#1300 Jim Al-Khalili - On Time: The Physics That Makes the Universe Tick

00:00 Ricardo Lopes: Hello, everyone. Welcome to a new episode of the Dissenter. I'm your host, as always, Ricardo Lops, and today I'm joined by Doctor Jim Valcalilli. He's a theoretical physicist who holds the position of Distinguished Professor Emeritus at the University. Of Surrey in the UK and today we're going to talk about his new book on time, the physics that makes the universe tick. So Dr. Alkhalili, welcome to the show. It's an honor to everyone.

00:28 Jim Al-Khalili: Hello, thank you very much for having me.

00:31 Ricardo Lopes: OK, great. So I imagine that the understanding of time that a physicist has is very different from how just a regular person understands time and also different from how we understand time or how we experience time psychologically, right? So according to physics, what is time?

00:57 Jim Al-Khalili: Uh, OK. Well, of course, physicists and philosophers have been worrying about the meaning of time for thousands of years and, and as, as have many people. The, the thing about time is it's, it's one of those concepts which everybody has an opinion on. Everyone knows, will, will say they know something about what it means. But you're right, psychological time. The way we experience it is not the same as the way time appears in, in, in physics, but even in physics, we can't agree on, on the meaning of time because different areas of physics have different definitions of what time is. So it's it's still a big mess.

01:39 Ricardo Lopes: Yes, and we're going to get into that later because in your book you also explore the understanding of time that we get from or we can get from general relativity, from quantum physics, from thermodynamics. But, uh, particularly when physicists talk about time as being one dimension of the space-time continuum, I mean, for a regular person, how should we understand that? Because of course, three dimensions of space, yeah, that's easy for us to understand, but time as another dimension. I mean, what the

02:18 Jim Al-Khalili: Yeah, you, you're right. I mean, we think of dimensions as directions, right? So, we, we know what we mean when we say we live in three dimensional space. There are 3 dimensions that are directions that are at right angles to each other, you know, I can move forwards, backwards, left, right, up, down. Those are the 3 dimensions of space. Um. In, in physics, even before Einstein told us about 4 dimensional space-time, we know that if we want to define what in physics we call an event, something that happens, we need 4 numbers to locate it in space and time. We have 3 numbers to locate it in space because space is 3 dimensional, but we also have to say when this event happened. So it's a moment in time as well. Uh, SO we know that we need 4 numbers, but that doesn't mean time is a dimension. It just means it's another label to define when things happen. Einstein, of course, changes this, and he's, and he says, well, in fact, it wasn't him who first came up with time as the 4th dimension, it was a, uh, another physicist, Herman Minkowski. But the idea in, in Einstein's relativity theory is that time is not separate from space. When you define. Things happening when you measure durations of time or even distances in space, we have different views depending on our perspective. Uh, AND Einstein said, you know, you know, Newton and Galileo and all these other great physicists before me, they were wrong if they think that time is separate from space. It's an absolute ticking by the the seconds, minutes, hours independently of space. He said no, time and space are mixed up together. And if time and space is mixed up together, that in a sense means time is another direction. But it's not a direction in space that's already taken by the three dimensions. It's a direction outside of our space in a sense that in a way we can never imagine. So, I understand why people find it so difficult to concept. What do you mean time is a dimension? Well, it's, it's mathematically, it makes sense if you study the mathematics and study the physics. But you can't ever imagine what time as 1/4 dimension means. It just, it's, it's what we need to, to, to, to, to use to, to explain reality accurately.

04:48 Ricardo Lopes: Talking about directions, is it really the case that according to physics, time moves in a particular direction, because of course we as humans and I imagine other animals as well, experience time as moving forward from the from the past into the present into the future. But is that the same understanding that we have of the movement of time in physics?

05:18 Jim Al-Khalili: In parts of physics, yes. In other parts, there seems to be an issue. I mean, this is again one of those concepts where people think, really, are you physicists really spending all your efforts to study something that is so obvious? Of course, the future comes after the past, of course, the past comes before the future. So this idea is that time has a directionality, we call it the arrow of time. And it points from the past to the future. What could be more obvious than that? How could it point from the future to the past? You know, time doesn't go backwards, it only goes forwards in our experience. The problem in physics is that, Apart from one area, you mentioned this at the start, we maybe we'll talk about it uh uh a little more uh in a minute, uh, this area of thermodynamics, where sure, it suggests that time really does have a direction exactly as we perceive it psychologically. But in other areas of physics, like Einstein's theory of relativity, like quantum mechanics, even like Newton's equations of motion that we learned at school, all of them seem to work. If you change the direction of time, they say it works in one direction, but it also works in another. We say they are symmetric in time. So, what is going on? If these fundamental important equations and laws of physics are symmetric in time, and yet we only see time going in one direction, how does this, what we call irreversibility of time, the arrow of time, come from. Fundamental laws and equations that are symmetric in time. So this has been a big puzzle and it's one that I tackle in my book and one that I argue I've, I, I think I know the answer to.

07:08 Ricardo Lopes: OK, we'll get more into that when we tackle thermodynamics then, but in my previous question, I mentioned the present, but does the present really exist? I mean, is it that at any moment in time we can really point at a present?

