Instant Genius - The nature of time, with Colin Stuart

Episode Date: September 19, 2021

Astronomy author and speaker Colin Stuart explains why time has an arrow, its intimate relationship with space, and why it's impossible to go back in time and kill Hitler. Once you’ve mastered the basics with Instant Genius, dive deeper with Instant Genius Extra, where you’ll find longer, richer discussions about the most exciting ideas in the world of science and technology. Only available on Apple Podcasts. Produced by the team behind BBC Science Focus Magazine. Visit our website: sciencefocus.com Hosted on Acast. See acast.com/privacy for more information. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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I'm Jason Goodyear, commissioning editor at BBC Science Focus magazine. In this episode, I talked to Colin Stewart, an astronomy writer and speaker about his new book, Time, Ten Things You Should Know. Your new book, Time, Ten Things You Should Know. you've written a whole book all about time. And I think it's not something that usually people think about very much. It's something that we experience and we think that we innately understand. But there's a lot more going on than just one increment, one unit of time, one second, say,

simply following the other. I mean, obviously, you cover a lot of ground in the book, but as a way of starting, one thing that really interests me when thinking about time on an analytical level is it's ability to connect us to timescales that are beyond the realm of human experience. So in the book, you start saying about how you can age things. So I thought we could start as you're from an astronomy cosmology background. A lot of people ask, how do we know, how do we go about calculating the age of something so old like the earth? Well, so the great news is that we have clocks littered around that we've worked out how to read. And so one of the chapters in the

is rocks or clocks, because inside many of the oldest rocks on the earth are slowly ticking clocks in the form of radioactivity. So a radioactive atom is one that's unstable, and over time it will start to decay and change into something else. But it does that in a really reliable and consistent way. So if we can count the number of changes that have taken place, we can then work back in time to see how long ago that that process started. So for living things, we use carbon, a rare form of carbon called carbon 14. But for really, really old things like the Earth, we're using uranium, which has a half-life of about four and a half billion years, which turns out to be about the same age as the Earth. So that's, it's one thing calculating

the age of the Earth. That's impressive enough. But how about the age of other objects in, in the solar system or even the universe? So how do we go about studying and calculating their ages? So for the solar system, it's kind of related. So we use a process called crater counting. And it's basically the idea that if a surface has loads of craters, it's old because it's been hit many times, it's been around for a long time. If it doesn't have many craters, then it's fresh and new. And we can calibrate the number of craters with the moon rocks that were brought back

by the Apollo astronauts. So we use the dating from the moon rocks, match that to the number of craters on the moon. and then if we see a surface somewhere else in the solar system, maybe on another moon of another planet, and it has a similar number of craters, we can say, well, it's a similar age to the moon, and so we can calibrate it that way. But once we move beyond the solar system,

and if we're talking about stars themselves, then we need yet another method. And this is the idea that when you break up light from the sun or another star, you get the rainbow, right? your famous spectrum of colors from red to violet. But what you notice, if you look carefully, is that there are different colors missing. You'll see black lines instead of certain colors.

And if you look at this spectrum, it looks a lot like a barcode. And really, in effect, that's what it is. By scanning this spectrum, we can see what a star is made of, because each of those black lines corresponds to a different chemical element. So we know, for example, that the sun is made of 67 different chemical elements. Whereas the oldest stars in the universe, the first ones to ever shine, they were only made of hydrogen and helium, the elements that the Big Bang made. So we can age a star by how pristine it is. If it's a really old star, it's only going to be made of what was available just after the Big Bang, hydrogen and helium.

If it's a much newer star like the sun, then it was born in the time when the universe was a lot more diverse and it had many more chemicals flying around. So taking the inventory of a star tells us its age, the more things it has in it, the newer it is. So just to sort of further explain these barcodes, these black lines, so that's due to different elements having different internal structures and absorbing different energies or wavelengths of light, right? Yeah, so each element has its own unique configuration of electrons whizzing around inside. And that governs the way that light gets swallowed. So each particular element swallows a very specific colour of light based on where its electrons are. And so if we see a particular colour missing, we know, okay, that must be because of hydrogen or helium or when you get to the sun, iron and oxygen and calcium and that sort of stuff.

