In Our Time - The Physics of Time

Episode Date: December 18, 2008

Melvyn Bragg and guests discuss the physics of time. When writing the Principia Mathematica, Isaac Newton declared his hand on most of the big questions in physics. He outlined the nature of space, explained the motions of the planets and conceived the operation of gravity. He also laid down the law on time declaring: “Absolute, true, and mathematical time, of itself and from its own nature, flows equably without relation to anything external.” For Newton time was absolute and set apart from the universe, but with the theories of Albert Einstein time became more complicated; it could be squeezed and distorted and was different in different places.Time is integral to our experience of things but we find it very difficult to think about. It may not even exist and yet seems written into the existence of absolutely everything. With Jim Al-Khalili, Professor of Theoretical Physics and Chair in the Public Engagement in Science at the University of Surrey; Monica Grady, Professor of Planetary and Space Sciences at the Open University and Ian Stewart, Professor of Mathematics at the University of Warwick.

This BBC podcast is supported by ads outside the UK. Thanks for downloading the In Our Time podcast. For more details about In Our Time and for our terms of use, please go to BBC.co.com.uk forward slash radio 4. I hope you enjoy the programme. Hello, when writing the Principia Mathematica, Isaac Newton declared his hand on most of the big questions in physics. He outlined the nature of space,

explain the motions of the planets and conceived the operation of gravity. He also laid down the law on time, declaring, Absolute, true and mathematical time of itself and from its own nature flows equably without relation to anything external. For Newton, time was absolute and set apart from the universe, but with the theories of Albert Einstein, time became more complicated. It could be squeezed and distorted and was different in different places. Time is integral to our experience of things,

but we find it very difficult to think about. It may not even exist, and yet seems written, into the existence of absolutely everything. With me to discuss time on Monica Grady, Professor of Planetary and Space Sciences at the Open University. Ian Stewart, Professor of Mathematics at the University of Warwick, and Jim Al-Khalili, Professor of Theoretical Physics and Chair in the Public Engagement in Science

at the University of Surrey. Jim Al-Kalili, let's start with a clock on the wall with its hour, minutes and seconds. What sort of time is that marking? Well, it's very interesting. A lot of people confuse time with clocks. and so without clocks, would there be any time? Clocks are just counters.

They're just our human invention, our way of recording how we perceive time to go by. So a clock is divided up into hours, minutes and seconds, but we could have divided it up into any other units. I mean, our day is how long it takes for the Earth to spin once around its axis. And dividing a day into 24 hours and an hour into 60 minutes and so on is a human invention. It goes back to ancient Babylonian times. And so clocks mark our own local time. And we'll get on later on to talk about times being different.

But as far as Newton was concerned, if there were clocks spread out across the universe, they could all in principle be synchronized and will all tick by at the same rate. You say it went back to the Babylonians. Just briefly, why did the Babylonians choose 60? This has got the hexagestimal system. They chose...

60 and then the Indians brought in the decimal system units of 10, which is much more convenient since we have 10 digits. But it's something that goes back to the second millennium BC. And it's mostly lost. We still keep hours divided into minutes and seconds in units of 60, and angles. A degree is divided out into 60 minutes of arc and a minute of arc into 60 seconds of arc. So we still retain this hexagessimal system.

but we find it much more convenient now having had decimal system introduced. Still, let's just pause for a second. It's been hanging around for a very long time. Sorry, but does it have any meaning being 60? Did they find a meaning in it, given that the earth goes around on its axis every 24 hours? Did they find 60 by accident?

Was it useful? Were they playing around with that number anyway? Or did it relate to something that mattered to the way they measured it? Well, Ian might be better explaining where the 60 comes from, but the division of an hour into minutes and seconds is much more recent than the ancient Babylonians. There were no timekeepers back then. There were no mechanical clocks that could measure time to an accuracy of within minutes. You know, getting it to within half an hour accuracy in a day was the best they could do with sundials and water clocks.

So the actual division into minutes and seconds is a much more modern idea. Because of the idea, do the clocks mark time or do they make time? Well, this is precisely the point. A lot of people think that clocks make time. And so without clocks ticking by, there would be no time. And the question in physics is, well, if there were no clocks, if there were no people, indeed, if there was nothing in the universe, if the universe was empty, then there will be no events.