07:32 Jim Al-Khalili: Well, we can psychologically because, you know, we can only ever experience the present, uh, you know, the future has yet to be, the past has happened and it's gone. We are only, you know, there's this, uh, certainly in English when we talk about these, uh, there are phrases like live in the moment, uh, you know, enjoy the present moment. We can only ever live in, in, in the present moment. So for us psychologically, the present is the only point in time that is real. And yet, uh um in, in, in, in physics, particularly in relativity theory, you know, there is nothing special about the present moment. All times coexist. All times are equally real. Uh, AND in fact, many have argued that this, this way of thinking means all of time is just an illusion. If the future hasn't happened yet, it doesn't exist. If the past is gone, it no longer exists. And the present moment is just the dividing line between the past and the future. It's the edge of the shadow between light and dark. So, in and of itself, the present moment has no real existence. So if the future doesn't exist, the past doesn't exists, and the present doesn't exist, no time exists, all time is an illusion. And there have been many philosophers over thousands, thousands of years going back to the ancient Greeks who've argued that time is an illusion. Of course, the opposite view is Einstein's picture, what's called the eternalist picture of time. That if time is a dimension, if it's an axis, then all times are equally real, just as all points in space are equally real. You know, I am here, uh, uh, in, in, in the UK talking to you. My, the position I'm in is real, but I know your position is also real. Uh I can go to other places and come back again. In Einstein's relativity, all times are equally real, not just the present moment. So there's these two extremes, no time exists, time is an illusion, and all times exist, all times are real. Is the truth somewhere in between or is one of them correct? We're still arguing about it.

09:59 Ricardo Lopes: Uh, SO going back for a second to relativity, is time relative, and if so, what does that mean for time to be relative?

10:12 Jim Al-Khalili: What it means is that we can't all agree on when things happen or how long something takes. So, um, there is no, uh, moment in time that I will call now that everyone else will agree with because this idea that, you know, time is relative, because time is mixed up with space in this four dimensional space-time. There is no absolute time. There's no clock that ticks at the same rate, everyone, uh, for everyone in the universe. Um, IF I measure something that lasts for 5 seconds, you could be flowing, moving past me at close to the speed of light relative to me. Even even speed is relative because you could say you're not moving, I'm moving past you at close to the speed of light in the opposite direction. But what I would measure as a time interval between two events of 5 seconds, you may see it as 3 seconds or 10 seconds or 15 seconds, so we can't agree on how long things last. We see each other's clocks running at different rates. This is what we mean by time is relative. There is no. Single universal clock that measures time constantly everywhere, it depends on your perspective.

11:38 Ricardo Lopes: And it depends also on the speed you're moving, right.

11:44 Jim Al-Khalili: Relative to somebody else. So, so if you're, you're moving, you look at your clock, your clock seems to be ticking by normally, but someone standing somewhere else watching you go past, they will see your clocks moving more slowly. For you, motion is relative, you see them moving and you could see their clocks moving slowly, so you can't agree on how fast time is going by between you.

12:10 Ricardo Lopes: Yeah, that's really fascinating. Uh, BUT if time is part of this four dimensional space-time continuum, can it also be affected by gravity, and if so, in what ways?

12:25 Jim Al-Khalili: Yes, this is the other way that time uh can be squeezed and stretched. Einstein had two theories of relativity. His first theory, special relativity, this is the famous one that we, you know, that gives us E equals MC2. That's the theory that says time is the 4th dimension, that time slows down when you travel close to the speed of light and so on. But 10 years after his theory of special relativity, special relativity, Einstein published in 1905. 10 years later in 1915, he completed what we now call his general theory of relativity. So that is special theory, more general, made broader, and it includes gravity in, in the description of, of the universe. So in general relativity, Einstein says, You know, when Newton teaches you, you know, where you still learn at school that gravity is this force, this invisible, uh, elastic band that pulls masses together and attractive force. It's not really a force at all. Einstein says, no, gravity is the shape of four-dimensional space-time. Space-time can be curved by gravity. So, the theory, the mathematics says space-time is curved. But for us, what does it mean? How do we measure it? Because for us, space and time are different things, right? So we can do experiments to measure distances in space, we can do experiments using clocks to measure time. And what we find is that gravity causes time to slow down. And this is not just a theory, this is in fact very well known and we can even make use of it technologically. The stronger gravity is, the slower time will run. When you're sitting in a gravitational field, you don't feel any different. You don't feel like, oh, time is going so slowly. Oh, this is because I'm in a gravitational field. Everything is normal for you. But outside of that gravitational field, time will be running more quickly. So this idea of time slowing down is a real effect. Now, in science fiction, it's been used a lot. The the most wonderful example I talk about this in the book is in the film Interstellar. Uh, Matthew McConaughey and his crew, uh, uh, in, in they, they fly off to orbit around this, uh, black hole, and, and, and for them, because the gravity of the black hole is so strong, time is running slower, and Matthew McConaughey's character knows that his young daughter back on Earth is waiting for him, but he knows every hour he spends in orbit around the black hole, 7 years are passing on Earth because his time is so slow. So he doesn't age, but of course, when he goes back to Earth, his young daughter's now an old lady because time has gone faster. It's, it's science fiction, but that is exactly what would happen if we could go and, and, and get close to a black hole with its strong gravity. We see that this has a much smaller effect, but more important here on Earth because it uh is part of GPS systems. So whenever anybody uses Google Maps, They are also having to acknowledge that Einstein was right about gravity slowing time down because the satellites in orbit, the global positioning satellites that send radio messages to your phone to tell you where you are, they are further away from the Earth's center, so they feel slightly weaker gravity. And so their time can run a bit faster than the the time at sea level, which is closer to the Earth's center where gravity is stronger. And scientists and engineers have to account for this. They have to slow the clocks, the atomic clocks, very accurate clocks, uh, that measure time on the satellites, so that they are running at the same speed as clocks on Earth. If we didn't believe time slows down due to gravity. Google Maps, GPS, satnav in your car would not work. Or it'd it'd be very, very inaccurate and very quickly would become completely uh useless. So, time really does slow down in gravity.

