Brian Greene Explains Time Like You've Never Seen It Before

[00:00:00]The true nature of time in relativity. Spacetime is a four-dimensional entity. Three dimensions of space: forward, backward, left, right, up, down, and one dimension of time.

[00:00:13]These four dimensions are interwoven into a single geometric structure. Events in spacetime have locations in all four dimensions. They occur somewhere within three spatial coordinates and somewhere within one temporal coordinate. Just as a location in space requires three numbers to be specified, one in spacetime requires four. This is where the key lies. In this four-dimensional space-time, all moments, past, present and future, exist just like all locations in space. Let's consider spatial distance. New York and Los Angeles both exist right now.

[00:00:58]Even though you're in New York and Los Angeles is far away, the fact that you 're here and not there doesn't mean it does n't exist. It just means you're in a different location in space. Similarly, in the image of space-time, yesterday and tomorrow exist even though you experience today.

[00:01:16]The fact that you are in this moment and not another does not mean that others do not exist. It just means you're in a different location in time. Just as being in New York doesn't make Los Angeles any less real, being in this moment doesn't make the others any less real. This is called the block universe or the eternalist view of time. All of space-time, all of history from the Big Bang to the distant future, exists as an immutable four-dimensional structure. You and I are four-dimensional objects, lines of the universe traversing this structure. Your universe line extends from your birth to your death. And all moments of your existence are equally real, equally present in the four-dimensional structure, even though your consciousness only experiences a thin slice at a time. Now I can feel the objections arising. This seems to contradict everything we've experienced. When I say now, I mean this moment, this present instant, not yesterday or tomorrow. They feel fundamentally different. The past is gone, it is fixed and immutable. The future has not yet happened. It's open, uncertain, it's not real yet. The present, the now, is what is really happening. What is real right now. But here's the worrying fact. There is no universal now. This was Einstein's great revelation and has been experimentally verified countless times. What counts as now depends on your state of movement. Events that are simultaneous for you, that occur at the same time from your perspective, are not simultaneous for someone who is moving relative to you. Let me give you a concrete example. Imagine you are standing still on Earth and your friend is in a spaceship traveling at high speed. They both look at their watches and notice that it is exactly noon. At that moment, they both think it is midday and believe they are experiencing the same instant, but that is not the case. At least not in an absolute sense. From your perspective on Earth, there are events happening right now on a distant planet, say 10 light-years away.

[00:03:34]You consider those events to be simultaneous with your noon. They are part of their present. But from his friend's perspective on the spaceship, different events on that distant planet are simultaneous to his noon. If the ship moves towards the planet, events that are in its future are in its friend's present. If he moves away from the planet, events that are in his past are in his friend's present. This is not a small effect or a measurement uncertainty. It is fundamental to the structure of space-time. There is no universal present moment, nor a cosmic now in which all observers agree.

[00:04:12]Simultaneity, which means that two events occur at the same time, is relative. It depends on its movement.

[00:04:20]This relativity of simultaneity completely destroys the common-sense notion of time as a universal flow. If there is no universal present moment, then it makes no sense to say that the entire universe is in a particular moment and moving on to the next one.

[00:04:39]Different observers divide space-time into past, present, and future in different ways. And there is no privileged division, nor an objective fact about which division is the correct one.

[00:04:51]Here's an even more surprising example. Right now, as you read this, events are happening on a planet orbiting a distant star. From his current perspective, being relatively still, those events could be in what he considers the present. But imagine someone else moving at high speed relative to you, passing by Earth right now. From their perspective, those same events on the distant planet could be in what they consider the past or the future, depending on the direction they are moving.

