The Theory About Time That Physicists Refuse to Believe | Kip Thorne

Added: 2026-05-09

[00:00:00]There's a memory I keep returning to more often than I probably should. It's the autumn of 1,972.

[00:00:10]I'm in John Wheeler's office at Princeton, the one with the tall, narrow windows that looked out over the bare branches of the elms and the chalkboard that always seemed to be covered in equations he had drawn the night before and forgotten about. John is pacing. He always paced when he was thinking in these short deliberate steps as though the answer were a small animal he was trying not to startle. I remember the sound of his shoes on the wooden floor.

[00:00:40]Uh I remember the smell of chalk dust and the radiator clicking in the corner and the late afternoon light coming in through those windows. Um I remember it the way you remember the moments in your life when without quite knowing it, the ground is shifting beneath you. And John turns to me and he says very quietly almost as though he didn't want the equations on the board to overhear. He says, "Kip, we may have made a mistake."

[00:01:07]Now, John Wheeler did not speak that way casually. This was the man who coined the term black hole. the man who had worked with Einstein at Princeton in the 1950s, with Boore at Copenhagen in the 1930s, who had trained Richard Fineman, who had spent his entire adult life in the company of the greatest minds physics had ever produced. When John Wheeler said we'd made a mistake, he meant the kind of mistake that rearranges the floor of physics underneath your feet. He meant the kind of mistake you spend the rest of your life trying to understand. and the mistake he was talking about, the one he and his student Bryce Dwit had stumbled into a few years earlier when they tried to do something everyone said couldn't be done. That mistake is the subject of what I want to tell you about today. Uh because uh here's the thing. For more than 50 years now, there has been an equation sitting on the dusty back shelves of theoretical physics. It's called the Wheeler Dwit equation. And uh if you ask most physicists about it, you'll get a polite nod, a vague reference to quantum gravity and a quick change of subject. You'll find it in textbooks, but only in the appendices.

[00:02:27]You'll find it in conferences, but only in the late evening sessions when the cameras are off and the wine is on the table. You'll find it referenced in technical papers almost always in the same way cautiously with the kind of language people use when they don't quite want to commit to what they're saying. And the reason the reason this beautiful austere equation has been kept at a kind of professional arms length for half a century is that when you actually sit down and read what it says about the universe, it tells you something that no physicist I have ever known has been able to look in the eye for very long. It tells you that time does not exist. I want you to sit with those four words for a moment. Time does not exist. Not time runs differently in different places. We already knew that.

[00:03:19]Einstein showed us that more than a hundred years ago. Not time is relative.

[00:03:24]Fine. Anyone who's gone to a science museum in the last 50 years has heard that phrase. We got comfortable with that decades ago. No. The Wheeler Dwit equation says something much much stranger than any of that. It says that at the deepest level, the level beneath spacetime, beneath gravity, beneath everything, we use the word real for there is no time at all. No flow, no clock, no before, no after. The universe at its most fundamental is frozen. And I want you to understand, I am not telling you this as some sort of metaphor. I'm not telling you this as a philosophers's flourish or as a poetic interpretation of the math or as the kind of thing professors say when they want to seem profound at dinner parties. I am telling you that when Bryce Dwit and John Wheeler combined Einstein's theory of gravity with the rules of quantum mechanics, the two greatest theories in the history of physics, the two theories on which our entire understanding of the cosmos rests, what came out of the equations in plain mathematical English was a universe with no time in it. The equation reads in its simplest form H * S= 0. The Hamiltonian which is the operator that in any normal quantum system tells you how things change with time acting on the wave function of the universe gives you nothing zero. The universe written this way does not evolve. It does not become. It simply is. Now you might reasonably ask and I would forgive you for asking it with a certain amount of skepticism. How is that possible? You're sitting somewhere listening to me. The clock on your wall is ticking. The sun rises and sets. Your coffee is getting cold. You have a memory of breakfast and an expectation of dinner. How can time not exist? That is exactly the right question. And the answer to that question is one of the most extraordinary detective stories in all of physics. A story that involves Einstein, Wheeler, Hawking, a quiet graduate student named Don Page, who in 1983 had what may have been the most important idea no one outside theoretical physics has ever heard of and an experiment carried out in a laboratory in Turin, Italy in the year 2014. An experiment that I think will eventually be remembered the way we remember the Michaelelsson Morley experiment of 1,887.

[00:06:15]An experiment that in a quiet way changed everything. But before we go there, and we are going there, I promise you I want to pause for just a moment because what we're about to walk into is the kind of territory where if you're not careful, you can lose your footing.

[00:06:34]The ideas are subtle. The implications are vast and I have learned over 50 years of trying to communicate this material that there is a particular kind of patience required to absorb it. So I want to mention something that I think will help. You see, the deeper I've gone into this material, and I've been thinking about it now on and off, uh, for the better part of 50 years, the more I found that the books that helped me most weren't the textbooks. The textbooks give you the equations, but they don't give you the feeling. They don't walk you slowly through the strangeness. They don't sit with you in the moment when the math first starts to whisper its secret. There's a wonderful little volume, a a quiet book, the kind that doesn't shout, that lays out exactly the puzzle we're about to dive into. It tells you what the equations are really saying about time, about reality, about what it means for the universe to exist without a before and an after. The kind of thing they don't teach you in undergraduate physics, frankly, the kind of thing they don't teach you in graduate physics either because most professors find it too uncomfortable to spend a lecture on.

[00:07:54]Most professors prefer to keep the equations clean and not ask the embarrassing questions about what they actually mean. Uh, this book asks those questions and it answers them with a kind of clarity that in my experience is genuinely rare. If you want to follow this thread further than I'm able to in the next hour, the link is below in the description. I'll mention it once more later. For now, I just wanted to say if what I'm about to tell you opens a door for you, there is more on the other side of that door, and it is well worth walking through. The book is for the curious. It assumes nothing. It rewards patience, and it goes places I cannot quite go in a single conversation. All right, let me take you back then to where the story really begins. Because to understand why the Wheeler Dit equation is so disturbing, you have to first understand why anyone tried to write it down in the first place. You have to understand the the long strange road that physics had to walk before this equation could even be conceived.

[00:09:02]And that road begins as so many things do with Albert Einstein in 1915 after nearly a decade of struggle. A decade in which he made wrong turns, abandoned promising paths, fell into despair more than once was nearly scooped by the great mathematician David Hilbert. In the final weeks, Einstein published the general theory of relativity. And what general relativity tells us is something so simple in retrospect and so profound that it took the genius of Einstein to see it. It tells us that gravity is not, as Newton thought, a force pulling objects toward each other across empty space. Gravity is geometry. Gravity is the curvature of spacetime itself. Imagine for a moment a bowling ball placed on the middle of a stretched rubber sheet.

