Jim Al-Khalili: My name is Jim Al-Khalili and I'm Emeritus Professor of Physics at the University of Surrey. My book is called 'On Time', the physics that makes the universe tick. If you look up the problem of time online, there will be various definitions of what is regarded as the problem of time. I like to lay out four different distinct problems. One is whether time flows. The second, how can we reconcile quantum field theory with Einstein's general theory of relativity? Third, what is special about now? The fourth and final problem of time. Where does this direction of time come from? At the end of this interview, hopefully we'll lay out the problems and the questions and the paradoxes, some of which we've figured out, others remain.

Jim Al-Khalili: For me as a physicist, I try to understand the external world, try and make sense of it. And to do that objectively, we have to sort of extract ourselves from the thing that we're studying. When it comes to time, there's a problem because we are unavoidably embedded within time. We can't extract ourselves from it and view it objectively. Very often, physicists and philosophers who study the nature of time make this distinction between physical time that is embedded within the laws of physics and our own perception of time, psychological time. I like to use the term manifest time. I think it was first used by the philosopher Craig Callender. So manifest time is a way of compartmentalizing our perception of time from time that is external to us.

Jim Al-Khalili: One of the most powerful distinctions between our perception of time, manifest time and physical time, time that appears in our laws of physics, external to our senses, is this notion that we feel very strongly that time is flowing. There's some continuum, there's change. We tend to think that time, and this is quite a strong feeling, we tend to think that time speeds up as we get older. Where did time go suddenly? I'm 10 to the last 10, 20 years ago, my kids have grown up. How did that happen? And yet, your childhood seemed to stretch on forever. So there's a theory that suggests it's to do with the laying down of new experiences. It's true that one year when you're five years old, the period between one birthday and the next is a really long time. One year when you're 50 years old goes by in a flash.

Jim Al-Khalili: There's the opposite property to time when you look at shorter intervals. So rather than the laying down of new experiences and being active, slowing time down, compare half an hour sitting in the dentist's waiting room when you have nothing to do. You haven't got your phone on, you're not reading anything with half an hour at a party where you're enjoying yourself. Now at the party, you're laying down your experiences. You're meeting people, you're chatting to them, you have a lot of input coming into your senses. And yet that half an hour at the party just goes by much more quickly than the time that drags when you're sitting in the dentist's waiting room.

Jim Al-Khalili: A concept closely related to the nature of time is the idea of change. When something changes, we think of it in a common sense way as changing over time, as time flowing and something changes. This idea goes all the way back to the ancient Greeks who couldn't agree on whether time was fundamental, that it had to exist and then change happens within it. Or whether change is the fundamental concept and time simply reflects that something changes. Physicists have argued about this and they argue about it to this day. So the great physicist Richard Feynman said that time is what happens when nothing else happens. That somehow time exists in and of itself.

Jim Al-Khalili: Isaac Newton had the view that time was external to the universe. External certainly to space and things that happen within space. That there's this cosmic clock that inexorably ticks by the seconds, the minutes, the hours, the years, regardless of what we are doing. We can't stop that time. We're just caught in the flow and time keeps on going. So that absolute notion of time is something that Isaac Newton explained and it's something that in everyday life, I guess, is the common sense view of time. That there is this external clock that's ticking by regardless of whether we think time is speeding up or slowing down.

Jim Al-Khalili: Of course, that idea of an absolute external time was thrown away entirely when Einstein developed his theories of relativity. Einstein explained there is no such thing as absolute time. Time is relative. And yet we still have this strong psychological feeling that there's a past, there's a future, there's a now, there's a flow. Our consciousness is moving along this river of time, gobbling up the future, spitting out the past. The present moment is ever changing. So that notion of change is embedded within this idea that time is flowing. And yet in the laws of physics, there is change, but there's no flow in and of itself to time.

Jim Al-Khalili: In most of our equations of physics that describe how something, a system changes, yes, it changes over time, but time comes in in a very almost trivial way, as a humble coordinate, a number, a parameter, a symbol, lowercase t that sits inside your equation. It's not changing. If you have an equation that describes a system, you can just look at the mass, look at the equation. Well, what does it mean when we evolve this equation in time? Well, it just means we're solving this equation for a value of this coordinate. What a system is doing now is described by what it's doing at this particular value of coordinate time. But we can also evolve it forward and see what it's doing at a different value of coordinate time in the future, or evolve it backwards to see what it's doing in the past. But there's no flow. There's no real change. Their time is simply a number that appears in the equations. And there must be more to time than that.

Jim Al-Khalili: Well, certainly that's our psychological feeling about time, that there is change, and we experience this change as a continuum. Where is that change? Where is that flow in the equations of physics? It doesn't exist. So it seems quite natural to us that time can speed up or slow down depending on what we're doing. If you're having fun, time goes by quickly, if you're bored, time drags. But we know that's just in our subjective experience. It's not what really happens to time. Surely, external to our experience, time continues at a constant rate everywhere in the universe. Well, of course, that's Newtonian time, and that's the time of our everyday experience.

Jim Al-Khalili: But Einstein comes along and says, "Actually, the idea that time speeds up, slows down isn't just subjective, isn't just in our heads. There is a real physical phenomenon whereby time can run at different rates for different observers." What we say in different frames of reference. That was a revolution in physics. Einstein, of course, had two theories of relativity.

Jim Al-Khalili: The first one in 1905 was forced upon him by not studying the nature of time and space, initially, but by studying the nature of light and the speed of light. Physicist the late 19th century had been searching for the medium that carries light. Now, light was known to be a wave. Light, therefore, physicists believed, needed something to travel through some medium, this ether. The fact that I couldn't find it was very puzzling, and Einstein overthrew the whole concept of what it means, what time and space means, by suggesting that light doesn't need an ether at all. Which means that however fast you're moving relative to someone else, you should still see light moving at the same speed. What does this mean?

