Instant Genius - Quantum physics – Everything You Wanted To Know About…Physics, episode three

Episode Date: April 26, 2020

Prof Jim Al-Khalili demystifies the strange world of quantum physics. We discuss the key experiments, how quantum effects play out in the real world and, of course, Schrödinger's infamous cat. Hosted on Acast. See acast.com/privacy for more information. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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so you can experience exceptional sound at home. Music just as the artist intended. Visit name audio.com to learn more. Hello 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 Alcalli. In this episode, we're exploring the bizarre world of the very, very small, the quantum

realm. Hopefully, Jim's going to help me get my head around quantum strangeness once and for all. And we're going to talk about what the laws of quantum physics tell us about the world. And of course, we'll talk about Schrodinger's infamous cat. So now we're on to the very, very, very, very, very tiny, but quite hard to understand stuff. Not that the other stuff is easier, which is the quantum realm. So, first off, why do we need quantum mechanics to understand the world? Well, I suppose we could understand a lot of the world without quantum mechanics. And I think it's probably safe to say that most people get on just fine without quantum mechanics. I mean, I'd give you a quick example.

You know, biologists have developed genetics and molecular biology and so many advances in the last few decades without really, the need for any quantum mechanics. I'm in fact actually working in an area which does overlap the two, but that's another story. But for a physicist, I'd say we need quantum mechanics because it gives us the rules that explains how the world of the very small behaves. The world of atoms and molecules and the particles that make up atoms

behaves in a very different way from the way the stuff around us, you know, bouncing balls and swinging pendulums and, you know, riding a bicycle or, you know, shooting a rocket, all the movements and forces and energy and momentum and all the stuff that we can use Newton's physics for to describe our everyday world all breaks down when you get down to the level of atoms and below. So to understand how they behave, we need this new type of mechanics, which we call quantum mechanics. And that's more than just so that, you know, who cares how atoms behave, you might argue. But without quantum mechanics, without an understanding of this behavior of the microscopic world, we would not understand how, for example, objects transmit electricity.

We wouldn't understand what semiconductors are. Therefore, we would never have invented silicon chips. We would not have developed modern electronics. We would not be recording this podcast. We would not have TVs and mobile phones and computers and computers. computers, and in fact, so much of modern technology, anything, in fact, that relies on a computer, anything that relies on electronics only works because we have understood the nature of the subatomic world through quantum mechanics. So I'd say pretty, pretty important.

And so the idea of a quantum, where did it all start? How did this idea of a separate set of rules to govern the very small. Where did it all start? It all began in 1900 and a German physicist by the name of Max Planck. So, you know, people may have heard of Planck's constant. Max Planck was something of a reluctant revolutionary. You know, he first proposed the earliest ideas in quantum theory that wasn't really happy with it himself because he couldn't really believe that it could be true. He was trying to understand the nature of how certain objects give off heat. And people had done the experiments and realized that the experiments didn't fit the theory that they had then about the way this radiation, heat radiation, was being emitted from warm objects. And he said,

well, there's a fix to it you can do. You can, you know, you use this new equation. And suddenly, the numbers you get from your calculations exactly match what you see in the laboratory. What he said was basically this heat radiation, the energy that's given off, comes ultimately in lumps, indivisible lumps, which he called quanta. And the size of each individual quantum of energy is governed by this number. It's one of the constants of nature. In our universe, it has a certain value. It's like the speed of light. And that became known as Planck's constant.

So Planck started it off by saying that heat radiation comes in lumps. Energy is lumpy. A few years later, in 1905, Einstein says actually all electromagnetic radiation, light, is also lumpy and made of little bundles of energy we now call photons. In fact, that idea of Einstein's, that light energy comes in tiny lumps, is what won him the Nobel Prize, not his theory of relativity, which he published in that same year. So that's just going to show how important it was. And gradually over the next couple of decades, physicists realized more and more that the rules that govern how the tiniest pieces of matter and energy behave are subject to this rather strange behavior, all arising from Max Planck's suggestion that heat is lumpy and everything else follows from there. And so this came up really high on the old Google search queries. I suspect because it's one of the most famously taught experiments in this area.

Can you tell me what is the double slit experiment? Yes, the double slit experiment, the infamous double slit experiment, the great American physicist Richard Feynman says this is the, it captures the central mystery of quantum mechanics. because quantum mechanics, although so successful and so powerful and underpinning most of physics and chemistry and electronics, nevertheless, at its heart, it is weird. And some physicists don't like the use of the word weird, but it is, right? It's counterintuitive to use maybe a better word. How can it be like that?

And the two-slit experiment really encapsulates this. I gave a lecture a few years back at the Royal Institution in London, which was recorded. And I talk about Tuesday's experiment. And I now realize mistakenly said, no one really has a common sense explanation for how this is possible. I'll say what it is in a moment. So if you do have any idea, and I thought I was just addressing the few hundred people in the audience. So this is safe. I say if you do have a logical explanation for this, give us a shout because the king of Sweden would like to know and he'll give you a Nobel Prize.

