In Our Time - The Proton

Episode Date: April 26, 2018

Melvyn Bragg and guests discuss the discovery and growing understanding of the Proton, formed from three quarks close to the Big Bang and found in the nuclei of all elements. The positive charges they emit means they attract the fundamental particles of negatively charged electrons, an attraction that leads to the creation of atoms which in turn leads to chemistry, biology and life itself. The Sun (in common with other stars) is a fusion engine that turn protons by a series of processes into helium, emitting energy in the process, with about half of the Sun's protons captured so far. Hydrogen atoms, stripped of electrons, are single protons which can be accelerated to smash other nuclei and have applications in proton therapy. Many questions remain, such as why are electrical charges for protons and electrons so perfectly balanced?WithFrank Close Professor Emeritus of Physics at the University of OxfordHelen Heath Reader in Physics at the University of BristolAndSimon Jolly Lecturer in High Energy Physics at University College LondonProducer: Simon Tillotson.

This is the BBC. Thanks for downloading this episode of In Our Time. There's a reading list to go with it on our website, and you can get news about our programs if you follow us on Twitter at BBC In Our Time. I hope you enjoy the programs. Hello, there are enough protons in the sun for it to last a thousand billion years, and it's only about halfway through them. So that's a relief.

The properties of protons there, as on Earth and throughout the universe, are those that make chemistry, biology and life itself possible. They've existed since a split second after the Big Bang and are found in a nuclei of all elements. Hydrogen, by far the most abundant element in the universe, is a single protein with one electron. Stripped of electrons, those protons can be accelerated to smash other nuclei to reveal more of the secrets of particle physics

and they can be used in the treatment of some cancers. And while much is known about protons, much remains to be discovered. With me to discuss the proton are Frank Close, Professor Emeritus of Physics at the University of Oxford Simon Jolly, lecturer in high energy physics at University College London and Helen Heath, reader in physics at the University of Bristol. Frank Close, what's the proton? Well, a proton is one of the seeds of atoms.

Probably the story really begins around the end of the 19th century, a time when the idea that matter is made of atoms was established, but the belief that atoms are the smallest pieces was beginning to fall apart, probably almost literally, because in Cambridge, JJ Thompson had discovered that inside atoms are little particles called electrons. Now, electrons are probably most familiar as the carriers of electric current, because electrons are electrically charged.

Electric charges come in two varieties, plus and minus, and electrons by convention are negatively charged. So inside all atoms, there's a lot of negatively charged particles, And yet atoms overall, matter overall, isn't electrically charged. So by the start of the 20th century, people realized there must be something positively charged inside atoms to counterbalance this negative stuff. And these positively charged carriers we now call protons. And the question was, what are they, where are they, how does it all work? Can you give this idea of the size?

Because I love this sort of astonish me stuff. The size of the things we're talking about the protons. and how people like you get to see them and know they're there? Well, the size, I mean, an example I gave a scale hydrogen atom up to the longest hole in the golf course. So the distance of the hole is about 400 metres that you're trying to get the ball in. And the size of the hole that you're trying to get the ball into, that is like the size of the proton if the teeing off point is where the electron on the outside is. So if you can imagine scaling the thing up to that, that shows you how small the problem,

proton is inside an atom. As to the atom itself, those of us who have hair, about a million atoms across one another will fill a single hair. You have to take a deep breath, don't you? And so this is technology at its finest getting you there? That's certainly true, but in fact, the ways that this was sussed out by Ernest Rutherford between about 1912 and 1920, just about a century ago, was with hindsight relatively simple, but the simplicity was in a way the genius. He realized that nature was very nice. It provided little subatomic bullets called alpha particles that we now know these are bits of atoms that are being spit off.