Jim Al-Khalili: If I shine a torch out into space, and then a friend jumps in a rocket and flies, let's say, at three quarters of the light speed. So I will see light overtaking them in their spacecraft at a quarter light speed. Einstein says, "You would expect that they would see light overtaking them at that same speed." But no, they would still see that light overtaking them at the same speed that you see it leaving you. All observers, however fast they're moving relative to each other, will see light moving at the same speed. It shouldn't make any sense at all, until you realize that they must have different concepts of space and time.

Jim Al-Khalili: And in fact, my friend travelling in the spacecraft trying to catch up with this light beam, if you were to look at their clock on board their spacecraft, you'd see it ticking by more slowly. That's the sun's workout. You change the nature of space and time in order to preserve the constancy of the speed of light for everyone. So Einstein came to this conclusion in his special theory of relativity due to the constancy of the speed of light and the fact that all motion is relative. So no one can say, "I am definitely standing still and you're moving." To come up with the idea that time and space get altered. And since we're talking here about the nature of time, this is what leads to the idea of time dilation. That time can be seen to run slower on a moving clock as it moves past you.

Jim Al-Khalili: Now, if someone's moving past me close to the speed of light, I will see them doing everything in slow motion. It's just not just that their clock is moving slower. Somehow their high speed is messing with the mechanics of the clock. That whole frame of reference I will see time running slower in it. However, since all motion is relative, the person board the spacecraft that I see behaving in slow motion would see me behaving in slow motion because for them, I'm doing all the moving. And so it's my time that is running slower.

Jim Al-Khalili: This idea of time dilation seems like just a thought experiment or some notion that Einstein's come up with surely doesn't happen in the real world. Well, it does. For example, muons are subatomic particles that are created in the upper atmosphere. Cosmic rays hit the molecules of air in the upper atmosphere and the energy from the collision creates a whole stream of particles. Among them is this particle called the muon, which is like the more massive cousin of the electron. Muons aren't like electrons in the sense that they're not stable. They only live for a fraction of a second. These muons that are created in the upper atmosphere then stream down to earth.

Jim Al-Khalili: Now, we can measure how fast they're moving close to the speed of light. Nevertheless, for the distance they have to cover at that speed, they would have to live a lot longer than their normal lifetime, the lifetime they would have at rest. But of course, they do manage to get down to the ground and we can capture muons in our detectors so we know they've arrived from the upper atmosphere. How do they do this? Well, because they're going close to the speed of light, their clocks, their internal clocks that tick by their lifetime slow down. So they are living their life in slow motion, which gives them enough time to reach the earth.

Jim Al-Khalili: Now, the counter explanation to this is how does the muon see things? Surely, if all motion is relative, the muon sees our clocks running slower. Well, of course, from the muon's perspective, something else happens when you go close to the speed of light, namely distances shrink. So along with what's called time dilation, the slowing down of time, there's length contraction. And the length contraction is the distance the muon has to travel from the upper atmosphere to sea level. The muon is moving so close to the speed of light that it sees this distance shrunk. And it says, "Well, I'm only living for two microseconds. I'm counting how long I'm living for. The reason I can get down to the ground is because I don't have to travel that far." So everything fits and works logically. But this idea that time slows down is real. This notion of time dilation is part of Einstein's special theory of relativity.

Jim Al-Khalili: There's another way that time slows down. And that is part of his general theory of relativity, which he developed a decade later. And that's where we have to bring gravity into the story. You see, in special relativity, it's all about things moving at constant velocities. No acceleration, no speeding up, slowing down, going around in circles. Things have to move in a straight line at a constant speed. And then you can use the rules of special relativity to talk about time intervals and lengths and how fast clocks tick and so on. Once Einstein wanted to generalize that, he realized he had to bring in acceleration.

Jim Al-Khalili: What he referred to as the happiest thought of his life was something we now teach as the principle of equivalence, which is to say that acceleration is equivalent to gravity. Now, we're sort of aware of this in everyday language because when a car or an aircraft is accelerating, we talk about the G-force. Now, that G is G for gravity. When you're pushed back in your seat as your car accelerates, that's exactly the same force as you feel when you're laying down with your head resting on the pillow, if you're accelerating at 1G.

Jim Al-Khalili: This principle of equivalence led to the idea that in order to explain the structure and nature of four-dimensional space-time, one has to come up with a different picture of gravity, since gravity is linked to acceleration. And that led to Einstein's general theory of relativity, which was a theory of gravity that replaced, yet again he's replacing something that Newton has given us. In general relativity, Einstein gives us a new picture of gravity. In that intervening 10-year period between special theory in 1905 and general relativity in 1915, Einstein and other physicists realize that time is the fourth dimension, and it's actually you have to talk about it in terms of four-dimensional space-time. His general theory says, "Yes, time is the fourth dimension, part of space-time, but gravity curves space-time. Gravity changes the shape of space-time."

Jim Al-Khalili: Now, we can't perceive space-time as a four-dimensional whole. We can only have instruments that measure distances in space or clocks that measure intervals in time, so we have to sort of separate the two. So how does that look to us? Well, time gets slowed down by gravity. And again, this is not an optical illusion. This is in a sense more fascinating, more fun than the time-dilation of special relativity, because special relativity, you only see that effect when you're going close to the speed of light.

Jim Al-Khalili: When I explain this, for example, when I give school's talks, and the kids will ask, "How do you know you're just making this up?" So, well, everyone of you that uses a smartphone, you use GPS, you use Google Maps, you are making use of the fact that time is running at different rates, depending on how strong gravity is. You see, the closer you are to the surface of the Earth, the stronger the gravitational field and the slower time runs. The further out you're going to space, the weaker gravity is, and the faster time can run. It's only just tiny fractions of a second, but it matters.