Of course, the Royal Institution put that lecture on YouTube and that clip, when I talk about that two slit experiment has gathered, I don't know, 2 million or something like that views. I still get regular emails from people claiming to have solved the two slit experiment problem, as do I've recently discovered the Royal Institution themselves. They're still regular. So it's the idea that matter particles behave like waves in the same way that Einstein showed that light waves can also behave like particles. And this is what's called wave particle duality. So the idea is that you fire particles, say atoms through a screen with two slits. And on the other side, you have another screen which captures those particles. But what builds up on the screen is what's called an interference pattern.

the sort of pattern that you would see, you know, sort of light and dark patches of light, if you've shined light through a diffraction grating, or that you'd see, you know, in a ripple tank experiment when waves of water start interfering with each other. So essentially what you see is the behavior that you'd expect from waves, and yet you're sending individual atom through the screen. And you can even send these atoms through one at a time. So you make it, you've got the atom, fire it, ping goes and hits the screen at a dot. So you know it's a part. And you know it's a particle. Somehow you'd say it's gone through one or two of the one of the two slits in the middle screen. Maybe it didn't. Maybe it got captured and then you wouldn't see a ping on the back

screen at all. But assuming it goes through one or the other, if it hits the back screen, but you send lots and lots of atoms through and you gradually get this interference pattern. It's the only way to explain it is that each atom goes through both slits at the same time. And that's where people get a headache because there is really no other way. I can dress it up and talk about wave functions and probability amplitudes and use lots of technical language, but ultimately each atom goes through both slits at the same time without actually breaking in parts.

That's what it's all about. That's the headache. And so do we know, do we just take it as, you know, we see the experimental observation of this happening, and so that is what is happening? Or do we understand it further? Do we understand how particles are able to behave like waves? Do we have any theories that could explain that?

Well, we have what are called interpretations. Now, for many years, approaching a century now since quantum mechanics was first developed in the 1920s, most physicists, practicing quantum physicists, have not worried too much about how the atom gets through the two slits. What they would say is the mathematics of quantum mechanics predicts that you would see this interference pattern. It predicts the probability that any given atom would land in any one place.

So the theory matches what you see when you do the experiments. The problem is, if you want to capture what that atom is doing as it's approaching the two slits to see which slit it goes through, and you figure out it goes through one slit or the other, you change the outcome of the experiment, and you no longer get the interference pattern. It's as though the atom knows you're spying on it, and it won't allow you to see how it carries out its trick.

And so most physicists say, well, look, all we can ever do is predict the results of experiments, and the theory of quantum mechanics is perfectly unambiguous. It says if you leave it to its own devices, you get an interference pattern. If you try and look, you won't get the interference pattern. This is what you'd expect to see. Go and do the experiment, sure enough. But it doesn't tell you.

Quantum mechanics doesn't tell you how that atom is doing what it's doing. And therefore, over the years, they've developed all sorts of interpretations to try and get round this idea. Each of them may regard the other interpretations as wacky and stupid, but each of them has its own wackiness hidden somewhere, whether it's the universe splits into multiple parts, whether there's an invisible interconnected field covering the whole universe, whether it signals going backwards in time,

all sorts of interpretations. None of them is exactly common sense logic, which was why I suggested to the audience. Maybe they could come up with an idea. Actually, probably quite confident in the knowledge that some very smart people have tried very hard, and we still haven't found the correct explanation to how that atom gets through both the slits at once.

Brilliant. So that brings me on to another thing. So, you know, these experiments are now almost law, I suppose, in the way that they have become associated with quantum mechanics. So it brings me on to the next one, which is Heidenberg's uncertainty principle, which had it really high search volume. Can you tell me about that? Werner Heisenberg was one of the great geniuses of the early part of the 20th century. He was only in his early 20s, early mid-20s, when he made a contribution to quantum mechanics that very few others had been able to match. And one of the

theories, the ideas that he developed is what's called the uncertainty principle. And this is, I mean, it's sort of linked to the two-set experiment in a way, because what it says is that you cannot know whether something, you cannot measure the particle nature of something and the wave nature of that same thing at the same time. It either behaves like a particle or it behaves like a wave. If you set up an experiment to see where, say, a photon,

a particle of light is, if you experiment is designed to locate it, you will locate it and it will behave like a particle. If you set up the experiment to measure its wavelength, which is, you know, it's wave-like spread-outness, you will measure that you will see that it's spread out, not a particle in a single point. And it does both things depending on how you want to look at it. But you can never design an experiment to see it being a wave and a particle at the same time. It's like you can never toss a coin and have it land, heads and tails at the same time. Neil's Boar, the other great, or the father of quantum mechanics,

referred to this idea as complementarity. And it's linked with the uncertainty principle. What a lot of people, even physicists actually, get wrong, is to assume that the uncertainty principle tells us that, say, an electron doesn't have a position and momentum at the same time. No, the uncertainty principle just tells us we cannot make. measure its position and momentum at the same time. There are certain ways of explaining quantum mechanics, something called hidden variables theories, which say, yes, these particles do have a position and momentum at the same time.

