Instant Genius - Space & Time – Everything You Wanted To Know About…Physics, episode two

Episode Date: April 25, 2020

Prof Jim Al-Khalili helps us get to grips with the big concepts in cosmology. We talk space time, relativity and, of course, the end of the Universe. 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 Dan Bennett, the magazine's editor, and today we're back, in Google's most popular search queries about physics with Professor Jim Al-Killian. In this episode, we're going to dive into some big cosmological concepts. We're talking space and time, the end of the universe, and Jim's going to explain why the Earth's core is actually two and a half years younger than its surface. So, Jim, today we're going to talk about the big stuff, space and time. Now, before we do that, I suppose we should probably talk about the R word. Could you explain to me what is relativity?

Okay, so I teach a course on relativity to my first year physics undergraduates, and it's 33 hours worth of lectures. So I'll give you the summary view here. So Einstein had two theories of relativity. His special theory in 1905 was the one that, so that's the one that gave us E equals MC squared, the most famous equation in physics, but in fact that equation isn't the most important thing about his special theory. What is important is that he showed that we can't talk about space and time separately.

You know, space is where stuff happens, you know, three dimensions, our space, we understand what that means. And we tend to think of time as just ticking by inexorably external. nothing we can do to change it. You know, we can have different time zones around the world and we can move an hour forward for British summertime, but that's not messing with time. That's just the way we, you know, where we fix our clocks.

But Einstein said time and space have to be combined together into four-dimensional space time. Otherwise, we can't understand, you know, relate events. Because without going to too much history, the idea was that, you know, Even Galileo and Newton understood that, for example, all motion is relative. So, you know, if I'm, you're moving relative to me, you can equally well say, no, I'm standing still and you, Jim, are the one who's moving. And all laws of physics, it turns out, you know, are independent of whether you're moving or not.

So I always give the example, when you're on board a plane and the air steward is pouring you a coffee, they don't start tipping the jug at the front of the plane when you're, sitting at the back, and by the time the coffee lands into your cup, the plane has moved forward enough for it to catch it. Everything happens on board the plane that's moving in the same way that it would if you were on the ground and not moving. Everything is, all motions relative. So this led Einstein to the idea that time and space, for reasons that are not obvious, that time and space has to be unified. The thing that he brought new to the story, beyond what Galileo and Newton said was that everything is relative or motion is relative, but the speed of light isn't relative. So however fast we're moving relative to each other, we would both measure light to have the

same speed. If I send a beam of light out into space, I see it moving away from me at a billion kilometres an hour. I think that's, no, no, is that right? Yeah, something like that. I think I always think of it as, well, physicists will talk about three times 10 to the power 8 meters per second, but I think it's easier to put it, a billion kilometers an hour. Yeah, very fast. I see it moving away at that speed, speed of light. Now, if you were to jump in a rocket and fly off to try and catch up with the light beam, let's say you're travelling at 99% speed of light, you would still see it moving past

your rocket window at the same speed that I see it moving away from me, which is a real, you know, it's mind bending. So that led Einstein to the idea that time and space are unified into four-dimensional space, time and from that all sorts of things follow. Time slows down when you go very fast, lengths contract, and then it leads on to sort of new equations for momentum and energy and so on. So that's the sort of stuff that you teach. It's a lot, it goes a lot further than E equals C squared. Brilliant. And you mentioned it there, the idea of space time, which I guess to the layperson is quite different to how we think about space and time. So can you tell me,

What is space time? Right. So we know when something happens, an event we call in physics, we need four numbers to define it. It's coordinates. We need the X, Y and Z coordinates to locate it in space, but it also need to say when it happened. An event, you know, you have to have the where and the when to fix it.

So in a sense, time adds another dimension anyway. But Einstein said, no, time is really like another axis. time really is a fourth dimension. And space time in Einstein's special theory was, you know, where stuff happens. Now, that isn't sexy. It isn't so interesting until you hit Einstein's second theory, his general theory of relativity, which he developed 10 years later in 1915. And there, he said, space time itself is affected by the matter

and energy that exists within it in a really, really profound way. So his general theory says that matter and energy curves space time around it, warps space and time. And space and time, when it's warped, it has an effect on the stuff that's within it. And that gave Einstein a new picture of what gravity is, the gravitational field. So now, today, when we talk about what is space time, we say it's the shape. of the gravitational field. It's, you know, if there was no, in a sense, if there was no matter and energy in the universe,

we wouldn't have empty space time. We wouldn't have space time at all because space time is defined by matter and energy, and matter and energy is defined by space time. Really, really profound ideas, which I'm not doing justice to in just a few remarks. No, I mean, you're doing a great job for just to explain it in a few seconds. So within that context, you just explain how space, time and matter are, you know, inseparably linked, and space time effect, in effect, creates or generates gravity. Can you just talk about, so even harder question, how does space time create gravity? Well, there's a very famous book, but probably not so well known, by Einstein himself, called Relativity, the Special and General Theory.