16:48 Ricardo Lopes: So let's say that, and also because you mentioned the movie Interstellar and the black hole, let's imagine that someone, a poor soul out there, is falling into a black hole from the standpoint of someone watching that happening. From outside that gravitational, the gravitational influence of the black hole, would that seem like uh the person was falling for eternity or or

17:20 Jim Al-Khalili: not? Yeah, yes, the, the closer they get to the black hole's event horizon, the slower their time will be running as perceived by someone watching them from far away. So, uh, uh, to the point where they get to the event horizon. And they will freeze in the sense that it looks like time is not running at all. Of course, we say this because that that this is technically what someone would see, but in reality, um, Because what you're seeing is light coming from the falling person and light, the frequency of light is a, is, is depends on how fast time is running. So as time slows down, the frequency of the light is going down, which means the color of the light is changing. So, in fact, before you get to the event horizon of the black hole, the light that's coming from your body into my eyes for me to see you goes beyond the visible. So you would in any case, disappear from view. But in reality, what's happening to the person falling, well, they don't stop at the event horizon, they accelerate and they go straight through the event horizon, inevitably inside to to inside the black hole, uh, never to, to reappear again. Uh, SO it's a, it's a matter of Who is looking at what, uh, according to what clocks, the clocks that they have next to them, which is measuring their time, uh, it, it's, it's a very confusing, uh, picture, but actually, the physics is very well understood now, even though unlike Matthew McConaughey, we have not actually ourselves ever visited anywhere near a black hole yet.

19:06 Ricardo Lopes: But of course this is not exactly about time, but just out of curiosity, do we have a good understanding of, uh, I mean, physically and biologically, what a person who would be falling into a black hole would experience? I mean, is it that their body would just progressively disintegrate or what would happen

19:30 Jim Al-Khalili: exactly? Well, it, it sort of depends on what sort, what type of black hole we're, you're falling into. Um, A supermassive black hole like the one in the film Interstellar is so big that the, the, uh, gravitational pull changes slowly, gently. So as you fall in beyond the event horizon, you won't know that you've gone beyond the event horizon. You wouldn't feel any difference. Uh, YOU just feel this sense of, of, of falling of freefall. Um, YOU would know, of course, if you try to come out again. If you, if you're in a rocket and you say, oh, no, no, no, I've had enough, I want to go home now. Forget it if you've gone past the event horizon, you're on a one-way trip to the, to the center of the black hole. But, uh, another type of black hole is what's called a stellar black hole, which, uh, is formed from a collapsing star. Those are much smaller. Those black holes, you know, could be just a few kilometers, uh uh in, in diameter as seen from outside. Uh, AND there, gravity changes very quickly. The, the, the strength of gravity, what's called the gravitational potential, changes very quickly as you get closer and closer to, to, to the black hole. And so if you're falling, for example, With your feet first, your feet pointing towards the black hole and your head pointing away, your feet will feel a stronger gravitational pull than your head. So your feet will be stretching, pulling, and making you longer. Uh, SO this is such a, an, a terrible, uh, uh, experience to have. It's been given the technical name in physics, spaghettification. Spaghettification means you become like spaghetti, you get pulled and stretched. So I always say, you know, falling into a black hole. If you're going to fall into a black hole, find a big one, find and you know, a supermassive black hole at the center of a galaxy, so at least you have the opportunity as you're falling into the black hole to maybe, you know, take some selfies and, and, and enjoy the ride. Uh, YOU, you're, you're gonna most likely get crushed to zero size anyway when you hit the singularity, but you might as well enjoy the ride down. If you're falling into a stellar black hole. Yeah, you're going to be ripped apart long before you, you get inside it.

22:00 Ricardo Lopes: But, but I mean, would the person be able to send the selfies back home? Would that information be able to escape the black hole or, or not?

22:10 Jim Al-Khalili: Not, not once you've gone past the event horizon. In fact, even before the event horizon, that information will take a very long time to get out because it's climbing out of the gravitational hole, uh, and it's, it's wavelength has been stretched. But in principle, yes, if you manage to send the signal just before you pass the event horizon, the point of no return, then, you know, you can say your goodbyes and you can send your photos and people receive them. Don't send any selfies after you've gone beyond the event horizon. It's a, it's a waste of time.

22:42 Ricardo Lopes: Uh, OK, so, uh, and of course we as humans have ways of, and over time, over our history, we've been developing different ways of timekeeping and of measuring time, keeping track of time. Do those ways that we have even the most technologically advanced ways have anything to do with how time works according to physics?

23:10 Jim Al-Khalili: Well, we certainly do need accurate timekeepers to measure all sorts of phenomena and mechanisms in physics. Um, AFTER all, time gives us a measure of change. And in physics, most of our equations of physics are really ways of Finding out how something changes from one state to another, how uh a particle moves from one place to another, how a system evolves in time. Uh, AND so we need to measure that time. That time is what's called coordinate time, you know, if a system is doing something, by system, I mean anything, you know, a particle, a box of gas, you know, a, a planet. If it's in a particular state at some moment in time. Then our dynamical equations of physics will allow us to compute what it will be doing at some point in the future or what it was doing at some point in the past. In order to check these uh equations, these theories, we need to do experiments. And to do experiments, we need accurate timekeepers. And the more accurate the timekeeper is, the more precise our theory can be checked. So, so yes, in physics, we do need clocks, and we do need clocks as accurate as possible, especially if we're measuring very, very fast. Changes, you know, chemical reactions that happen very fast or, or, or, or changes that are happening at a tiny, you know, a short duration of time, you know, fractions and fractions of a second, uh, then we need, Accurate, you know, to my, my, my, my big clock on the wall in my kitchen is not going to be able to measure how fast a proton is moving inside an atomic nucleus, for example, so we need lots of different clocks of increasing accuracy to measure change in the universe.

25:13 Ricardo Lopes: And what is the shortest time scale that it would be possible for us to measure? So we have seconds, milliseconds, nanoseconds. Is something even shorter than that? Possible for us to measure.

