2:34:40
  Speaker 1: predictions involving temporal evolution. General relativity describes the geometry governing
• 2:34:46
  Speaker 1: clock rates and causal structure with remarkable success. Those are
• 2:34:51
  Speaker 1: firm achievements. The unresolved questions lie deeper. Does time exist
• 2:34:56
  Speaker 1: if nothing changes? Is there an objective present? Does time
• 2:35:00
  Speaker 1: literally flow, or is flow part of our experience of
• 2:35:05
  Speaker 1: physical change? Why did the universe begin in the low
• 2:35:09
  Speaker 1: entropy condition required for the thermodynamic arrow? Is time fundamental
• 2:35:14
  Speaker 1: in quantum gravity, or does the familiar temporal world emerge
• 2:35:18
  Speaker 1: only when deeper physical relationships are viewed on the right scale?
• 2:35:23
  Speaker 1: At present, There is no single experimentally confirmed answer binding
• 2:35:27
  Speaker 1: all of these questions together. That is why physicists can
• 2:35:31
  Speaker 1: disagree about what time actually is without disagreeing about how
• 2:35:35
  Speaker 1: to use it. A satellite navigation system does not need
• 2:35:39
  Speaker 1: a final philosophy of time to correct its clocks. An
• 2:35:43
  Speaker 1: atomic physicist does not need to settle eternalism before calculating
• 2:35:47
  Speaker 1: a transition. A cosmologist can estimate the age of the
• 2:35:51
  Speaker 1: universe without deciding whether temporal passage is fundamental. The calculations
• 2:35:56
  Speaker 1: still work. Perhaps that is what makes time such an
• 2:35:59
  Speaker 1: unusual concept. Almost every experiment in physics depends on knowing
• 2:36:05
  Speaker 1: when something happens. Every measurement of motion, decay, oscillation, propagation
• 2:36:10
  Speaker 1: or change relies on some temporal structure. And yet, if
• 2:36:15
  Speaker 1: we keep asking what that structure ultimately represents, The question
• 2:36:19
  Speaker 1: leads away from the face of an ordinary clock and
• 2:36:22
  Speaker 1: towards the unfinished foundations of physics. We can measure time
• 2:36:26
  Speaker 1: extraordinarily well. We can predict what clocks will do. We
• 2:36:30
  Speaker 1: can watch the universe change. But the final answer to
• 2:36:34
  Speaker 1: what time itself is remains just beyond the reach of
• 2:36:37
  Speaker 1: our best theories.