[00:05:27]This means that what counts as having already happened, happening now, or not happening yet, depends on one's movement. There is no absolute division of space-time into past, present, and future. These categories depend on the observer, such as left and right or up and down. Just as there is no absolute fact about which direction is up in space, it depends on your orientation. There is no absolute fact about what events are the now in time. It depends on its movement. And if there is no universal now, then in what direction does time flow? Flow implies movement from one moment to another. From the past, through the present, towards the future. But if these divisions are not absolute, if different observers divide space-time into past, present, and future in different ways, then what is it that flows? What is it that moves? The image of the block universe suggests that nothing flows. All of spacetime simply exists eternally as a four-dimensional structure. Your experience of flow, of moving through time, is a product of how your consciousness processes the information encoded in your world line. You are not moving through time. You are a static pattern in four-dimensional space-time that contains encoded memories of past moments and anticipations of future moments. The experience of flow arises from the structure of that information, not from any actual movement. This is deeply counterintuitive, and I don't blame you if you find it hard to grasp. Our entire experience is saturated with the feeling that time flows, that the present moment is special, that the future is fundamentally different from the past. How can physics tell us that all of this is an illusion? Let me elaborate on why physicists take this view seriously, because it's not just philosophical speculation. It is based on our best theories and the strongest evidence about how the universe actually works. First, let's consider the fundamental equations of physics. Whether we're talking about Newton's laws, Maxwell's equations for electromagnetism, Einstein's equations for gravity, or Schrödinger's equation in quantum mechanics, these equations are time-symmetric or nearly so. That means they work equally well moving forward or backward in time. If you take a solution to these equations and reverse the direction of time, swapping the past and the future, you get another perfectly valid solution; There is no arrow of time integrated into the fundamental laws. They do not distinguish between the past and the future. They do not favor one direction over the other. If you filmed fundamental physical processes and played the film back, the reverse processes would obey the same laws as the original processes. Now, you might object, but obviously time has a direction. Eggs break but do not spontaneously reassemble themselves. The coffee cools down but does not heat up spontaneously.

[00:08:58]People get older, but they don't get younger. Clearly there is an arrow of time in the world we observe. And you would be right. There is an arrow of time in the macroscopic world we experience. But fundamentally, this arrow comes from thermodynamics, from the increase in entropy, not from the fundamental laws that govern individual particles. The second law of thermodynamics states that entropy—roughly speaking, disorder or the number of possible microscopic arrangements consistent with macroscopic observations— tends to increase over time. This gives us a direction. Entropy was lower in the past and will be higher in the future. That's why eggs break but don't put themselves back together. Why does coffee get cold? Why do we age? All of these are processes that increase entropy, that go from more ordered states to less ordered states. But here 's the key point. The thermodynamic arrow of time is statistical, not fundamental. It arises from the fact that there are far more disordered arrangements of particles than ordered ones. So systems naturally evolve towards disorder simply because there are more ways to be disordered. It is not that the fundamental laws favor one direction of time over the other. They don't. Given certain initial conditions, the universe began in a state of very low entropy near the Big Bang.

[00:10:26]Entropy increases are overwhelmingly likely as you move away from those initial conditions in any time direction. The arrow of thermodynamics gives us a direction that we can call future versus past. But it doesn't give us a flow. It does not give us a present moment in motion. It simply labels a direction through the structure of four-dimensional spacetime as the direction of increasing entropy.

[00:10:48]The structure itself remains static, eternal, and immutable. Furthermore, the thermodynamic arrow depends on special initial conditions. The fact that the universe began in a state of low entropy. If the universe had started in a state of high entropy or if we lived in an eternal universe without a beginning, there might not be a consistent arrow of thermodynamics. And yet, spacetime would still exist as a four-dimensional structure. The arrow of time is a feature of our particular universe, not a necessary feature of time itself. Now, let me address another aspect of our experience that seems incompatible with the block universe. The apparent openness of the future versus the fixity of the past. It feels as if the past is predetermined, written in stone, immutable. What happened, happened, and nothing can change it. But the future feels open, uncertain, not yet determined. We make decisions that affect the future, but we cannot change the past. However, in the image of the block universe, this asymmetry between past and future is also an illusion or, at least, depends on the observer. From the four-dimensional perspective, the future is as fixed as the past.

[00:12:15]Your entire worldline from birth to death exists in the space-time structure. Everything you will ever do is already there, just as everything you have already done is there. The future is no more open than the past. Both are equally real, equally determined by the structure of space-time. But wait, you could protest. Does n't quantum mechanics introduce genuine randomness, genuine indeterminacy into the future? Does n't that mean the future is open in a way that the past is not? This is a good question and it leads us to the relationship between quantum mechanics and time, which is complicated. Quantum mechanics does introduce an apparent randomness into physical processes. When you measure a quantum system, you get a random result from a range of possibilities with probabilities determined by the system's wave function, but without a deterministic prediction of what result you will get. However, and this is crucial, even quantum randomness does not necessarily mean that the future is open in an absolute sense. There are interpretations of quantum mechanics such as the many-worlds theory, where the apparent randomness is just an artifact of our limited perspective. In the many- worlds view, all possible measurement outcomes actually occur in different branches of reality and the wave function never collapses.