So nothing to mark the passage of time. Would there then still be time? and that becomes a very interesting philosophical question. If we go into the clock and think about the actual materials it's made of the atoms, are they also marking time? They are, yes. I mean, our clocks, the clock on the wall is a mechanical device that we can control how fast it ticks by and counts the units.

But down at the atomic level, nature counts time independently of humans. And that actually gives us a much more precise, an accurate way of measuring time. And we have developed things called atomic clocks, which are based on the frequency of light that's given off by atoms. And that frequency is another way of measuring time. It's a way of counting how quickly an atom vibrates, how quickly an atom, how much energy emits.

And it's down to the laws of quantum physics now, and it's extremely precise. So atomic clocks are accurate to within, a thousandth of a, I don't know what, is it ridiculous numbers of accuracy, a thousandth of a second accuracy in a million years. But you're still talking about clocks? Well, we can call it a clock in the sense that what I call a clock is something that counts units of equally spaced units

throughout what we perceive as the passage of time. Monica Grady, is it possible to think about the whole of nature of a series of clocks? How is time built into nature and to geography? geological feature, sorry, and so on. Can you give us some idea of different times there? Yes, it all comes back to what Jim referred to as, you know, a basic unit. One basic unit for us is the time it takes for the earth to go around the sun, which is a year, and we have the time it takes the earth to spin around once on its axis, which is a day.

And once 24 hours, and so once you've got those units of time, then you've got those units of time, then you can start dividing them up. But we can also look at time in terms of astronomical processes. The time it takes for light to travel is a distance. So we talk about a light year. And now usually we think of a year as time, but in astronomical times, light year is a distance. and it's the length or the extent which light has travelled in that particular space of time.

And when we look at stars in the sky, we're not looking at them as they are now, we're looking at them as they were when the light left them. Now that might be several hundreds of thousands of millions of years ago because they are vast distances away, which is why we use this very strange measure of distance, the light year. So in terms of astronomical times and distances, they're all interlinked. The time, the distance, the whole map of the universe is interlinked.

But to take a more home an example, we see the sun's light eight minutes late, as it were. That's correct, because the sun is, 150 million kilometres away, and it takes eight minutes for light to travel that distance. And nothing in our physical universe, as far as we know, goes faster than the speed of light. So that just gives an idea where eight light seconds away from the sun. That's what 150 million kilometres is. Can we talk a little about geological time, Monica, though?

Yes, geological time is not quite as astronomical as astronomical time. that again we use atoms again to measure geological time we use the decay of a radioactive isotope to measure time

so we look at the age of our own planet which is 4,560 million years old and we can divide the age of the rocks into different units or eras or periods depending on the age of those rocks

and we measure the age of the rocks by how much time it's taken for a radioactive isotope that used to be in that rock to decay. It is becoming an extraordinary exact science. Can you give the listeners one or two specific examples of how this is measured and how old we can thereby work out

that part of the earth at that stage of its development? Right. Okay, well, we can say that the solar system, system was born 4,500 and 58 million years ago, which is an incredibly precise measurements and we think it's also an accurate measurement as well. And the way we measure that is looking at meteorites, which are bits of material that were formed when the solar system was born from a cloud of gas and dust. And they have these radioactive clocks in them. So we think of radioactivity in a nuclear reactor as an isotope, which decays and gives off heat and energy, and leaves behind something called a daughter isotope.

So, for instance, uranium decays to lead. And we can look at the amount of lead that is left in a rock and calculate how much uranium there must have been to start with, and so how long it's taken to produce that lead. And that gives you this very accurate scale that you can divide the rocks into. And we can look throughout geological time and we can say, right, okay, the oldest mineral that we have on the earth

is 4,520 million years old. We can say the oldest rocks with fossils in are 2,700 million years old and so on. And using that, you know, we can make a timeline for when different things happen within the solar system on Earth when our atmosphere changed, when life got going, when humans walk the Earth,

and we can make a timeline like that looking how things have changed. Ian Stewart, if we think of the Earth as a clock, can we think of the universe as a clock, unwinding from 13.5 billion years ago, if I've got that right? This is a very interesting question because once you start thinking about the origins of the universe, then there seem to be two possibilities for the nature of time,