He first published it in German, around about the time he came up with a general theory. So in 1916, it got translated into English and it's had many editions. And he would keep adding appendices to this book. It's a tiniest book. It's almost like a popular science book. The last appendix he added, Appendix 5, he added a year before he died in 1954. And in it, he gives the most beautiful explanation of what gravity is. or how gravity affects space time.

And he says basically there's no such thing as empty space time with no matter and energy. He says space time does not claim existence on its own, but only as a structural quality of the gravitational field. Basically what he's saying is space time is a thing, right? It's stuff, as it were. and it is the gravitational field. So the idea that far from any matter galaxies or stars, I think, out in deep, empty space, we say that's empty flat space time. No, space time wouldn't exist at all, were it not for matter and energy.

So the matter and energy creates a gravitational field, which is itself synonymous with space and time itself. Space and time is the essence of our universe. It's the fabric of the universe. and you can't disentangle it from the stuff that the universe contains. It's one and the same thing. Perfect. Now, we talked about relativity, and we talked about how the speed of light is universal,

but everything else is subject to relativity. Sorry, have I put that the right way around? Yes, the speed of light is, yes, that's right, yeah. So everything is subject. Okay, so relativity makes some strange predictions. And in your book, The World According to Physics, you picked out one that I really liked. You explained that the Earth's core is actually younger than the surface. So how is that so?

Yeah. So one of the predictions of Einstein's general theory, the second one, is that because gravity, gravitational field is space time, gravity, we see it as changing space. and changing time. In fact, gravity slows time down. And so the stronger the gravitational field, the slower clocks will run. And the earth has its own gravitation. It's like a gravitational well. So the deeper you get into the earth, the deeper into the well you go, and the slower time will run. So at the center of the earth, clocks are ticking by, counting the seconds, the minutes, the years, the billions of years, at a slightly slower rate than clocks on the surface of the Earth.

And you can do the calculations and physicists have done this calculation. We haven't actually sent a clock to the core of the Earth, but we so trust Einstein's theory, because we've tested it so carefully that we believe this is correct, done the calculations, and it works out that over the course of the Earth's life, five or so billion years, the Earth's core, because time is running by ever slightly more slowly there on the surface, it will have aged cumulatively about, I think it's two and a half years less than the surface. Now, two and a half years out of five billion is not much, but it's still there. It's a measurable thing.

And this idea of time slowing down is more than a theory, because after all, it is the basis on which our GPS system works. The mobile phone that you use to locate your position only works because scientists and engineers have had to deliberately slow clocks down on satellites, because the clocks on their own devices would tick by slightly more quickly than clocks on the surface of the earth, because they're further away from the core of the earth, so they feel slightly weaker gravity. So they can tick by more quickly, and then they'll be out of sync with the clocks on the earth. so you wouldn't be able to use the satellite to locate your position. So this is more than theory.

This is actually part of the technology that we all use without realizing. Great. And so you mentioned that previously we have a universal speed limit, which is the speed of light. How do we know this? Well, it's not obvious. And Einstein's special theory of relativity, I mean, you have to take. take it in sort of logical stages because if you say, well, you know, 300 million meters per second, well, that's a number. Surely just because light travels at that speed, maybe you physicists just haven't

been imaginative enough to discover something that can go a bit faster. What's wrong with 301 million meters per second? You know, it's not infinity. It's still a number. But the speed of light is special. It's not the light is special, is that this speed is special because it's woven into the fabric of space and time. Einstein unified time and space, the thing that the glue that pulls them together is the speed of light. So the speed of light is a property of our universe. It doesn't make sense to say that something could go faster than it. In fact, when I teach my physicists, my students this, I give them lots of examples. One example is that the faster you've got to make something go, the more energy you've got to put into it. And the closer, so it's like a train.