But all that he needed to know was they've got positive electrical charge and because positive charges repel, positive charges, light charges repel, if these alpha particles were fired towards atoms, then the positive charges inside those atoms would repel the alpha particles. And so he... How did he fire them at the atoms? Nature did that. for him, if you like. What he was wanting to see

was what happened after they had passed by the atom. Sorry, Frank. How did nature do it for him? Nature randomly in some cases causes radioactivity to happen, which means an atom can spontaneously change from an unstable form to a more

stable form. In the process it emits stuff. In particular, it might emit these alpha particles. How and why it happens is the deepness of quantum mechanics which I hope you'll not ask me about. But for him, what he did was he first of all

fired these things at the atoms of heavy elements like gold. And all we need to know about alpha particles is that they're very light on the scale of a gold atom but they're much heavier than a hydrogen atom. So when you're firing an alpha particle at gold, it's like firing a little elastic ball at a big football. It bounces off, it bounces back at you.

And that was the surprise that Rutherford had that these things bounced back. so he realized that the positive charges in gold must be concentrated in a big lump in the middle. That was the discovery of the nucleus. Then he went to the other end of the periodic table, such as hydrogen that you mentioned in the introduction. The lightest.

The lightest. And in the case of hydrogen, we now know there's a single proton, which is only about one quarter the mass of an alpha particle. So on this scale, the alpha particles like the big football, and the protons like the little elastic ball. So when the alpha particle arrived, it kicked the proton out. And Rutherford called these things H particles, H for hydrogen. And then the next step was he discovered that if he fired things like alpha particles of oxygen and nitrogen, which were also light,

these H particles got chipped out of there as well. So his final insight was these things, which he then called proton, are the fundamental carriers of the positive charge. You cluster more and more of them together, and you get more and more. positive seed to attract electrons to make heavy and heavy elements. Thank you very much indeed for that. I'm much relieved now. We're on our way. Simon. Can you tell us how you would describe an atom? It depends on how sophisticated you want your picture to be. Prior to Rutherford's discovery, it was thought that there was this plum pudding model,

this uniform collection of positive and negative charges. rather as discovery led us down the route to the model of the atom where you have this cluster of positive charges at the core and this orbiting halo of electrons around that positively charged core. So the simple way to imagine it is like a tiny planetary system with the sun is the nucleus and then the planets are the orbiting electrons. The complexity starts to come. in when quantum mechanics falls into the picture because you start to describe some atomic

particles not as little hard objects but as actually tiny wave packets so they're not localized in space they start to have a distribution so the electron cloud around a nucleus is exactly that it's not discrete objects as they orbit the atom it actually forms this halo around the atom so the point at which our macroscopic picture of the world of imagining solid objects orbiting other solid objects starts to break down a little is with the electrons as they orbit the nucleus. But a simple way of picturing it is the Rutherford model of the atom with a solid nucleus, with protons and neutrons and then an orbiting cloud of electrons around it. So what's inside the atom?

You have these two parts. So at the core, the nucleus is made up of positively charged protons and neutral neutrons and they're bound together extremely tightly and then this cloud of electrons which which orbit around the nucleus are held there by the attraction the electrical attraction between the negative charge on the electrons and the positive charge from from the protons the nucleus itself is not actually bound together by the electromagnetic force. There's another force called the strong nuclear force

which helps to bind positively charge protons together because otherwise if there wasn't another force they would simply force each other apart by electrostatic repulsion. So the nucleus is bound extremely tightly with the

strong force and then the electrons in air orbit are held in by the electrical attraction. It's a little world isn't it? It's a little universe down there or in there all around That's correct, yes. Billions and billions and billions of little universes in the studio. More than you can imagine.

Yes, I can't imagine even billions and billions. What's the link between the proton and differences between chemical elements? The proton is really the particle that defines the chemical element. So what sets one chemical element apart from another, is not just the mass, the fact that hydrogen is the lightest, helium is heavier and then up to lithium and so on. It's also how they form bonds. And the way that one atom bonds with another most commonly is through the sharing of electrons. Now, the number of electrons that each atom has has to match exactly the number of protons in a stable atom.