that time didn't exist in any sense until the universe came into being. It's very hard to find the right words in our language to describe this, and then space and time come into existence together. And from that moment on, time is ticking in the universe according to various, you know, whatever rules, the laws of nature that things run on. the other way is to say well time was always around the universe then appears at some instant of that time and the time that we're experiencing inside our universe is a kind of

and it's it's showing that there was this universal time going on so the universe is existing in time rather than time being something that comes out of the universe and there's a lot of until recently this wasn't a big issue because people weren't too worried about the beginnings of things. So when the universe is up and running,

time is ticking along. All of the particles and so on are vibrating away, doing nice regular oscillations, and all of this stuff correlates well with everything else. If you measure time by lots of different methods,

all of these methods are consistent with each other. And that I think is why we have this feeling there is a sort of single universal time that's running. and so once the universe gets going, the internal functioning of the thing, everything acts like a clock. I think that's right.

So the universe has this sort of, as it develops, as it evolves, the history of how it does that gives a very good measure of time passing. But it could be that down on a fundamental level, the particles themselves aren't actually obeying laws that have time built in as some explicit mathematical variable,

that the time emerges from the way they interact with each other. And this is a fairly modern view, in fact, very modern view. One way to think of this, suppose you've got a film. It's a short film of an animal moving. Okay, and all of the frames of the film are arranged in sequence, and as you run your eye along those frames, it's like time passing. Now suppose I took some scissors and chopped that up into all the separate frames and just jumbled them together.

In a sense it's still the same film, but there's no explicit time there in the way they're arranged. But you could reconstruct the time by saying successive frames in the movie will be very similar to the previous frame. There are very small changes from one frame to the next. And if you made a big list of which frames look very close to other frames, you could reconstruct the entire movie with some considerable accuracy.

So this does mean that time doesn't have to be fundamental. It could be a kind of high-level emergent property, as we say. It could be a property of the universe as a whole, which is created from the way all the bits and pieces interact. But if you write down the rules for how the bits and pieces interact, you don't need explicit time in there. Does this, you'll have to help me from now on you guys

Does this mean that time exists independent, doesn't exist independently as Newton said? It could well be that it is not independent that in a sense Universes, space, time, gravity matter, all of these things they're a package and they all influence each other

and they all grow out of each other. Can we move into this area now, Jim, Jim Carilli? Let's have the Newton idea of what time is and then let's bring in what Einstein did. These are the two big bowl figures and we'll start with them. So can you just say, what do Newton say

and what did Einstein say that affected what Newton had said? Okay, well, Newton believed that time was absolute, that time ticks by universally, there's a cosmic clock that counts by the seconds, minutes, hours, years. God created it. God created it. And it sits outside the universe.

He also believed that space was the stage on which everything happened. And so time and space were there and the universe of objects of things of particles and people and so on. They played out their drama on the stage with time ticking by outside. And so we had no control over time. We might have our own subjective feeling about how quickly time goes by. I mean, a nice example is two rival football supporters watching a football game. And if one team's in the lead, of course, that team supporter wants the game to finished quickly, but time goes by very slowly.

For the guy hoping his team can equalize, time runs out very quickly. They're not measuring different times. It's our own subjective view of how fast time goes by. But Newton's view is, and it's the view that we still hold, you know, the common sense view is that if time goes by fast or slow, that's our own subjective view of it. Now, when Einstein came along in the early 20th century, with the help of other physicists and mathematicians at the time, but essentially Einstein, made the big leap forward, which was that time isn't absolute,

that time, as Ian mentioned, is somehow woven into the fabric of everything in the universe, space and time and everything within the universe. So time has created in the universe. And because it's no longer absolute, it's now something that we can control. So time and space become something more complete. Monica Grady, what are the consequences of that then? Well, some of the consequences of this are that we really can link very closely what we know about planets orbiting a star,

what we know about stars within a galaxy orbiting a black hole. And we can say, right, okay, the analogy that's almost always used is that of a sheet with something dropped into it. A torch sheet, a held torch sheet, yes, with something dropped into it to represent either a star or a black hole. And then if you then roll a ball around it, you can see the fact that the sheet is sloping, is pulled down, but you get the force pulling this little object going round and around.

