Instant Genius - Why understanding how time works is the biggest mystery in science

Episode Date: September 3, 2026

When most of us think about time our minds likely turn to thoughts of the second hand that ticks around a clock, or maybe the occasions we’ve been late to catch a bus. But if we stop to think about what time actually is things quickly get more subtle and complicated. Does time only exist in our minds as a psychological illusion, does it really only run forwards and not backwards, and how close is modern physics to helping us to figure out its true nature? In this episode we’re joined by physicist, best-selling author and BBC presenter Prof Jim Al-Khalili to talk about his latest book, On Time – The Physics That Makes the Universe Tick. He tells us how thinkers going back as far as ancient Greece have struggled to define what time really is and how it works, how Einstein’s theories tell us that time can run at different rates in different circumstances, and how understanding its true nature maybe key in our search for a theory of everything. Take your curiosity further with a subscription to BBC Science Focus magazine. Every issue is packed with fascinating insights into the science behind everyday life, the latest breakthroughs and expert analysis, delivered straight to your door. Receive an extra £5 when you subscribe using the code SF5OFF, (minimum spend £20, see full terms and conditions on our website). https://www.ourmediashop.com/bbc-science-focus-magazine-pod30 Learn more about your ad choices. Visit podcastchoices.com/adchoices

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Whatever lies ahead, don't change your morning. Let your morning change you. Discover coffee plus on espresso.com. Hello and welcome to Instant Genius, a bite-sized masterclass in podcast form. Every Monday and Friday, you'll hear world-leading scientists and experts talking about the most fascinating ideas

in science and technology today. I'm Jason Goodyear, commissioning editor, a BBC science focus. When most of us think about time, our minds likely turn to thoughts of the second hand that ticks around a clock, maybe the occasions have been late to catch a bus, or the reason why we get more wrinkles and grey hairs as we age. But if we stop to think about what time actually is, things quickly get more subtle and complicated. Does time only exist in our minds as a psychological illusion? Does it really only run forwards, and not backwards.

And how close is modern physics to helping us to figure out its true nature? In this episode, we're joined by physicist, best-selling author and BBC presenter, Professor Jim Al-Khalili, to talk about his latest book, On Time, the Physics that makes the universe tick. He tells us how thinkers going back as far as ancient Greece

have struggled to define what time really is and how it works. How Einstein's theories tell us that time can run at different rates, in different circumstances, and how understanding its true nature may be key in our search for a theory of everything. So welcome to the podcast. Thanks so much for joining us. Pleasure to be here.

So today we're talking about your latest book, On Time, the physics that makes the universe tick. So I think when it comes to the way that we think about time, you can maybe split people into two categories, like some of them think, well, it's the ticking of the second hand on a clock. it can help me cook the perfect soft boiled egg, you know, track my progress on how quickly I can run a 10K or, you know, it's why I get more wrinkles and grey hairs. But then we have another category, which is what the book's about, which is the preserve of physicists and also philosophers, which see something more and they see time as a sort of mysterious fundamental puzzle of nature.

So having gone through the book, it seems like this is a project you've really wanted to take on for quite a while. So, you know, why is that? Yeah, well, I think, I mean, time enters into our equations of physics and laws of physics all over the place. And even really among physicists, very often, it's just along for the ride. You know, people say, it's time really just an illusion. It's time, as you say, it's time just that thing that clocks measure or something

to make sure that we, you know, keep our schedules and, you know, events happening in some order, even in physics, you know, that the, the, the, what's called the dynamical equations are these, these are the equations that describe how something, some system, some physical entity changes over time, but it's just a number that comes in. My passion has been in trying to see whether we have a unified picture of what time is, because even in physics, I mean, I should backtrack it just quickly, say, you mentioned the two types of people and how they perceive time. The way I describe it in the book as this, what's called manifest time or psychological time. So that's the time of our perception. And then there's physical time. Each have their problems.

