Instant Genius - Mysteries in physics – Everything You Wanted To Know About…Physics, episode five

Episode Date: April 28, 2020

Prof Jim Al-Khalili reveals some of the biggest unsolved mysteries. We talk about the plausibility of time travel, whether there are multiple universes and what we need to discover a ‘theory of everything’. 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 name audio.com to learn more. Hi, and welcome back once again 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 Alcalli. Now, in today's episode, we're going to wade into the great sea of mysteries in the world of physics. If you're starting here, and I don't blame you, please be sure to check out the previous four episodes where Jim explains the key concepts in the world of physics after you've listened to this one. But for now, here's what Jim has to say about time travel, multiple universes, and the search for a theory of everything. Okay, so let's talk about the stuff that's still left to figure out in physics, which, you know, in some respects, it, is, you know, in some respects, it

it seems like we've figured out a lot. Then when you start to really think about it, it seems like there's still quite a long way to go. The first one, which was the absolute highest search thing that I found on Google, and I'm sure you probably get this a lot in your talks as well, is time travel possible? Yes, I'm sort of rather disappointed

that it's still the highest search for question on Google in terms of physics, because I wrote a book on this 21 years ago. And so people clearly are not reading my book. Otherwise, they wouldn't have to ask this question. But that's okay. That's fine. I've given a talk for many years to school kids about time travel.

And I sort of relate it to science fiction movies, Doctor Who. If you Google time travel movies, it's amazing just how many. many they've been over the years and they vary very widely in their quality. Some are intelligent and really smart and you think, you know, I remember, so was it, looper, which is a really great, great movement. It really gets your thinking, going back and, you know, and it sort of almost, it sort of makes sense. And then you get others, what was that?

Oh, hot tub time machine is like, and believe it or not, there's a hot tub time machine two, which is two hours of my life, I will never get back. I don't know why I watched that to the end, but it was awful. Anyway, so science fiction has tackled the question of time travel, but in real physics, it also is a valid area of research. We have to divide it up into two. There's time travel into the future and time travel into the past. Time travel into the future is actually possible, and we know it. We do experiment. us all the time to show this because it relies on Einstein's theory of relativity. Einstein says, in fact, his two theories of relativity, his special theory, the E equals MC squared one, the time is the

fourth dimension one, he says that when something travels very close to the speed of light, its time will slow down. I'm saying this very sort of broadly, I'm not getting into all the technical detail, but if you fly off in a rocket and you're traveling at, say, 99% the speed of light, travel around for two weeks of your time, your spacecraft computer says, yep, two weeks have elapsed before you come back home, you zip around the galaxy. You could come back to Earth and find a hundred years have gone by on Earth. So because your clock has been running more slowly, clocks on Earth have been running faster, and so more time would have elapsed on Earth. So essentially, you've traveled close to the speed of light in your rocket, which has

behaved like a time machine. You've got to the future. Another way in Einstein's general theory of relativity is another way of slowing time down, which is using gravity. The stronger gravity is the slower time runs. We even see that on Earth. That's how GPS clocks work. I talked about this in a previous episode, that we have to slow down the atomic clocks on board satellites artificially, so they synchronize with the clocks on Earth because time is running slightly more quickly on the satellites because it's feeling weaker gravity. So the Earth's gravity slows time down, but if you really want to get to the future,

if you really want to slow time down, you want to go and orbit around a supermassive black hole. It's the sort of thing that Matthew McConaughey does in Interstellar, which, by the way, is one of the best sci-fi movies covering time travel. So you can get to the future by slowing time down. Getting to the past, time travel to the past, is trickier. But not ruled out according to our current laws of physics. Einstein's general theory says, theoretically you could travel to the past. The reason most physicists

don't think this is possible is because it leads to logical issues going back and killing your younger self so you never grow up to be a time-traveling murderer and therefore you don't kill yourself, therefore you do go back and kill yourself, and so on. So is time travel possible?