25:31 Jim Al-Khalili: Yes, absolutely, um, uh, a lot of, uh, uh, modern, uh, timekeepers use things like, um, lasers, so laser pulses of short durations. Um, uh, THERE are all sorts of clever tricks that can be done to measure intervals of time far shorter even than a nanosecond. A nanosecond is 1 billionth of a second. At the moment, I think the shortest time measured is of the order of a few, I think it's a few what's called zeptoseconds. A zeptosecond is is a trillionth of a trillionth of a 2nd, 0.000, so 23 zeros, then a 1. That's such, such a tiny interval of time. It's not the shortest time that we know of in nature. There are things that happen much quicker than that, but we're not able to measure them. So things like certain particles have very short lifetimes. They only exist for a tiny fraction, much less even than a zeptosecond. Uh, WE only know they exist because we're not. Measuring them, we're measuring what they er change into, what they decay into. Uh, LIKE, you know, when we say we've discovered the Higgs boson, uh, the Large Hadron Collider, no physicists actually saw the Higgs boson particle. No, they saw the particles that it became after it existed for this tiny fraction of, of a second. So in nature, things can happen very, very quickly, certainly down at the subatomic scale. Uh, BUT our clocks are getting. More and more accurate and able to measure shorter and shorter intervals of time, and when you talk about trillionth of a trillionth of a second, you know, that's I, I, I give examples in the book how to compare what, what is a 1 trillion trillions a second compared with a second or uh is as you know, how does it compare with a second compared with the age of the entire universe and you realize these are just your brain explodes when you try and, and imagine how short a time we're talking about.

27:49 Ricardo Lopes: But theoretically, would there be a limit to the shortest possible time scale that exists, or could it be that time is theoretically infinitely divisible?

28:05 Jim Al-Khalili: We don't know is the honest answer. Um, THERE are current ideas and theories, in particular, uh, uh, theories of what's called quantum gravity, uh, which are theories that try to combine quantum mechanics, the theory, uh, of the very small, with Einstein's general theory of relativity. Uh, SO, a quantum version of gravity. Some Uh, ideas in quantum gravity suggest that yes, time ultimately, when you get small enough, it becomes discrete, it becomes lumpy. Uh, WE reach what's called the Planck scale, uh, and the Planck time is therefore then the smallest interval of time that is meaningful. Anything less than the Planck scale and time just doesn't have a meaning anymore. Um. A lot of physicists favor the idea that there should be a smallest interval of time, just as there should be a smallest, uh, length distance, the, the, the Planck length. Um, BUT then, you know, there are other theories that suggest that time may just continue, as you say, be continuous all the way and, and isn't, isn't discrete, it isn't quantized. Um, WE'VE yet to find the correct theory of gravity. Uh, OF quantum gravity, so we've yet to know the answer to whether time itself is, is, uh, discrete or not.

29:32 Ricardo Lopes: So earlier I asked you about the past, the present, the future, the directionality of time. Does time have a beginning and an end?

29:45 Jim Al-Khalili: Another wonderful question that doesn't have a definite answer. If you only apply Einstein's general theory of relativity, then you would say that the Big Bang was the origin of everything, the origin of the universe. The Big Bang didn't happen at a moment in time, just as it didn't happen at a point in space. It happened everywhere in space. And it marked the beginning of time. It's, it was, is when space-time itself, uh, uh, uh uh was born. So the, the, the usual, uh, um, analogy that people use that I always like to use is when people say, well, the Big Bang happened. You say it's the beginning of time, but what caused the Big Bang? There must have been a time before for there to be a cause. There cannot be a first cause, and, you know, philosophers have argued about this for, for, for, for millennia. But the analogy is, you know, if I say to you, walk to the South Pole and keep walking, and when you reach the South Pole, keep heading south. Of course, you can't because once you're at the South Pole, any step you take in any direction will take you back north again. Uh, AND so there is no point further south than the South Pole. Therefore, there is no moment in time before the Big Bang, before the moment that time actually existed. That there's no time before the Big Bangs. There's no point in saying what happened before because there was no time for the word before to exist in. But, but there's the big but here, which makes this whole subject so fascinating. Einstein's general theory of relativity, which says the Big Bang is the birth of time, may not be the last word on the subject. It may be that the Big Bang was simply the moment that our universe was, was created, our bubble within some, some multiverse. Maybe time is eternal, maybe time goes all the way back to infinity and at some moment within the multiverse, our universe popped into existence. So it did happen at a moment in time. There are other ideas which suggest the Big Bang. Uh, MIGHT have been the birth of our universe, but the universe cycles, it, it, it, it is born, it expands, it recollapses, it's born again. So this cyclic picture of time repeating, not repeating itself because the, the next generation universe will be very different presumably to ours, certainly in terms of what happens in it. Um. So we don't know whether time had a beginning. When it comes to whether time has, will have an end, again, we don't know for sure. We talk about time. If you fall into a black hole, then as you fall towards the center, what what's called the singularity of the black hole, that point marks the end of space and time. It's the edge of space and time, just like the Big Bang is the singularity for the beginning of the universe, which marks the beginning of time. So the interior of a black hole could be the end of time, but that doesn't mean that's the fate of our universe that is going to like it'll be like falling into a black hole. The Current's idea is that our universe will most likely continue to expand forever. How it expands the nature of that expansion because we're still trying to understand uh what's called dark energy, which is this mysterious thing that's making the universe expand ever more quickly. We don't understand the nature of dark energy, so we don't know exactly how the universe will expand, what the fate will be. But in any case, that in that scenario, time will continue forever. It maybe nothing happens and the universe just dies, the heat, uh, heat death and it just, it keeps on expanding, getting colder and colder. Everything is spread out and it reaches what's called thermal equilibrium. Nothing changes. But in my view, time still exists, even though there's no measure of change to. Tell us that time still exists. It's still passing by, it's still pointing to the future, from an earlier moment all the way back to the Big Bang. So, If I were Uh, to make a guess, I would say the most likely thing for me is that time did have a beginning, but it won't have an end. Uh, BUT, you know, I could very well be wrong. Yeah, and I'm sure there would be people who would say, yes, Jim, you are wrong. Well, uh,

34:37 Ricardo Lopes: let's also see if we are still here to really experience whether time has an end or not. Well,

34:46 Jim Al-Khalili: well, well, OK, so that that is the big problem. We're not going to be around to be able to check that idea, so I can say anything, prove me wrong.