You're trying to make a train go to the speed of light and exceed the speed of light. What happens in relativity is as masses get close to the speed of light, they become more massive. They become heavier. So it's like adding another carriage to the train. Every time you want to make it go faster, you've got to add another carriage, making it even harder to accelerate it further. And the train will become infinitely long, infinitely massive at the speed of light. So basically all the energy you're putting in to make this train go faster is being converted into its mass. That's your E equals MC squared, rather than being used to make it go faster.

So there are lots of examples like that to show how the speed of light is the universe. There's another example I give from Star Trek where I say, actually, if you did have something going faster than light, then according to some observers, it would appear as though it was travelling backwards in time. And I give a very nice paradox to make, to show how ridiculous it will be. So it's logically impossible to go faster than light if I were to explain that example. example in full, which I'm not going to do here. Great. Fantastic. So that brings us on quite nicely to the next thing, which is the idea that the universe

is expanding all the time. How do we know this? Well, it was first predicted, again, by Einstein's general theory of relativity. In fact, it wasn't Einstein who first discovered it, but other physicists, cosmologists who were looking at the maths, realized that this suggested the universe could be expanding and therefore must have been starting from a point when everything was all stuck together, very close together and high density. But it wasn't until the late 1920s when the American astronomer Edwin Hubble actually saw the universe expanding.

He was looking at distant galaxies way beyond the Milky Way, and he saw that the lights coming from those galaxies was redshifted, Doppler shifted. So in the same way that a car, very fast car that goes past you, as it goes past you and recedes into the distance, the pitch of the engine drops. So that drop in pitch is because the waves of sound waves from the car are being stretched as it moves away from you. The same thing happens with light,

and these distant galaxies, the light coming from them is stretched because they're moving away. So he sees the light move towards the red end of the visible spectrum, which means it's getting longer and longer wavelength, and that allowed him to figure out that in fact that's because these galaxies are moving away from us and in every direction we look in space things are moving away from us and that's not because we're at the middle of the galaxy everything at the middle of the universe i beg your pardon everything is moving from everything away from everything else because the space in between the galaxies itself

that that is what is stretching it's not objects moving through the universe through space they're sitting still in their part of space it's the the the bit in the middle between us and distant galaxy that is actually stretching. And we have ample evidence that this is actually happening now. And so do we know, essentially, earlier you explained about how space time is one thing, does that also mean that time is expanding? No. I mean, this is quite a subtle point, in fact, because yeah, exactly, you'd think, hang on a minute, we've just unified space and time. So when you say the universe is expanding, then surely space time is expanding. In fact, no. You can actually play around with the mathematics in Einstein's equations.

And what you see, in fact, the expansion of the universe is the expansion of space alone. So it's almost as though we're having to disentangle space and time again, which we're told can't be done. But, you know, you mess around with the equation. So the three-dimensional space is expanding, but time itself doesn't. So over time, space gets bigger. Okay. And then this one was a great one on Google.

This had lots of search volume behind it. What is the universe expanding into? Yeah, this is the question I get asked most often when I give talks on this subject. The trick is not to think of the universe expanding into something, because that suggests there is something outside that would then presumably, just still be part of our universe. You know, we don't have an edge to space. If space goes on forever, it goes on forever. There's nothing beyond it. The trick is to say, it's not that the universe has an edge and that edge is moving into uncharted territory, as it were. Everything in the universe

sits exactly where it is. It's not moving, or not moving much, but the space in between is stretching, like a rubber band. So, you know, you can imagine, you know, two points, draw two sort of dots on a rubber band. Those dots are not moving along the rubber band. They staying where they are, but the rubber band itself is stretching. So space is everything,

and that everything is stretching. It's not moving into anything because it contains everything by definition. And I suppose you could legitimately say, well, what's the thing that you, universe is expanding into, expanding into, expanding into and go on forever and ever if you kind of yeah, yeah. And I should say there are, there are speculative ideas on what's called multiverse theory, the idea that our universe is a bubble universe within some higher dimension. And in that

scenario, you can sort of think of our space expanding into something else, something called the inflaton field. But again, that was something. It's a theory called eternal inflation theory, but it's speculative, but at least it sort of gives an answer that our universe is a bubble that's getting bigger within some higher dimensional multiverse. Okay. And just, I mean, we're going to touch on this a little bit later, so I don't want to get too into it. But I think it's probably, we'd be remiss if I didn't ask, what do we think is driving this expansion? Well, until about 20 or so years ago, the idea was that the only thing driving the expansion was the Big Bang itself, what we call the initial conditions of the universe, whatever it was that caused the universe to be the way it was at the start, that initial energy pushed everything apart and caused space to expand. And we therefore assume that all the matter and energy in the universe, because it exerts a gravitational attraction on every thing.