[00:09:57]The ball makes a depression. Now roll a marble across the sheet. The marble doesn't travel in a straight line. It curves toward the bowling ball. Not because the ball is pulling it, but because the sheet itself has been warped and the marble is just following the natural path through the warped sheet.

[00:10:19]That is uh that is gravity in Einstein's picture. The presence of mass and energy bends the fabric of spacetime. And what we feel as gravity is just objects following the natural paths, what physicists call geodics through that curved geometry. Wheeler put it more elegantly than anyone in a phrase I still remember him saying with that quiet half smile of his. Mass tells spacetime how to curve. Spacetime tells mass how to move. 11 words. The whole of general relativity in 11 words. I have spent my entire career trying to live up to the precision of that sentence. Now, if that were the whole story, life would be simple. But it isn't. Because at almost the exact same moment Einstein was finishing general relativity, a different group of physicists, Heisenberg, Schroinger, Bore, Bourne, Durac, Paulie were building something that on its surface seemed to have nothing to do with gravity at all. They were building quantum mechanics. And quantum mechanics is the theory of the very small. It governs atoms, electrons, photons, the tiny machinery underneath everything we see. And what quantum mechanics tells us is that at the smallest scales, things don't have definite positions. They don't have definite trajectories. They exist as waves of probability in superp positions of possibilities until we measure them and the wave collapses into a single outcome. These two theories, general relativity and quantum mechanics, are the two pillars of 20th century physics.

[00:12:09]Each one has been tested to absurd precision. General relativity predicted the bending of starlight around the sun, the procession of Mercury's orbit, the existence of black holes, the slowing of clocks and gravitational fields, the expansion of the universe, gravitational waves. We confirmed every single one.

[00:12:30]Quantum mechanics gave us the laser, the transistor, the entire electronics industry, modern chemistry, the structure of the atomic nucleus.

[00:12:40]It predicted the exact spectrum of hydrogen to 14 decimal places. We have never, not once in a hundred years of trying with the most sensitive instruments human beings have ever built, found a single experiment in which either theory failed. And yet, and yet they are completely, irreconcilably, mathematically incompatible. This is the great open wound in physics. The two theories that describe everything we know cannot both be right at the same time in the same place. General relativity treats spacetime as smooth and continuous, a four-dimensional fabric that responds in deterministic, beautifully geometric ways to the matter inside it. Quantum mechanics treats reality as fundamentally probabilistic, fundamentally discreet, fundamentally indeterminate. General relativity says the geometry of spaceime is determined exactly by the mass and energy in it.

[00:13:39]Quantum mechanics says nothing is determined exactly. Everything is a smear of probabilities until it isn't.

[00:13:47]When you try to put the two together, when you try to write down a quantum theory of gravity, the equations explode. infinities pour out of them.

[00:13:56]The mathematics frankly screams. For most of the 20th century, physicists could ignore this. The reason they could ignore it is that quantum effects only matter at very small scales and gravity only matters at very large scales and there's almost no overlap between those two regimes. You don't need quantum mechanics to send a rocket to the moon.

[00:14:19]You don't need general relativity to design a transistor. The two theories live in different neighborhoods of the universe. And as long as you stay in your own neighborhood, you don't have to think about it almost. There are exactly two places in the entire universe where the two regimes meet where gravity is so strong and the scales are so small that you cannot use one theory without the other. One is the singularity at the center of a black hole. the point where according to general relativity all the matter that fell in collapses to infinite density. The other and this is the one that interested Wheeler and Dit the most is the very beginning of the universe. The big bang the moment when the entire observable cosmos was compressed into a region smaller than an atom, smaller than a proton, smaller than anything we have a word for. If you want to understand the first instant of time, you cannot use general relativity alone. The whole universe was at that moment a quantum object. The whole universe, every galaxy that would ever exist, every star, every planet, every grain of sand on every beach that would ever be was at that moment governed by the same probabilistic smeared out quantum laws that govern the electron in a hydrogen atom. You need a quantum theory of gravity to even begin to talk about it. And so in the 1960s, John Wheeler and Bryce Dit and Dwit was, I should say, a remarkable physicist, one of the unsung heroes of 20th century theoretical work. He was the one who really did the heavy lifting on the technical side, while Wheeler provided the philosophical engine that drove the whole enterprise. Wheeler and Dit sat down and tried to do the impossible.

[00:16:20]They tried to write down an equation for the wave function of the entire universe. Now when I say wave function of the universe, I want you to understand what that phrase means because it is not a phrase to be tossed around lightly. In ordinary quantum mechanics, you have a wave function usually written as the Greek letter sigh that describes the state of a particle or an atom or a small system of particles. It tells you the probability of finding the particle in different places with different velocities, different spins. The wave function evolves over time according to the Schroinger equation. That's the heart of quantum mechanics written down by Irwin Schroinger in 1,926 inches a hotel in the Swiss Alps in what was either the most productive vacation in the history of science or the most embarrassing depending on whom you ask and how much you know about Schroinger's personal life. What Wheeler and Dit wanted to do was scale this idea up to its absolute mathematical maximum. Not the wave function of a particle, not the wave function of an atom, not the wave function of a planet or a star or a galaxy, the wave function of everything.

[00:17:36]A single mathematical object that contained all the information about every particle, every field, every photon, every nutrino, every black hole, every galaxy cluster, every cubic centimeter of space, the entire cosmos treated as one quantum system. It sounds insane. I know it sounds insane. It is in some sense insane, but the mathematics doesn't care whether it sounds insane. The mathematics just asks if you take general relativity and you quantize it the same way we quantized everything else, what equation do you get? And the equation they got after years of struggle, after enormous technical difficulty, after a process Wheeler later described to me as feeling our way through a dark room and finding that the walls were not where we expected them to be, was the Wheeler Dit equation HSI equals zero. And there sitting on the chalkboard was the problem because you see that zero on the right side of the equation is not just a number. It is a catastrophe. In ordinary quantum mechanics, the Schroinger equation has a sigh on one side and an I* H bar times the time derivative of SI on the other side. That time derivative is what tells the wave function how to change with time. It's the heartbeat of the equation. It's what makes the universe go. Without that term, quantum mechanics is just a still photograph.

[00:19:13]With that term, it is a moving picture.

[00:19:16]In the Wheeler Dwit equation, that term is absent. It's not small. It's not approximately zero. It's not zero up to some small correction. It is exactly mathematically structurally zero. The wave function of the universe does not change with time. There is no Schroinger evolution. There is no flow. There is no clock ticking outside the universe to tell it what time it is because the universe is everything. There is nothing outside it. There is no platform from which to observe it. There is no external chronometer. And so the equation faithful only to its own logic simply removes time from the picture entirely.

[00:20:02]When Dwit first showed this result to Wheeler, the story I heard, and Wheeler told me this himself sitting in his office at the University of Texas decades later, with the same expression he'd worn that day in 1972.