[00:13:45]It evolves deterministically according to the Schrödinger equation. In this interpretation, the complete quantum state of the universe, the universal wave function, evolves deterministically through time. There is no fundamental randomness, nor a genuine openness to the future.

[00:14:05]What we experience as random is just our subjective uncertainty about which branch of the wave function we are on. All branches exist in the four-dimensional structure. We simply don't have access to the other branches.

[00:14:20]Even in interpretations where the collapse of the wave function is real and introduces genuine randomness, it is not clear that this gives us the kind of opening we think we experience.

[00:14:32]Randomness occurs at the quantum level, affecting individual particles or small systems. But our decisions, our choices, our sense of agency, are macroscopic phenomena involving trillions of particles. It is not obvious that quantum randomness at the microscopic level translates into a significant opening at the level of human decisions. Furthermore, randomness is not the same as openness or free will. If my choices are determined by random quantum fluctuations, that doesn't make them free. He makes them random. I have no more control over randomness than I have over determinism. So, even if quantum mechanics introduces indeterminacy, it is not clear that this rescues our intuition that the future is open in a way that allows for meaningful choice. Now, I want to be careful here. I am not claiming that free will definitely does not exist, that our choices do not matter, or that we are just puppets acting out a predetermined script.

[00:15:38]The relationship between physics and free will is complex and philosophically controversial. And there are several ways to reconcile our sense of agency with the laws of physics. What I'm saying is that the block universe image, where all of spacetime exists as an immutable four-dimensional structure, without a universal present and where past and future are equally real, is what our best physics suggests is the case. And this image is radically different from our everyday experience of time as something that flows, with a special present moment and an open future. Let me give you another piece of evidence in favor of the block universe. Temporal dilation.

[00:16:22]This is a well-confirmed prediction of relativity that shows that time is not absolute or universal, but depends on motion and gravity. If you are traveling at high speed relative to someone else, time passes more slowly for you than for that person. This is not just an appearance or a measuring device.

[00:16:42]It's real. If you take a round trip in a fast spaceship while your twin stays on Earth, you will actually age less than him. When you return, less time will have passed for you. You could have experienced one year while your twin experienced 10 years. This has been confirmed countless times in experiments with atomic clocks in airplanes, satellites, particle accelerators, and GPS systems. Time really does pass at different rates for observers in different states of motion. Similarly, time passes more slowly in stronger gravitational fields. An atomic clock at sea level runs slower than an identical one on a mountaintop because the gravitational field is stronger at sea level. Again, this has been measured and experimentally confirmed many times. These time dilation effects are incompatible with the idea of ​​a universal time flow. If time flows, how can it flow at different rates for different observers? What does it even mean for time to flow faster or slower? Flow relative to what? The block universe offers a natural explanation. Spacetime is a four-dimensional geometry, and different paths through that geometry have different lengths measured by proper time. The time experienced by an observer following that path.

[00:18:14]Just as different paths between two points in space can have different lengths. Different universe lines between two events in spacetime can have different durations. There is no universal time. There are only several proper times measured along various universe lines. This geometric image of space-time, where time is just another dimension where the past, present, and future exist equally, where there is no universal "now" or temporal flow, is not philosophical speculation. It is the image that emerges from relativity. A theory that has been tested with extraordinary accuracy and has passed all tests. Let me address one more aspect of our temporal experience.

[00:18:57]Memory and causality. We remember the past but not the future. Causes precede effects. These asymmetries seem to require a fundamental difference between the past and the future. A real flow of time from the previous to the subsequent. But again, these asymmetries can be explained without invoking a flowing time or a moving present moment. Both memory and causality have thermodynamic explanations related to the increase in entropy. Memories are states of low entropy, structured information encoded in your brain. Creating memories increases the entropy of the surrounding environment because recording information requires dissipating energy. The fact that we remember the past but not the future is a consequence of the arrow of thermodynamics. We can create records of states of lower entropy, the past, close to the Big Bang, but not states of higher entropy, the future, further from the Big Bang.