It's just that we're unable to measure it. So it's something we have to live with. Heisenberg's ancestry principle says we cannot know everything about the quantum world at the same time. We have to pick and choose what we want to know. So I'm going to perhaps sound like one of your students now, but do we, so do we understand, again, why, or what it is that means that we cannot observe both? Do we understand what it is? No, I guess the answer to that is no. I mean, it's a property of the quantum world. And I think we would not, we are unable to, to, to, to, explain why it is so, why it is the way it is, without an interpretation. Now, many physicists will say, well, you know, we have interpreted. You know, I, I'm a many worlds enthusiast, or I'm a

spontaneous collapse enthusiast, or I'm a, I'm a cubism enthusiast. These are all sorts different ways of explaining the weirdness. But we don't yet know which of them is the correct one. And my view, my personal view, is that although we have an embarrassment of different ways of trying to understand the deeper sort of narrative, the deeper meaning of how the quantum world behaves, the mathematics is unambiguous, but the narrative that goes along with the mathematics. Nature itself isn't ambiguous. Nature does things one way or the other. The way that atom gets the two, either there are parallel universes or there aren't, right? You know, we can't just out of matter of philosophical. taste. Today is, you know, is a Monday, therefore I believe in many worlds. Tomorrow I is, you know, that's not science. Believing, following a philosophical viewpoint is not science. So my ambition, my wish is that before I die, we, probably not me, we will finally hit upon the correct interpretation of quantum mechanics, because thus far, it's the only theory in all

of science that seems to have got away, Scott-free, without having an interpretation. No other theory in science can get away with it. Quantum mechanics seem to be, yeah, whatever, the maths works. Take your pick how you want to explain why it works. So then another, it brings us on to another famous sort of paradigm, which is, you know, strewn throughout pop culture now, which is Schrodinger's cat. Can you explain that to us briefly and tell us what it tells us about quantum physics? I'll explain Schrodinger's thought experiment very briefly. So Erwin Schrodinger, another one of the the great founders of quantum mechanics, was himself unhappy with some of the implications of what the theory he helped develop were telling us about the quantum world. So in the mid-1930s, he came up

with this thought experiment. His basic idea was, you know, look, atoms can be, go through two slits at the same time, can be in two places at once, can do two things at once, but cats are made of atoms, ultimately. So surely they would also exhibit this weird two things at once, what we call superposition. He said put a cat in a box with some radioactive material that has, say, a 50-50 chance of emitting a particle that would hit some vial of poison, release the poison, kill the cat. He said, in the quantum mechanics tells us unambiguously that that radioactive atom, within the space of an hour, until we open the box to check, we can't say that it has or hasn't released this particle. We have to say it's in a combination, a superposition of both having and not having

released the particle. Therefore, until you open the box, the cat is also in a supposition of being dead and alive. And Schrodinger was highlighting this to show how ridiculous the implications of quantum accounts were. We don't see cats dead and alive. You didn't force the cat to make up its mind where they want to be dead or alive when you open the box. There's nothing special about you, just because you have a PhD and the cat doesn't. You know, the cat should know if it's dead or alive. So it became very famous. I mean, I think we now sort of understand that, of course, cats are not dead and alive at the same time, because quantum effects leak away very quickly in a physical process called decoherence.

The quantum system becomes entangled with its surroundings, and it loses this both at once property. So really, Schrodinger's cat's paradox isn't a paradox. We've resolved it. but it retains this, a sort of magic in popular culture. I suppose it expresses the kind of the weirdness of it all quite well. So that brings me actually quite nicely

to one of the fascinating fields that you work in that's kind of emerging now, which is called quantum biology. So what quantum effects do we think are at play in biology at the moment? Well, this is a very, it's a relative new field, it's quite speculative, it's controversial. Physicists throughout the last century, since the birth of quantum mechanics, have arrogantly assumed that biologists will need our help.

You know, by the late 20s physicists, it's out, well, okay, so we've sold physics and therefore all of chemistry, those biologists probably need a bit of a helping hand to explain the meaning of life. Turns out they didn't need our help. But no, quantum biology is not what some people might think. It's not the fact that ultimately all living things are made of atoms and atoms behave according to the rules of quantum mechanics, so quantum mechanics must play a role in life.