So each of these, each element has its own sort of fingerprint in this way. Exactly. And so the more fingerprints you see from more elements the younger the star must be. And so you say about the different stars having different chemical compositions, so with the sun having heavier metals as opposed to the, well, the older stars. So why is that? So it's that over time the stars churn hydrogen and helium into those heavier elements. So the first stars would have been made of only hydrogen and helium. But over the first stars would have been made of only hydrogen and helium. But over time, the nuclear process is going on in the core, fusion, starts to turn the helium into carbon and then nitrogen and then oxygen. And then as the star ages and dies, it makes neon and silicon and iron.

And then when the stars explode, they spread those elements out throughout the universe. And they end up inside new stars. So when the sun was made, it already had some of these heavy elements, like laced inside it from the dead stars that had gone before it. So stars are essentially the sort of chemical factories of the universe. If we had no stars, yeah, the universe would be hydrogen and helium alone. So sort of taking a different tack now. We're talking about stars, we're talking about astronomy. So one thing that I find quite poetic is when we're looking, even with their own eyes into space,

we're actually looking back in time in an odd way. Could you explain that concept for me, please? Yeah, sure. It's just to do with the fact that the speed of light is fixed. It takes time for light to get from one place to another. And light is how we see things. So we don't even have to talk about planets or stars to start with. If we just talk about our everyday lives,

if we're in the same room and looking at each other, it takes about a billionth of a second for the light to get from your face to my eyes. So I'm not seeing you as you are when the light hits my eyes. I'm seeing you as you were when the light left your face. which was a billionth of a second ago. So there is never a now. Whenever we see something,

we are always seeing it in the past because it takes time for light to get to us. And my favorite analogy for it, and the one I use in the book, is it's like a postcard. I mean, we don't really get postcards anymore, right? We have email, but you imagine a postcard drops on your doormat.

It never tells you what the writer is doing when it arrives on your dormant. It always tells you what the writer was doing a couple of days ago when they wrote the postcard. It takes time for the message to get to you. And so it's the same with light. Your face is a billionth of a second ago. The moon is just over one second ago.

The sun eight minutes ago. And when you start getting out to the stars, you're talking hundreds of years. And then when you get to galaxies, you're talking millions and billions of years ago. So it always takes time for light to get to us. And we're never seeing the present. We're always seeing the past.

I think that's a really a point worth sort of hammering in home that actually the light from the sun takes eight minutes to reach the earth. And if you put that into context and you think about how distant stars are, as you said, that's really something to think about. It's really mind-blowing, in my opinion. It is. Although with the sun, there's an extra thing to add in, is that the light from the sun is made in the core of the sun.

So yes, it takes eight minutes to get from the surface of the sun. the sun to the earth. But it actually takes 170,000 years to get to the surface from the core. So weirdly, you could have a star that is 100,000 light years away, and the light from that star would get to the earth quicker than it would from the core of the sun. Because in empty space, you've got the, you know, it's empty, you can stream across, you've got nothing to bump into. inside the sun is so chaotic that that it takes 170,000 years to reach the surface and then yes, eight minutes to reach the earth.

So we're talking about things that we can see with our own eyes now, of course, for however many hundreds of years, we've been able to look further by the use of telescopes. So what are this sort of current limits of our ability to look into deep space and into what I don't know,

what you call it, deep time? Well, both, yeah. So we normally say deep space, but you're right, it's deep time. And that's such a useful thing for astronomers, because we get to see what the universe was like, two, three, four, five billion years ago, because that light is just getting here now. So it's such a plus that we get to track the universe's history this way. I mean, if you're talking about visible light, right, the same light that our eyes can see, we can look back to a couple of hundred million years after the Big Bang.

The trouble is that if you want to look back further, the universe has been expanding ever since the Big Back, and space has been stretching. And any light in the universe has also been stretching. So any light emitted by a galaxy back in the very early days of the universe, even if it was visible like when it left the galaxy, it's been stretched so much in the in the time between that it would now be in the infrared part of the spectrum

which incidentally is exactly why the James Webb Space Telescope is going up in the next few weeks because it's that it's that infrared light that it's trying to see but it's a subtle thing but it wasn't infrared light when it left the galaxies it was visible light and it's been stretched since so that's the the so-called redshift or the Doppler effect yeah exactly as the universe stretches so does light and red light is a bit more stretched than blue light. Yeah, so essentially anything that's moving away,

because it's moving away, its wavelength is stretched. So it appears a different wavelength when it meets us, right? Yeah, and it's something we're used to. It's the same reason why if an ambulance or a police car screeches past you, as it's moving away, you hear that tail-tow's change in the pitch of the siren. And it's because the sound waves are being stretched as the ambulance drags them away from you. It's the same as the galaxy and light.

