Instant Genius - Energy – Everything You Wanted To Know About…Physics, episode four

Episode Date: April 27, 2020

Prof Jim Al-Khalili tackles thermodynamics – the study of energy. Together, we unravel the idea of entropy, talk about the direction of time and muse upon the inevitable heat death 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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alongside French acoustic specialist focal, combine handcrafted tradition with cutting-edge innovation and high-end materials, delivering digital precision with analogue warmth, so you can experience exceptional sound at home. Music just as the artist intended. Visit nameadio.com to learn more. and welcome back to everything you wanted to know about physics,

a new kind of podcast from the team behind BBC Science Focus magazine. I'm Dan Bennett, the magazine's editor, and today we're back answering Google's most popular search queries about physics with Professor Jim out clearly. In this episode, we're talking about energy. More specifically, Jim's going to talk us through the laws of thermodynamics and explain how they provide. the era of time and what they tell us about the universe and how it's all going to end.

Surprisingly, this was quite high up on Google search ranking, actually, because it's something I obviously studied at college, but then didn't really talk about it much until I got into this job at the magazine. So, can you tell me what is thermodynamics, and why does it matter how hot things are? Yeah, thermodynamics. is sort of the poor relation of some of the other big areas of physics. So, you know, relativity

theory in cosmology, particle physics, quantum mechanics, they're sort of sexier areas. Thermodynamics doesn't sound as exciting, but in fact, I think increasingly we're realizing that it's playing a vital role in understanding,

you know, trying to unify all our phenomena and ideas and theories and physics. Thermodynamics is essentially the theory that describes heat and energy transfer and the way objects behave on this large macro scale. So it's not quantum mechanics, it's not down at the particle level. It tends not to be at the cosmological scales, but it's the everyday scales. And thermodynamics, I think, plays a very important role, particularly, potentially fundamentally, in terms of understanding the nature of time itself. Okay. And so thermodynamics has its own set of laws. Could you, this is a big ask, but could you try and explain the laws to a lay person?

Okay. So, yeah, so thermodynamics was developed in the late 19th century. People like Ludwig Boltzman, Max Planck and James Clark Maxwell helped develop it. And it's linked with another area called statistical mechanics. which we learn at school in physics in something called kinetic theory, understanding the nature of pressure and temperature in terms of particles and molecules bouncing about. But essentially thermodynamics, as I said, is about energy. And so there are four laws. The first law is one of the most important in science. It's the law of conservation of energy.

So it means that you can change energy from one form to another. But you never lose energy. It's always conserved. The total amount of energy in a system is conserved. The second law, which in a sense has sort of become even more famous. The second law of thermodynamics is something that people may have heard the phrase, even if they don't know what it is. But what it says is that systems inevitably unwind.

They wear out. run out of steam. You cannot maintain 100% efficiency. And the way we measure this wearing out, running down is through a quantity called entropy. So the second law says that in any enclosed system, entropy inevitably increases. You know, your kid's bedroom inevitably gets more untidy over time. That's entropy increasing. A tidy bedroom has low entropy, and untidy messy bedroom has high entropy, and untidy, messy bedroom has high entropy. And unless you do something to change it, so you sort of intervene to tidy up the bedroom, for example,

then you can't stop this inexorable increase in entropy. And the whole universe has an entropy associated with it, which is also increasing. The third law says that that's a bit more sort of obscure, but it says that entropy gets, less as you drop the temperature. So entropy goes down as a system gets colder and colder. And entropy goes to zero when the temperature is zero. Which is sort of an interesting, you know, when everything is slowed down, down at the tiniest level, dropping temperature means calming things down. When everything

is not moving at all at absolute zero degrees, the third law said entropy is zero. And then it was discover that there was a fourth law that should have been put in, but actually it's a law that the others rely on. So rather than call it the fourth law of thermodynamics, it became known as the zero's law of thermodynamics. And that is a really sort of basic one based on logic. And all it says is that it defines essentially the concept of temperature. It says, you know, if body A is in thermal equilibrium with body B, and it's also in thermal equilibrium, remove body C, then B and C will also be in thermal equilibrium, which is another way of saying the same temperature. So we needed the zero law to define what temperature is in order to make sense