34:56 Ricardo Lopes: But, but when it comes to the origins of our universe, and I mean if that also marks the origins of time, uh, sometimes I hear physicists and philosophers of science debate whether the universe came from something or came from nothing. I mean, at this, at this moment. In time, er, is there enough evidence to say whether one of those hypotheses is more supportive than the other?

35:36 Jim Al-Khalili: Well, it depends what we mean by by saying it comes from something, you know, we can, we can think of time as coming from. Uh, A, a quantum fluctuation, uh, uh, some, somehow down in the quantum realm, time itself. Doesn't exist. Time is emergent, uh, uh, uh, an emergent property, it's not a fundamental concept of reality. Um, OUR universe could have emerged from a quantum fluctuation. People say, you know, where does the energy of, you know, all the, the stuff of the universe come from? Well, you, you, you can argue that, uh, the positive energy of that created all the matter. Uh, IS balanced by the negative gravitational, uh, uh, energy, and, and there are technical, um, explanations for this, but in terms of, um, Whether our universe had a cause, you know, what caused the universe? Could it have just appeared spontaneously? Out, out of, out of nothing. Um, IN a sense, this is more of a philosophical question or metaphysical question because we are reaching the point where physicists start to get nervous about, about trying to come up with explanations. Um, PHYSICS is a is an empirical, uh, uh, uh field. It's an experimental field. You, you have a theory or hypothesis, you have to test it to see if it's right or not. We have theories at the moment, which many would argue are not real science because they can't be tested, you know, theories of quantum gravity, for example, like string theory. Uh, BUT I, but I, they are real scientific theories because it's not that we can't experimentally test them, it's just that we don't currently have the means to do it, you know, you, you can't build an accelerator big enough to, to, to probe such a small dimensions of space and time. Um. But when it comes to, well, did the universe have a a first cause. Or was there an earlier generation universe before ours? Some physicists argue so that, uh, Roger Penrose, a Nobel Prize winner, um, colleague of Stephen Hawking's, uh, Roger Penrose is a, is a, is a genius, mathematical physicist. He's, he's, uh, he's 95 now. I mean, I know this because he's the same age as my father, so I'm always, you know, thinking about him when his birthday comes by. I mean, he's still active. Uh, HE has this idea of what's called the, um, Uh, uh, a cyclic universe that repeats itself, you know, it, it, the Big Bang expands and then it becomes uh, uh, the Big Bang for another universe. Um, SO he would argue that there wasn't a first cause, there was an earlier generation universe, a, a previous epoch. Um, AND he would argue actually there may be ways of of testing that idea, uh, somehow that the signature or the imprint or evidence of an earlier universe might still exist out in, in our universe if we could measure certain properties of space, uh, uh, very, very carefully. Um, SO, yeah, uh, what did our universe have a cause, or did it come from nothing? Uh, I, for me, this is still a philosophical, metaphysical question, uh, uh, and not really yet something that physicists can address seriously.

39:00 Ricardo Lopes: Mhm. Uh, IN regards to some other metaphysical questions that many times people, uh, tie in with, uh, physics, does the way that physics understand time. Uh, UNDERSTANDS time have any bearing on questions surrounding determinism and free will. I mean, is there any link there?

39:26 Jim Al-Khalili: Oh, absolutely, a very strong link, uh, um, I, I have, I devote a chapter in the book to, to this idea, um. The great French mathematician, uh, uh, Laplace, uh, came up with uh um this thought experiment, which we now call Laplace's demon. He said, if you could imagine a, a super intelligent being that could know the positions and, and the movements of every particle in the universe very precisely. Then they, in principle should be able to compute what will happen to those particles uh uh in the future, how they will collide, how they will interact, and therefore this super being would be able to predict, who would be able to see the future, um. Of course, I, and I, as I explained in the book, there are many reasons why this is wrong and, and it would never be able to do that. But that doesn't mean that we don't live in what's called a deterministic universe. A determinism simply means that the future is, is set. It's not that the future is existing waiting for us, but what will be will be. The fate of the universe is already. Determined in advance. The interesting thing here, and this is how it links with free will, is that we can never predict that future. You can't see the future of the universe of space-time unless you could be outside of the universe, what's called the God's eye view, to see the universe from outside. From within the universe, it's impossible to predict the future. Even if when the future happens, you say, ah, of course this was always going to happen, you know, uh, I had no control to to to to make it otherwise. But while we're living only in the present moment. Because we can't predict or see the future. So when people say, you know, I, I can see the future, I predict this, no, you can't. You're a charlatan. Um, BECAUSE we can't predict the future, even in principle, I mean, we can make some. Predictions a little bit into the future, I can say that, for example, tomorrow I'll be a day older. The sun will shine, will rise. Um, uh, I can predict that even though if I toss one coin, I don't know if it's going to be heads or tails, but I can predict if I toss 1000 coins, half of them will be heads, half will be tails. Our computer models of the climate are predicting that the the Earth's climate is changing due to uh humanity's actions. So those are predictions we can make based on what we understand now. But predicting what will happen next becomes more and more difficult, the more complex the system is that we're trying to describe and the further in in the future we're trying to describe it. For that reason, I always argue that even though we may live in a deterministic universe, which we may not, by the way. But even if we do live in a deterministic universe, because we can't predict that future for all intents and purposes, we have free will, we have free agency because it, it may be an illusion if viewed from outside the universe, but from within the universe, our free will is real, you know, we can choose what we want to do, even if afterwards we say, ah, but I didn't really have a choice, this was always going to happen. At the time, we are making free choices. This is what's called compatibleism. And, uh, and, and, and many philosophers don't like it. They say, oh, you want determinism, the future's fixed, but you also want free will. No, no, that's cheating. Yeah, tough luck. I, I, I, I think it's a, a, a valid and a reasonable view.