[00:20:00]Similarly, our concept of causality, where causes precede effects, is linked to the entropy gradient. We identify as causes those previous events that are correlated with subsequent events in ways that imply an increase in entropy. But in the fundamental laws of physics, there is no asymmetry between cause and effect. The laws work equally well in both time directions. Thus, memory and causality, although they give us a strong subjective sense of directed time, are actually consequences of the arrow of thermodynamics, which is a statistical phenomenon dependent on special initial conditions, not a fundamental characteristic of time itself. Now, having laid out the argument in favor of the block universe, let me acknowledge that this image is unsettling and counterintuitive. It seems to deny the most immediate and undeniable aspect of our experience: that time flows, that the present moment is special, and that the future is open. If physics says otherwise, isn't there something wrong with physics? Shouldn't we trust our direct experience more than abstract equations? This is a reasonable reaction, but I believe that, ultimately, it is wrong.

[00:21:19]Our intuitions about time were shaped by evolution to help us navigate the macroscopic world on human timescales. They were not designed to reveal the deep structure of reality. Just as our perceptual system deceives us about the movement of the sun, which appears to move across the sky, even though it is the Earth that rotates, our temporal phenomenology could deceive us about the nature of time. We should not expect reality at fundamental levels to coincide with our intuitions formed at macroscopic levels. Quantum mechanics violates our intuitions that particles have definite positions and velocities. Relativity violates our intuitions that space and time are absolute and universal. The deep structure of reality has no obligation to coincide with our everyday experience. Furthermore, the block universe does not actually contradict our experiences. She simply reinterprets them. You continue to experience a flow of time, a succession of moments, a distinction between past and future. The block universe explains why you have these experiences, because you are a four-dimensional structure with memory records and causal connections arranged in a particular way. And your consciousness processes this information sequentially, creating the subjective experience of flow even though the underlying reality is static. It's like watching a movie.

[00:22:50]The film reel contains all the frames at once. Scenes from the past, present, and future exist simultaneously in the film. But when you watch the film, you experience a flow, a progression from one scene to the next, even though nothing is actually flowing. The experience of temporal flow can arise from the structure of static information processed in a particular way.

[00:23:14]But this is where things get even stranger. Because I have presented the block universe as if it were the last word on time, the ultimate truth revealed by physics. But it isn't. There are deeper enigmas, unresolved tensions between different parts of physics. Suggestions that our current understanding of time is incomplete. And exploring these enigmas will reveal that the situation is even more perplexing than the simple block universe panorama suggests.

[00:23:45]In the next part, I want to explore the tension between the block universe of relativity and the apparent collapse of the wave function of quantum mechanics, the puzzle of why time has a direction if the fundamental laws are symmetric, and the deep mystery of where the "now" comes from if it is not in the equations. Because although the block universe solves some puzzles about time, it creates others. And the whole story is more complex and mysterious than I have suggested so far.

[00:24:21]So, we have established that relativity gives us a block universe where all space-time exists equally, where there is no universal now, where time does not flow in any fundamental sense. But this poses a profound enigma. If the block universe is correct, if all moments exist equally within a static, four- dimensional structure, why does it feel so powerfully like time is flowing?

[00:24:52]Why do we experience a present moment in motion? Where does the now come from? This is sometimes called the stream of consciousness problem or the mystery of the present moment. And it is one of the deepest unsolved problems in our understanding of time. Because here's the point.

[00:25:09]Even if we accept the block universe, even if we recognize that physics gives us no evidence of flowing time or a special present moment, we still experience these things.

[00:25:23]The experience itself demands an explanation. Let me be more precise about what needs to be explained. When you experience this present moment right now, as you read these words, there is a quality to that experience, a sense of immediacy that seems undeniable. This moment feels qualitatively different from your memories of past moments or your anticipations of future moments. It feels present, immediate, actually happening in a way that the past and future do not. Furthermore, this sense of now seems to move or shift. Just a moment ago, you were experiencing a different now. Perhaps the beginning of this sentence instead of the end. And in a moment, you will experience another now. So there is not only a sense of immediacy, but a sense of flow of the present moment moving through time, of consciousness moving from one moment to the next. Where did this come from? If the universe block is correct, your entire universe line, your entire four-dimensional existence from birth to death, exists as a static and immutable structure.