So just to apply for Eddington, I think, who did this first with the tablecloth. You put a melon and then put a walnut, and the walnut circled the melon, and there was in a dip there. So the idea of gravity was that it was because space itself was the sheet. And so it's a sheet full of things and it's the things themselves that are the other things to happen. The sheet is space-time and the mass, the weight, the bigness, the heaviness, the denseness of an object warps that space-time. So if you think of this torch sheet and you throw something big in it, then the sheet buckles and it warps. especially if you've marked the sheet with squares,

you can see that the squares then become distorted and get sort of pulled in to elongated as they get pulled towards that heavy object. In what way is this a development of Einstein, and in what way is it important for your physics? Who would to take this on? Well, we'll come to Ian first and then to Jim. Okay, yeah.

I mean, one of the things that that example shows in Newtonian physics, as well as space and time, you've got the idea of a force. A force is something that pushes objects around and makes them deviate from straight line paths. Now, in the Einstein picture, for very good reasons, that's reformulated. Forces are not real.

What is real is a curvature of space time, the sagging of your sheet. And what you perceive as a force is in a sense an illusion created by the geometry of space. and time. Illusion is the wrong word because it's perfectly real. We don't have very good

word. Can I just pop in here? Is this very different from Newton's idea of gravity? I mean, gravity I know is the great unexamined. We're not going to solve it even on in our time at 20 past 9 this morning, but nevertheless is it very different from, and in what way, importantly different? It is, yeah, there was a big problem for Newton

which the philosophers at the time picked up on, which is Newtonian gravity is a force that acts at a distance. The sun pulls the earth, but the sun is 93 million miles away and there doesn't seem to be a piece of elastic or anything in between holding them together. Now in the Einstein picture that's no longer a problem because space has a different shape. The earth feels that shape. There's no action at a distance. It's the geometry of the whole solar system that's affecting the way the earth moves. Monica,

and it happens instantaneously as well. So if you think of dropping something big into this this torch sheet. Something, you know, you might be dropping it at this end, but something at the other end instantly will move because of the buckling, the warping, the curvature of that space time. So we're talking of space and time together. They're now linked.

It does this, as it were, oust Newton's idea of the apple? It does. I mean, I think we're sort of getting ahead of ourselves a bit talking about space time being warped in gravity, because that actually belonged to Einstein's general theory of relativity, which came later. His earlier social theory was the one in which he said time is another dimension, almost like space.

Now we sort of know that if you want to define an event happening, you have to say where it happened and when it happened. So time is another number that you have to give to define an event. But Einstein went further. He said time is like another direction. It's not a direction within our three dimensions of space because you can't imagine any direction. It has to be outside our space.

And he unified time together with space. So Einstein had this idea of what's called the Block Universe, whereby time is just another axis. So in the same way that you can imagine different places in space, you can imagine different points in time, past, present and future. And that's a very weird concept, because it suggests that what we think of as the present moment now is just our very special localised point of view,

that past and future are just as real as the present. Ian? Yes, I mean it's sort of as if... Well, in the Newtonian picture, of course, you can still separate the time dimension... Excuse me, from the space dimension. So it's a bit... Think of a slice loaf.

You've got a whole pile of slices of bread. Those are space. And you stack them along the length of the loaf and each slice is the next instant of time, what's happening to the universe as you go. And the space is... Space is... and time are not mixed up in that picture.

The slices are space directions and along the loaf is the time direction. But then Einstein went further. Because of the way the physics works, you have to actually be able to mix up space and time. In a sense, whenever you move anything, you're mixing up space and time.

If an object moves, then in a certain period of time, it changes its location in space. What this means is that two different observers, two different people, say one on Earth and one moving by in a rocket, won't agree on, say, how long an object is or how far two points are in space. And they won't agree on the interval of time between two events.

Can you even more specific? Okay, so we see two flashes of light, for instance. One on, someone on Earth... At a distance from each other. At a distance from each other. The observer on Earth will say those two flashes of light happened at the same time and they're a mile apart. the observer on the rocket moving past and the faster that rocket moves the bigger the effect

because as you approach the speed of light these effects become more important the observer on the rocket may say no in fact those two events are a millisecond apart that one happened before the other and in fact they're not a mile apart they're some other distance more or less apart so they won't agree on distances

and they won't agree on times and time this is when time and space become relative and it's only when you can combine time and space together in these four dimensions that you can invent a new distance that's a mixture of space and time that everyone will agree on. That's why it's important to have time as the fourth dimension. I wanted to make the same point that you have got to have a frame of reference now. You've got to be able to anchor something and say,

okay, in the frame of reference in which I am standing, this is relative to that, if you like. happened before that other event. But if you are outside that frame of reference, you will see something different. So in order to be able to compare events, we have to have this common frame of reference, which we can define.