It's possible to add or remove electrons and then the atom becomes charged, and that's what we call an ion. But in the most common type of atomic bonding called covalent bonding, it depends on the number of electrons that you have in the outer part of the atom. Now electrons actually like to cluster together into groups within the atoms. So at the lowest energy level within an atom, you have two electrons. Now, if you think of something like hydrogen, hydrogen is now only got one proton in the centre and one electron orbiting, which means there's a gap. So that means hydrogen actually likes to form a single bond. And then covalent bonding is the exchange of that electron with another chemical. And so the number of bonds that each chemical can form is defined by the number

of protons that you have in the nucleus. All right. I think I'm still on board. Helen Heath, it was one thought that the protons were fundamental particles. How and when did that change? Well, there are lots of pieces of evidence that the proton isn't fundamental. Initially, there was work done with cosmic rays. So cosmic rays are naturally accelerated particles that come into the upper atmosphere. And there was a lot of early work done studying their interactions with matter. I feel obliged to mention people like Cecil Powell,

who was at Bristol University, who did a lot of the early work on this. and in those collisions they found lots of particles essentially a zoo of different particles and in trying to understand those they started grouping them together particles that had similar properties for example similar masses and that led people to try and think about

why there were these patterns in this zoo of particles that you could see and Murray-Gale Man and George Weig came up with this idea of a quark model. Quark, yes. So that these this zoo of particles were actually different combinations

of smaller particles called quarks. And the quarks had some unusual properties or they would have had to have some unusual properties, one of which is that they would have a charge that was a fraction

of either the proton or the electrons charge. And we've never seen a particle with a fraction of the proton electrons charge in a laboratory experiment. We've never been able to measure

something with a fractional charge moving through our labs. You've never seen a quark? Well, it's not quite true. We've never, we've never seen a quark out on its own. They like to go around together and they like to go around in particular combinations. And the combination, there are a number of combinations that we've seen now, but until quite recently the combinations that we'd seen were three quarks together, which is called a barion, or three anti-quarts together, which is.

an anti-barion and a combination of a quark and an anti-quark which we call a meson. And the proton is the lightest mass barion. And it consists of two types of quark that give it its external properties such as its charge and those are the up quark, which has a charge of plus two-thirds of the electron charge and the down quark which is charge of minus a third. So we've got plus two-thirds, plus two-thirds, minus a third, which makes plus one. And that's our proton. So can we go back to what does that make our view of the proton now?

So our view of the proton now is that we've got these three quarks, two ups and a down, which are what we call the valence quarks. So essentially, it's an analogy with chemistry. So in chemistry, the valence electrons are the ones on the outside that give the atom its chemical properties. And the valence quarks of the proton give us the outward-looking properties of the proton, for example. so it's electric charge. Heavy nudge from the two gentlemen on my right. Well, that's encouraging.

But one of the interesting things about the proton is if you start to look into it, you find it's actually a lot more complicated than that. So first of all, there's the particles that hold those quarks together, which are called gluons. And then those gluons themselves can split to make quag-canti-quart pairs

which appear and disappear. And we can actually start to see those. So the first experiment's, really seeing the objects inside the proton were in the 1960s in Stanford in California. If you scatter electrons from protons, a bit like Rutherford scattering, Frank was talking about,

if you scatter electrons from protons, then you start to see not just a whole big object, and proton is big on particle physics scales, but you start to see individual tiny scattering centers inside point-like things inside the proton. So we can actually confirm that there are objects inside the proton. Do you think you've reached a centroid yet? Well, that's a very big question.

Have we got the, is what we think of now as fundamental, truly fundamental? I think I'd like to think not, but we have to keep looking. I suppose the best you can say is that at the present state of knowledge, we know of nothing smaller than the quark scale, and we know nothing smaller than the electron scale. They are, in an analogy, the fundamental letters of nature's alphabet. That doesn't mean to say that in the future we might have more powerful microscopes that can resolve internal structure in them,

analogies to the way that Rutherford discovered internal structure in the atom a century ago, but for the moment that's where we are. Let's go back to the beginning, back to anana second after the Big Bang, and then back to the sun. Come on, Frank, you're the man for this. What's the role of the proton in the sun? Well, at the... The sun came a long time after the Big Bang,

but that's true. There's a direct connection. But protons came quite soon after the Big Bang, about a second or so. So protons were the first of the now existing particles, if you like, that emerged out of the Big Bang. And they are, to the best of our experiments, stable. And so protons have been around ever since then. When you say stable, do you mean not moving,