[00:26:37]All moments in your life are equally real, equally present in four-dimensional space-time. So why do you experience only a thin slice at a time? Why does that slice seem to move? What distinguishes a particular moment like now? This is what philosophers call the problem of the passage of time or the problem of temporal experience. And it is extraordinarily difficult to solve within the framework of the block universe. Let me go through some of the proposed solutions and explain why each one is problematic. The first approach is to say that the experience of the now is just one feature of how consciousness works. How our brains process temporal information. At every moment along your timeline of the universe, your brain contains certain information. Memories of past moments, perceptions of current stimuli, anticipations of future moments. The processing of this information creates the subjective experience of being in a particular now. From this point of view, each moment in your universe timeline has its own associated "now".

[00:27:50]When you experience Monday, that moment along your universe line contains the experience: now is Monday. When you experience Tuesday, that moment contains the experience. It 's Tuesday now. Both experiences exist in the four-dimensional structure. You simply cannot access them simultaneously because consciousness is necessarily sequential. It is part of the structure of a moment to experience oneself as now and other moments as past or future. This is sometimes called the many-nows vision.

[00:28:24]Each moment is experienced as it is now, from its own perspective. And there is no objective fact about what moment is the true now. Just as every place in space is "here " from the perspective of someone in that place, every moment in time is "now" from the perspective of someone at that moment. This solution has the advantage of being consistent with the block universe. It does not require adding anything to the four-dimensional structure, only recognizing that each moment in that structure contains as part of its content a subjective experience of being. Therefore, the flow of time is explained as a structural feature of how moments relate to each other, not as an actual movement. But, and this is an important "but", this solution feels deeply unsatisfactory to many people, including many philosophers and physicists, because it seems to deny the very thing we are trying to explain. The experience that this moment, this now, is special, is what is really happening. The many-nows view says that all moments are equally " now" from their own perspectives, which seems to strip the concept of now of any real meaning.

[00:29:40]Furthermore, this view does not truly explain the experience of the flow of the present moving from one moment to the next. It simply says that every moment contains the experience of being a now. But that doesn't explain why it feels like the present is moving forward, progressing, flowing. There is a difference between a series of static snapshots, each labeled "now", and the dynamic experience of moving through them. Think of it this way. Imagine an animated book. Each page displays a slightly different image. And when you flip through the pages quickly, you see an apparent movement. Movement is an illusion. Each page is static and all exist simultaneously in the book.

[00:30:27]But there is still a difference between seeing the book as a static object and turning the pages to create the experience of movement. The vision of the many "nows" gives us the static book, but it does not explain the act of turning the pages. It does not explain why consciousness experiences the pages sequentially rather than all at once, or why there is a direction to the sequence. These might seem like questions of psychology or neuroscience rather than physics, but they are questions about the fundamental nature of consciousness and its relationship to time, and we don't have good answers. A second approach is to invoke quantum mechanics and suggest that the now is related to the collapse of the wave function.

[00:31:14]In standard quantum mechanics, systems exist in superpositions of multiple states until they are measured, at which point the wave function collapses to a single defined state. Perhaps this collapse is what creates the now. Each collapse is a moment of definiteness and reality that divides the collapsed and defined past from the uncollapsed and undefined future.

[00:31:41]This is an intriguing idea because it would give the now a physical basis instead of just a psychological one. The present moment would be where the quantum indefiniteness of the future becomes the quantum definition of the past. Time would flow in the sense that collapse events continually turn possibilities into realities. But this approach faces serious problems.

[00:32:07]First, the collapse of the wave function is itself deeply mysterious and controversial. Many interpretations of quantum mechanics deny that collapse is a real physical process. In the many-worlds interpretation, for example, the wave function never collapses. All possibilities occur in different branches and what we experience as collapse is just our branch of the wave function effectively isolating itself from the others.