[00:20:17]The story I heard was that Wheeler simply stared at the equation for a long time. The light moved across the floor.

[00:20:25]The afternoon got later and then finally he said, "Bryce, I think we've broken physics." Now, here is the thing that most people when they first hear about this don't quite grasp. They assume that this must be some sort of technical artifact, a a mathematical glitch, uh, a sign that we did the calculation wrong, uh, a temporary embarrassment that will eventually be cleaned up. They assume that with enough cleverness, we'll be able to put time back into the equation, that somewhere there's a missing term, a forgotten variable, a clever substitution that will rescue our intuition. But that is not what the last 50 years of work have shown. What the last 50 years of work have shown is that the Wheeler Dwit equation is in a deep sense mathematically correct. The absence of time is not a bug. It is a feature. It is what general relativity taken seriously and combined with quantum mechanics demands. The reason there is no time in the equation is that in general relativity, time is not a fundamental thing. Time is not a backdrop against which physics happens.

[00:21:41]Time is part of the geometry. It is one of the things that has to be quantized along with everything else. And when you quantize it, when you treat it the way quantum mechanics requires you to treat all dynamical variables, it stops being something that flows and becomes something that just is. I want to give you an analogy because this is the kind of thing that if you don't have a picture for it, sounds like nonsense.

[00:22:07]Imagine you're watching a film, not streaming a film. I mean, an actual film, the old-fashioned kind on a strip of celluloid, the kind I grew up watching at the small movie house in Logan, Utah when I was a boy. The film has thousands of individual frames. When you run it through a projector at 24 frames a second, you see motion. You see Humphrey Bogart walk across the room and light a cigarette. You see Lauren Beall give him that look. You see time passing. But now imagine you take the film off the projector. You unspool it.

[00:22:43]You lay it out flat on a long long table all the way down a hallway all the way out the door. Every frame is there.

[00:22:51]Bogart in his trench coat at the door.

[00:22:53]Bogart three steps in. Bogart at the bar. Bogart with the cigarette to his lips. Bogart with his mouth open to speak. His hand reaching for the matches. His eyes meeting.

[00:23:07]Every moment of his entrance, every microssecond of his walk is there simultaneously on the table. There's no now, there's no next. There's just the whole film, all of it existing at once.

[00:23:22]The Wheeler Dit equation says the universe is the film on the table, not the film in the projector. Every moment that has ever happened, every moment that ever will happen, your birth, you're reading these words, your last breath, the death of the sun, the collapse of the last galaxy, the cold dark end of the cosmos, they are all there in the wave function uh simultaneously equally real. The flow we feel, the sense that now is special, that the past is gone and the future hasn't arrived, is according to this equation an illusion. Not in some loose philosophical sense, in a precise mathematical physical sense. The film is on the table. The projector is in our heads. Einstein, in fact, said almost exactly this. In the spring of 1,955, just a few weeks before his own death, he wrote a letter to the family of his oldest friend, Michelle Besso, who had just died. Besso had been Einstein's closest confidant. The two of them had walked together in burn half a century earlier when Einstein was still working at the patent office, and it was in those walks that special relativity took its final shape. When Besso died, Einstein wrote to his family. And in that letter, which I have always found one of the most haunting things Einstein ever wrote, he said something to the effect that for those of us who believe in physics, the distinction between past, present, and future is a stubbornly persistent illusion. A stubbornly persistent illusion. Three weeks later, Einstein himself was gone.

[00:25:11]Those words have stayed with me for 50 years. A stubbornly persistent illusion.

[00:25:16]He didn't say interesting illusion. He didn't say mild illusion. He said stubbornly persistent. Because that's what the experience of time is, isn't it? It is stubborn. It refuses to dissolve even when we know intellectually that the equations don't support it. We feel the flow even as the math denies it. But here's the thing.

[00:25:44]Einstein wrote that line because of special and general relativity. He had already convinced himself by 1955 that time was not a separate flowing thing that it was woven together with space into a four-dimensional structure and that the now we feel is not an objective feature of that structure but a perspectival one. That was a radical view in 1955. It is still a radical view today. But the Wheeler Dwit equation goes much, much further. Einstein still believed time was a dimension, a fourth dimension woven together with space, but a dimension nonetheless, something real even if not flowing. The Wheeler Dit equation goes one step beyond Einstein. It says time is not even a dimension at the deepest level.

[00:26:33]Time is something we read off the relationships between things inside the universe. It's not part of the bedrock.

[00:26:41]It's something that emerges from the structure the way a melody emerges from the relationships between individual notes. Now, when this result first started circulating in the physics community in the late 1960s and early 1970s, the reaction was, how shall I put it? mostly silence. A great polite professional silence. Physicists didn't deny the equation. They couldn't. The math was right. Anyone who checked the math could verify it. But they also didn't know what to do with it. So they tucked it away. They said, "Well, this is a problem for the future. This is a problem for when we have a complete theory of quantum gravity. For now, we have practical work to do. And the practical work was real. There was particle physics to do. There was cosmology. There was the discovery of the cosmic microwave background in 1964, which had to be understood. There was the puzzle of the missing dark matter.

[00:27:48]There was the rise of inflation theory in the early 1980s. There was eventually the dream of detecting gravitational waves, which would consume the next 40 years of my own life. The field was busy. The field had places to be. And the Wheeler Dit equation with its troubling absence of time sat on the shelf tactfully ignored. Stephven Hawking and Steven and I were friends and rivals for nearly 50 years. He was without question the the most relentless mind I ever encountered. Steven would sometimes mention the Wheeler Dwit equation in his in his lectures and you could see him sort of work around it the way you'd work around a piece of furniture in a dark room you didn't want to bump into. He understood it perfectly. He'd written papers using it.

[00:28:36]The famous Hardle Hawking no boundary proposal for the wave function of the universe is in essence a particular solution to the Wheeler Dit equation.

[00:28:46]But Steven didn't dwell on the missing time issue. He focused on the parts of the equation he could use. None of us quite knew how to talk about the missing time without sounding like a mystic. I um remember once um having dinner with Steven in Cambridge um in the early 1980s.

[00:29:09]We were talking about the wave function of the universe and I asked him more or less directly what he thought it meant that there was no time in the equation.

[00:29:19]And Steven uh through that voice synthesizer with the long pauses he had to use to compose his thoughts, Steven said, "Kip, I think we should not think about that question. I think we should solve the equation first." Which was Steven all over. Get the math right, figure out the philosophy later. and he was probably right. But there are some of us and I count myself among them perhaps to my detriment who could never quite stop thinking about that question.

[00:29:48]That more or less is where things stood for 15 years. The Wheeler Dwit equation sat on the shelf. Time officially was a problem postponed. And then in 1983, something extraordinary happened. A quiet, unassuming theoretical physicist named Don Page, a former student of Stephven Hawkings working at Penn State University at the time, wrote a paper with his colleague William Wooders. The paper was on the surface technical. It was published in a respectable journal.