Into the future, yes, by slowing time down, into the past, maybe, according to our current theories, but there may be a loophole there that we have to close up to avoid these paradoxes. Great. So the next thing is a really, really popular topic for us. Whenever we cover it, readers really engage with it. We get loads of letters and ideas,

and it definitely helps move the magazine off the shop shelves. And that is the idea of a theory of everything. So I'm going to just simply ask, what do we mean when we say, you know, a unified theory, which we've touched on a little bit, but it'd be great to go over again. Why do we need one and how do we get there? We've discovered over the history of physics, certainly of the last few hundred years, that phenomena and concepts that on the face of it may seem not to be connected with each other end up being part of the same broader, concept of phenomenon or law. Newton was the first person to discover this when he realized that the

apple falling to the ground and the moon and orbit around the earth are subject to the same force of gravity and he came up with his universal law of gravity. So that's in a sense unifying two different concepts. James Clark Maxwell unifies electricity and magnetism into electromagnetic theory. And over the course of physics, we've done more and more of this. So Einstein unified unifies space and time, Paul Dirac unified quantum mechanics with special relativity. And so gradually, these different branches describing very, very different phenomena,

are sort of merging together and giving us a simpler, I guess, picture of reality. So a unified theory is one that unifies different concepts and phenomena. When we talk specifically in particle physics of what's called a grand unified theory, a GUT, what we mean is unifying the forces of the subatomic world, bringing together electromagnism, the weak force and the strong forces, these are the nuclear forces, all into one theory.

A grand unified theory doesn't include gravity, by the way, and that's still sitting out there. But in quantum mechanics, what we currently have, what's called the standard model of particle physics, covers those three forces, but it's not really a grand unified theory, because it doesn't connect them all up in one law, one equation. You still have to talk about the strong nuclear force separately from the weak nuclear force, for example. A theory of everything is one step beyond a grand unified theory. A theory of everything brings gravity into the fold as well. So it would, one way of talking about it is saying it unifies quantum mechanics with Einstein's general theory of relativity. Another way of talking about a theory of everything is that it unifies the

forces of nature, the four forces of nature, and we're still looking for such a theory of everything. We're a long way, I think, from finding one. Now, we thought we were getting close. By the end of the 20th century, you know, people like Stephen Hawking were saying, you know, the end of theoretical physics is in sight. We're almost there. There was great hope for what's called string theory, super string theory. There are still many people working in string theory who believe that is the best possible hope for a theory of everything. And string theory is a theory of 10 dimensions and particles are really vibrating strings. The mathematics is very beautiful. It's very powerful. But there are other physicists who are saying, look, you've been thinking about this for decades now. It may be very

useful maths, very pretty maths. It might have helped us understand other things, but it doesn't look like it's going to lead to a theory of everything anytime soon. In any case, how would you even test it? So we are some way away from a theory of everything. But to address your final question, why do we think we need one? Why do we feel it's important to unify quantum mechanics and relativity? And if I get my way and thermodynamics as well, well, you know, you think, well, maybe each works in its own domain. You know, if you're describing subatomic particles, you use quantum mechanics. If you're describing galaxies, you use general relativity.

Never the twain shall meet. But there are ways of explaining why it's important. You know, Einstein says that space and time curve or are warped in the presence of matter. Now, if that matter happens to be a quantum particle, like an electron, which can be in a superposition of, say, being in two places at once, then each of those two places will will have a tiny effect on the space time around it. So space time will be, will also be put in a superposition. Therefore, there you have it. You've got space time, that's general relativity, but you've also got quantum superposition, which is quantum mechanics. You need to bring those two