[00:32:37]Second, even if the collapse were real, it is not clear that it would occur in a way that could explain our macroscopic experience of a moving now. The collapse is typically associated with measurements or interactions that occur constantly throughout the universe at all scales. If every collapse creates a now, then there would be countless nows constantly happening everywhere, which does not match our experience of a single, coherent present moment. Third, and most fundamentally, this approach does not actually solve the problem. It just moves it back one level.

[00:33:15]Even if collapse creates defining moments, we still need to explain why consciousness experiences these moments sequentially.

[00:33:24]Why is there a flow from one collapse to the next? Why does a particular collapse event feel like the now, instead of all of them existing equally in the four-dimensional structure? A third, more radical approach is to deny the block universe altogether and argue that the flow of time is fundamental. That the present moment is a basic characteristic of reality that our current physical theories fail to capture.

[00:33:50]From this perspective, the future does not genuinely exist yet. Only the past and the present are real.

[00:33:57]Time really does flow in some objective sense, giving existence to new moments as it progresses.

[00:34:06]This is called presentism in philosophy, and it has the advantage of perfectly coinciding with our subjective experience. If only the present exists, then of course the present moment feels special. It is the only moment that is real. If the future does not yet exist, then of course, the future feels open and uncertain. If the past has ceased to exist or only exists as records in the present, then, of course, it feels fixed and immutable. But presentism faces devastating problems when confronted with relativity. Remember that in relativity there is no universal now. Different observers in different states of motion do not agree on what events are simultaneous or on what exists at this moment. If only the present exists and there is no objective fact about what the present is, then there is no objective fact about what exists. Reality becomes observer-dependent in an extreme way that most physicists consider unacceptable. Furthermore, presentism seems incompatible with the basic structure of space-time described by relativity. The equations of general relativity treat spacetime as a single four-dimensional geometric object. They do not describe a three-dimensional present moment moving through time. They describe the entire four-dimensional structure at once.

[00:35:34]For presentism to work, you would need to fundamentally revise relativity. And there is no empirical motivation to do so. Relativity works remarkably well as it is. Some philosophers have attempted to develop versions of presentism compatible with relativity, but these tend to be very complicated and ad hoc, requiring special structures or preferred frames of reference that the theory gives us no reason to postulate. The simplest interpretation of relativity is the block universe, not presentism. So, we're trapped. The block universe appears to be what physics tells us is real. But it fails to explain our most immediate and undeniable experience.

[00:36:20]The feeling of being in a particular now, the experience of time flowing from the past through the present into the future. We have a deep tension between physics and phenomenology, between what our theories say and what we experience, without a completely satisfactory resolution. Let me make this even more perplexing by introducing another level. The relationship between consciousness and time. Because the mystery of the now is intimately connected with the mystery of consciousness itself. Consider this. When you experience a moment, you 're not just experiencing a single snapshot. You are experiencing a temporary duration. What William James called the specious present. A short interval, perhaps from a few hundred milliseconds to a few seconds, that is experienced as unified.

[00:37:13]When you listen to a melody, you don't just hear one note at a time. You hear several notes together as a phrase, as a musical unit that spans time. This means that the now is not truly instantaneous. It has a finite duration. But how long does it last?

[00:37:32]And what determines that duration? Is it a characteristic of consciousness, of how our brains integrate information over time? Or is there something more fundamental? Furthermore, the contents of consciousness at any given moment include not only present perceptions, but also memories and anticipations, connections with the past and future that are part of the current experience. When you are conscious now, part of what you are conscious of is that this moment is connected to previous moments that you remember and later moments that you anticipate. Your present experience includes temporal structure, temporal relationships, a feeling of being immersed in a flow even though the experience itself is, in a sense, always. This raises a profound question. Is consciousness itself inherently temporal?

[00:38:28]Does consciousness require a flow, a moving present in a way that makes the block universe incompatible with it? Or can consciousness exist within the block universe, with the experience of flow emerging from the static information structure even if there is no actual flow? This brings us to what is called the hard problem of consciousness. The puzzle of why and how physical processes in the brain give rise to subjective experiences at all. And the temporal aspects of consciousness could be intimately related to this difficult problem. Perhaps we cannot understand the present until we understand consciousness. And perhaps we cannot understand consciousness until we understand time. Now, let me introduce yet another complication. The arrow of time and the puzzle of why it even exists. I mentioned earlier that the fundamental laws of physics are symmetric in time. They work the same forwards or backwards.