If the galaxy is running away from the Milky Way, which most are, then it also gets stretched and that makes it appear redder. So you mentioned the James Webb Telescope there. I just wanted to talk about briefly because this launch is imminent. And a lot of people call it the natural success to Hubble, because Hubble's been such a great success story. So why is it so important? Why is the James Webb telescope so important?

Why is it so important to study these infrared, this infrared light? Well, one of the things is that you always hear it called a successor to Hubble, and that annoys me a little bit because it's a very different. I'm not for you. I just mean, it's a common thing. Because it isn't really. I mean, it is in the sense that it's the next big NASA flagship telescope, but it's just such a different beast to Hubble.

It works in a very different way. For starters, it's huge. It's 6.5 meters in diameter, and it has to be to collect the infrared light. And also, the Earth is so hot, you know, so emitting so much of its own infrared radiation, that we have to go almost five times further than the moon with this telescope to get to somewhere that's cold enough. But then you are able to look back into the very early universe. And it's because we want to know when the first stars lit up,

and we want to know when the first galaxies appeared. because of that story of it being a circle of life, those first stars and galaxies were a crucial step to get to us. We want to know how that happened and how quickly that happened. But it also allows us to do other things. For example, those dark lines we were talking about earlier that tell us what a star is made of. We can use the same technique to work out what a planet's atmosphere is made of too.

Except some of those absorption lines, some of those dark bands, they don't happen in the visible. They only happen in the infrared. And so to work out whether, so water, for example, is a great absorber of infrared radiation. So if we look at an exoplanet,

for example, an alien planet, the atmosphere, we can use James Webb to look, look for those crucial chemicals too like water. So sort of going back to what I originally said about how, as human, human beings, as, you know, living entities, we have a sort of innate understanding of time.

And one that we take for granted, and one thing that's really essential to that is the fact that time only runs forwards. So at first glance, that seems like quite a simple thing, but actually there's quite a lot going on there, isn't there? There is. It doesn't have to run forwards. So, for example, if you were to watch a film of a ball, tennis ball or something flying through the air,

I think you'd really struggle to know whether you were watching it forwards or backwards. There's nothing in that that tells you whether it was thrown from left to right or right to left. It looks the same both ways. And in fact, most of the laws of physics are time symmetric. They don't really care whether time is running arbitrarily what we say forwards or backwards.

They work the same way. Except there is a situation where you really know something's wrong, and that's if you watch something break. let's say you watch a mug drop on the floor and it cracks into many pieces. If you were shown that backwards, you'd know that you were watching it backwards. You know, broken mugs don't suddenly reform. So we say that there's an arrow of time. And it was actually an astronomer, Arthur Eddington,

who came up with this idea or this phrase, an arrow of time. So time seems to point in one direction from the past and to the future. it's such a a key part of our everyday lives but we never really stopped to think about why it runs that way and it was only maybe 200 years ago that we started to get an idea of why it seems to run

from past the future and not the other way around so the history of this is really interesting for me so this idea originally stems from perhaps if you don't you haven't studied physics and you don't know this from thermodynamics So yeah, thermodynamics is, I've got to say, if someone who studied physics, a pretty dull part of physics, but it is the secret to why time runs in the way that it does.

It is that things just get more disordered over time. So that's the second law of thermodynamics. It says that entropy always increases. Entropy means messiness or disorder. So if you were to leave your garden alone for a couple of years, it's going to get messier. If you don't tidy up at home, you're in your kitchen and your bedroom alone, it's going to get messier. It doesn't have to, though. So let's imagine a box in the room that you're in, and this box measures a meter on each side.