It's not that, because everything in animate or animate matter is ultimately made of quantum stuff, if you burrow down deep enough. No, the idea here is that there are certain phenomena and mechanisms that have been discovered over the last 10, 20 years inside living cells that look like they need non-trivial quantum mechanics to explain them. By non-trivial, I don't just mean quantum mechanics that describes the rules of how atoms bond together. That's standard chemistry.

Things like quantum entanglement particles being interconnected across space, quantum tunneling particles being able to do the equivalent of a ghost moving through a brick wall, quantum superposition being in, you know, in two places at once, two states at once. So certain phenomena like photosynthesis, the way enzymes catalyzer reactions inside living cells, examples like that look like they need quantum mechanics to help them out. So quantum biology is a coming together of quantum physics, computational chemistry, molecular biology, to see whether these phenomena really do play a role. Can life exist without quantum mechanics? Maybe quantum mechanics is just going along for the ride,

in which case it would be a bit boring. But is there some functional advantage? That life evolved the ability to utilize the trickery of the quantum world to give it a leg up. That's what makes it fascinating. And it's still an open question. Perfect. So I'm now going to just move on to some sort of odds and ends that I found high up on Google

and one I had after reading your book. So first of, another fameless term in popular culture is the Quantum Leap, which is often misused actually by people like myself, writers. I think I'm did. definitely have fallen victim to that. Can you tell me what is a quantum leap? Well, the glib answer is quantum leap was a very entertaining late 1980s TV series in which what's his name, Sam Beckett, quantum physicist jumps through time, but no, it's not that. More correctly, I think physicists would refer to it as a quantum jump. And this is the idea

that in the early quantum theory, people like Niels Bohr, the Danish physicist, was trying to understand how electrons behave within atoms. Because the idea that people like Ernest Rutherford had developed was that the atoms like a miniature solar system. And so electrons go around the atomic nucleus in the same way that planets orbit the sun. But then they seem to be in these fixed orbits. They couldn't just orbit around at any distance from the nucleus.

They had to be in these fixed orbits. But if you gave the electron energy, then, you know, if you gave the Earth, say, lots of energy, made it go faster, it would gradually spiral out to move into an outer orbit. If the Earth were to lose energy, it would spiral in towards the sun. Electrons don't behave in that way when you give them energy. Because energy is lumpy, it's quantized, there are certain discrete amounts of energy you can give electrons. And you give them the right energy, they will quantum jump to a high, what's called a higher energy level. In a way, it's like an outer orbit, but we should be very

careful not to think of electrons as tiny particles buzzing around the nucleus. They are spread out clouds of probability is probably a more accurate way of describing them. But nevertheless, a quantum jump is an instantaneous change of state of an electron, say, from a lower energy to a high energy or jump back down again from a higher energy state to a lower energy state in which it gets rid of the excess energy again in a lump, a photon of lights, for example. Perfect. And then what's missing? What's missing from our understanding of the quantum world and how it works?

What have we left to understand or figure out? We've come a long way since the early pioneers, since quantum mechanics was developed in the 1920, Of course, you know, throughout the 20th century, what then physicists, particularly people like Paul Dirac, were able to do, was unify quantum mechanics with Einstein's special theory of relativity to talk about particles that move close to the speed of light. What then developed was what's called quantum field theory. and we've arrived at what we now call the standard model of particle physics, which is basically all the building blocks of matter, underpinned by the rules of quantum mechanics. But we still, well, what's missing, of course, for me is what's the correct interpretation.

So at the foundational level, what does it all mean? Some physicists will argue that's more of a philosophical question. But also, I think, you know, the Holy Grail, of course, is to try and unify quantum mechanics with Einstein's general theory of relativity. And there are really some fundamental differences between those two approaches to reality. And indeed, if you bring in what I describe in the book as the third pillar of physics, thermodynamics, then, you know, we don't even understand the nature of time in physics. You know, because quantum mechanics, relativity theory, thermodynamics,

each of them gives us a different definition of what time is, whether it's a dimension, whether it's an arrow pointing from past to future, or whether it's simply just a number that you plug into an equation. So I think there are still some very, despite the tremendous success we've had with quantum mechanics and the remarkable applications we have developed based on our understanding of the quantum world,

we are still not there yet. I think we're further away from a full understanding than we probably thought we were 20 years ago. In a sense, that's sort of exciting, you know, because it means there's more work to be done. But we shouldn't fall ourselves into thinking that, you know, physics is going to come to an end anytime soon. Brilliant. Well, that's a good place as any to finish up.

So in the next episode, Jim and I are going to be talking about energy. More specifically, we're looking at thermostom. These are the laws that give us the arrow of time and predict the inevitable heat death of the universe. So if you're looking forward to that and you've enjoyed these last two episodes, please do subscribe. And if you can spare a minute, leave us a review and let us know what subject you want us to tackle 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 Alcali's new book, The World According to Physics, published by Princeton University Press, is the perfect place 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.

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