[00:39:27]But we observe a thermodynamic arrow where entropy increases from the past to the future. But this raises a profound question.

[00:39:36]Why did the universe begin in such a low state of entropy? The Big Bang appears to have been an extraordinarily special low-entropy configuration. All the matter and energy of the universe were concentrated in a tiny volume in a very particular, highly ordered state. Since then, entropy has increased and will continue to increase until it reaches heat death, maximum entropy, total disorder. But why this initial condition? Why did the universe begin in such an improbable state of low entropy? If we think of the block universe, where all of spacetime exists as a complete four-dimensional structure, this question becomes : why does that structure have such a drastic entropy gradient, with very low entropy at one end near the Big Bang and very high entropy at the other end in the far future? This is one of the deepest unsolved problems in cosmology and statistical mechanics.

[00:40:50]There are several proposals. Perhaps it is related to quantum gravity near the Big Bang. Perhaps there is a multiverse where different regions have different entropy gradients, and we exist in one with a strong gradient because only such regions allow life.

[00:41:05]Perhaps there is some deeper law that we do not yet understand that explains the special initial conditions.

[00:41:11]But here's why this matters for our understanding of time.

[00:41:15]The arrow of thermodynamics is the only physical basis we have to distinguish the past from the future. It is what gives time direction. This allows us to label one direction towards the future and the other towards the past. If the universe had started in a state of high entropy or did not have a clear entropy gradient, we would not have a defined arrow of time. So the fact that we experience time flowing in a particular direction, with the past being different from the future, depends on the arrow of thermodynamics, which in turn depends on special initial conditions that we do not understand. The directionality of time, one of its most basic characteristics, rests on a mystery. And here's an even more disturbing thought. In the distant future, when the universe reaches heat death and maximum entropy, there will be no entropy gradient, nor a thermodynamic arrow. Does that mean that time will cease to have a direction? Does it mean that time will stop in some sense? Or will the four-dimensional structure continue to exist, but without a physical basis to distinguish the past from the future within regions of high entropy? These questions do not have clear answers. We are at the limits of our current understanding, where cosmology, thermodynamics, quantum mechanics, and the nature of time intersect in ways we do not fully comprehend. Let me now address a different riddle about time. The relationship between time and causality. We usually think that causes precede effects. If A causes B, then A occurs before B. This seems like a necessary truth, a basic feature of how causality works. But in relativity, the situation is more complex. For events close enough in spacetime for light to travel from one to the other, there is an objective fact about which occurred first. These are called temporally separated events, and all observers agree on their temporal order. If A and B are temporally separated and A occurred before B in one frame of reference, then A occurred before B in all frames. This is what allows causality to function consistently. Causes can only influence effects with temporal separation. But for events that are sufficiently distant in spacetime that light cannot travel from one to the other, there is no objective fact about which occurred first. These are called spatially separated events. And different observers in different states of motion will not agree on their temporal order. For one observer, A occurs before B.

[00:44:08]For another observer moving differently, B occurs before A. For a third observer, they occur simultaneously. This means that the temporal order is neither absolute nor universal. It depends on the observer for spatially separated events. Only for events that can causally influence each other, those separated in time, is there an objective temporal order. This is pretty strange, but this is where it gets really weird. In quantum mechanics, there are phenomena that appear to involve correlations between spatially separated events, correlations that cannot be explained by signals traveling at the speed of light or less. This is quantum entanglement. And it has been experimentally verified countless times.

[00:44:55]When two particles are entangled, measuring one instantly affects the state of the other, no matter how far apart they are. This does not violate relativity.

[00:45:06]Entanglement cannot be used to send signals faster than light. But it does create correlations between spatially separated events that seem to require a connection that transcends the normal causal structure of space-time. Some interpretations of quantum mechanics handle this by saying that the correlations are already built into the initial quantum state. The entangled particles were correlated from the beginning, and the measurement only reveals that pre-existing correlation. But other interpretations, such as certain versions of collapse theory, seem to require some kind of instantaneous influence across space. If instantaneous influences exist