And the other feature you have to throw in to get the real Einstein picture is there's one thing that doesn't depend on your frame of reference, which is the speed of light. Light seems to be different from other moving particles. if I'm travelling in a car at 100 miles an hour and light is shining towards me, it doesn't hit me 100 miles an hour faster

than light would if I was stationary. It hits me at the same speed. And that's a big puzzle, but it's what leads to the whole mathematical structure that Einstein came up with. And so the geometry of space and time really does have space and time combined together in a kind of irreducible way.

You can't just separate them out into two different things. Jim, I think, I hope we're still talking about this. I understand that Einstein saw time as static. If anything flowed, we flowed through time. That was the way it went. Can you make that rather more intelligible? Yes, because he imagined time as another length, in a sense, almost like a distance in space,

but it was in a direction that we could never imagine because our brains are three-dimensional. and we can't break out from this our own perception of time. But he said, no, time, just as an axis, a direction in space exists, all points along that distance all coexist and are equally important. And you may happen to be at one place, but it doesn't mean another place further along that length axis is just as real. Likewise, time, all points along the time axis coexist.

So what we call our present now is just a cross-exis. section, a slice of Ian's loaf, you know, through space at a particular moment. But Einstein, I think he famously tried to console a widow of a friend of his, you know, who just passed away, saying that all times coexist, all times are equally important. He, you know, he was with us in the past, but the past is just as real as now and just as real as the future. So it's only our conscious perception that we are creeping along the time axis. But that the last, but that's the the whole of time is real and co-exist. Sorry, Monarchy, can I say something before you do?

I'm sure what you're going to say is a lot more interesting than me, but there you go. For example, Jim said, are we therefore kidding ourselves? Is our instinct about, is our innate instinct about time, something that's convenient for us or has been nurtured in us since the Babylonians? And in fact, it isn't like that at all. Well, I would say time is very convenient for us, because we see time passing. We see we get greyer, we get balder, we get fatter. You've just defined the guests on the programme.

Which one's the bald one, Jim, and which one's the bad one? But so we see in very physical ways, we see time passing. And we do, we mark the years in terms of birthdays and celebrations. So time is very important to all of us, in a physical way. And so it does have that reality. So are we talking about different sorts of reality here? The reality of you lot out there in the universe

with Einstein and post-Einstein, one of the current physicists says, another said that time does not exist at all. And the rest of us sitting here looking at clocks and trying to be punctual. I think there's always a problem with the way you interpret a mathematical description of a physical system because there are often many different mathematically equivalent ways of saying the same thing,

logically equivalent ways of saying the same thing, but when you interpret them, they come out completely differently. And so this Einstein picture of a particle moving in space and time traces out a curve, a world line he called it, in the space time diagram. If you put your pencil on a piece of paper and the horizontal direction is space and the vertical direction is time, and you wiggle your pencil to and fro in space and move it up the page in time, you draw a curve.

That curve is a mathematically equivalent description to my description of moving the pencil. But the difference is that the mathematical curve is a finished object running all the way up the time axis. It's the whole history in one go, whereas while I was drawing it, only the beginning of the curve existed. The new bit didn't exist yet.

Now, because the laws of nature in relativity predict the future uniquely in terms of the present, there is only one way that path could go, that pencil, there's only one curve it could draw under particular physical conditions, rules on how it moves. So we have two different descriptions that it's moving and it's creating its future as it moves. Or no, the future is sitting there waiting already, it's already there, we're just not seeing it. those are two different physical statements of the same mathematics. Are we still talking about mathematics in relation to what could be called diurnal reality? We are. I mean, this idea of Einstein's that all-time coexist is a useful way of solving certain problems in physics.

I don't want to push it too far. I don't believe that the past still exists out there, or the future is there waiting for us. I mean, that has several philosophical implications, such as whether we actually have free will or not. Do we just think that the future is open for us to make free choices, whereas in fact, if you could stand outside the universe, God, you would be omniscient and you would see that everything is already preordained. I don't believe the future is preordained.