[00:30:28]It was titled modestly evolution without evolution. It was almost immediately ignored. the major journals had passed on it. Uh the conferences didn't feature it. Uh for about 20 years, almost nobody talked about it. But that paper I have come to believe was one of the great quiet revolutions in 20th century physics. And in it, Paige and Wooders proposed something so audacious, so unexpected that it took the entire field a generation to digest. They said yes the universe has no time. The Wheeler Dwit equation is correct. The whole cosmos as a single quantum system is static, frozen, unchanging.

[00:31:17]But and this is the key. The universe contains things. It contains subsystems.

[00:31:24]It contains parts. It contains among other things observers.

[00:31:29]And those subsystems can be quantum mechanically entangled with each other.

[00:31:35]And if you have two things that are entangled, then from the inside, from the perspective of one of those things looking at the other, the other thing looks like a clock. Time they said, is not a fundamental property of the universe. Time is a property that emerges from the entanglement between subsystems. It is a relationship, not a thing. The universe as a whole is timeless but the parts of the universe as they correlate with each other generate the appearance of time. Now I want to pause here because this is the conceptual hinge of the entire story and uh before I keep going I want to mention one more time and I won't keep doing this I promise the book I referenced earlier because this idea this page wooters mechanism is exactly the kind of thing that takes pages and pages to really sit with. I've given you the headline. The book takes you through the actual physics, what entanglement means, why it gives rise to time, how the math works, why it took 30 years to be experimentally confirmed, and it does the thing that physics popularizations almost never do, which is treat the reader as an intelligent adult who is capable of being shown the stranges directly. It doesn't water it down. It doesn't pretend the implications are smaller than they are. It walks you through slowly, carefully, the way a good teacher would. I've been recommending it to my own graduate students at Caltech and I've been recommending it to friends who have nothing to do with physics. I've been recommending it to people who like you maybe watched a video like this one and wanted to follow the thread further. The link is in the description below. If at any point in what I'm telling you, you find yourself thinking, "I need to see this for myself." If you find yourself thinking, "This is the kind of thing I want to spend a long quiet evening with." That's where you go. The book is for the curious. It assumes nothing. It rewards patience. And it answers slowly and carefully the questions that I am in this hour only able to gesture at. All right, back to Paige and Wooters. The way I like to explain their idea is with a story. Imagine two people in a closed room. There's no clock on the wall.

[00:34:03]There's no window. There's no way for either of them to know what time it is from any external source. There is only the room and the two of them and nothing else. The room is for the purposes of the story a self-contained universe.

[00:34:20]Now, imagine that one of these people is holding a stopwatch. Not a clock that knows the time, but a simple counter that starts at zero and increases. Tick, tick, tick. And the other person is doing some activity, reading a book, let's say, drinking a cup of coffee, watching a candle burn down. Neither of them alone has access to time in any absolute sense. There is no master clock outside the room. The first person's stopwatch does not know the universal time because there is no universal time.

[00:34:58]The second person drinking the coffee is not aging according to some external schedule because there is no external schedule. The room is a closed system.

[00:35:09]The room has no outside. But here is the magic from the perspective of the second person. The first person's stopwatch is a clock. Because as the first person's stopwatch ticks from 0 to 1 to 2 to 3, the second person's coffee cools, the second person turns the page of their book and so on. The two systems viewed from the inside appear to be evolving in time relative to each other. Even though from a hypothetical outside perspective, from the perspective of the universe as a whole, neither of them is evolving at all. The whole room is frozen. The whole story is as I said before the film on the table, not the film and the projector. But there is a relationship between the parts of the film that when you look from the inside generates the experience of motion. That in essence is what Paige and Wooders showed mathematically.

[00:36:07]They showed that if you take a quantum universe with no time and you split it into two subsystems that are quantum mechanically entangled with each other, then from the perspective of one subsystem, the other subsystem will appear to be evolving in time. Not as an approximation, not as a useful fiction, as a precise consequence of the mathematics of entanglement. Time in their picture is a kind of correlation.

[00:36:40]It is what one part of the universe sees when it looks at the rest of the universe. And the more entanglement there is, the more deeply the parts are correlated with each other, the sharper, the cleaner, the more timelike the apparent flow becomes. When I first really sat with this idea, um, I'll be honest with you, I had to read it three times. The first time I read it, I thought it was a clever trick, a mathematical slight of hand. The second time, I thought it might be a deep result, but I wasn't sure I believed it.

[00:37:17]I wasn't sure it wasn't just a way of redefining time, so that the question of time disappeared. The third time, something clicked. I understood and I remember I got up from my desk in my office at Caltech and I walked outside and I stood for a while looking at the San Gabriel Mountains because I needed to be somewhere where I could see the horizon. I needed to be somewhere where I could remind myself that there was a world out there and not just the chalkboard in my office because the implication of what uh Paige and Wooders had done, if it was right, was that time itself, the river that we all assumed we were swimming in, was not the river. It was the swimming. Time wasn't the medium. Time was the relationship between us and the medium. For about two decades, this was a beautiful idea on a chalkboard. Most physicists who heard about it nodded and said, "That's clever." and went back to their own work. There was no way to test it. It was at best a deep philosophical reframing. Maybe true, maybe just a mathematical curiosity. Maybe one of those tantalizing ideas that look profound until you try to do something with them and they slip through your fingers. And then in 2013 and 2014, a research group at the National Institute of Metrological Research in Turin, Italy, led by a young experimental physicist named Ekatarina Morava working with Marco Genevves, Georgio Breida, Marco Gagna, and a few others. They did something extraordinary. They built an experiment to test the Page Wooders mechanism in a laboratory. I want to be careful here about exactly what I claim.

[00:39:07]They did not test it on the universe. We can't do that. We don't have a second universe to compare to and we don't have a way to step outside the one we're in.

[00:39:17]What they did was build a small isolated quantum system, two photons prepared in an entangled state. And they asked from inside this little universe of two photons, does time appear to flow? Even though from the outside the system as a whole is in a stationary timeindependent quantum state, the setup is in its details technically intricate, but the core of it is beautifully simple. They took a single photon and split it using a process called spontaneous parametric down conversion into two daughter photons that were entangled in their polarization. One of these photons would serve as the clock. Its polarization would be measured at different angles simulating different times. The other photon would serve as the system being observed. Its polarization would be the thing that from the inside appeared to evolve. From an outside perspective, from the perspective of someone standing in the laboratory observing the entire two photon system as a whole, the system was in a stationary state. The total quantum state did not evolve. The wave function of the two photons considered together was timeindependent.

[00:40:33]It was a Wheeler Dwitlike situation scaled down to the smallest possible universe. But when the experimenters used the first photon as a clock, when they conditioned their measurements of the second photon on different polarization states of the first, they saw something remarkable. They saw the second photon's polarization rotate.