43:19 Ricardo Lopes: Uh, BUT, uh, that question surrounding determinism and free will, does it also apply? At a psychological level, I mean, what I mean is our understanding of how time works from a physics perspective, apart from whether apart from allowing us to answer or potentially answer whether the universe itself is deterministic or not, can it also have any bearing on whether psychologically we have free will or not?

43:59 Jim Al-Khalili: I think it's certainly uh the fact that we can't predict the future. Uh, uh, uh, MEANS we, I mean, I certainly can't predict what you will do next. I'm, I, I may be, you know, basing it on, you know, our brief conversation so far for this podcast, and I, I make a prediction you're going to ask me another question about the nature of time. Um, BUT we can't read someone's mind, we can't predict what they're doing, even though our brains. You know, our psychology is based on our consciousness. Our consciousness is really the software of the brain. It's, it's the firing and connections of, of neurons in, in tens of billions of neurons in the brain. Um, SO there's nothing magical about, Uh, consciousness and, and, and therefore our behavior, and yet it's far too complex for us ever to predict what someone will do for, for, for, for in for certainty, uh, in, in the next moment or or or or or beyond it because it's too, too complex, too many parameters to, to, to. Keep hold of. So, not predicting the future also means not being able to predict what someone else would do, and indeed what not to predict what we ourselves may may do given certain circumstances, because we are complex systems embedded within a complex world. Uh, AND so I think Having determinism doesn't rule out. Acting freely and therefore, having moral and and ethical responsibilities. You know, you can't say, well, you know, there's no such thing as free will, the universe is deterministic. So it wasn't my fault that I committed this crime, you know, it was, it was, this is just, this is just the laws of physics, you know, making, no, we still have moral and free uh uh will to behave in a certain way, and it doesn't matter. That in the future, we might look back and say we couldn't have behaved otherwise at at the time, psychologically. The future is open to us.

46:09 Ricardo Lopes: So over the course of our conversation we've been talking about the understanding we have of how time works from the perspective of quantum mechanics and general relativity. Could you tell us more about the thermodynamics? I mean, what understanding of time we can get from that framework, and then do you think it's, it would be possible to reconcile all of these three different frameworks?

46:38 Jim Al-Khalili: This is the area of my book that I spent the longest thinking about and and and and writing about because it's, it's the most fascinating. Thermodynamics goes back to the 19th century. It began as a, as the field of study of the nature of energy and heat and work, uh, and, and it linked in the development of, of, of engines in the industrial age. Um, BY the second half of the 19th century, uh, and, and a related field also was developed called statistical mechanics. So, essentially, thermodynamics and statistical mechanics are about the properties of matter. In bulk, in large quantities. So, concepts like pressure or temperature, uh, and, and heat are ideas in thermodynamics and statistical mechanics that emerge when you have lots and lots of, uh, subsystems moving parts. So, uh, a box of, of gas, you can say it has a temperature, it has a pressure pushing on the, on the, uh, walls of the box. But if you look down at the level of individual molecules, there is no such thing as pressure. There's no such thing as temperature. It's just the movement of of the kinetic energy of the molecules. But what's fascinating when it comes to the nature of time is that in thermodynamics and statistical mechanics, um, time does seem to have a direction, a directionality. It's irreversible, um. In its simplest form, it's what I call, I'm not the only one, but what physicists might call statistical inevitability. That if you have a box of gas and all the molecules of gas are pushed into one corner, then over time, those molecules will spread out to fill the whole box. So, there's a direction to time from the molecules being uh all clumped together to the molecules being spread out evenly in in equilibrium. That directionality of time, uh, is, is encompassed in what we call the second law of thermodynamics. It's always amusing that it's a very, very important law in physics, but it's still #2 in laws of thermodynamics. It's not the first law, it's only the 2nd law. Um, BUT it's, it's very, uh, interesting that the 2nd law of thermodynamics seems to apply so widely to so many phenomena. Uh, IT'S really a statement about the increase of what's called entropy. Uh, uh, AND entropy is a, is a concept that has many different meanings and definitions depending on, on, on what area of physics or what, what the thing is that you want to explain. One simple example of entropy increasing is molecules of gas spreading out to equilibrium. Another favorite example of mine is a pack of playing cards. Uh, THE pack of cards, when you get them new from the box, they're all arranged hearts, diamonds, spades, and clubs, all, you know, uh, 23456, all the way jack, queen, king, ace, organized in, in suits and in ascending order. When you shuffle the deck of cards, they become more disordered. So one of the most common ways of of defining entropy and therefore the second law of thermodynamics, which says entropy always increases, is to say it's disorder that increases. Shuffling a deck of cards will make it more disordered. It won't make it more ordered. Things run down, things become messier. Um, SO this is, this is the directionality of time in, in, in thermodynamics, the entropy always, uh, increases. But this is in thermodynamics and of course, as we mentioned earlier, in other areas of physics like relativity theory, like quantum mechanics, even like Newton's mechanics, there isn't this directionality to time. There seems to, time seems to be able to work equally well in both directions. So, this, this has been a long standing problem in physics. How does the irreversibility, the arrow of time, which is so obvious to us in everyday life and so obvious from the Second law of thermodynamics. Yet doesn't seem to exist in the other areas of physics. How do we get one from the other? And this is a one of the big puzzles that I, I tackle, uh, later on in my book.

51:18 Ricardo Lopes: Uh, BUT do you think it would be theoretically possible to reconcile all of these different frameworks?