Jim Al-Khalili: GPS, the satellites that are sending radio signals down to our phones, they are in orbit around the Earth. They're feeling slightly weaker gravity than the clocks on Earth. And so time is running faster on board those satellites than it is on Earth. So we have to take this into account, in a sense, slow down the clocks on board the satellites so they synchronize with clocks on Earth. Because if we don't do that, satellites would not be able to locate our position accurately. So it's a real effect. It's true, you know, that my head is aging faster than my feet. But I'm tiny, tiny fractions of a second, but it's there. And of course it gets much more dramatic if you go and find a much stronger gravitational field. It sounds ridiculous, but actually the physics is all correct.

Jim Al-Khalili: We're used to describing events that are happening at a point in space and a moment in time. So in physics, when you want to define an event, say the snapping of my fingers, it happened at this point in space and at that moment in time, we need four numbers to describe it. We know that. Even Isaac Newton would have understood that. In special relativity, once we unify time and space, time is not just another disconnected number, a place in space and a moment in time. It's a point in four-dimensional space time. Time is this fourth dimension.

Jim Al-Khalili: Now, saying time is the fourth dimension is easy enough, but what does it actually mean? We can't visualize four dimensions. We can only visualize three dimensions. Our brains are three-dimensional. There's a trick we can do here. So I have a three-dimensional construct. What if time replaced the third spatial dimension? Now I have a three-dimensional construct, two of space and one of time, that I can visualize. And this leads to an idea very useful in physics called the block universe.

Jim Al-Khalili: In a sense, it's a bit like sheets of paper in a book. So each sheet or slice is all of space at one moment in time. At the one end is time at the earliest moment. The direction is the time axis. And then you have these slices of space for each moment in time. Now, we experience time as going by continuously, but you can imagine there's a slice for every moment in time. The idea of a block universe is very useful because we can now draw a picture of it and think about space time having thrown away one of the dimensions of space.

Jim Al-Khalili: What does it then mean for us to be moving through space time? While here, physicists use the idea of a world line. So this is a line that moves through this block universe. It can start at a particular point, which coincides to a point in space, at a moment in time, and ends with another point in space at a later moment in time. Every one of us has a world line. My world line began when I was born. I mean, let's make an approximate point about that being the moment of my birth, being the beginning of my world line. And it will end when I die. That world line isn't a straight line. It's wavy because although I'm moving through time, I'm also moving through space.

Jim Al-Khalili: Now, if I were to spend my whole life without moving, then I would have a straight line, world line. But actually, it waves as I move through space while traveling through time. And physicists use world lines very usefully when it comes to describing things like particles colliding with each other and how they move and the speed that they're moving at.

Jim Al-Khalili: Probably the most ambitious project in physics over the past, certainly half a century or more, has been to reconcile quantum mechanics, or more correctly, one should say quantum field theory, which is the more modern reincarnation of the quantum mechanics of 100 years ago, quantum field theory with Einstein's general theory of relativity, namely, the theory of the very small that has time as a coordinate and the theory that describes the structure of space-time itself.

Jim Al-Khalili: One early attempt at doing this led to what we call the Wheeler-Dewitt equation. Wheeler, John Wheeler and DeWitt, were two cosmologists who try to come up with a way of unifying quantum mechanics and relativity theory and devise this equation, which has a very remarkable and strange feature, namely that it has no time in it. It just is. And this is an equation that one can argue describes the state of the entire universe, the quantum state of the universe, and there's no time in it. So this would then add support to the notion that at a fundamental level, time is an illusion, time doesn't really exist at all.

Jim Al-Khalili: Now, of course, the Wheeler-Dewitt equation describes the universe as a whole, and you can only really appreciate the universe as a whole if you were able to extract yourself from outside it. We only ever view the universe from within. But the Wheeler-Dewitt equation has led to the idea that since we perceive time, it seems very real and other areas of physics, maybe time is a so-called emergent property of reality.

Jim Al-Khalili: Now, emergence is a rather complex notion. There's what you might call weak emergence, which is the idea that, for example, an example of weak emergence is the wetness of water. You can't appreciate the wetness of water just by examining one H2O molecule. You need trillions and trillions of water molecules to come together, and the notion of wetness appears. Or temperature is another example. There is no such thing as temperature when you're just looking at one or two molecules bouncing around and bumping into each other. But zoom out and look at a box of gas molecules bumping into each other, and suddenly you get this notion of temperature that you can ascribe to it.

Jim Al-Khalili: We could, if we studied the microscopic structure of a system enough, determine, deduce that once you scale it up, you will see properties like temperature or the wetness of water emerging. Then there's a notion of strong emergence, which is, so consciousness is a good example of that, that you could never hit upon the idea of consciousness just by looking at the interaction of individual neurons in the brain. So, emergence is something that appears, grows out of something more fundamental.

Jim Al-Khalili: And there's been this argument that time is also an emergent property, it grows out of something deeper, something more fundamental. And the suggestion is that that deeper thing is embedded within the quantum realm. So, in quantum, even though in quantum mechanics and the famous equation of quantum mechanics, Schrodinger's equation, we're Schrodinger developed this equation 100 years ago, and it describes how the quantum state of a, say, an atom evolves in time. That's an equation that does have time in it. We teach physicists at university that this Schrodinger equation that depends on time can be tweaked and modified, and you can arrive at a new version of it, which is called the time independent Schrodinger equation, where time doesn't exist at all.

Jim Al-Khalili: The wheeler-do-it equation for describing the whole universe, which brings gravity into quantum mechanics, behaves in a similar way. You've extracted time from it entirely. So, how is it then that time emerges from a timeless universe? And that is still a subject that's being studied and debated and argued about today.