In that box, there are trillions and trillions of air molecules. Now, they could, and as they move around, they could all gather, let's say, in one half of the box. And that would have made them more ordered. They would have got more organized than they were. before. But there are so many other ways that they could be disordered, that they can move and become more erratic, that it's just far more likely that they do. So in the book, I talk about some examples, actually. One is just a set of six dice. Let's say you set the dice up so that you have the same number showing on each of them. And then you roll those dice. I mean, they could

go back to the way that they were. They could stick in that highly ordered, organized. way, but the chances are they're going to get more disordered. And it's the same with the pack of new playing cards. When you get them, they're all lovely and organized into suit and numerical order. If you shuffle them, there's no way you're going to get them back the way that they were. They're always going to get more disordered and disorganized. And so it's the same with the universe. We always see order deteriorate.

And that's what gives us this arrow that points from the past. where things were more organized to the future where things are less organized. Now, I have a three-year-old, and we're getting a lot of questions right now. It's like, why, why, why, why, why? And to be honest, it's surprising how often the answer is the second law of thermodynamics. You know, why does that break, second-law of thermodynamics? Why does this happen, second-law of third dynamics? So I say it in jest, right?

But you'd be surprised that from the answer is actually the second-law of thermodynamics. That's pretty heady stuff for a three-year-old, I reckon. But like one thing that whenever I've spoken to people about entropy and as you say, disorder just naturally increases, that people will often say to me, okay, so as an example, we're playing a game with snooker. We finished the game and then it's obviously completely disordered from the original arrangement. The balls are all in different pockets. So when I re-rack the table and put the balls back into their original position, why doesn't that decrease the entropy?

It decreases the entropy of the snooker balls. You've made the snooker balls more organized, but it doesn't decrease the entropy of the entire universe because in order to do that, you've got to put in some effort. You've got to pick the balls out the pockets. You've got to, you might lose some energy by knocking the balls together so that they make a sound. So overall, it takes energy to make that a more ordered system.

And so the entropy of the entire system, you, the room, the snookable, the universe, still, the entropy still increases. And so overall, that you maintain that sacred law of thermodynamics. It always takes effort to undo what's happened. So obviously we're talking about the overriding theme of time here. So if we're speaking about entropy or disorder, always increasing, what does this mean for the lifespan of the entire universe? Well, eventually, they'll come a time where there is no more order. We'll reach a point of maximum disorder. And then you can't borrow energy from anywhere in order to put the snookable

back the way they were. And so you won't be able to organise anything into anything. We call it the heat death of the universe. And so the universe will just become a cold and static and unchanging heat. I mean, the good news is this is a very long way away. We're talking trillions and trillions and trillions of years, if that's the only thing at play, which you might not be. So sort of looking at that from kind of the reverse point then.

So you say, as you said, the universe is expanding. That's why we get the red shift and the entropy is increasing. If we take that back to the beginning of the universe, what can we learn about, you know, I guess I want to say the big bang and the beginning of the universe, the beginning of time and the state of entropy and the state of the universe then? Well, the fact that we still haven't reached

the heat death of the universe yet, and we're not even close to reaching it, tells us that the entropy at the beginning of the universe just after the Big Bang must have been very high. So it must have been very ordered at the beginning, and it's been deteriorating ever since, but we still haven't reached the point of maximum disorder. So we don't know why the Big Bang

and the early universe appeared to be in this incredibly highly ordered state. It's one of the biggest mysteries in cosmology, and it stood for at least 50 years. And so if we could answer that, I think I said in the book that we wouldn't just tell us why time has an arrow. It might even tell us why we're here to experience time at all. Because if it started off, the universe, very disordered,

it might have reached that heat death before any stars, galaxies, people, life could have ever emerged. It took us a long time. The Earth didn't appear until the universe was about, what, nine billion years old. We didn't appear until a couple of hundred thousand years ago. So it took a long time to get to us

and it might not have ever had the chance if the Big Bang hadn't, for whatever reason, spawned a highly ordered universe. So in the book, you have some really good analogies about the time scale of the existence of the universe of the earth

and early humans, homo species. Could you just let our listeners know that, please? Yeah, well, we knock around these numbers, all the time. We've said that's four and a half billion years old. The universe is 13.8 billion years old. And it's quite casual to do that without actually thinking about what that's a huge amount of time. So some of the analogies were that if you had, this is it for the age of the earth, if you reduced each year to a second. So rather than having four and a half billion years,