ideas together in order to understand that concept. So it's not just a vanity project that we'd like to have one equation to stick on the T-shirt because it's neat and it's cool and we can show how clever we are. The laws of nature must be unified because there are phenomena and examples where you can only understand them if you bring together all the laws of physics. So it's not all about the merchandise. I'm going to jump ahead a little bit here because this ties in quite nicely to something you sort of alight to in your book. Are we do what you sort of loosely describe as an Einstein moment? I it's difficult to know I mean I think we we we feel that quantum mechanics is right we feel that general relativity is right we're not going to overthrow them we're not going to get rid of them but in the same way that Newton's mechanics or so Newton's law of gravity is not wrong it's just an approximation Einstein came along with his general theory of relativity and said no it's Newton's ideology

of gravity as being this invisible force that just pulls stuff together, that acts instantaneously across distance, it's fine as an approximation. It works, but here's a better, more profound, more deep picture of what gravity really is. It may be that we need a similar revolution that will bring together our current understanding quantum mechanics of relativity theory. So it was thought that maybe super string theory was this, the Einstein moment. The person who maybe who possibly contributed most of this is the American physicist Ed Witten. So Ed Witten in the mid-90s

came up with what we now call M-theory, which was a major advance. It was known as the second revolution in string theory, the first one being in the mid-80s. And it was thought that maybe Ed Witten, you know, he'd done what was needed to get to a theory of everything. But it hasn't led to the... the success that many people hoped.

So maybe we are still waiting for that Einstein to come along and a revision. It won't overthrow relativity in quantum mechanics, but we'll just go beyond them to find a deeper explanation to reality. And maybe it's too difficult for humans. Maybe the next Einstein will be artificial intelligence. You know, maybe an algorithm is needed to solve such a difficult problem. Okay. So I'm going to move back into the realm.

of science fiction here, which I suspect is what puts these, plants these query so highly up with Google search rankings. So the first one is we've talked before about bubble universes and other universes. Do we know, or do we think there might be other universes out there? And if so, can we say how many? We think there might be other universes out there, yes. I can, I can. even go as far as say, you know, many cosmologists believe that the multiverse exists and are very fond of that idea. But the words I'm using here, fond of the idea or believing it, that's not science. That's not, that's not physics, believing something, as I might as well say, I believe

there are fairies at the bottom of my garden. You know, that's, I don't have, you know, where's the scientific evidence for that? There are reasons why a multiverse, a higher dimension containing lots of universes, including our own, is an attractive concept. For example, we don't understand why the conditions in our universe are just right for matter to exist in the form it does, in the form of atoms that can join together to make molecule, to make complex structures, including life, including us. So how come the ingredients of the universe are tweaked so perfectly? to enable life to exist and contemplate the question in the first place. It's a difficult notion.

I mean, if you have a religious faith, you say, well, you know, there's a supreme power, intelligence, divine creator who set the dials in the first place, whether or not that that God intervenes in the meantime or just set dials going and then sat back and admired his handiwork. That's a way of explaining it away. But for a physicist, particularly, you know, I mean, I'm not really. religious. But I think even those physicists who do have a faith, we want to try and understand the reasons why these things happen. Why is our universe so special? If the law of gravity was,

the force of gravity was ever so slightly weaker than it is, then matter wouldn't have clumped together to form galaxies, then stars, then planets, at all. If the electric charge on the electron was ever so slightly stronger, atoms wouldn't exist. because electrons would just collapse into the nucleus. So everything is very, very delicately balanced. And you think, well, how come? You know, we live in a Goldilocks universe. And we talk about the Goldilocks effect.

You know, everything is just right. Like the porridge, baby bears porridge is just right for Goldilocks. Our universe is just right for us. How come? Now, if you think of the lottery. If, say, one week, only one lottery ticket, some random number, only one lottery ticket was printed. They didn't bother printing any of the others.