I believe, because for other reasons, other laws of nature like quantum mechanics, that the future is still yet to unfold. Monica, you were... Yeah, I didn't want to go down the future line, but I wanted to take issue with what you said about the past not existing altogether. I can't remember what exactly the phrase it was you used,

but immediately it triggered something in me which said, yes, we can see the past all the time. Go out at night, look at the stars. You're looking at light from a star which has... That's travelled 60 light years, that's travelled 10 light years, that's travelled eight light minutes of the sun's shining, if it's daytime. So you are actually seeing lots of different pasts all in one go

when you look at the night sky. You see a supernova which exploded 1900 years ago, and you see the light you see it looks as if the star has exploded tonight. But it didn't it? It happened 2,000 years ago. Does this translate into the way we think and constitute our lives here. I'm just trying to get a relationship between you out there in space and not sitting here inside, trying to work it out.

Is there a relationship, or are you on this huge mathematical adventure which occasionally find ourselves in the slipstream? Come back to the point I was making about the pencil moving up the paper. Now think of that as an analogy for a human consciousness moving through the person's life. are we the moving point of the pencil which is leaving a trace behind this is what we did during our lives

in our past as we grew up and in our brains is the memory of that and the future is still to unfold that pencil point is going to keep moving and we find out where it goes as the life happens is it like that or is that curve

already drawn and we are just travelling along it and as we travel along it, we find out where it goes. These feel very different to us, and the second one feels rather horrible. We have no free will. It's all preordained. It's all

the will of the great deity. What will happen will happen. And yet, the description in terms of the mathematics and the physics, is essentially the same. Can I just plug away? I know it's probably boring for you guys, and then we'll move on, I promise you. I think that most people think in

all three tenses, all the time, the past, the present and the future. It's all going on. You look up, you read, great works of literature, you just think about your own thinking. That's what's going on. In my head, in your three heads, I bet everybody listen, there's a bit of the past, there's a bit of the present,

there's a bit of the future, the future speculative, but a lot of it is worked out already and so and so and so forth. So in that sense, does that correspond with what you're finding out, about the physics of time out there? Which one are you going to have a go to? I think that's an

an illusion that our, you know, we have our memories. We have, you know, we look and read a book or we look out into the night sky and we see what happened in the past and it's impacting on our consciousness now. So we're seeing the past now. I think in physics, when you look at the equations of physics describing how things work, there's time in there. And if you want to know what's happening, you have to plug in a number of value for time. That's the now.

and then you crank the handle of these equations to work out what will be happening at another specific moment in time in the future and the equation will tell you what's going to happen if it's deterministic but you can't predict what the future is so I think it's a lot of it is an illusion Can we come to the idea of the arrow of time

Ian Stewart I was coined by Arthur Eddington in 1927? What's all that about? We have this very clear feeling we've been discussing it throughout the programme that we move in a particular direction through time. We start as little babies, we grow bigger, we grow older, we grow fatter, we go boreder, we grow grayer. And we do not, on the whole, find people who start fully developed as adults

and get smaller and smaller and smaller until they disappear. So time seems to us to move in one direction. And that all sounds entirely reasonable. Why should it be otherwise? You know, it goes from the past to the future. until you start looking at these mathematical equations for the physics. And for example, if you take the equation for a ticking pendulum, left, then right, then left, then right.

Well, if I looked at that at a slightly different moment, it would be right, then left, then right, then left. It's actually running backwards. So the pendulum ticking and the pendulum ticking, if you made a movie of a pendulum ticking and ran it backwards, it would look like a pendulum ticking. And that exemplifies a feature of the physics, which is if you change the sign of time from plus to minus,

run the time backwards in the equations, solutions of the equations turn into valid solutions of the equations. But for many systems, the classic one is boiling an egg. We boil an egg and it runs very sensibly and you start with an egg and you end up with a boiled egg. We can't unboil an egg by putting it into boiling water and then turning off the gas and letting it all cool down. So something funny is happening to the time reverse.

Monica. Yes. The mathematical equations might change if you just change the sign. But when you look at the physical reality, which you were talking about with the boiled egg, it does seem to all go in one direction. I mean, if we look on the really grand canvas and start with the big bang when space and time and matter were produced, eventually hydrogen atoms were produced, then stars which started to burn the hydrogen

to other elements. Then the stars exploded and produced other elements and those other elements then get wrapped up in rocks and dust and then they change and so there is a huge life cycle of stars

constantly converting all the hydrogen that started out into heavier and heavier elements and the stars change and some of them explode and some of them just shrivel and die. So eventually all the stars in our galaxy are going to either explode or burn out and shrivel.