51:28 Jim Al-Khalili: I think it's it's possible to reconcile how time. Uh, uh, HAS a directionality in thermodynamics, but not in other areas of physics. I, I think that is, is perfectly possible. I mean, certainly, uh, uh, what we would argue is that, Uh, in an isolated system like a box of gas, um. The, the, the system always wants to move towards equilibrium from being not in equilibrium like all the all the particles or the molecules in one corner of the box to being spread out. Um, THE dynamical equations, if you're, if you look at individual molecules and how they move, what they would say is if you could run the movie backwards. You should equally well see. Uh, uh, ENTROPY increasing into the past. So entropy is increasing into the future because the molecules are spreading out. But the equations say you can also run entropy backwards and, and it will increase the time backwards and entropy will increase. This is the symmetry of time. I argue that you can't do that because the box of gas is not an isolated system. Well, it it could be an isolated system while those molecules are spreading out, but you had to intervene and push the molecules in the corner from outside to get that going. You had to push the system away from equilibrium, and that's because the system is not isolated. So, for me, this idea that time is symmetric in the dynamical equations of physics is an idealization. It only applies in isolated systems. And there's no such thing as a truly isolated system in our universe. Everything is interacting with everything else and therefore time is always flowing in one direction. The only system that is truly isolated, that doesn't have an outside is the whole universe. And that's the only place we have to worry about reconciling. The arrow of time from thermodynamics with this time symmetric equations of physics. And even there, physicists and cosmologists have come up with an answer. They say this. They say, if today, uh, entropy of the universe is some value, there's a, the universe has a certain level of order or disorder. Tomorrow and next week, it will be more disordered. Stars will be shining more and spreading out their energy just like the molecules of gas are spreading in the box. So tomorrow entropy will be higher, so there's an arrow of time pointing from today through to the future, but. If the universe is truly isolated and and subject to the symmetric equations of physics, then yesterday and last week, entropy should be much higher than today in violation of the Second Law of thermodynamics. You can't have entropy increasing to the future from today, but you can pass from today. But is today a special moment? OK, maybe not today, maybe last week or maybe a month ago. Then all moments from a month ago to the future, entropy's increasing. Great, we've solved the problem of last week being higher entropy than today. But what about 2 months ago? I, so the only way to solve this is, hey, let's move that special moment all the way back to the Big Bang. And then every moment after the Big Bang, entropy's increasing. But what about before the Big Bang? Ah, there's no before the Big Bang, so we don't have to worry about that problem. Therefore, we've solved it. This is what's called the past hypothesis. It's a crazy idea. It seems like cheating, but it works. But it's only needed for the whole universe. Within the universe, there is an arrow of time.

55:26 Ricardo Lopes: So I have two more questions, and of course, and of course the next one, I mean I really have to ask it because otherwise I would be publicly executed by my audience because it's

55:38 Jim Al-Khalili: I even know what you're going to ask me, but go ahead yeah.

55:43 Ricardo Lopes: OK, so about uh time travel. Uh YEAH, so, of course, er there's, um, I er er there's possibly different ways that we can tackle this question in, in the popular understanding, I mean maybe if people would be thinking about time traveling as they see in some movies or books that like, I don't know, in movies like. Uh, Donnie Darko, and these people are more into independent slash cult movies, movies like Prime and things like that, and then of course, uh, they can get into, uh, time paradoxes. I don't know, let's say, oh, I, I want to, I want to jump into a time machine and go back in time to kill Hitler to prevent World War II, but maybe then. The consequence of that is that my parents won't meet and I won't be there in the future. So if I'm not there in the future, how am I back here in the past, you know, those kinds of those kinds of, those kinds of funny things. But I mean those kinds. Of explorations that we have in sci-fi about surrounding time traveling, are those legitimately scientific ways of approaching it or not? And if not, how do physicists approach it?

57:06 Jim Al-Khalili: Yeah, the notion of time travel does seem to, it, it doesn't have the respectability of other areas studying the nature of time. I mean, for that reason, my, my chapter on time travel, I, I left to the very end of the book. I called it a bonus chapter because I didn't want to have it included in all the serious stuff about entropy and thermodynamics and space-time and so on. But you're right, you know, you have to address this question. Well, actually, Our current best theory of the nature of time is Einstein's general theory of relativity. And the way we test theories is to push them to the limit. So we are trying to study them under extreme scenarios, which may not be realistic, but we want to see if we can break the theory. And one way of testing how time uh is is defined in general relativity is to consider what are called closed time-like curves. Essentially ways of traveling back in time. It's not like time running backwards. So it's so if you travel through a time-like curve or a longer time-like curve, it's not that you will, you will check your, your, your, your, your watch to see the tick, the seconds ticking backwards. You're moving forward in time. But it's a bit like on, on a, in, in a theme park ride on the roller coaster, and you're looping the loop. So you, you, you're going around, you're moving forward, but you're doubling back and returning to, to, to your original position in space. In this case, it'll be returning to your position in time or even to an earlier moment from when you, you left. You're right, it leaves all sorts of paradoxes that we have to solve. But the, the laws of physics themselves don't forbid time travel. Uh, AND all the time they don't forbid it, it gives us a way of testing our theories. So there are very respectable, um, scientific research papers and journals over many years studying. The nature of time like curves uh and the possibility of traveling to the past. They're they're not talking about building a time machine to, you know, to go back and visit your grandparents or to visit your younger self, that's left to the science fiction movies, but they do try and and deal with these things, uh, mathematically. My view is that, I think if we had a better theory of time, maybe a theory of quantum gravity, which also included some aspects of thermodynamics, of course, because we need to include all our current ideas in physics uh uh uh uniformly, um, then we might find that closed time-like curves are impossible, that they are forbidden by the laws of physics. At the moment, they're not forbidden, right? So all the time, they're still possible. It's a serious area of research, but it's also a fun thing to speculate on whether we can travel back in time. Traveling to the future, in a sense, you can do that. Well, you can travel to the future just by sitting still, you know, tomorrow will come, right? Um, BUT you can travel to the future, if you travel close to the speed of light or if you sit in a gravitational field to slow your time down. That's not real time travel to the future because real time travel to the future would expect, The future to already exist waiting for you. Now, what's happening here is you're just moving out of everyone else's timeline, getting to the future before everybody else, so it's not real time travel. Time travel to the past would require real time travel. I, I don't think it will be possible, but at the moment, I can't rule it out. And there are possibilities, ways of getting around the paradoxes, by the way, such as, uh, in, in if we subscribe to the many worlds interpretation of quantum mechanics and there are multiple realities, you could go back to the past, which will be a past in another. Uh, uh, TIMELINE in which you don't have to cause any paradoxes. So, who knows, possibly.