Jim Al-Khalili: In order to then try and have some concrete idea of what time is, philosophers have come up with different ways of describing, you know, whether there was a special present moment that devised the past from the future. Einstein tells us that time is the fourth dimension, and others will argue that time doesn't exist at all. It's all just an illusion. It's a construct. It's something that we've invented, or it's the thing that clocks measure.

Jim Al-Khalili: So, for example, if we divide time up into three parts, the past, the present, and the future, one might argue that the past has gone. We no longer have access to it. All we have access to is records of the past, but these are records that we don't access in the present moment. The actual events of the past are gone. They don't exist. The future has yet to be, even in a deterministic universe where everything is preordained, nevertheless, the future hasn't happened yet, so it doesn't exist. All that leaves is the present moment. But the present moment isn't extended. It's the dividing line between the past and the future. It's the shadow between the light and the dark. It doesn't have an existence in and of itself. It's just the gap between them.

Jim Al-Khalili: So, if the past doesn't exist, the future doesn't exist, and the present doesn't have any extent that it doesn't exist either, we've done away with all of time, then time is an illusion. Now, that sounds a bit like sort of a philosophical trickery, and in a sense it is, because there's the opposite view which is the one that's given to us by Einstein, which is to say that if time is the fourth dimension, then just like the dimensions of space, all points in space exist and are equally real. If time is also a dimension, then all points in time exist and are equally real.

Jim Al-Khalili: Therefore, rather than the past, present, and future all being an illusion, they are all equally real. They all exist. Time is, in a sense, frozen. We experience a present moment. We are sort of, you can think of it in terms of our consciousness drifting along that time axis, and we're stuck on that rail, we're moving along. But the time itself, for all eternity, just is. All times coexist. So, this is an idea called eternalism, and I think probably if push comes to shove, most physicists would subscribe to it, because we know that Einstein's description of time is the best one we have.

Jim Al-Khalili: The idea of eternalism, that all times are equally real in the block universe, can seem quite bleak. Now, we already had a notion that reality is rather bleak if we follow what Newton taught us, namely that we might well live in what's called a deterministic universe, where the future is predetermined, preordained, even though it hasn't happened yet. That's what's called the clockwork universe, where everything is ticking by mechanistically and everything. Causes lead to effects, and if we were powerful enough, we could work out what those effects would be. We could be able, we could predict the future.

Jim Al-Khalili: In reality, we can't predict the future for a number of reasons. But in Newton's clockwork universe, even though that future is preordained, it's destined to be, it still doesn't exist. And so, we can still imagine that we have free will, free agency. You know, maybe you look back after something has happened, you say, well, it was ever thus that was ever going to be the way it has, because the universe has evolved deterministically.

Jim Al-Khalili: But when it comes to the block universe and eternalism, that future, it's not that that future has yet to unfold, it's already there waiting for us. Somehow that is a more stark description of determinism and a stronger assault on free will. What's the point of thinking that I'm making free choices if that future is already out there? Now, of course, what saves us is that we are embedded within the universe, within the block universe. If you could pull yourself outside of space-time, the so-called God-Zai view, you could see all times mapped out. You could see exactly past, present, and future all equally real. But we don't have that privilege, vantage point.

Jim Al-Khalili: Embedded within the universe and even in a fully deterministic universe, the fact is we can never predict what is going to happen in advance. We are unable to predict this. So you may refer to that as just the illusion of free will, but for me, I think that's good enough. So I subscribe to a philosophical view called compatibilism, which says that, yes, I have free will even though I live in a deterministic universe. And the reason I say that is because even if the future is preordained and set out in the block universe in this eternalist picture, I'm never able to predict it in advance. So I'm thinking of making that I'm making free choices, and that's good enough.

Jim Al-Khalili: Of course, eternalism, the idea that all times past, present, and future all coexist, according to relativity, isn't the only option available. Philosophers have also argued that maybe an alternative idea is what's called presentism, which is that only the present moment exists, or past presentism, the past and the present exist, or the growing universe model where the past and present exist but slowly gobble up the future. There's something called the spotlight theory, the idea that it's a light shining on one moment that's changing along the time axis. But the problem with all these alternative theories to eternalism, presentism, past presentism, and so on, is that they come into conflict with all our current attempts at coming up with a theory of quantum gravity,

Jim Al-Khalili: whether one subscribes to things like string theory or loop quantum gravity, all these mathematical ideas that unify quantum mechanics of relativity all seem to require the eternalist picture. It's the one that makes most sense when we're trying to combine quantum mechanics and relativity to solve this problem, the first problem of physical time. And of course, most importantly here is that the eternalist picture says there is nothing special about the present moment. There's nothing special about now. We experience now subjectively and think it's special, dividing the past from the future, but in physics there's nothing special about this moment. It's just a point on the time axis in four-dimensional spacetime.

Jim Al-Khalili: Even in Einstein's special theory of relativity, we can appreciate the concept of a now, an absolute universal present moment simply doesn't make sense. One can imagine two events. I can experience two events that to me seem to have happened at the same time. They're simultaneous events. Now, if I know they took place at an equal distance from me, then even though it's taken some fraction of a second for the light from those two events, the signal to reach my eyes, I can work back with and say, "Well, if I saw them happening at the same time, going back, they must have actually like flashes of light. Those flashes of light will have happened simultaneously because I'm halfway between them and I saw them happen at the same time." But for another observer moving past me at close to light speed, they will not look as though they've happened at the same time.

Jim Al-Khalili: This is something that generations of physics students have to learn, something called the relativity of simultaneity, which very clearly shows that what one observer regards as now, as two events happening at some moment in time that they say is happened now, another observer will disagree. So, no one event happened before the other. In fact, we can even imagine a scenario where I see one event happening, let's call it event A, happening just before event B. For another observer moving very fast relative to me, they may see event B happening before event A. So, where is now, if our past and future are mixed up?