So I started off by thinking my grand challenge is to combine the two. How does physical time relate to the time that we experience? That isn't a problem that has baffled philosophers and scientists for millennia going all the way back to the ancient Greeks. But even within each category, there are problems, famous problems of time that I try and talk about. So it's just been a fascinating journey. It's been an itch that I've wanted to scratch for many years. Yeah, that definitely comes across. So you mentioned they're the ancient Greeks. So I think this is really interesting because generally, if we start thinking about time, inevitably almost, the notion of change arises. And you talk about, you know, different people who've had ideas about this, Parmenides and Heraclitus, the ancient Greek thinkers.

And so one Heraclitus quote that I really like personally is like, because he always talks about rivers and things. The rivers of time, yes. Yeah. The second time a man steps into the river, not only is the river not the same, the man himself isn't the same. You know, it's really, I love that quote. So what can we say about these? Because people do have competing ideas about this. Yeah, and that's a really lovely example, that these competing ideas about,

the nature of time, go all the way back to the ancient Greeks, hundreds of years BC. As you say, Heraclitus is the guy who coined the term the river of time. For Heraclitus, time is real. It flows past us. We might think that, you know, we're being carried on along the river of time, or we're standing with time flowing past us. And it's real and it's all about change. Whereas Parmenides, who is a contemporary of Heraclitus, argued that time was just. an illusion. There is no such thing as time. And even more famously, I'm sure if quite his student or younger colleague was a guy called Zeno. We know Zeno's paradoxes are quite, you know, quite famous. Again, like Permanides, he believed that time was an illusion. And that struggle between

is time a real thing, is it an illusion? Is there something there? With time being something real that flows, even today, you know, there's a whole discipline called the philosophy of time where people are still trying to understand it. So, you know, there's a big challenge in my book to rather arrogant to assume that I could solve all these problems after so many clever people have tried. So let's have a look at physics there, you know, your bread and butter. You sort of touched on that. So I studied physics myself a long time ago at university. You know, like you bet you're dead, sort of to my shame, I did often see time as just a variable in equations. without really thinking much about what it actually meant.

Yeah, yeah, I think we said there are different ways of defining time depending on what area of physics you're studying. So if you're talking about the dynamical equations of motion, so examples are like Newton's equations of motion in classical mechanics, or Schrodinger's equation describing the evolution of the quantum state in quantum mechanics, there are lots of equations describe how things change over time. And in those equations, you've got this symbol little T. What something is doing at a particular value of little T,

and you sort of crank the handle and solve the equation, and we learn all these tricks as physicists how to do that, and you can work out what that thing is doing at some later time, later value of T, or even crank it backwards and work out what it was doing at an earlier time. But in all those cases, T is just this parameter in the equation. We call it coordinate time. And you're right, it is, in that case, just a thing. number. It's not a physical thing in and of itself. It's a number you plug into the equation

to tell you what the real thing, the system you're trying to study, is actually doing. But of course, that's not the whole of physics. And probably our best theory on the nature of time is Einstein's general theory of relativity. And even before that, his special theory, special relativity in 1905 was what led to the notion that time isn't absolute, that time isn't just this clock that ticks by at the same rate ever in the universe which Newton believed. No, time, according to Einstein, and we now know that we believe that's the correct picture of it, is a dimension of space time. It's a fourth dimension. That's a very different from coordinate time. In Einstein relativity, all times coexist, just as all points.

points in space exist. I can be here there. They're different. Just because I'm here doesn't mean the place that I'm existing in is the only real place in space. Lots of other places are real. Einstein says, well, the moment in time that you regard as your present moment, you're now, is no more real than all other moments in time. So that's a very different picture of time coming from relativity. And then it gets even more complicated because you have another area of physics, thermodynamics, which says no, time isn't just a coordinate, nor is it just a dimension. It's an arrow. It's a direction pointing from past to future. Nowhere else in physics do you ever see an arrow of time? So even in physics, there's this whole confusion about how we define time.