[00:40:55]They saw it move. They saw inside this tiny laboratory analog of the timeless universe. Time emerging out of entanglement exactly the way Paige and Wooters had predicted 30 years before.

[00:41:10]Let me say that again because it is genuinely one of the strangest experimental results of the 21st century. From the outside, nothing was happening. The system was frozen, static, timeindependent from the inside using one of the entangled photons as a clock. The other photon's polarization was rotating in time. There was a flow. There was a before and an after. There was inside this little quantum bubble the unmistakable appearance of evolution.

[00:41:47]Now that uh uh experiment did not prove the universe has no time. It would be far too much to claim that. But it did prove uh something I think is just as important. It proved that the mechanism, the mathematical machinery by which time can emerge from a fundamentally timeless reality actually works. It is not a fantasy. It is not a thought experiment.

[00:42:09]It is something you can do in a basement laboratory in northern Italy with a couple of lasers and a polarization analyzer and some patience. It is a real repeatable physical phenomenon. And that I think was the moment when the Wheeler Dit equation stopped being an embarrassment and started being a frontier. After 50 years of professional silence, after 50 years of polite avoidance, the equation could finally be looked at directly because we now had evidence, actual laboratory evidence that its strangest consequence was not a paradox but a real feature of how quantum systems behave. I want to mention here one of the things I find most humbling about this entire story.

[00:42:58]The Paige Wooders paper when it was first published in 1983 was not taken seriously by most of the physics community. Don Page in particular struggled for years to get attention for the idea. He was not a self-promoter. He was a quiet, deeply religious man. and he'd been an evangelical Christian his whole career, which was unusual in academic physics, and he had a tendency to publish important ideas in venues where they got buried. He once told me with a kind of resigned smile that he thought of the page mechanism as the result that nobody wanted to be true, right? It implied things about the nature of time that most physicists were not ready to accept. So they didn't it took the experimental confirmation in 2014, three decades later for the physics community to really wake up and even then the wake up was slow. The Morava experiment was published in physical review a a major journal but it didn't make headlines. It didn't win the Nobel Prize. It is still today not as well known as I think it deserves to be.

[00:44:13]Most working physicists I talked to have heard of it vaguely. Most physics students have not heard of it at all.

[00:44:20]This is I have come to believe one of the great patterns in the history of physics. The most important results are often the quietest. They sit on shelves for decades while the field catches up to them. The Michaelelsson Mley experiment in 1887, which would eventually demolish the idea of the luminiferous ether and pave the way for special relativity, was initially considered a failure. They didn't find what they were looking for. So, they thought their apparatus must be broken.

[00:44:51]It took 18 years until Einstein's 1905 paper for the field to understand what Michaelelsson and Mley had actually shown. The page wooders mechanism may follow the same pattern. We may look back 50 or 100 years from now and say that was the moment time as we knew it ended. We just didn't notice at the time. And it leads to a question that I want you to sit with for a moment because we are going to spend the rest of our time wrestling with it. The question is this. If time is emergent, if time is what one part of the universe sees when it correlates with another part, then what is happening at the deepest level? What is the universe actually doing beneath time? What is the timeless reality out of which time arises? Because the answer to that question, and I'll tell you, it is an answer my colleagues and I have been arguing about for 40 years with no resolution in sight. The answer to that question is going to take us into territory stranger than anything we've covered so far. We're going to have to talk about what it means for the past and the future to coexist. We're going to have to talk about why we remember the past and not the future when the equations don't distinguish between them. We're going to have to talk about whether the present moment is real or whether it's a kind of illusion the brain manufactures because we are entangled with our own memories. We're going to have to talk about what physicists call the block universe. And we're going to have to ask the question Einstein asked, the question Wheeler asked, the question every honest physicist eventually has to ask, and not be able to look away from if the universe is timeless, what does that mean for me, for you? For the lives we live, the choices we make, the futures we imagine. Let me start from the most surprising place. When I was a graduate student at Princeton in the 1960s, I was taught, as every physicist is taught, that the laws of physics are time symmetric. What that means in plain English is that if you wrote down the equations of motion for any physical system and you replace the variable t for time with minus t, the equations would look exactly the same. Newton's laws are time symmetric. Maxwell's equations of electromagnetism are time symmetric. Einstein's equations of general relativity are time symmetric.

[00:47:25]Even the Schroinger equation of quantum mechanics with one small caveat involving the weak nuclear force which we won't get into here is time symmetric. What that means is that according to fundamental physics there is no preferred direction of time. The universe should not in principle distinguish between past and future. If you took a film of any fundamental physical process, two particles colliding, an electron orbiting a nucleus, a gravitational wave rippling through space, and you ran it backwards, the backwards version would also obey the laws of physics. It would be a perfectly legitimate thing for the universe to do. It would not look weird.

[00:48:08]It would not violate any rule. And yet we do not live in a time symmetric world. We live in a world where eggs break but do not unbreak. Where coffee cools but does not spontaneously reheat.

[00:48:24]Where memory runs from past to future.

[00:48:27]Where we grow older and not younger.

[00:48:30]Where civilizations rise and fall and do not having fallen rise again in reverse.

[00:48:36]The arrow of time is the most obvious feature of our experience. and it is nowhere in the fundamental laws. This is a paradox so deep that it has tormented physicists for over a century. Ludvig Boltzman in the late 1800s gave us the beginning of an answer with the concept of entropy. He showed that there is a quantity entropy roughly the measure of disorder in a system that that statistically tends to increase. Not because the laws of physics demand it, but because there are vastly more disordered configurations than ordered ones. If you take a deck of cards and shuffle it, you almost always get a disordered arrangement because there are many, many more disordered arrangements than ordered ones. The system left to itself will with overwhelming probability drift from order toward disorder. The arrow of time in Boltzman's picture is the arrow of increasing entropy. But this only pushed the question back a step. Why is entropy lower in the past than in the future?

[00:49:46]Why was the universe in a state of low entropy at the beginning? This is what cosmologists call the past hypothesis.

[00:49:53]the assumption that the universe started at the big bang in an extraordinarily low entropy highly ordered state and has been increasing in entropy ever since.

[00:50:04]We don't know why. We have models. We have inflation theory which suggests the early universe went through a brief period of exponential expansion that smoothed it out and set up the initial conditions. But why inflation happened and why the boundary condition was what it was is still fundamentally unknown.

[00:50:27]We just know it was. And that one initial condition 14 billion years ago is what gives us the arrow of time we experience today. Roger Penrose, the great British mathematical physicist who shared the 2020 Nobel Prize with two other colleagues, has spent much of his career arguing that the low entropy initial state of the universe is the most extraordinary most unexplained fact in all of cosmology.

[00:50:57]He has calculated that the probability of the universe having started in the state it did given a random selection from all possible starting states is approximately 1 in 10 to the 10^ the 123rd.