What do you mean? They've always been the same for 13 and a half billion years. I see. They move around, but an individual proton does not spontaneously decay and convert into something else. It is, as Helen said, it's the bottom of that particular family. Other more unstable barons can decay and come down the ladder, if you like, but nature likes to find the lowest energy state, and the proton is the lowest energy state of three quarks. And so the proton, to the best of our experiments, is absolutely stable. So the protons that were created within a second of the Big Bang, if you like, they're all around

and they gravitationally attract one another until there's big huge clumps of protons, which we call stars like the sun. Now, our sun is an example of a star which is dominantly made of protons and electrons, but the nuclear particles are protons. And in the heart of the sun, the protons, of course, being positively charged,

like to keep away from one of the sun. another. But in the heart of the sun, the temperature is of the order of 10 million degrees. And at those temperatures, the protons are sufficiently agitated that they can occasionally bump into one another. And when that happens, a series of processes takes place called fusion. The protons turn eventually into four protons by a series of processes come together and turn into a nucleus of helium, the next element in the periodic table. Now, a nucleus of helium is slightly lighter in mass than the four protons that made it. And that mass difference by Einstein's famous E, it was MC squared, says, oh, that mass can give you energy. So the energy that the sun is radiating, as light and other forms,

ultimately comes from the protons at its heart going through this cooking process and turning into helium. Simon, John, there are some fundamental forces in physics, which I'm read about from your notes, some of the particular play with protons. Can you tell us about that? The most important one that we've talked about so far is electromagnetism, so electric charge. So we know that the proton has a positive electric charge and the electron has a negative

electric charge and it's the attraction between the two of those that holds the atom together and then allows atoms to bond. But there are other forces at play that allow the atom itself to remain bound together. So as Frank alluded to, when you cluster protons together in a star to try and convert hydrogen into helium, you need to overcome this electrostatic repulsion between all of these equally charged protons. There's a much stronger force, which fortunately we call the strong force. So the strong nuclear force is what binds the nucleus together. Now, the third force is almost completely irrelevant to particle physicists, which is gravity. So protons have a mass, and that means that they experience

gravity. In a star, it is the battle between the gravitational pressure that's squeezing all of these hydrogen atoms together that gives you the energy to fuse them into helium. The fourth force is kind of a funny one. It's something that we call the weak nuclear force. Unlike the other forces, it doesn't act at a distance. It's what we call a contact interaction. So the weak nuclear force, we only see evidence of it on a day-to-day basis in certain types of radioactive decay, where a neutron will decay into a proton and then emit an electron and a particle that we don't see very easily called a neutrino. That interaction is governed by the weak nuclear force. The reason why it's important to this story is if you have four protons, that's not actually helium.

Helium needs two protons and two neutrons. The energy of the gravitational compression in a star gives a proton enough energy via the weak, nuclear force to convert into a neutron which normally it wouldn't do because it's completely stable and that gives you the strong nuclear force from the neutrons to bind those two protons into a helium atom and the very small mass difference then gives you the heat that comes out in a star. In fact it's the weak force Simon alludes to that is at work in that first stage in the Sun's process where protons turn into helium and an example

of how weak it is, it's that 5,000 million years after the sun first started burning, if you were a proton in there, there's still today only a 50-50 chance that you've taken that first step in turning into helium because the force that does it is so feeble.

Halony, we now come to gluons and pions. It's wonderful. I like the words very much indeed. Could you tell people what they do these words? Well, gluons essentially glue the quarks together inside the proton or indeed inside the neutrons. How do they do then? Well, all our forces at the fundamental level arise because you have particles being exchanged. You think of these objects, quarks, that as far as we know, they've got no extents. They're not touching.