Yeah, so let's get on to relativity in a moment, but you mentioned their thermodynamics, and it's the arrow of time. And this is sort of, as far as I could tell, bound up in the idea of Boltzman's theory of entropy. So how does that fit into this picture? Well, that was the earliest notion within physics that time has a direction, that it's irreversible. Boltzmann and other people like Maxwell were studying this idea that this quantity called entropy always seems to increase. So it's basically, there's lots of ways of defining entry. It's really messy, but the simplest way is to say that it's a measure of disorder. So you take a pack of cards that's ordered in suits and ascending numbers, you know,

straight out of the packet, we say that's highly ordered. It's a special arrangement of the cards. It has low entropy. You shuffle the pack, you increase its entropy, you increase the messiness of disorder, entropy is going up. And in thermodynamics, famously, in fact, the second law of them dynamics says in a closed system that's not interacting with its surroundings, entropy can never go down. It always goes up or stays the same. There might be possible fluctuations, but on the whole, entropy always rises, and that gives a direction in time, that the arrow of time is fixed by thermodynamics. Now, of course, in my book, I say, well, that's actually very naive and messy and subjective and not a good, there are lots of, since thermodynamics, since Boltzman, we now know of

lots of different arrows of time. The causal arrow, you know, cause coming, having to come before its effect, not after its effect. The cosmological arrow pointing in the direction of the expanding universe. The quantum was called the entanglement arrow or decoherence arrow, which says that, you know, it's famously Schrodinger opening the box of the cat dead and alive at the same time. Before he opens the box, it's dead and alive. When he opens it, it's one or the other. You can't reverse that time. You can't go from, you know, backwards once something is decaying. So there's irreversibility in time all around us. It's not just thermodynamics. Yeah. So let's have a look at relativity, then, as you said, like one of the sort of greatest ideas in physics.

And so in this theory, time is sort of intimately bound to the fabric of the universe itself. And also it has a really fascinating relationship with gravity. Yes. So Einstein had two theories of relativity. His first one, his special theory. That's the one that most people will have heard of the famous equation. E equals MC squared. But that's actually a sort of a byproduct.

That's not the most interesting thing about special relativity. Most interestingly is that it says that when you move close to the speed of light, things change. Time runs at different rates, lengths get squashed. So that's the theory that first gave us time being the fourth dimension of space time. Much more fascinating is Einstein's general theory of relativity, which is his theory of gravity that supersedes Newton's law of gravitation. And in it, he says, gravity isn't this invisible force that pulls objects together. Masses attract each other, keeps us stuck to the ground, keeps the Earth in orbit around the

sun, the moon around the Earth, and so on. Although that's a good approximate picture of it, but what's really happening is that a gravitational field is causing space time to curve. And the mathematics are beautiful, and it's borne out by lots of examples. experiments that show that's correct. But of course, the difficulty is that we don't have some instrument that measures curvature of space time. We have instruments that measure lengths in space, and we have instruments that measure time, clocks. And so we can look at how Einstein's general theory or his theory of gravity affect time and space. And what we've realized and experiments

prove this back in the mid-20th century, that gravity slows time down. So not only a time real, and as you say, part of the fabric of reality, fabric of space time, but it's also something that can be influenced by gravity, by a gravitational field. And this is the most fascinating and lots of examples of how gravity does this, but it's a real thing. It's not just theory. So just sticking with that, like one example is the twin experiment where one person, one twin experiences a different speed of different gravity. The other remains on earth and they age at different rates. Yes.

So that's an example, the twins paradox. And you were telling me earlier, Jason, you're a twin. So, you know, maybe you can empathize more than other than this example. But that's an example I've talked to my students for many, many years. Actually, you can explain that using just special relativity, not using any ideas or notions of gravity. And that's the idea that if one twin flies off, close to the speed of light, turns around and comes back again, they will come back.

longer than the twin that stayed at home and how can that be if all motion is relative and so on. And there are different ways of explaining it. Ultimately, I guess you'd say, well, you'd probably have to explain it because the twin that travels away has to speed up, get up to speed, get to the distant point, slow down, turn around, accelerate and come back again. And that acceleration, Einstein tells us, is equivalent to gravity. This is called the principle of equivalence. So in a sense, it is like gravity slowing time down. but in this case it's acceleration slowing time down. But you don't have to travel anywhere.