else, it should be slowing the expansion down, putting the brakes on the expansion. But back in the late 90s, it was discovered that in fact there's something else, which we now call dark energy, which is driving this expansion ever more rapidly. So it wasn't just the big bang that got things going in the first place. There's something else that's giving it a helping hand now that's starting to win against the attractive pull of gravity. So dark energy is almost like anti-gravity. It's pushing things apart at the largest length scales. So in answer, two things are driving expansion. The big bang, the initial big bang, and now, as we know it, dark energy. And this was, this was surprise me. This was quite popular on Google. How do we know how old the universe really is? And how are we certain?

Well, yes, we're sort of narrowing it down, making it ever more accurate, 13.82 billion years, give or take, is the current estimate. I mean, the simple answer is that we know it can't be any younger than the older stars that we know of stars in our galaxy, for example, and the older stars of order 12 billion years. So the universe itself must be older than that. but we can also put another limit on how all the universe is because of what's called the cosmic microwave background. So this is radiation in deep space. In a sense, it gives us the temperature of empty space, which is just under 3 degrees Kelvin,

3 degrees above absolute zero. So about minus 270 degrees Celsius. And the Big Bang theory predicts that. It predicts exactly that's what the temperature of the units would be if we understand what that cosmic microwave background is. And what it is is light that was released in the very early universe when atoms first formed. And we know from the Big Bang theory,

that must have happened a few hundred thousand years after the Big Bang itself. Now, a few hundred thousand years is small change by sort of the 13.8 billion years age of the universe. So for all intents and purposes, we can say the cosmic microwave radiation was released just after the Big Bang. And we can work out how old that radiation is from the temperature that it has now. Because as the universe expands, that light, that radiation is stretching as well and cooling the universe, cooling space down. So by measuring the temperature,

that can tell us how long ago atoms first formed and released this radiation. So that gives us another handle on the age of the universe. And there's lots of other pieces of evidence from astronomy, but those are probably the most obvious ones. Okay. And then this is another one, I'm sure you get quite often in your talks.

What happened before the Big Bang? Yeah, yeah. This is a very popular one. Well, the standard answer, certainly until recent years, would have been, you know, there is no before because the Big Bang marks the birth of space and time. So there was no before the Big Bang because there was no time to embed the word before in. It doesn't make sense to say before when there was no time itself. And, you know, the nice way of explaining that is saying, if you were to walk down to the South Pole and I told you, when you get to the South Pole, you know, when you get to the South Pole, keep walking south, it's meaningless, you know, because any step you take from the South Pole

will take you back north again. There is no further south than the South Pole. In that sense, there is no time earlier than the Big Bang. Now, I said that was until recently. Now cosmologists are starting to speculate about the possibility that there might have been before the Big Bang. It may be, for example, in this speculative idea called Eternal Inflation, that our, our our universe was born in our local Big Bang, but that wasn't the birth of everything. There are other parallel bubble universes with their own Big Bangs popping in and out of existence, but all these universes sit within this multiverse. So before our universe burst into existence, there was still something else. There was still something that we can call

existence or reality. So maybe there was it before the Big Bang. The difficulty is how do you test an idea like that? Because how do you do an experiment to test the existence of the multiverse? Quite. And then that brings us nicely, well, we fast forward a little bit. We know back in time, what's going backwards, we have a very good picture of how it all came to be. What's our understanding like of the end and how certain are we of the sort of scenarios that will emerge? Well, thanks to dark energy, we sort of can narrow it down a bit because before the discovery of dark energy in the late 90s, I mentioned earlier that we would assume that gravity, the combined cumulative pull of all the matter and energy in the universe should be slowing its expansion down. And there were three possible scenarios. One was that it would, you know, it's like stretching a spring.

and at some point you can't stretch it any further. Now with the spring, you know what happens if you release it. It comes back together again. So that was one option that there was enough matter and energy in the universe that would slow the expansion of the universe down and then cause it to recalapse on itself and what would end up as what we would call the big crunch. But if there's not enough matter and energy to do that,

it could do one of two other things. It could either slow the expansion down just to a steady rate and then stay that way. It'll just drift apart steadily. Or it could slow it down so that it's what we say, what we call asymptotically, reaches zero. So it slows down slower and slower and slower. And after an infinite amount of time, it's stopped.