[00:51:14]That is a number so large that I cannot describe it. It is a number larger than the total number of particles in the observable universe raised to itself raised to itself many times over. It is for all practical purposes infinite. And yet somehow that is the state the universe started in. Here is what I want you to notice. The arrow of time in this picture is not a fundamental feature of physics. It is a contingent feature of our particular universe. It comes from a boundary condition. The absurdly low entropy state at the beginning, not from the laws themselves. The laws by themselves don't care about the direction of time. Now combine that with the Wheeler Dit equation. combine the idea that the laws of physics are time symmetric. The idea that the arrow of time comes from a boundary condition and the idea that at the deepest quantum gravitational level, time doesn't even exist as a fundamental variable. What picture do you get? You get a picture in which our entire experience of time, the flow, the direction, the sense of now is fundamentally an emergent property. It arises from three things layered on top of each other like geological strata.

[00:52:40]First, the structure of the wave function of the universe which by itself contains no time. Second, the entanglement between subsystems within that wave function which generates the appearance of time relative to internal clocks. Third, the asymmetric distribution of entropy across the universe which gives the emergent time a direction. In other words, time is not something the universe has. Time is something certain parts of the universe see. And here is where the philosophical implications become genuinely vertigenous.

[00:53:19]Because if time is emergent, if time is something that arises from internal correlations rather than something the universe fundamentally has, then the universe uh taken as a whole must contain the past and the future on equal footing. This is what physicists call the block universe interpretation. And it goes back to Einstein and his teacher Herman Mancowski.

[00:53:43]Manowski in 1908 gave a famous lecture in which he said that henceforth space by itself and time by itself were doomed to fade away into mere shadows. And only a kind of union of the two would preserve an independent reality. He called this union spacetime. And in spacetime, every event, every moment of every object's history has a fixed location. Your birth is at one set of coordinates. Your reading these words is at another. Your death is at a third.

[00:54:14]The supernova explosion that billions of years from now will end the sun is at a fourth. They are all there in the four-dimensional block, equally real, equally present in the mathematical sense. Now, the block universe interpretation has been around for over a hundred years. Many physicists believe it. Many philosophers find it disturbing. The disturbing thing about it, of course, is that it seems to deny the special status of now. If your birth and your death are both points in the block, equally fixed, equally real, then in what sense is the present moment privileged? In what sense is now different from any other point in your life? The Wheeler Dit equation taken seriously says it isn't. There is no privileged now. The block is real. The flow is illusion. We feel like we're moving through time, but we're not. We are all of us, every moment of us, just there in the four-dimensional block. All of it at once. Your three-year-old self is still there at her own coordinates.

[00:55:19]Your 80-year-old self is there, too, at her coordinates. Every moment of your life is permanent in the block. It does not go away. It does not fade into the past. It is right there real eternal embedded in the mathematics. I want to tell you and this is one of the things I find genuinely difficult to convey that when you really sit with this idea it has a very particular emotional flavor.

[00:55:50]It is not depressing exactly. It is not um uh exhilarating exactly. Um it is uh something else. It is a a kind of vertigo uh a standing on the edge of a very high cliff and looking down. The world doesn't feel different. Your coffee still cools. Your watch still ticks. You still feel like you are reading these words now and not five minutes ago. But underneath the experience, there's a new awareness that none of that is what it seems. That the flow is a story your brain tells. That the now you feel is just one frame in the strip of film. not different from any other. Wheeler used to say that uh the most amazing thing about reality is not that it exists but that it can be understood. Uh I think about that line a lot because here is a piece of reality that we can almost but not quite understand. We have the mathematics, we have the experiments, we have the framework. What we don't have yet is the intuition. We are all of us even those of us who have spent decades on this still trying to feel our way through the intuition I sometimes think is the last thing to arrive. The math comes first, the experiments come second. And the gut feeling, the way of seeing the world that makes the new picture obvious that takes generations.

[00:57:22]There's another piece of evidence I want to mention u because it's directly relevant and because it comes out of work that I was peripherilally involved in. The detection of gravitational waves at LIGO on the 14th of September 2015, a moment I will not forget for the rest of my life, gave us for the first time a direct view of the curvature of spaceime in the most extreme conditions. Two black holes spiraling together and merging. I remember where I was that morning. I was at home in Pasadena on the phone with Ry Weiss who was at MIT and Barry Barish who was on his way to a meeting. The signal had come in during the night. Both of the LIGO observatories, the one in Livingston, Louisiana and the one in Hanford, Washington, had recorded it 3,000 m apart, separated by 7 milliseconds, exactly as you would expect from a gravitational wave traveling at the speed of light. The waveform matched to extraordinary precision what general relativity predicted for the merger of two black holes. One of about 36 solar masses and one of about 29 spiraling together a billion300 million lightyears away from Earth. We measured the strain of spaceime to a precision of one part in 10 to the 21st. To give you a sense of that number, that's like measuring the distance to the nearest star over four light years away to within the width of a human hair. It is the most precise measurement human beings have ever made. And what we measured was spacetime itself rippling, not space rippling through time.

[00:59:06]Spacetime, the unified four-dimensional fabric rippling. The equations that describe what we saw don't distinguish between the time direction and the space directions in any fundamental way. The wave was in spaceime period. The detection confirmed in the most direct possible way that Einstein's vision of a unified four-dimensional structure was right. And every observation we have ever made in every laboratory, with every telescope, with every detector is consistent with the idea that time is part of spaceime. That spacetime is a unified four-dimensional structure and that now is not a special slice of reality, but simply where we happen to be situated within the block. So if you ask me, and people sometimes do often at dinner parties, sometimes at airports, whether I believe in the block universe, whether I believe the Wheeler Dit equation is telling us the truth. My answer is I believe it the way I believe in general relativity, which is to say I believe it as far as the evidence permits and not one inch further. And what the evidence permits right now is a picture in which the universe is timeless at its deepest level. Time emerges from entanglement. The arrow of time emerges from entropy. And our subjective experience of now is the brain's way of reading the local correlations between memory, sensation, and prediction. That last part, the brain's way of reading correlations is itself a deep question. And it brings us to the part of this story I find most thrilling. Because if our subjective experience of time is itself a kind of computation, a kind of interpretation that the brain performs on a fundamentally timeless physical substrate, then questions about consciousness and questions about time become entangled in a way they never have been before. What is a moment of consciousness? What is a now? If you take the Wheeler Dwit equation at face value, every moment of your conscious experience is a particular configuration of neurons, electromagnetic fields, ion channels in a particular region of the four-dimensional block. The U of five minutes ago is a different configuration in a different region of the block. The U of five minutes from now is a third configuration. From the outside, all three are equally real, equally present, equally fixed. But each one of those configurations, and this is the strange and beautiful part, has encoded within it the experience of being now. Because the you of five minutes ago experience that moment as a now. The you of five minutes from now will experience that moment as a now. Each frame of the film when you look inside it contains a small consciousness that thinks this is the present. This is the moment that is real. And in the block universe, every one of those frames is equally correct in that judgment. Every now is the now from inside its own frame. This is what Carlo Ralli, a wonderful Italian physicist, one of the great minds working on this problem today. What Carlo calls the relational view of time.