Jim Al-Khalili: Now, of course, what we're forbidden from doing is something called the violation of causality. So, if event A was the cause of event B, then no one can see B happening before A. You know, if event A is me shooting a gun and event B is someone being shot and falling down, you're not going to see them being shot before I fired the gun, because you can imagine them then stopping me from firing the gun, even though they've already been shot. I know that's a rather violent example. I don't tend to use violent examples in physics, but there we go. I can't illustrate it, but the fact is if one event can affect the other, then there is an order that you can't mess with. Cause has to come before effect.

Jim Al-Khalili: But if those two events are far enough apart and close enough in time such that there's not enough time for a light signal to transfer between them, then we say they are not causally connected. And in physics, we talk about them as being space-like separated. So in that case, events A and B can have their order fuzzy. Someone can see A before B, someone else can see B before A. And once you realize this, you realize you cannot pinpoint a universal present moment if we can switch past and future around.

Jim Al-Khalili: So that present moment becomes rather fuzzy according to relativity theory. And while in relativity theory, we clearly see there's a fuzziness about what we would refer to as a universal now. No such thing exists in relativity. Even in manifest time, our psychological or experienced time, there's a fuzziness about what it means to say now. Apart from the fact that it's ever-changing, you pinpoint a moment in time as now, but it's already in the past by the time you've said it.

Jim Al-Khalili: But the notion that now is a moment is also really not something that we see in psychological time. It's an extended present. It has a thickness to it. To begin with, when an event happens, of course in relativity, I said an event, events A and B happen, and there's a certain finite time for the light to reach my eyes from the events. There's a further time for that light to enter my eyes, travel to my brain, be processed, and for me to be conscious of that event has taken place.

Jim Al-Khalili: So there's what's called perceptual latency, a delay between an event happening and us being conscious of it. And that can be anything from up to a third or more of a second later. So when is it that we should regard something as happening? When it's happened or when we're conscious of it? There's already that fuzziness there. But let's say that's the moment that something happens. What about when does the now start for us? Well again, it's rather fuzzy.

Jim Al-Khalili: An example is how we appreciate a piece of music. We don't just hear one note at a time that replaces the previous note because that's gone and it's in our past. No, we experience music as a continuum. And the way we do this is through what's called episodic memories. We are storing memories of events in our brain that are then stitched together in a continuum so that it's not just the present moment note that we are conscious of. We're conscious of some finite time in the past. We've together to give us the music that we appreciate.

Jim Al-Khalili: Added to that the fact that we anticipate where the music is going in the future, even though we haven't heard it yet. And so what we regard as the present now is really an extended period of time that relies on past events that are still stored in our memory that we have access to. Even though we are only ever accessing any moment in the past in the present moment. It's still there and it gives us this sense that we're experiencing a flow of time or a finite duration of a present moment.

Jim Al-Khalili: One if there's nothing special about now, about the present moment, we still have to admit that it divides up the future from the past. So what is it that's different between the past and the future? Certainly, it's almost tautological to say the past comes before the future. That's what the word before means. But there's this asymmetry. They are different, certainly in the way we perceive them. And the flow, even though we might decide that the flow of time is an illusion, there's certainly a directionality to time pointing from the past to the future.

Jim Al-Khalili: Many physicists will argue this is due to the error of time, that the difference between the past and the future is due to what's called the error of time. But you don't need an arrow to talk about what's different between them. Any more than you need an arrow to point from the shallow end of a swimming pool to the deep end to tell you there's a difference between the shallow end of the deep end. They are just different. Nevertheless, we experience time moving only in one direction. Time is irreversible. Now, is that irreversibility just something again that we can regard as an illusion in an eternalist picture? Or is there really an arrow of time?

Jim Al-Khalili: Well, the third area of physics that gives us a direction to time really stems from 19th century work of people like Ludwig Boltzmann and the laws of thermodynamics. One of the most famous ideas in physics is the second law of thermodynamics. It's always amusing that it doesn't even make it to the top of the list of laws of thermodynamics. It's only number two. And yet, it is regarded as almost sacred in how true it must be.

Jim Al-Khalili: Even Einstein said if you can't reconcile time as a coordinate in quantum mechanics and time as a dimension in relativity theory and time as a direction or arrow in thermodynamics, then it's the arrow of thermodynamics that's the one that's going to survive. The other two may have to go if we want to come up with some theory that unifies all these concepts. So where does this arrow of time come from? Is it real or not? Now, the reason why physicists hesitate with this is that nowhere else other than in thermodynamics is there this notion that time has a directionality?

Jim Al-Khalili: All our fundamental equations of physics are time symmetric. We say they're time reversal and variant. If you change the sign of the symbol time, the coordinate time in the equations, if you make t minus t, and more often than not, you have to make a few other tweaks as well, which we don't need to go into, then you realize that the system that this equation describes evolves perfectly well in the opposite direction. So time can move forwards or backwards, and these equations would describe physical reality very sensibly. Time is symmetric according to the laws of physics, and yet we see irreversibility all around us.

Jim Al-Khalili: I get older. All these walls roll down hills. Batteries run out of charge. Things decay, get older. The universe gets bigger. Wherever you look, you see things happening in one direction of time, but not in another. And this is the fourth problem of time, the second problem of physical time, namely, where does the directionality of time come from if all our fundamental equations of physics are symmetric in time?

Jim Al-Khalili: When we talk about the arrow of time, the direction of time, pointing from past to future, we have to explain the concept of entropy. Now, entropy is not a vague concept. It's just a very versatile concept because it can mean very many different things. The simplest way of describing entropy is to say that it's what follows from thermodynamics and statistical mechanics, that things move from higher-ordered states to less-ordered states. Disorder increases.

Jim Al-Khalili: A box of gas, where all the molecules of the gas are all squashed together in one corner of the box, we say that has high order. It's neat and tidy, and it's a very special state. But as the molecules diffuse and fill up the box, evenly, entropy increases. So there are many ways of describing entropy, but basically we say entropy always either stays the same or increases in an isolated system. This is what the second law of thermodynamics tells us.