And then they carry out the draw and that number comes out. I think, what are the chances that there was only one ticket in existence and that's the number that came out? That's what we feel like with our universe. Now, the multiverse idea, if there are lots and lots of universes around, then it's not surprising that at least one of them would be appropriate for life to exist and for us to be asking a question. With millions of lottery tickets printed,

it's not surprising that one of them will have the number. You know, someone has to win. They may feel special, but there are lots of other people who didn't win. So there are lots of other universes, maybe in the multiverse that have different forces of gravity, different speed of light, different charge on the electron,

different fundamental constants. You know, there'd be universes that sort of formed in their own Big Bang, but weren't appropriate, and they just collapsed in on themselves. Some universities expanded too quickly, nothing of interest happened within them. So the multiverse idea is attractive because it gets over this idea called the anthropic idea. How come our universe is so special? Well, maybe because it's not the only one.

And then there are other ideas in cosmology that suggest maybe there's a multiverse. For example, there's an idea called eternal inflation, whereby our Big Bang is only a little bubble appearing within, a larger space and you know so inflation is continuing forever outside of our universe and we've got these all bubble universes appearing within this this field that have their own existence, their own reality. The problem with the multiverse idea, attractive though it is, is that we still don't have any way of testing whether it's right or wrong. And many, many scientists, many physicists will say that is not proper science. If you can't falsify the theory,

if you can't test it and verify it, then it's not a scientific theory. It's the same as fairies at the bottom of my garden. However attractive that idea might be, the multiverse, not the fairies idea. I understand. Perfect. And moving on to another sort of totem pole

of science fiction. Antimatter. I hear about it all the time. And it's in every film set in the future. They have lots of it to send them flying around, space and galaxies and doing all sorts of incredible stuff. So back in the real world, what is it and where is it? Antimatter sounds very exotic.

It sounds very sci-fi. And therefore, a lot of people assume it's not real. It's not, you know, antimatter is a made-up thing like warp drives. But antimatter is real. It was predicted by Paul Dirac, English physicist in the late 1920s, and it was discovered a few years later. Antimatter essentially is like normal matter, but it has the opposite properties. So the simplest example is the electron, negatively charged particle orbiting in atoms.

It has an antimatter partner called the positron. Why? Because from the name, it has positive electric charge. In other ways, it's identical to the electron. One has a negative, one has positive charge. And most matter particles have their antimatter counterparts. Now, the thing is, the thing that people like Paul Dirac realized, is that when you bring matter and antimatter together,

all those opposite properties they have combine and cancel out. Their matter, their mass doesn't cancel out because antimatter still has mass. It still, as far as we can know, falls in, you drop a positron, it'll fall to the ground in the same way the electron will fall to the ground. So they both have mass. But you bring them together and they, what we call, annihilate each other. Essentially what happens is that Einstein's equation, E equals M. C squared kicks in, and the stuff, the M, the matter, the combined matter of the electron

of the electron, turn into pure energy. The amount of energy, well, that's governed by this equation. However much mass you have, the sum of the two masses of the particles, times that by the square of the speed of light, your C squared, and that gives you the amount of energy. So matter and antimatter will turn into pure energy. The opposite can also happen down at the quantum level. Pure energy can form matter as long as it also forms the equal amount of antimatter. So a photon, which is a particle of light energy, can transform into an electron and a positron. So it's creating matter out of nothing. And that's why we now, we don't talk about law of conservation of energy. We have to talk about law of conservation of mass and energy,

because matter and energy are interchangeable in this way. If people think it's science fiction, Well, I can just remind listeners that anyone who's ever been into a hospital and had a PET scan, PET stands for positron emission tomography. Positrons, particles of antimatter. So if you go and have a PET scan, what's happening is that you are injected with these radio nuclei, It's a radioactive material, which the nuclei of these radioactive atoms spit out positrons. They spit out antimatter particles. Now, that positron doesn't travel very far before it hits an electron.

They annihilate. They create two photons of light, which head off back to back, and are captured in this scanner, in these photon detectors. And what you can do then is track back the tracks of where these photons came from and pinpoint where that positron hit the electron, which was round about the place where that metatine was found itself within a cell. And so that's the way you image things. So you image things like our brains using particles of antimatter. That's not science fiction. That's very important medicine.