[01:02:38]There is no universal now. There are only local nows. Each one defined by the correlations between a particular conscious system and the rest of the universe at that location in the block.

[01:02:50]Your now is real to you. My now is real to me. The now of someone living a thousand years from now on a planet around a star that hasn't yet been discovered is real to them. They are all simultaneously real in the timeless block. And the only thing that distinguishes them is where they are in the four-dimensional structure. I find this both terrifying and liberating.

[01:03:18]Terrifying because it dissolves the special privilege of the present moment.

[01:03:23]liberating because it means nothing is ever truly lost. Every moment of joy you have ever had, your wedding day, your child's birth, the first time you saw the ocean, the laughter of friends now gone, is still there in the block exactly as it was, equally real to the moment you are living right now. Time in this picture doesn't take things from you. Time only seems to take things from you because you only experience your own slice of the block at any given moment.

[01:03:57]But the rest of the block is still there forever, untouched, permanent. When I think about my friends who have died, Wheeler, Hawking, my wife, I take a strange comfort in this. The Wheeler Dit equation in its austere mathematical way tells me that they are still there. Not in some metaphorical sense in the sense that the equations describe a four-dimensional block in which their lives are still happening in their own frames with their own felt nows. I cannot reach them. I cannot speak to them. The structure of the block forbids that there are paths through the block, the geodics I mentioned earlier, that connect events to other events, and only some events can causally influence others. The arrow of causation is what makes the past unreachable from the future. But the events themselves, the moments themselves are all there. I want to be careful here because this is the kind of statement that can easily become sentimental and I am trying to stay rigorous. The mathematics does not say that consciousness persists in any active sense. It does not say that the deceased live on in any spiritual way.

[01:05:12]What it says is that the four-dimensional structure of the universe contains every event that has ever occurred or will ever occur and that those events have within them whatever qualities they had when they were experienced. That is a statement about the structure of physics. Whether to take comfort from it is a personal matter. Each person, I think, has to find their own way of being with this idea. Now, I want to address the most obvious objection because if you've been following along, you have surely been thinking it. The objection goes like this. Kip, this is very interesting, but my experience is overwhelming. I feel time passing. I remember the past. I anticipate the future. I make choices.

[01:06:02]The block universe denies all of that.

[01:06:04]It says I am a fixed pattern in a four-dimensional structure with no real freedom, no real becoming. How can that be right? And here is what I want to say to that objection, which is one of the most important things I can say in this entire conversation. The block universe does not deny your experience. Your experience is a real feature of the block. The feeling of time passing is a real feature of the block. The feeling of making a choice is a real feature of the block. None of those things go away when you adopt the block universe interpretation.

[01:06:42]What changes is your understanding of what those features are. The flow of time in the block universe is not the universe moving through some external time. It is the local correlation between your memory and your sensations embedded in the larger structure. The feeling of choice is not an uncaused intervention from outside the block. It is a complex computational process happening in your brain embedded in the block in which various possibilities are weighed against each other and one is selected from the outside. That process is fully described by physics. From the inside, it feels like deliberation. It feels like freedom. And in a real sense, it is freedom. It is the freedom of a complex system to compute its own response to its environment, to draw on its memories, to imagine its possible futures, to choose. The fact that the entire process is also encoded in the block does not diminish the inside experience. It just adds a layer of of description. The point is that the block universe does not eliminate any of the features of your experience. It reframes them. It says yes, you experience flow, choice, meaning, love, loss, hope, regret, all of these things, they are all really happening. They are all really part of reality. They are not, however, happening in the way you naively imagine they are. They are happening as patterns in a timeless structure and the experience of them happening in time is itself one of the patterns. Wheeler near the end of his life used to say that the universe is not something that is out there being observed. The universe he said is something we participate in. He had a phrase he liked participatory universe and uh I think he was groping toward an idea that the Wheeler dwit equation makes precise. We are not observers of the universe from outside it. We are correlations within it. We are entanglements.

[01:09:01]We are the relationships that the universe has with itself. And when we feel time passing, what we are feeling is one part of the universe. The part that is us being correlated with another part of the universe, the part that is everything else. The flow is the correlation. We don't move through time.

[01:09:20]We are time. Or to put it the way Wheeler liked to put it, we are the universe trying to know itself. I want to tell you about one more piece of this puzzle because it brings the story full circle in a way I find genuinely beautiful. In 2017, a group of theoretical physicists, including some old colleagues of mine at Caltech and some younger uh researchers in Italy and Vienna published a series of papers showing that the Paige Wooders mechanism is not just a mathematical curiosity, but a deep and inevitable feature of any quantum theory of gravity. They showed that if you take any closed quantum system that includes its own observers, that is observers who are inside the system, not outside it, and you ask what does time look like for these internal observers, you find that it always emerges from entanglement in exactly the way Paige and Wutters described. It is in some sense mathematically inevitable.

[01:10:23]There is no way to write down a sensible quantum theory of a closed universe without this property. What that means is that the Wheeler Dwit equation with its absence of time is not an exception.

[01:10:38]It is the rule. Any complete quantum theory of any closed universe will have this property. The universe at the deepest level is timeless. Time is what the inside looks like. And there have been other developments too that I want to mention briefly because they are part of the larger story. Lucien Hardy at the perimeter institute has been working on what he calls the causaloid framework which generalizes the page wooters idea further and asks whether causality itself is fundamental or emergent.

[01:11:10]uh some of my colleagues working on the uh ads CFT correspondence that is the relationship between certain theories of gravity and certain quantum field theories on the boundary of spaceime have found to their surprise that time on the inside of a curved spaceime corresponds to entanglement structure on the boundary. The deeper you look, the more time looks like a thing that emerges from quantum correlations. And then there is the question of black holes which I cannot leave without mentioning because black holes are where all of this becomes most acute. When something falls into a black hole from the perspective of someone watching from outside, it never quite gets there. Time slows down for the falling object asmtoically approaching but never reaching the moment of crossing the event horizon. From the inside, however, from the perspective of the falling object itself, there is no such barrier.

[01:12:12]The crossing happens in finite time. The two perspectives are radically different. The two observers experience radically different times and both perspectives are correct simultaneously in the same universe. This is general relativity at its strangest and it foreshadows the timelessness of the deeper theory. Different observers see different times depending on their location and motion. There is no universal clock. There is only the local correlation between an observer and their environment. When we combine that observation with the Wheeler Dwit equation, we begin to see that the absence of universal time is not an exotic theoretical possibility. It is the natural extension of what general relativity has been telling us for a hundred years. Einstein already showed that there is no universal. Now the Wheeler Dwit equation just takes that one step further. There is no universal time at all. There are only local times generated by the correlations between subsystems of a fundamentally timeless universe. And that I think is the deepest insight of the entire 50-year journey. We started with general relativity. We added quantum mechanics.