Jim Al-Khalili: But what happens when entropy has reached a maximum? In a box of gas, what happens when all those molecules of gas have spread out evenly? Now, if you were to zoom in and look at the detailed motion of the molecules, you'll see them all jiggling about. But if you were to film them jiggling about and then run the film backwards, you wouldn't be able to tell the difference. Because at what's called thermal equilibrium, things look exactly the same forwards and backwards in time.

Jim Al-Khalili: I've had this argument with other physicists who will say the hour of time disappears at thermal equilibrium. Well, sure, it disappears in the sense that we cannot perceive a direction to time. We can't tell the difference between a movie running forwards and backwards, but that doesn't mean that time has ceased to go on. Of course, if you could extract yourself from outside of this box, then you will see entropy continue to exist outside.

Jim Al-Khalili: But what if the whole universe reaches thermal equilibrium? In some distant future, the heat death of the universe, where everything is dispersed, all matter has fallen to black holes and black holes have evaporated out and everything is just thermal radiation or sort of moving apart as the universe expands. At thermal equilibrium, does time cease to exist? Well, I would argue no. Just because you can't perceive a direction to time doesn't mean that it's not going by. Indeed, if the universe continues to expand forever, as our current theories would suggest, then time is pointing in the direction of expansion of space, because mological arrow, so-called, of time.

Jim Al-Khalili: And in any case, we would never be able to check whether there's time going on or not, because at thermal equilibrium, we can't exist. We are special states, that that's what defines life. We are low entropy constructs that have some complexity. At thermal equilibrium, we have to be part of this thermal spread, even spread of everything in the universe.

Jim Al-Khalili: Now, of course, what's interesting is that even at thermal equilibrium, maybe entropy can fluctuate, maybe just randomly, in the same way that you can shuffle a pack of cards. And when it's fully shuffled, you really can't tell whether it's any more shuffled or less shuffled, but maybe now and again, just through chance, very unlikely, but just through chance, you may shuffle it and retrieve some order. You'll get a running flush as a poker player would say. You know, some you get cards that are ordered just through sheer chance. What happens in a universe at thermal equilibrium, where just through sheer chance, you suddenly get temporary isolated bits of the universe that have lower entropy, that have moved away from equilibrium? This is something called Boltzmann-Brains, the idea that there is a non-zero, you know, highly unlikely, a non-zero chance that's out of thermal equilibrium at maximum entropy, some lower entropy state can appear temporarily and then disappear again.

Jim Al-Khalili: So things can still happen at thermal equilibrium, but certainly time, just because we can't perceive which direction is going, doesn't mean it isn't continuing to evolve. One area I've been interested in in recent years is what's called open systems or more specifically open quantum systems. So the idea that the dynamical equations of physics that we know and love really almost all apply to what are called isolated systems, systems that are isolated from their surroundings.

吉姆·阿尔-卡利利: 但我们知道，没有任何事物是完全与周围环境隔离的。有一个绝妙的例子：盒子里的气体分子相互碰撞，它们在多次碰撞后随时间演化的方式取决于许多因素。我的意思是，这起源于像蝴蝶效应这样的事物，一只蝴蝶在世界的一侧扇动翅膀，最终会在世界的另一侧引发一场风暴。所以实际的初始条件可以增长。嗯，在可见宇宙的另一侧是否存在单个电子，将影响这些分子的动力学，其引力效应，在大约第50次碰撞后改变这些分子的动力学，这是已经完成的计算。没有任何事物可以完全与周围环境隔离。

吉姆·阿尔-卡利利: 这意味着当涉及到调和时间之箭与时间对称方程时，时间对称方程必须让步。通常，物理学家会反过来思考，他们会说：“看，时间的方向性是如何从时间对称的物理定律中产生的呢？”嗯，我将换个角度来说。时间对称的物理定律是理想化的，它们只适用于真正的孤立系统。只有一个真正的孤立系统，那就是整个宇宙。宇宙中的一切最终都会以某种方式与周围环境相互作用，而这种与周围环境的相互作用会消除任何时间对称性的概念。它带来了时间的方向性，而在量子层面，这 действительно具有深远的影响，因为一个量子系统不仅受到周围环境的干扰，而且它还与周围环境纠缠在一起。

吉姆·阿尔-卡利利: 这是一个真正不可逆的过程。嗯，更确切地说，当它与周围环境充分纠缠时，就会导致所谓的“退相干”。量子性消散到环境中。你无法找回它。所以，退相干被认为是自然界中唯一真正不可逆的过程。但由于它在宇宙中无处不在，退相干一直在发生，这带来了时间的方向性，这是根本性的。

吉姆·阿尔-卡利利: 所以，当人们争论或询问，在存在时间对称方程的情况下，时间之箭从何而来时，时间之箭其实早已存在。我会争辩说，时间之箭是嵌入现实中的。它从根本上嵌入宇宙之中，是由于量子纠缠和量子退相干一直在增加。所以，就像熵增加一样，我们可以描述纠缠的增加。我们甚至可以定义我们近年来尝试做的事情，定义纠缠熵，而不是热力学熵中无序的增加，例如冰块在水杯中融化，洗牌，气体分子在盒子中消散，气体分子在盒子中扩散。

吉姆·阿尔-卡利利: 这都是热力学熵。还有我们可能认为是纠缠熵的东西。一个系统与其周围环境的纠缠不可避免地增加，这给我们提供了时间的方向。我们熟悉和喜爱的时间对称方程，只是当我们能够忽略我们感兴趣的系统周围环境时的理想化极限。