you had four and a half billion seconds, that's still 144 years. So you'd have to live two lifetimes, pretty much, in order to get through as many seconds as there have been years in the Earth's history. If you were to collect a penny for each year of Earth's history, then you'd be a millionaire, as you may expect, but also your pile of money, your pile of metal would weigh more than the Eiffel Tower. But then there are other things that were in the book in the end, but show you how we struggle with time. Now, the idea that the Tyrannosaurus Rex live nearer in time to the iPhone than it did to the Stegosaurus, or the fact that Cleopatra lived closer in time to the moon landings than to the Great Pyramids. Once you go back, you know, not so far into the past,

it all kind of begins to blur into one. And we forget that the different species of dinosaurs lived hundreds of millions of years apart from each other. Yeah, it all goes back to sort of how we started with the sort of innate human conception of time and its passage, which I think is really interesting. Well, I guess it comes from the way our brains have evolved, right? It doesn't really confer you much evolutionary advantage to contemplate four and a half billion years. It's useful, it's fun, right? But it's not going to be getting eaten by a tiger or, you know, or finding a mate or finding food. If you can contemplate the changing of the seasons with the moon and the phases of the moon and start to have a calendar,

and you know when to plant and you know when to harvest. And that's a lot more useful. So thinking in days and weeks and months and years probably is a useful thing to do, which is why we do it reasonably easily. Talking in terms of millennia and billions of years, our brains are, you know, our kind of primitive monkey brains weren't really built for that.

So it's still possible, but it just takes a bit of extra contemplation, I think, doesn't come as easily. So as we're talking about time, I think what most people would, one of the, I think, top five questions would be, what about travelling back in time? It's such a sort of sci-fi trope. It's captured so many people's imagination. But before we start to talk about slightly more complicated physics, I'd just like to have a look at the, something I find really fascinating, which is the time travel paradoxes. Yeah, this is the really fun stuff. This is the stuff where you start to sort of really question your sanity after a while.

If you think it through, it does kind of make sense. There are several that you mentioned, you know, several famous ones that you mentioned in the book. But there are some that always come up, which probably chief among is the grandfather paradox. So can you tell us a bit about that? Yeah, that's definitely the classic one. That's your back to the future, Martin McFly at the prom idea. So it's that. If you go back in time and kill your grandfather when he was a young man,

then he never grows up to have whichever parent it was that he had. And so that parent never grows up to have you. If your grandfather dies as a 10-year-old, you could never have been born. And so if you could never have been born, who is standing there with a gun shooting him in the first place? So there's a logical inconsistency there. There's something has to go awry. if you are standing there with a gun at your grandfather's head, you cannot kill him. It's just a logical impossibility that he is going to die because as soon as he dies,

you never exist. So something has to go wrong. Maybe you try and fire the gun and you wound him instead of killing him, or maybe you miss. It's called the Novakov Consistency Principle, right? The thing that causes the paradox to happen necessarily cannot happen for you to be there. And the kind of related version to that is the famous Hitler assassination, which is another example of the grandfather paradox. There was a New York Times magazine poll that asked people, would you go back in time and kill a baby Hitler if you could? And the majority people said they would. But you can't. Because if you go back in time and kill Hitler again, let's say when he's 10, he never grows up to be the leader of the Nazis. And so he never does the awful things that he's known

for. So you never learn about him at school. And so you can never make the decision to go back and kill him because how do you even know about him in the first place? He's this like Austrian boy that has no significance to anyone. So again, you can't kill Hitler because if he did, your reason for being there vanishes and so you can't be there. So that's the other sort of version of the grandfather. So I'm sure they were well meaning these people that said they go back in time and kill Hitler. Unfortunately, unfortunately, you can't do it. Thank you for listening to this episode of Instant Genius. That was Astronomy writer and speaker, Colin Stewart.

If you wanted to know more about the nature of time, check out his book. Time, 10 Things You Should Know. Or, if you liked what you heard, check out his online beginner's astrophysics course at Colinstirot.net. Or, to hear him tell me more about space time, black holes and wormholes, head over to the Instant Genius Extra podcast. The September issue of BBC Science Focus magazine is out now. Pick up a copy in store or visit ScienceFocus.com.

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