Great. Okay, last question. You've touched on dark energy and dark matter and explained what they are quite well, I think. Can you tell me, as a journalist, it always feels like we are a decade away from finding it, from finding this force and this matter that is ripping the universe apart? Well, we always have to be careful between dark energy and dark matter, because although the word dark is there, neither of them,

you know, I think the word dark is not really suitable for either concept. Dark matter probably would be better called invisible matter. And dark energy,

well, that's just, that's just, it's meaningless. You know, I think at the time when it was discovered just over 20 years ago, there was always

competition within physics as to what to call it. I'd have preferred to call it quintessence, which was a lovely, lovely word, but that didn't stick. Dark energy,

we're, sort of getting closer to try and understand its origins, its nature, it's down to what's called the quantum vacuum. But I think when you talk about we're always 10 years away, I think that's referring to dark matter. And dark energy is ripping the universe apart. It's pushing everything, causing space to expand ever more quickly. Dark matter, no, dark matter has a gravitational pull. It's attractive. It's like normal matter. The thing is we don't know what it's made of. We know it's out there because there's some invisible stuff. out in space that's causing matter to be to move in a certain way actually even

bending light because gravitation we now know from Einstein that gravity is basically the curvature of space time and so light beams light from distant galaxies gets bent by gravitational fields and sometimes you see light being bent in a way that can't be accounted for by the matter that you can see so they say okay there must be invisible stuff out there that's causing that light to bend we also know that dark matter is necessary to hold galaxies together. So we know it's there because we can see its gravitational effect, but we don't know what it's made of.

It's clearly not made of any of the particles we already know about. So it must be made of something else. And so for many years now, we've been trying to understand what its constituents are. And it's a bit like nuclear fusion. You're right. It's always, you know, 10 years away. We're building ever more elaborate, ever more sensitive instruments

to try and capture it. dark matter if it's made of particles which we think it should be then those particles should be streaming through the earth all the time they they see the earth as transparent because dark matter doesn't interact with normal matter other than via gravity and at the particle level that's that's not going to have much effect so we're still nevertheless designing instruments that might try and capture every now and again one of these particles of dark matter we're trying to make them in places like the large hadron collider and so far we still haven't found them. There are candidates. There's like half a dozen different types of hypothetical particles that fit the bill that we're still, you know, but we still don't know which one. I wrote a novel a couple of years ago called Sunfall in which I suggested that it's set in 2041. So by that point, to 20 years from now, we will have discovered dark matter. And it plays a big role in my story.

And I suggest that dark matter is made of particles called Neutralina. which are a type of particle called super symmetric particles, which may be candidates for dark matter. And I was sort of worried that either neutralineers will be ruled out before my book came out or that they would be discovered before my book came out. Either way, I wouldn't be so proud. But all the time it's possible, I could be like Arthur C. Clark, you know, sort of presciently predicting the future if it turns out I was right.

Okay, brilliant. That's it for today's episode. thank you for listening to our new podcast. If you've enjoyed the last few episodes, please make sure you subscribe because the next episode is a special one. In the sixth and final episode, we're going to answer your questions

sent to us on Twitter and via email. Of course, if you want to send us more questions for the next and future episodes, be sure to follow us on Twitter at Science Focus. 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 handle next. And of course, if you want more primers on the big ideas in science, 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, then Professor Jim Alcaloly's new book, The World According to Physics, published by Princeton University Press, is the perfect fit places to start. It's a concise introduction to the most important ideas in physics now. And Jim is a wonderfully clear writer who takes the grandest of ideas and makes them simple to understand. Thank you for listening to the Science Focus podcast from the BBC Science Focus magazine team. With the UK's bestselling sites and technology monthly, available in print and in several digital formats throughout the world. Find out more at sciencefocus.com or look out for us in your App Store.

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