吉姆·阿尔-卡利利: 所以，如果我相信时间之箭的方向是嵌入现实之中的，那么没有时间本身，你就不能有时间的方向。这就像柴郡猫的微笑。在这种情况下，你需要柴郡猫。所以，如果时间的方向是真实的，那么我会认为时间本身是真实的。它是否从更基本的事物中涌现，那是另一回事，但我们并非生活在一个时间是幻觉的无时间宇宙中。时间确实存在，而且它确实有一个方向。

时间的起点与终点

吉姆·阿尔-卡利利: 现在，如果时间是真实的，如果时间有一个从过去到未来的方向，一个箭头，那么我想下一个问题将是，这个箭头有起点和终点吗？时间在某个点开始了吗，会在某个点结束吗？那么，让我们来探讨时间有起点这个想法。根据广义相对论，时间始于大爆炸。那是最早的时刻。人们经常会说，你怎么知道在那之前没有时间呢？

吉姆·阿尔-卡利利: 我的意思是，基本的广义相对论的答案是，你说的“之前”是什么意思？你知道，必须在大爆炸之前有一个时间来嵌入“之前”这个词。这有点像我让你走到南极，当你到达南极后，继续往南走。这是毫无意义的。一旦你到达南极，你走的每一步都会让你回到北方。那是你能到达的最南端。在地球表面，没有比南极更南的点。

吉姆·阿尔-卡利利: 在大爆炸中，时间与空间、物质和能量一同被创造出来，没有比那一刻更早的时间点。事实上，像斯蒂芬·霍金这样的人甚至提出了一些想法，认为时空具有平滑性，当你越来越接近大爆炸时，时间的意义就会消失。它变得像另一个空间维度。所以，从这个意义上说，时间确实有一个起点。它就是大爆炸的时刻。

吉姆·阿尔-卡利利: 当然，如果人们没有提出其他建议，那就太无聊了，而他们当然提出了投机性的想法。例如，大爆炸只是我们宇宙诞生时刻的概念。如果我们的宇宙只是一个更大的多重宇宙中的一个气泡呢？这个多重宇宙正在不断经历所谓的“永恒暴胀”，不断膨胀，并且时不时地在这个所谓的“暴胀场”中出现气泡。我们的宇宙是一个气泡，我们的大爆炸是我们宇宙的第一个时刻，但时间在其之前就已经存在。存在某种多重宇宙的时间，它与大爆炸的时间以及我们在宇宙中体验到的时间是不同的。

吉姆·阿尔-卡利利: 这是一个时间确实可以延伸到无限的可能性。还有其他观点认为，也许大爆炸是我们宇宙的开端，但存在一个镜像宇宙，它朝着相反的方向发展。所以，对我们来说，时间向前流动。有一个我们宇宙的镜像朝着相反的方向移动。如果那个宇宙中有智慧生命，他们会认为他们的时间向前移动，而我们宇宙的时间向后移动，尽管我们永远无法接触彼此的宇宙。

吉姆·阿尔-卡利利: 然后还有一些概念，比如时间是循环的。大爆炸将随时间向前演化，宇宙将再次大挤压，形成下一代，另一个宇宙。这再次表明，时间，宇宙，一直以来都在不断地被创造和毁灭。所以再次，大爆炸可能并不是时间的第一个时刻。当然，还应该说，一旦你将量子力学引入画面，那么那个独特的时刻，时间的开端，即所谓的时空奇点，就会变得模糊。量子力学带来了模糊性。它带来了不确定性，暗示着就像相对论在“现在”这个概念上带来模糊性一样，因为同时性的相对性。

吉姆·阿尔-卡利利: 量子力学为空间和时间创造的最早时刻带来了模糊性。这些都是理论猜测。甚至有人可能争辩说它们是否是真正的科学理论，因为我们还不知道如何检验它们。我们能做哪些观察来证明这些理论是正确的？也许未来的某个时候能够告诉我们。但目前，大多数物理学家、宇宙学家会认为大爆炸是时间的开始，就此打住。

吉姆·阿尔-卡利利: 当然，另一个问题是时间有终点吗？那个箭头会持续到无限吗，还是时间会终结？同样，有各种选择。所以一个选择确实是有一天宇宙会达到最大膨胀点，然后自身坍缩成一次大挤压，那将是时间的终结。也许那次大挤压随后会成为下一代宇宙的大爆炸，时间将变成循环的。

吉姆·阿尔-卡利利: 但我们现在认为宇宙将永远膨胀。最终以所谓的“热寂”告终。这将是未来数十亿、数万亿年之后的事情。但这将暗示时间将永远持续下去。现在，原因在于，仅仅在四分之一个世纪前，天文学家发现，我们原以为会发生的情况（即由于宇宙中所有物质和能量的引力，空间膨胀正在减速），但没有人预料到它会以越来越快的速度膨胀。

吉姆·阿尔-卡利利: 这就是1998年天文学家通过观察非常遥远的星系并回溯，发现宇宙现在膨胀的速度比过去更快时所发现的。这表明它永远不会自行坍缩。它将永远膨胀。我们被留下了这个，嗯，我可能会争辩说，黯淡的场景。尽管我不知道为什么，为什么宇宙的热寂是一个更黯淡的场景？为什么它比在我看来相当灾难性的大挤压更黯淡？但宇宙的热寂将是我们宇宙的最终命运。

吉姆·阿尔-卡利利: 当所有恒星停止发光，它们耗尽所有核燃料并死亡。一些会坍缩成黑洞。物质会逐渐变冷。黑洞会根据斯蒂芬·霍金的理论蒸发，即所谓的霍金辐射。只是放出热辐射，黑洞随后会收缩并消失。最终，宇宙只会变得越来越空旷，充满了越来越冷的热辐射。