You don't have to get in a rocket, go anywhere near the speed of light, in order to experience time slowing down, like in the Twins paradox. The best example, because it's the one that people can probably most readily appreciate, is the fact that your sat nav in the car or GPS or Google Maps on your phone

would not work if we didn't take into account the fact that Earth's gravity slows time down. The problem is that the satellites, the global positioning satellites, they're further away from the Earth than we are, and so they feel slightly weaker gravity, and so gravity doesn't slow those clocks down so much, so they run faster than clocks on Earth. And we have to correct for that if we want GPS to work.

If we didn't believe that gravity slows time down, Google Maps would not work. I love that example. Two and five Canadians will hear the words you have cancer. That's why every step and dollar raised matters. On September 19th, join thousands in Toronto for the Princess Margaret Cancer Foundation Walk. Challenge yourself, friends, and family to walk 21 kilometers in support of life-saving research. Together, we can carry the fire and help create a world free from the fear of cancer.

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and coffee plus B6 to keep your day moving. Or go with the flow and choose ginseng delight. Our new double espresso with ginseng extract. Whatever lies ahead, don't change your morning. Let your morning change you. Discover coffee plus on espresso.com. Yeah, so we also talk about time being something called an emergent property. Maybe some people will have heard of that term.

But what does it mean and how does it apply? in this context? Well, of course, there are these two camps that say, one says at one end, time is just an illusion, it doesn't really exist, it's just in our heads. The other extremes that time is real. In fact, all times coexist and it's a physical thing. In the middle, there's this idea that acknowledges that time is real, but that it's not fundamental, right?

That if you dig down deep enough into the nature of reality, you find that time is something that emerges. Now, what does this mean? Well, the best examples of emergence are simple things like, my example has always been the wetness of water. You would not appreciate the property of wetness if you just look at one molecule of H2O. You need to zoom out and look at lots of them to see that happening. Another example is the notion of temperature. Temperature doesn't exist when you're looking at individual atoms and molecules, say, of a gas bouncing around. It's just their vibrations is what's called kinetic theory. It's their kinetic energy that is the thing.

But you zoom out to trillions and trillions of molecules and you start to see things like temperature and indeed things like pressure emerges. So these are emergent properties from some underlying deeper reality. And physicists today research has been carried out is looking into where the time itself and maybe even space are not fundamental. They're not base level part of reality.

They emerge from something more abstract, something deeper, something maybe down in the quantum realm. We don't know. It's speculation, but it's fun speculation. So another sort of inevitable question that comes up when we start thinking properly, deeply about time, is, you know, what did time have a beginning? So we know the Big Bang occurred about 13.8 billion years ago. But then, you know, the universe started expanding from that point.

But, you know, was that the start of time? What can we say about that? General relativity, which is the theory that actually explains the notion of the Big Bang, the beginning of our space and time, would say, yes, time exactly began at the Big Bang. There's no time before the Big Bang, in the same way that there's no point on the surface of the Earth further south than the South Pole. Right. So there is no before the Big Bang because there's no time to embed the word before in. That, you might think, is the end of the story.

If you've got nothing better than general relativity, then don't even ask the question. It doesn't make sense. But there are theories in modern cosmology that are looking into whether maybe the Big Bang was just the start of our universe. maybe there's a bigger reality, a multiverse, in which time was around long before our universe popped into existence. So even the notion of a beginning of time is something we're now actually thinking about seriously in physics. Yeah, so sort of on the other side of the coin is if we're thinking about the beginning of time, how about the end of time? Like, as you said, the universe is expanding. But there are several different scenarios.

scenarios in which we think this will proceed. Yes, yes. And this is a more active area of research because this is something that we can actually study seriously. We can speculate on what may have been before the Big Bang and suggestions to how we might look for clues. But the end of the universe is fascinating because it relies on this idea of dark energy. This thing that's causing the universe to expand ever more quickly rather than slow down

because of the gravity is putting the brakes on all the matter and energy in the universe. And we don't know enough about dark energy to know how it's going to affect the universe very far in the future. So one option may just be the universe will continue to expound forever, cooling down, spreading out, its entropy increasing until it reaches what we call thermal equilibrium and nothing happens. But I would argue time is still going by. It's just that you have no way of measuring that passing of time because there's no change. But it may be that the universe will rip itself apart more quickly.