But it takes an infinity of time to stop expanding. So those are the scenarios and standard books in physics will have plotting a graph of how the universe, whether it carries on expanding, whether it collapses on itself or whatever. Now with dark energy, we know the expansion of space is accelerating. And so rather than slowing down, it's just getting faster and faster and faster. So now it becomes the question, is dark energy going to carry on ripping the universe apart? What happens? Does it just get bigger and bigger and bigger ever more quickly?

Is it going to start ripping even matter apart eventually, something called the big rip? we don't actually quite understand the nature of dark energy itself to be able to give a definitive answer. We do know that for the first half of the universe's life thus far, the expansion was slowing down because gravity was winning that battle. Stuff wasn't so far apart from everything else, so gravity could do its job of putting the brakes on. But halfway through, about seven billion years ago or so, the universe got so big and matter so far apart from everything else, that dark energy kicked in and started to win, and it's now causing space to expand ever more quickly. What it ends up, the most likely scenario is something called the heat death of the universe,

which will be in billions, trillions of years from now, and space is just empty and almost infinitely large, and all matter and stuff has evaporated away, and it's just thermal radiation, just photons floating around. It would be a very, very boring universe indeed. Something to look forward to you then compared to now. That's a very, an uplifting note to finish on. Okay, so one analogy I really liked in the book was about what you called the sort of Loth universe

or what physicists called the Block Universe. I thought that was a really eloquent way of explaining how, what space and time look like in a sense. Could you describe that to me? Yes, so the Block Universe, idea was actually, didn't originate with Einstein himself. It originated with one of his old university teachers, Herman Minkowski, who read Einstein's special theory of relativity, and in around about 1908 came up with this idea that if time really is the fourth dimension, then

it's just another axis. Now, it's really difficult for us to visualize and imagine what a fourth dimensional thing would look like. We know we live in three dimensions. So any solid object has three directions at right angles to each other. So, you know, you can move forwards backwards, left and right, up and down. So those three directions, each of them is at right angles to each of the other two. No other direction that you could point to in space would be at right angles to all those three, right? So it'll have to be sort of somehow a combination of components from those three directions. But time is the fourth dimension. So time has to be at right angles to all of them. And so the trick that we do in physics is to say, well, imagine space wasn't three-dimensional,

it was only two-dimensional. So imagine we live in a cardboard cut-out flat universe. We just ignore one of the axes. That means our universe is a surface, and then time becomes the line that is perpendicular to that surface. So it replaces the old space third dimension and we use that direction for time. And then you imagine the whole of space time as a block, like a loaf of bread. So the length of the loaf running from one end to the other, that's the time direction. And every slice of the loaf is space at a moment in time. So consecutive slices are different moments in time. And the beauty of this picture is that it says that what we regard as our current now moment, you know, we remember the past, we predict the future, but we only live in the current.

If you could step outside of the block universe, you would see all times coexisting frozen. So there is no past, present and future. All times are equally real. And it turns out this is a really useful technique to use when we're trying to understand some concepts in relativity theory. I think it's not a good idea to take it too literally because we don't understand the nature of time itself and how time flows and there are directions to time and so on but in relativity theory certainly the block universe is quite useful

Einstein even actually used it to console the widow of a friend of his who went at the funeral when his friend died and he tried to persuade her that somehow it wasn't so bad because the present moment in which she was grieving was really just no different from any other moment along the time axis. You know, past, present and future are all equally real. So it's equally still with us as being sort of departed. Whether that can solve her at all, I'm not so sure.

Perfect. Let's wrap it up there. Now, in the next episode, Jim is going to help me demystify the quantum world. We'll be talking about why it's so bizarre and yet so important. We'll be unraveling the discoveries that shone a light on this weird world. And Jim's going to discuss how quantum strangeness might be playing out inside our bodies. So if you've enjoyed the last two episodes and we'll be tuning into the next one, please do subscribe.

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