1:01:13 Ricardo Lopes: OK, so just one final question then. Among the still unanswered questions regarding time from the standpoint of physics, which is the one that you would love the most to see answered? I mean, which is the one that you would be, oh my God, if this was answered in my lifetime, it would be amazing.

1:01:38 Jim Al-Khalili: Normally when I'm asked this question, it's, it's, it's broader than that. What question in physics would I like answered? You see, I, and I have a good answer for that, uh, which, which is that I would like to know what is the correct interpretation of quantum mechanics. But that answering that question would also help with trying to understand the nature of time because in some interpretations of quantum chaos, like uh Everett's many worlds interpretation, there are these parallel realities within the multiverse that would suggest that time travel is allowed, that that uh we we may not in practice be able to move from one timeline to another. Uh, BUT in principle, mathematically, that allows for the possibility of time travel, even if we're, we never technologically developed, uh, uh, build a time machine. However, if another interpretation is true that, for example, what's called the Bohmian mechanics or pilot wave theory mechanics, uh, then there is only one universe and, and the paradoxes of time travel are real. Uh, uh, AND therefore that I would argue would rule out time travel into the past. Time has to continue forward. Um, FOR me, that will be the, the biggest, uh, uh, uh, uh, uh, question to answer. I think, um, ideas about arrows of time and does an arrow of time emerge from time symmetric equations or are time symmetric equations merely idealizations in the universe that already has an arrow. Uh, I don't know if we'll ever have a satisfactory, definite answer to that. I certainly Pick a side in the book. I, I, I have a perspective on it and I argue the case for that. And you know, I think I'm, I, I, I'd like to think I'm right in the way I'm thinking. I want to persuade other physicists that I'm right. Uh, BUT I'm, I'm not sure there will be a big discovery in physics that will say, Jim was right all the time. Uh, IN any case, I was not the first person to, to, to, to have this view. So I wouldn't, I wouldn't take credit for it even if there was some proof.

1:03:47 Ricardo Lopes: Uh, GREAT. So the book is again on time, the physics that makes the universe tick, and of course I will be leaving a link to it in the description of the interview. And Dr. Alkhalili, it's been a great conversation, uh, very fascinating. Talk. So thank you so much for coming on the show, and I've been a great admirer of yours, so it's been an honor to everyone. Oh,

1:04:10 Jim Al-Khalili: thank you, thank you very much, Ricardo. It's been, it's been great fun. I, I, as you can probably tell, I like talking about this stuff, so it's been a pleasure being on the program.

1:04:19 Ricardo Lopes: Hi guys, thank you for watching this interview until the end. If you liked it, please share it, leave a like and hit the subscription button. The show is brought to you by Enlights Learning and Development done differently. Check their website at enlights.com and also please consider supporting the show on Patreon or PayPal. I would also like to give a huge thank you to my main patrons and PayPal supporters, Perergo Larsson, Jerry Muller, Frederick Sundo, Bernard Seyaz Olaf, Alex, Adam Cassel, Matthew Whittingbird, Arnaud Wolff, Tim Hollis, Eric Elena, John Connors, Philip Forrest Connolly. Then Dmitri Robert Windegerru Inasi Zu Mark Nevs, Colin Holbrookfield, Governor, Michel Stormir, Samuel Andrea, Francis Forti Agnun, Svergoro, and Hal Herz Agnon, Michel Jonathan Labrarith, John Yardston, and Samuel Curric Hines, Mark Smith, John Ware, Tom Hammel, Sardusran, David Sloan Wilson, Yasilla Dezara Romain Roach, Diego Londono Correa. Yannik Punteran Ruzmani, Charlotte Blis Nicole Barbaro, Adam Hunt, Pavlostazevski, Alekbaka, Madison, Gary G. Alman, Semov, Zal Adrian Yei Poltonin, John Barboza, Julian Price, Edward Hall, Eddin Bronner, Douglas Fry, Franco Bartolatti, Gabriel Pan Scortez or Suliliski, Scott Zachary Fish, Tim Duffyanny Smith, and Wisman. Daniel Friedman, William Buckner, Paul Georg Jarno, Luke Lovai, Georgios Theophanous, Chris Williamson, Peter Wolozin, David Williams, Di Acosta, Anton Ericsson, Charles Murray, Alex Shaw, Marie Martinez, Coralli Chevalier, Bangalore atheists, Larry D. Lee Junior. Old Eringbon. Esterri, Michael Bailey, then Spurber, Robert Grassy, Zigoren, Jeff McMahon, Jake Zul, Barnabas Raddix, Mark Kempel, Thomas Dovner, Luke Neeson, Chris Story, Kimberly Johnson, Benjamin Galbert, Jessica Nowicki, Linda Brendan, Nicholas Carlson, Ismael Bensleyman. George Ekoriati, Valentine Steinmann, Per Crawley, Kate Van Goler, Alexander Obert, Liam Dunaway, BR, Massoud Ali Mohammadi, Perpendicular, Janaertner, Ursula Guinov, Gregory Hastings, David Pinsov, Sean Nelson, Mike Levin, and Jos Necht. A special thanks to my producers Iar Webb, Jim Frank, Lucas Stink, Tom Vanneden, Bernardine Curtis Dixon, Benedict Mueller, Thomas Trumbull, Catherine and Patrick Tobin, John Carlo Montenegro, Al Nick Cortiz, and Nick Golden, and to my executive producers, Matthew Lavender, Sergio Quadrian, Bogdan Kanis, and Rosie. Thank you for all.