Maybe it might even stop expanding and recalapse on itself, in which case is the end of the universe the big crunch. So even there, you know, we have every possible scenario. Time may go on forever or it may come to an end, just as it may have had a beginning or may have existed forever. All options are on the table. Let's move on to quantum physics then. You mentioned there the famous thorpexbron Schrodinger's cat.

So we know that quantum physics tells us that particles and, you know, even now bigger systems can exist in multiple different states at the same time. But then how does time feed into this idea? You know, that seems very difficult to wrap your head around. Well, what's fascinating there is the notion that there's an arrow of time down in the quantum realm. We learn in physics classes at university about the Schroding equation, which is the equation that describes how a quantum system evolves in time. The thing about the Schroding equation is that it's an idealized situation because it only deals with isolated systems, a system that's not interacting with the outside world. Once a system becomes open to its external environment, we have to go beyond the Schroding equation.

We have to use what's called a master equation, very grand sounding name. In a master equation, time has a direction. It has an arrow. It points in the direction of a system becoming more entangled with its environment and ultimately decohering. This is what the old guys, the original founders of quantum mechanics would have called the measurement problem. We now call it de coherence theory. So it's the Schrodinger opening the box to see the cat dead and alive that you can't reverse. In a sense, it's a bit like the thermodynamics arrow of time. you put a hot cup of coffee in in the fridge, it will cool down. Heat dissipates from hot to cold. That's the increase in entropy. That's the second law. In a similar sort of way, a quantum system

put in a quantum fridge will leak away its quantum information. It will decoher. So there's this lovely irreversibility, this hour of time that emerges from the quantum world that for me is more fundamental than Boltzman's thermodynamic arrow of time. So you also talk about, so I, what I find really interesting about this, another thing that ties my head in knots is how do we define now? Because we mentioned sort of physics equations earlier, and, you know, as you say, they're sort of just exist by themselves and it doesn't really matter when now is. So how do we go about saying, you know, when exactly?

is now, what do we mean by that? Well, we only can make sense of now in manifest time, psychological time. There is no notion of now in physical time. As you say, you know, dynamical equations, you stick in a value of T and any T is as good as any other. The notion of the present moment is only, only makes any sense at all to us in our perception psychologically. In physics, in relativity theory, all points along the time axis are equal. real. We happen to favour one point which we call the present. All points beyond it is the future, which is unknowable to us. All points behind it is the past, which has gone and lost us. And that moment is creeping along the time axis as our present moment evolves and changes. There's no notion of a

now or a flowing moment in time in the laws of physics. That in a sense is probably the deepest worry about how we connect time in physics with the time of our perception. So sort of by way of summing up, let's go back to the start, which, you know, seems like the right thing to do given the topic. So we've talked about all of the strange things we've discovered about relativity, you know, quantum physics. We talked about how the nature of time has been a question for philosophers and scientists for thousands of years. Would you say it is still one of the biggest mysteries out there? I would, yes.

I don't know if all other physicists and philosophers would agree, but I think it is. You know, the big, the Holy Grail in physics is to find a theory of everything, certainly to unify quantum mechanics with general relativity, to get a theory of quantum gravity, and there are candidate ideas like string theory, for example. But they tend not to worry too much

about the fact that time enters in rather arbitrary ways. In my view, unless we have a unified or complete description of time, we're not going to have a complete description of reality, a theory of quantum gravity, for example. And so for me, yes, it is still the deepest outstanding mystery, maybe rivaled only by the meaning of the nature of consciousness. The nature of consciousness or the definition of life itself. But in physics, understanding the notion of time is still that mystery. that we have yet to solve.

Thank you for listening to this episode of Instant Genius, brought to you from the team behind BBC Science Focus. That was Professor Jim Alcalili. To discover more about the topics we've just discussed, check out his book, On Time, the physics that makes the universe tick. If you liked what you just heard, then please do consider subscribing to Instant Genius

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