吉姆·阿尔-卡利利: 当然，这将是一个处于热平衡的宇宙，这又回到了另一个观点，即时间会在热平衡时停止存在吗？嗯，不会。如果一个玻尔兹曼大脑突然从热平衡的波动中出现，而那个玻尔兹曼大脑能够思考，并且纯粹是偶然地，它演化出某种观察周围宇宙的方式，它会看到宇宙的其余部分处于热平衡状态，并且无法分辨出时间的流向。但时间仍然会存在。

吉姆·阿尔-卡利利: 如果宇宙将永远膨胀，它将继续膨胀，并且时间会沿着宇宙膨胀的方向，沿着宇宙中剩余辐射进一步冷却的方向，具有方向性。所以，从这个意义上说，时间将永远持续下去。这将是一个非常无聊的宇宙。什么都不会发生。但仅仅因为什么都没有发生，并不意味着时间没有持续，因为那将由空间持续膨胀来定义。

吉姆·阿尔-卡利利: 当然，还有第三种情况，即空间的膨胀，我们现在理解这是由所谓的暗能量引起的，其性质我们仍在努力确定。暗能量正在使宇宙膨胀得越来越快。它正在赢得与引力的对抗。所以，宇宙中所有物质的引力都在试图减缓空间的拉伸。暗能量正在使空间拉伸和膨胀得越来越快。我们仍然不了解暗能量的细节，它可能是一种影响力会随时间增长的事物。

吉姆·阿尔-卡利利: 所以，膨胀会越来越快，并在越来越小的尺度上变得明显，最终导致所谓的“大撕裂”，届时不仅是星系团之间的空间，宇宙的空虚在拉伸，甚至星系内部、恒星内部、行星内部、原子内部的空间最终都会开始扩散、增加，并撕裂宇宙。所有这些我们都不必担心。我们没有生存威胁。我们应该更担心我们的太阳何时停止发光，或者更担心在不久的将来照顾好我们的星球，而不是担心数万亿年后宇宙会发生什么。

吉姆·阿尔-卡利利: 但所有这些选择都是开放的。我个人的看法，嗯，这不是物理学家应该争论的方式，只是我的一种感觉或观点，但我的观点是宇宙确实有一个开端，但不会有终结。它不会永远持续下去。当我思考这个问题时，我也会意识到这可能也不太合理。也许如果它没有终结，它也不应该有一个开端。也许时间在两个方向上都是永恒的。就是这样。这是我们耸耸肩，转向哲学家寻求帮助的地方。

时间旅行：现实与悖论

吉姆·阿尔-卡利利: 好的，如果你跟着我走到这里，我们已经经历了一些沉重的话题，谈论了时间之箭和统一物理定律等等，也许现在是时候放松一下，来点甜点了。当涉及到时间的本质时，我经常被问到的一个问题是：我们能穿越时间吗？毕竟，如果你根据爱因斯坦的理论思考时间，将其视为四维时空的一部分，那么我可以在空间中移动。我可以在空间的这一点或那一点存在。现在，我可以回到这一点，我可以在周围旅行。这是为什么？为什么我们不能对时间做同样的事情？为什么我们不能沿着时间轴上下移动？为什么我们被困在沿着时间轴不可避免地移动，而所有时间 supposedly 都存在呢？

吉姆·阿尔-卡利利: 当然，时间并不完全像另一个空间维度。它有所不同。它是特殊的。爱因斯坦的相对论可能不是关于我们是否能穿越时间的最终定论，但它们是我们目前拥有的最好理论，它们确实告诉我们一些关于时间旅行可能性的信息。当然，我们都看过许多涉及时间旅行的电影和电视剧。我不想公开批评那些糟糕的时间旅行电影，比如《热浴盆时光机》，但也有一些非常精彩的。

吉姆·阿尔-卡利利: 但真实的物理学告诉我们，当谈论时间旅行时，我们必须区分未来时间旅行和过去时间旅行的可能性。事实证明，其中一种比另一种更容易。未来时间旅行很容易，我指的不仅仅是我坐着不动明天最终会到来。我正在向明天进行时间旅行。现在，我所说的未来时间旅行是指在其他人之前到达那里。相对论说这是可能的，通过减慢时间。

吉姆·阿尔-卡利利: 所以，如果我乘坐火箭以接近光速的速度在银河系中穿梭，比如说一周，当我回到地球时，由于高速旅行，而且还要说因为我正在加速和改变方向，因此加速度等同于引力，这意味着我的时间运行得更慢。本质上，当我回到地球时，我所经历的时间会比地球上没有我的时间流逝得少。所以我可能只经历了几周的时间旅行，但我可能在未来的几年后回到地球。

吉姆·阿尔-卡利利: 从某种意义上说，这不是真正的时间旅行，因为未来并非已经在那里等待着我。我只是“快进”到了未来。我沿着一条不同的时间线，一条不同的世界线穿梭于时空，以比其他人更少的时间流逝抵达未来。但就我而言，我可以在遥远的未来任意时刻抵达，这取决于我接近光速的程度或者我感受到的引力有多强。

吉姆·阿尔-卡利利: 所以在电影《星际穿越》中，马修·麦康纳饰演的角色是一位宇航员。他们降落在一个围绕超大质量黑洞运行的水行星上。那里的引力极大地减慢了时间。电影中的宇航员知道，他们在那个行星上每待一小时，地球上就过去了七年。果然，马修·麦康纳的角色穿越到了未来，虽然他没有老太多，他也没有离开太久，但他的小女儿现在已经是一位老妇人了。

吉姆·阿尔-卡利利: 你是否认为那算是真正的时间旅行，这取决于你。当然，更有趣的是我们能否回到过去进行时间旅行。而广义相对论同样不排除这种可能性。理论上，我们可以设想这种被称为“闭合类时曲线”的概念，它相当于乘坐过山车进行一个环形回路。当你沿着它旅行时，你的时间正常展开。但你会回到过去，你可以回到你离开的空间点，但时间更早。你可以通过这种方式回到过去。