of the other three laws. Okay. And so we talked to us about temperature, and that is, I suppose, a rough way of describing energy, precisely, I suppose. So when we talk about temperature, we don't just mean heat. And when we talk about heat, we don't just mean heat. We mean how much energy something has, is that correct? That's right. Heat is a form of energy. And you can measure heat, you know, the amount of heat, the amount of energy in different ways. One way of measuring it and the way it will sort of exchange its energy, a system will exchange

its energy through heat with another body is in terms of what its temperature is. Ultimately, temperatures down to how quickly the atoms and molecules of a body, are actually jiggling and vibrating about. The more they move about, the faster they move, the higher the temperature is. Okay, so this, these laws sort of help us to make predictions about energy. So I suppose that's a rather simple question, but actually is a lot more complicated than sounds. What is energy? When we talk about energy, there's lots of different types.

What is it exactly? Yes, and energy, it's one of those concepts, you know, a word that we all think we know the meaning of. Actually, it's quite a slippery concept in physics. We think we understand what energy is intuitively. So, you know, I might say I feel low on energy this morning if I'm tired or hungry or not feeling very well. If you're fit and well, you might feel energetic and enough to go to the day. gym. So we just mean we have that capacity to do stuff. We've got some umph. Sometimes people use energy in a very unscientific way, which is in a sense quite silly. They will use phrases like,

oh, I felt the positive energy when I entered this room. Nonsense. Or you're giving off a lot of negative energy. There is a concept called negative energy in physics, but that is not what these people meals mean. So there is really no such thing. But in physics, energy is essentially the capacity to do work. The more energy something has, the more able it is to do something, whether that doing means moving about, moving other objects around for one place to another. It might mean heating up. You know, if it's got a lot of heat, that means it's got a lot of energy. A battery has energy in the sense that it has potential to do work when you sort of connect it up to a circuit. It can do stuff, run an electronic device. So energy comes in all these different forms, energy of motion, gravitational

energy. Light is energy. And at the quantum level, down at the tiniest scale, energy really is composed of particles. So particles of energy, like the photon, the energy of the electromagnetic field. So all sorts of ways of describing energy. And then Einstein comes along and says, energy and mass are interchangeable. So mass is like frozen energy. You can convert energy into matter.

You can convert matter into energy. And we know we do this now in experiments. And so energy also has a gravitational field. You know, something with lots of energy will also have a gravitational sort of attraction to it. It can pull stuff to it like matter can pull stuff to it. So there, that's a rather sort of convoluted way of trying to get across the concept of energy. I think that was pretty good for such a massive field in a couple minutes.

And so you touched on it there and you touched it there a little bit before. So we have the key idea there of energy and then a very important concept that I think you get across really interestingly. in your book. And again, with entropy, it's quite highly searched for as well. So could you just, you know, go back and take a moment to explain the idea of entropy? So entropy, it's even more slippery a concept than energy, in fact. But so at a very basic level, I would say entropy is a measure of how disordered a system is. So I use the example of a child. A child. child's bedroom, for example, how untidy it is as a measure of its entropy. But another example is a pack of cards, a neatly ordered pack of cards into all the different

suits and numbers running in order is highly ordered, therefore low entropy. As you shuffle a pack of cards, you increase the entropy. Similarly, as an object loses, you know, you put a ball on top of a hill, it has potential energy, so the potential to do work, we talk about it as also as having lower entropy. So entropy is, again, it's a measure of the ability of something to do work. So a ball rolling down the hill increases its entropy, loses the capacity to do useful work. Entropy is now we're discovering that actually might play a very fundamental role in helping us unify the laws of physics. So there are different kinds of entry. One of the areas of research

that I'm interested at the moment in quantum mechanics, in particular an area called open quantum systems, which describes how a quantum system, like a particle, interacts with its surroundings. There are definitions of entropy involved there. So we talk about something called Shannon entropy or von Neumann entropy. So entropy doesn't just have one definition. It's one of those sort of umbrella sort of all-encompassing concepts that means different things depending on you're looking at. And you touch on there that entropy happens in one direction. So we go from ordered to disordered. Do we know why that is the case? We don't know why that is the case. I don't know what it is the case. Maybe some other people do, you know, who thought more deeply about