[01:13:28]We got an equation with no time in it.

[01:13:32]We thought that was a paradox. It turned out to be a revelation. Time is not the stage on which the universe acts. Time is what the universe sounds like when you listen to it from the inside. The ancients had an intuition that some of this might be true. Plato in the Timus said that time was the moving image of eternity. Augustine in his confessions wrote that he knew what time was as long as no one asked him. But the moment he was asked, he no longer knew. The Buddhist traditions have spoken for thousands of years about the illusory nature of duration about the present moment as the only real thing about the constructed nature of past and future.

[01:14:17]None of them had the equations. They had only the intuition and it turns out the intuition was in some deep sense correct. What the modern physics adds is precision. We can now say with mathematical care in what sense time is illusory and in what sense it is real.

[01:14:38]It is illusory as a fundamental feature of the universe. It is real as an emergent feature of the correlations between subsystems.

[01:14:48]It is illusory as an external clock that ticks for everything. It is real as a relationship between you and the world that your senses parse into a stream of moments. The Wheeler Dwit equation gives us the framework. The Paige Wooders mechanism gives us the bridge. The more of an experiment in tin gives us the empirical foothold and the block universe interpretation gives us the picture that ties it all together. Let me say a few things now about the implications of all this for how we live because I think there are implications and I think it is worth sitting with them if time is emergent. If every moment of your life is permanent in the block, if now is just a feature of how a particular pattern of correlations parses itself, what does that mean for how to live? What does that mean for grief, for regret, for hope? I have thought about this a great deal and I want to be careful because physics does not by itself tell you how to live.

[01:15:53]Physics describes the structure of reality. It does not prescribe how to feel about that structure. Different people encountering the same physics will draw different lessons from it.

[01:16:08]Some will find the block universe terrifying because it seems to undermine the agency of the present. Some will find it comforting because it seems to make every moment eternal. Some will simply shrug and continue with their lives because the equations are far away and the dishes still need to be done.

[01:16:28]But here is what I have come to believe after decades of thinking about it. The Wheeler Dit equation does not tell us that our lives are without meaning. It tells us if anything that our lives are more meaningful than we had supposed.

[01:16:44]Every moment of consciousness, every moment of love, every moment of wonder, every moment of grief, every one of these moments is permanent in the block.

[01:16:52]They do not fade. They are not erased by time. They are in the deepest mathematical sense eternal features of the universe. The fact that you experienced something, that you had that conversation, that you saw that sunset, that you held that child, that fact is sewn into the four-dimensional fabric of reality. It will always have happened.

[01:17:16]It will always be there. And the moments you have not yet experienced, the moments that are from your current vantage point in your future, those are also there, also waiting, also real. The block contains them. You will arrive at them in your local sequence of correlations and they will become nows for you in their own time. The future is not less real than the past. It is just less accessible to your memory, but it is no less real to the universe. I have a daughter, her name is Brett. She is now an adult, but I remember her as a child uh running through the backyard in Pasadena, chasing the dog, laughing in that particular way that only children laugh. That moment is in the block. It is permanent. The block contains the laugh of my daughter as a child forever.

[01:18:11]And I find when I think about that that the block universe is not cold. It is not impersonal. It is in its way the most generous interpretation of reality I have ever encountered. It says nothing is lost. Nothing is ever lost. Every moment of your life is preserved exactly in the structure of spaceime for as long as the universe exists which is mathematically forever. What I want to leave you with at the end of this long meditation is something Wheeler said to me one afternoon in his office near the end of his life. Uh the chalkboard was as always uh covered in equations. Uh the afternoon light was coming in through the high windows. We had been talking about the Wheeler Dit equation, about Paige and Wooters, about all of it. And Wheeler turned to me and he said in that quiet way he had with the slight smile he had when he was about to say something he had been thinking about for a long time. Kip, we are the universe waking up. We are the universe noticing itself. And the strangest thing of all is that the universe when it notices itself finds that it is older and stranger than it thought. I have carried that line with me ever since because what Wheeler was getting at, I think, is that the discovery of the timeless universe is not a discovery that diminishes us. It is a discovery that locates us. It tells us where we are. It tells us that we are not little observers perched outside the cosmos watching the parade of events go by. We are inside the cosmos. We are part of the structure. The flow we feel is the universe feeling itself through us from the inside. The Wheeler Dit equation, that quiet, austere half- buried equation that physicists for 50 years didn't quite know what to do with is I have come to believe not the catastrophe Wheeler feared it was that day in 1972.

[01:20:32]It is the deepest love letter the universe ever wrote to itself. It says, "I am everything. I have no outside. I have no clock. I am vast and I am still.

[01:20:46]And when I look at myself, when one of my parts looks at another of my parts, time emerges and a story begins and a consciousness wakes up. And that consciousness for a brief shining moment in the four-dimensional block gets to ask the question what is real and the answer I think is the question itself.

[01:21:08]The asking is the realness the looking is the time the wondering is the whole point. So the next time you feel that time is moving too fast or too slow I want you to remember that what you are feeling is not a flaw in the universe.

[01:21:24]It is the universe doing exactly what it does. It is the universe correlating with itself. It is in some real and not metaphorical sense the universe loving itself into existence. One moment, one entanglement, one frame of the great timeless film at a time. We are the moving image of eternity. We are the place where the timeless block knows itself. We are the way the universe learns its own name. And the equation that tells us this, the equation that for 50 years uh my colleagues and I have circled cautiously the way you circle something sacred and dangerous at the same time. That equation is not I now believe a wound in physics. It is the deepest answer physics has ever given to the oldest question we have ever asked.

[01:22:17]What is time? It is what we are. It is what we do. It is the music our consciousness makes when it sings itself into the structure of the cosmos. And it is in the end what we leave behind for the universe to remember itself by.

[01:22:34]Thank you for spending this hour with me. Thank you for letting me share something that has shaped my life as a physicist, as a thinker, and in the end as a human being. Keep looking at the sky. Keep asking the strange questions.

[01:22:52]Keep being curious about the parts of physics that the textbooks don't quite know how to talk about. Those are usually the parts that 50 years from now we will look back on and say that was where the next revolution was hiding.

[01:23:08]The universe, I promise you, is much much older and much much stranger than anyone ever told you in school. It is timeless at its heart. It is woven from entanglement.

[01:23:22]It is a four-dimensional block in which every moment you have ever lived and every moment you will ever live is permanent. And it is every moment asking itself through you what it is. That is not a small thing. That is everything.