And that'll happen to every galaxy. The galaxies are moving further and further out. Everything is eventually going to get colder and colder and colder. And you can't conceive of a universe in which suddenly everything will start again under the conditions that we know about at the moment. And this sounds so logical and reasonable that you'd be surprised to know that physicists, have struggled over it, even in recent years. Stephen Hawking, as recently in the mid-1980s, wrote a paper about whether the direction of time will flip over.

I mean, what we're describing here is what's called the Second Law of Thermodynamics, that time goes in one direction, which is in the direction of things becoming more disordered, more decayed, run down, older and so on. But there are other ways of defining a direction in time. One is what we call the psychological era of time, which is our own perception. of remembering our past and not knowing, anticipating our future. But that tends to run in the same directions, this thermodynamic arrow conveniently for us, so we think that everything's consistent.

But physicists, cosmologists have also wondered whether, for instance, we know the universe is getting bigger. We know now we're almost sure that it'll never reach a maximum size and then recalapse on itself because of the speed that it's actually, things are flying apart. But not too long ago, we didn't know this. for sure. And so it was suggested that when the universe reaches maximum

expansion and re-collapses again back down to a big bang, does everything then reverse in time? And will we, in fact, if everything's flipped over in time, will even our own arrows and our heads flip over? And so we'd still think the universe is getting

bigger, even though it's fit. It's actually not a silly concept to try and think about. Monica wants to come, and then, Ian, I'd like you to answer Jim's question. Right. I was just going to say, that what controls whether the universe is going to keep expanding forever or whether it'll stop and suddenly contract is something called the cosmological constant,

which was defined by Einstein, and he thought it was a mistake. He just shoved it in there to make his equations a balance, and we now know that it's a real thing. We call it dark energy. He was clever. What about this idea that there is no time, which is what a British physicist, still extant,

is propagating? Now what about that? If you look, on the big scale, time seems pretty much to work the way we think it does give or take some relativistic messing around and so forth. but if you look down on the quantum level, very tiny particles, it starts to become very questionable whether on that level of description, time exists as a recognisable concept that works for a fundamental particle

the way it works for the whole universe. One of the analogies here is the temperature of a gas. A gas is a lot of molecules moving around. If you look at an individual molecule, it's just moving around. It doesn't know what temperature it should be. But if you average the amount of movement, if they're moving very fast and bouncing off each other, then the temperature is high. And if they're moving very slowly, the temperature is low.

So some physicists would say there that temperature is not a concept that actually means anything. Others would say, look, it's a system-level concept, but it's not something that works on the level of the individual molecules. Now there's a feeling that time works like temperature for fundamental particles, that if you write down, there are mathematical models of particle interactions which don't have time built into them, but when you let a lot of particles interact

and see what the resulting universe does, it develops its own kind of time. So time could be an emergent property. Finally, we scarcely mentioned Newton, which is a great shame, but Newton for centuries dominated thought in physics and directed thought in physics. Then along came Einstein. He didn't displace Newton.

He added to Newton and so and so forth. Do you think it's possible that there will be as great a shift by another group of people, movement in physics and so on? Or have we got there? I think not only is it possible, but it's absolutely necessary. We are stuck with these ideas in physics, and there are problems such as the nature of time, which we haven't sorted out yet.

And there are various candidate ideas around, but we still don't know yet which one of them is going to do it. Or maybe a new Einstein or Newton is going to come along and then we'll realise how silly we've all been. Monica? One of the things is a string theory and the 11 dimensions. I mean, we've talked about time as a fourth, but there are other theories which have many more dimensions,

which I haven't a clue what it means. I don't understand it. Well, we'll have to wait until another year. Thank you very much, Jim Alcalili, Monica Grady, Ian Stewart. Next week, it's Christmas Day. We won't be there then. We'll be back on New Year's Day with the program about Boethius and the Consolation of Philosophy. So thanks for listening and Merry Christmas.

We hope you've enjoyed this Radio 4 podcast. You can find hundreds of other programmes about history, science and philosophy at BBC.com.com. com.com.com.