this than me. But this is the second law of thermodynamics. And the second law of thermodynamics says entropy always increases. And it's a fundamental law of nature. In a sense, it's down to, I guess, statistics and probability. Imagine you have a box that's partitioned into two halves. In one half, you've got lots of hot air and the other half you've got cold air. So the hot air has particles, molecules moving around very quickly. If you open a hole in the box and allow the particles to move, backwards and forwards, inevitably they're going to start to mix. So the hot molecules will move towards the cold side and the cold molecules move towards the hot side. And so it'll gradually reach thermal equilibrium. On the entropy, you know, the second law of thermodynamics way

of explaining it, we say it started off in lower entropy because it was highly ordered, divided up into hot and cold sections. But inevitably, it's statistically. It's statistically inevitable that it will mix. So it's much more likely. And the same with shuffling a pack of cards. It's much more likely as you shuffle a pack of cards, it will get more disordered than become ordered again. So the direction of entropy, and hence the direction of time itself, is really down to statistical inevitability. But the lovely thing is, of course, it gives a direction to time from past to future, which the other laws of physics don't do. Quantum mechanics doesn't give us a past or a future. You can run the equations of quantum mechanics forwards or backwards,

and it doesn't make any difference. The second law tells us there's a past and a future, and we think that has a very fundamental role to play in unifying the laws of nature. So can you elaborate on that a little? Can you tell me how entropy gives us a direction for the arrow of time, and what that tells us, or could hint to? for a unification theory for physics. Yes, the traditional way of talking about unifying the laws of physics and finding a theory of everything, you know, the equation that you can wear on your t-shirt, normally talks about unifying quantum mechanics and general relativity.

So quantum mechanics, the theory of the very small and general relativity, Einstein's theory of the very large and gravity. I mean, think of it as unifying the forces of nature. where relativity describes the force of gravity and quantum mechanics can account for and explain the other three of the four forces of nature. But we tend not to use, to call upon thermodynamics and the second law and entropy in helping us with this unifying quest. I think my hunch is that we're going to need to bring thermodynamics into the fold as well if we're ever going to go. going to have a hope of unifying the laws of nature. And the nicest example as to why it's important is because of how it defines time.

So, you know, we know so much about the universe and the workings of the laws of physics, but we still don't really understand the nature of time. We have relativity theory, which tells us time is the fourth dimension. It's a dimension, you know, it's like dimensions of space. We have quantum mechanics that tells us time is just, is a number, a parameter. It's something you stick in to your equation of equations of quantum mechanics, Schrodinger's equation, as it's known, to say, if I know the state of a system now,

I can crank the handle and work out what the state will be at some future time. But I could equally crank the handle backwards and work out what it was in the past. So time goes backwards and forwards. It's just a number you plug in. Then you have thermodynamics, which says, no, time isn't a dimension. It isn't a number. it's an arrow. It points from the past to the future. So these are three very different ways of looking at the meaning of time. And if we are going to have any hope of unifying the laws of physics, we need to bring together these three concepts of time. So that's why I think thermodynamics and entropy and the second law are fundamental in our understanding, our deep understanding of physics. And so with entropy, what does it tell us perhaps about our future, not mine in your future, but the future of the universe?

The very distant future of the universe. Well, of course, we have no reason to doubt that the second law will continue to apply throughout the history of the universe and throughout into the future of the universe. And so entropy, we would think, will continue inexorably to increase. And we're talking not just, you know, a million years from now or a billion years, but maybe trillions of years from now. Of course, it depends on the nature of dark energy, if we fully understand it, because it's causing the universe to expand ever more rapidly. But if dark energy is here to stay and the universe continues to expand,

entropy will continue to increase. The universe will become more and more disordered. The issue, of course, is that as you can do first expands, it gets cooler and gets colder. And there's that third law of thermodynamics that tells us that, you know, if as temperature approaches zero, entropy approaches zero. But at the same time, we have the second law telling us that, in the distant future of the universe,

entropy will increase. Of course, the reason that there is no conflict between those is that the universe will increase to, as near as damn it, infinite size if it keeps on expanding. Of course, stars and galaxies and planets and certainly all life will gradually decay,

will gradually fragment, disintegrate, matter may collapse into black holes, which will slowly evaporate away, In the end, nothing will be left, we think, other than just an infinite ocean of photons. Thermal energy, just particles of light just blowing around in empty space. And it would be a system which in thermodynamics would call the universe being in thermal equilibrium, which just means it's maximized its entropy, it's no good for anything. It has no information content.