It's not like they push each other by contact. So they have to interact somehow and they interact via fields. people will be familiar with fields because you can feel a magnetic field so you don't need magnets to touch for them to move each other. So our quarks are exchanging these glue-ons all the time and that's responsible for the attractive force that pulls them together. Can you, again, I'm just a simple question,

but can you see the gluons? Can we see the gluons? Well, we can't see glue-ons directly in that we can't sort of get them out in the lab and watch a gluon propagate. What we do see is in some interactions, a gluon is produced, and gluons and quarks, indeed,

can't get out of hadrons. Within about 10 to the minus 24 seconds of being expelled from a hadron, they convert themselves into more hadrons. So what we can see is the evidence of the gluon having, we call, fragmented, converted into hadrons, and we can look at the hadrons it's converted into these particles with quarks in.

and infer the properties of the glue-on from it. C was probably the right word. I mean, probably a better word. You can measure the effect of the gluons. You can measure the effects of the gluons, yes. Yes. What about pions? So in order that the gluons are exchanged,

the quarks have to have a charge, so forces come from charges. So we're familiar with it, many people familiar with the electric charge, which is, and the electrostatic force comes from that. The charge that quarks have is something called colour and there are three colours, the quarks have three colours and it's that charge that means that they interact via gluons. But we've never seen an object that's got overall colour out in the

world. Our protons and neutrons are held together by what we've referred to as the strong nuclear force. There has to be something that's exchanged, a particle that's causing that. And that particle is the pion. So the pion is a combination of ion. either an up quark with an anti-down or a down quark with an anti-up or up-anty-up-down-and-down combination. So these themselves are colourless objects, and their exchange between protons and neutrons is responsible for the force that holds the protons and neutrons together in the nucleus.

So that is also going on there? That's also going on there, yes. So apart from being the proton being a simple, single, it's busy as anything down there. proton is a very, very complicated object indeed. Working its socks off, right. Frank, why are electrons and protons so perfectly balanced as they seem to have been from the very beginning,

from the post-nano second? Well, the... Oh, well, just after they started going after, after the subatomic particles are settled a bit. I mean, when you say perfectly balanced, I mean, when it comes to mass, they're very, very different. The proton is about 2,000 times more heavy

than the electron. But when it comes to electric charge, then they are perfectly balanced. That's what I meant, yes. I missed that bit out. And I was sort of saying that because if that is a very good question, Melvin, then can we move on?

If I had the answer to that. But it is a very interesting question because I use this as an example of how something which is so self-evident that matter is overall electrically not charged, even though there are electric charges inside atoms. The fact that electrons charges

perfectly balance the protons charges is a question at the frontiers of physics, the frontiers of knowledge. And you can get right to it just from that self-evident fact that it is so obvious. You don't have to have higher mathematics to get to this one. It's right there staring you in the face. And it's a very profound question.

And it tells us that either this is a coincidence and as scientists we don't like the idea of coincidence as we look for reasons. But coincidence is going to have rules underneath them if you're dug far enough. And that's what we're trying to find. And to add to the mystery,

I mean, if the proton, if we'd asked this question 70 years ago before the idea of quartz came along, I might have said, well, there's something, whatever electric charges, I don't know, but you can add it to things or take it away from things. And so the electron has lost it and the protons got it and that's why they balance.

But now, as Helen has just told us, protons are made of quarks. And these quarks carry funny fractions of electric charge. on the average one-third of a charge, and they come together in threes, not fours or sevens or twelfths or things, but threes. So the fact that three times one-third gives you one,

which perfectly counterbalances the minus one of the electron, is either a miracle or there's something going on. And we think, well, this is evidence that something is going on. Why it's a great puzzle is because, as we said earlier, as far as we can tell, the electron is one of the fundamental letters of nature's alphabet, And the quarks we now see are fundamental letters of nature's alphabet. We know of nothing smaller. And yet this conspiracy of electric charges

seems to say that they somehow know about each other. And that... Do you think they have many, many, many billion-sized brains of some sort? Maybe. Well, if they know, you're using the word, not me. For which one? You use the word no, if they know about each other. Did I use the word know in the sense of N-O or K-N-O-W?

You did K-N-O-W, right? That was the mistake, wasn't it? You can pronounce a K. Yes, right. So, where were we when we got this dove version of this one? So this gives us a clue that somehow electrons and quarks are not completely independent. At some deeper level, and whether that deeper level means lower, further constituents we have yet defined, I mean, in a more profound sense, there is a theory out there waiting to be discovered,

which unites these different forms of matter. Simon, Zahman, Jolly. How are the properties of protons applied to the treatment of some cancers, which is a developing field? The way that we treat cancer is largely a mixture of three modalities, so surgery, chemotherapy and radiotherapy. With radiotherapy, what you're trying to do is use x-ray photons to irradiate the tumour. The way that you actually kill the cancer is you're trying to do. to attack the DNA strands within the nucleus of a cell as it divides. So as a cell undergoes division and the DNA strands unwrap themselves, if you can break those DNA strands at that point,

then the cell will fall apart and the cell division stops. If you can preferentially do that to cancer cells, then you can eradicate the cancer cells and leave the palsy tissue spared. Well, the way that you do that with x-rays is that the x-ray will come into the body and occasionally it will crash into one of the electrons surrounding all of the atoms either in the DNA strand itself, which means you lose an electron, which means the bond then breaks and that DNA strand just pops apart. Or you create free radicals, so you end up with these charged ions, which a bit like Pac-Man will go through and then bite and collect electrons in the nearby DNA strands.

The difficulty with x-rays is that, as most people know, x-rays pass all the way through the body. So if they didn't, then you wouldn't be able to take an x-ray of someone. But what that means is that the damage you are doing to the body happens all the way along the x-rays path, though, from the entry to the exit. The key part about protons is, as Frank said, is that fundamentally they are heavy sumatomic particles compared to electrons. So if you take a proton and you accelerate it up and fire it into the body, rather than just undergoing a single interaction where it will crash into one electron and that electron pops out, the proton is charged. So it's feeling the charges of all of the electrons of all of the atomic clouds as it passes through. And it's kind of rattling as it goes through little bump, bump, bump, bump, bump, interactions. It's not losing too much energy because it's so big and it's so heavy.

However, as it starts to slow down, it then deposits more and more energy. It's interacting more often with those electrons, so it's doing more damage, so it slows down more, so it does more damage, and in the end it comes to a screeching halt and does most of its damage in the last few millimeters of its path. Now, that spike in the damage, that spike in the dose is called the Bragg-Peak. After a certain William Bragg. it's the reason why we can treat cancers preferentially with protons as opposed to x-rays because we have this known range so long as we know the energy, we know how far it's going to travel,

so that means you can spare healthy tissue behind the brag peak, and also you can do damage preferentially in the tumour rather than in front of it. Thank you very much. Thank you very much. Halen Heath, particle accelerators are used to learn more about particle physics. What physics? What makes the proton such a good particle to accelerate? There are two main advantages to the proton over, say, the electron.

I mean, ideally you want a stable particle because otherwise when you've got a beam, they just disappear. So the two main possibilities are electron or its antiparticle and a proton or its antiparticle. The advantages the protons have, particularly for making new discoveries, is that it's easier to get higher energy protons. And the reason for that is our discovery machines are colliders in which the particles are moving in circles.

And in order to keep something moving in a circle, you are continually having to accelerate it towards the centre of the circle. That's how you make it move in a circular path. And when you accelerate a charged particle, it radiates energy. And you have to replace that energy if you want your beam to keep. going around with the same energy. And the rate at which you radiate that energy

depends on one over the mass to the fourth. And Frank already said that the electron is about 2,000 times lighter than the proton. So you need 2,000 squared times more energy input to keep the beams having the energy you need. And you need the energy because, as he also said, we're trying to convert the energy of the beams into the mass of new particles.

So that's one advantage of protons have. You need less energy just to keep them going round. And the second advantage that they have for discoveries is because the protons are complex object, and we're colliding pieces of the proton, we're actually having, if you think about it, we have three quarks and they have some share of the protons energy. If they're colliding, we're actually not always colliding with exactly the same energy. So compare with an electron positron where you've got the... and positron, and all that energy is in points.

So you've only got one energy, if you like. So you can only make particles of one mass, whereas the proton collisions allow us to make, have the possibility to make particles of lots of different masses. Frank, just to clear something up, thank you very much. When protons were created right over the big man, there were also antiprotons, which were there to wipe out the protons. Why didn't they succeed?

You're only asking the big questions. That's right. As you said, the big bang, the energy of the big bang turned into matter and antimatter, in particular protons and antiprotons, in equal amounts, according to our best theories and observations. But today, 13.8 billion years later, matter is made of atoms containing protons, not anti-atoms made of antiprotons. And the question of, so where did all the antimatter go to? is another of the frontier questions. And again, if I had the answer to that, I wouldn't be spending my time here today. I'd be off there getting a prize for it.