It just looks blur, you know, everywhere in the universe. So often this is what we also call the heat death of the universe, a very boring end where nothing can survive. There you go. I've cheered you up. Yeah, but better than now, I suppose. So that brings you on quite nicely to something. I asked my team if they had any questions for you.

And all the physics grads had this one, strangely. which I think I understand what they're getting at, but maybe you'll tell me, which is where do all the photons go? So I think by that they mean that, you know, we see all this light, which is made up of photons, and they bounce around.

And as we talked about, the laws of thermodynamics, they stick around somewhere. So where does it all go? Do you mean where does it all go at the end of time? I suppose from now until then, I suppose. Yeah, I think they mean from now until then. Well, one of the difficulties in conceptualising photons that lose energy is that a photon is a particle. We think of it as a lump, a discrete little dot in space. But if it's behaving like a wave, then it's also spread out.

And as a photon travels through empty space and space is expanding, its wavelength increases. So you can't talk about a photon being in one place. It's a wave. It's stretched. It's a really difficult concept. It's to imagine a photon, to talk about, you know, a photon having a definite color. Well, that is actually wrong in physics because,

color is associated with the frequency or the wavelength of the light, which is a wave-like property. So if it's a spread-out wave, it can't also be a point particle at the same time. This is Heisenberg's uncertainty principle in a sense. And so as the universe expands and light travels around, light always travels at the same speed, the speed of light, but we have to stop thinking about photons as tiny little point particles zipping around in empty space and bumping into each other. As the universe expands and cools ever further,

the light becomes stretched to longer and longer and longer wavelengths. And so if something has a wavelength of, I don't know, a kilometer between two crests, you can't talk about it as being a tiny quantum particle at a little point in time. So it's almost impossible to try and visualize have those two pictures of what photons are like at the same time, both particle and wave. So that's a great explanation. We did a lot better than my colleagues. And then this brings you on to just one other that I missed out.

So this is another one from the team. So we have this idea of absolute zero, which is the coldest, anything. think can be, which is an expression of the amount of energy it has. Is absolute zero essentially when nothing is moving? Is that a fair description? Yes, absolute zero temperature. If you think of temperature down at a fundamental level as vibrations of particles, of molecules or atoms or all the particles that make them up,

then when you drop something down to getting closer and closer to absolute zero, you're calming down their motion more and more. You're slowing them down until they're not moving. Now, the point is we can never reach absolute zero, or sort of in a similar way that we can never travel at the speed of light, if matter particles can't quite attain the speed of light. They can get closer and closer to it, but never quite reach there.

We can get closer and closer to absolute zero, but ultimately we get down to the quantum scale where you can't, you can have the lowest possible quantum of energy and you can't get below that. So there will always be some sort of quantum fluctuation, quantum vibrations that is the lowest, what's called the ground state,

the lowest possible energy that you can have. And you can't get below it. You can never have absolute no motion, absolute zero. There's always a little bit of fuzziness, thanks to quantum mechanics. Wow, brilliant. Well, I think that's a wrap for this episode. Thank you for listening to our new podcast. If you've enjoyed the last few episodes,

make sure you subscribe because the next episode we're going to talk about the biggest mysteries in physics. We'll be discussing time travel, multiple universes, antimatter, and the search for a theory of everything. Also, if you can spare a minute, please do leave us a review. We love your feedback, and we want to know what subjects you want us to tackle next. And of course, if you want more primers on the big ideas in science and technology, head to our website, sciencefocus.com, or find us on Twitter, Facebook or Instagram. And if you want to dive deeper into any of the topics covered,

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