But it is one of the big questions. You can get a price for explaining it. Oh, right, let's carry on there. And it is one of the big challenges to the imbalance between matter and antimatter is as big as you can imagine. I mean, everything that we know materially in the universe is matter. There is no evidence for antimatter in bulk.

We can make it, as Helen alluded to, an antiparticle at a time and use them in experiments and control them. They're certainly there. We know their properties. We can use them to do other things. But nature in bulk does not seem to make use of them. And the cause of this imbalance, we don't know.

Simon Jolie, particle physics and our understanding of the protein, is founded on quantum mechanics. How firm a basis is that? Quantum mechanics, and I'm probably presumptuous as a particle physicist by saying this, but quantum mechanics is both the best theory that we have

ever come up with and the worst. The reason I say that is that a scientific theory, really you need two parts to it. It's trying to do two things. One of which is give you a prediction. If I know a certain set of circumstances, what is the outcome? Is the sun going to rise tomorrow? Is it going to rain? The other part is to give you some kind of insight, to understand what the process is that you're seeing in front of you, not just because we desire this intuitive understanding of nature, but also that helps us build a further picture. Okay, if the sun comes up tomorrow, is it going to be sunny or rainy? We need some information on the model that we don't yet have.

The problem with quantum mechanics is it does one of those extraordinarily well, and the other one is terrible. So the predictions that you get from experiments based on quantum mechanics are some of the most accurate known to man. So the particle physics theories of interaction, so the electromagnetic interaction, the quantum theory of that interaction is called quantum electrodynamics. That is founded on quantum mechanics. Those are the most accurate scientific measurements that we have ever made. And then you have a quantum theory of the strong interaction, which is, called quantum chromodynamics. The problem with quantum mechanics is the picture that it gives you is absolutely awful. It tells you that at some fundamental level, a particle is not really a particle.

It's a wave. It's both here and there. It's mostly over there. It's somewhere over here. And now you have to try and conceptualize that image and go from predictions that match that picture perfectly to the solidity that we have in our day-to-day existence. The problem being that then if you want to make a further prediction and add further insight to the picture, how do you do it when you have no idea what's going on? I know what's going on mathematically because I can make these wonderfully accurate predictions. But the picture is awful, and that really is one of our fundamental limitations of understanding. Helen, Helen Heath, the proton is not a fundamental particle, it's made of smaller parts.

how does that affect what happens in the accelerators? Well, what happens is that you've got, as I said, you've got collisions just between parts of the proton rather than the whole proton. So you never have all the energy of the proton available. And the technical implication for that is that we can't control the energy exactly of the collision. So we actually have to throw an awful lot of protons at each other

before we get the energies that we want. So that's a big technical challenge. in itself, just the sheer number of collisions that you have to have in order to see something new, because most of those collisions are through very low energy parts of the proton, and they're really not very interesting from our point of view. Frank, are there any circumstances in which protons might decay? Well, there are, if protons are trapped inside a nucleus, individual protons, as in hydrogen, do not decay.

That's really, I think, the background to your question. but to clear things out, there are circumstances when protons decay in a form of radioactivity. That if you've got a lot of protons together in a heavy nucleus, as Simon alluded very early on in the program, their electrical charges make them very reluctant to be there. They're trying to force each other apart. So if there are too many protons there, there's too much energy contained in that electrostatic field. And it pays for one of those protons to change into a neutron. in that nucleus, and to balance the charge, it emits a positron, which is an antipartical

version of the electron. It's called a positron emitter, because by doing that, the proton has got rid of some electrical energy and turn into a neutron and change the nucleus. So positron emitters exist. Protons can decay in certain circumstances, but protons on their own, as in hydrogen, to the best experience we have, are stable. And there is a number. We know that if the proton does decay on the average it's only once in about 10 with 33 zeros of years that's I think a billion

billion billion times longer than the universe and you asked me next I can see you coming how do we know that well it touches on this quantum that's swap jobs you answer the question Melvin no no no no

it's like the half spin that they all do right so it touches on the quantum mechanics this thing that We do not know what an individual part was going to do, but we know if we've got enough of them that after a certain amount of time, half of them will have done something. So if we have a swimming pool full of water

with billions and billions of protons in it and wait long enough, do any of them decay? And the answer is no. So protons appear to be totally stable. You alluded, Simon, earlier on, to questions, to be asked, of quantum mechanics. What are the big questions you're still looking for?

I'm going to ask all three of you. So if you could be brisk because we're near the end, unfortunately. Ways to break quantum mechanics would be my, and that sounds like a funny thing to say, but one of my favourite quotes is from Isaac Asimov, I think, he said the most important words in science are not eureka, but that's funny. So you're always looking for things that don't fit your picture.

So what we need is some information that will allow, us to give more insight into the foundations of quantum mechanics because the fundamental particle physics is built on built on that theory. Helen? Well in terms of protons I think we're going to continue colliding that Large Hadron Collider and what we're hoping to see is something new coming out of that and that we're not expecting the what was that type moment that Simon's alluding to. And in our beginnings is our endings. Finally you, Frank. Well obviously something we haven't thought of is the simple answer. But what? we just mentioned, the stability of the proton.

I would love to see the explanation of how it is that electrons and protons can be united together in a theoretical sense, while at the same time keeping the proton stable. That appears to be the barrier. Well, thank you very much. I hope I can remember a lot of it. It was excellent. I really enjoyed that. Thanks, Frank Close, Helen Heath and Simon John. Next week we'll be

discussing the Alma Rabbit Empire in the Maghreb and Muslim Spain in the 11th and 12th centuries. Thank you. very much by listening. And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Melvin and his guests. I like this quantum mechanics thing. I think that we have this worry about quantum mechanics all wrong.

We're so used to the macroscopic world. We then try to understand it in terms of quantum. I think actually it's the quantum world is, if you like, the fundamental and correct one. And when you're thinking about one atomic particle, it does all these weird things. when you've got enough of them together, then they start doing the things that we're used to.

So we are made of lots of them, and that is the key thing. And you see it every time when you start a game of snooker, you split the pack. If you've got one ball on one ball, and you play the film backwards, you can't tell which way it's going. But you only need a couple,

two or three balls in the pack, and you can tell straight away which way it's going. Things change the moment a few particles work together. I think Newton's laws are for big things with lots of particles, and they emerge out of this quantum vagueness. So you're linking the two? Yeah, I think that quantum is the way things really are, and what we thought was fundamental,

like Newton's laws actually emerge out of this more fundamental stuff. And we get into problems when we think that Newton is fundamental and try to understand the fundamental stuff starting from Newton. the thing that I am leaning towards in terms of having the the picture frank is right in a sense is we used to do day-to-day experience we used to do solidity and ground and table and you know these things do not fall apart in a dissolution of waves partly it's the fact that in order for me to understand something that I can't see at such a small scale I need to in terms of a picture of it and the difficulty is when you take the quantum world and you're trying to internalize the picture you end up with this difference between the wave-like behavior and the particle-like behavior and fundamentally that may be a projection simply of the fact that we have two halves to our brain we have the instinctive emotional part and we have the rational intellectual part so we simply don't have the capacity to imagine

nature in any other way. But it's the resolution of those things. If I think about electric charge, electric charge is a single discrete thing that we always find in this confined lumps. We always find it in discrete lumps. And yet we're describing fundamental particles as being somewhere here and somewhere there. We're talking about a distribution of this wave packet. So it's, it's how you join those two together. Maybe the fundamental issue is that I don't have enough imagination to work out how you join those two parts.

But there's always this friction between these two parts of the theory. Alan, you would say something. Oh, I wanted to give you a wow number because it's one that always amazes me. What's that? Well, if you think, we talked

about the strong force holding the proton together and the, so the mass of the proton is 1.6 times 10 to the minus 27 kilograms. That's 1.67 divided by one with 10, 27 zeros after it. Tiny, tiny, tiny mass. And if you think about the protons pushing each other apart inside the nucleus, or indeed the upquarts pushing each other as apart inside the proton, then the forces that are feeling the repulsive forces are of the order of tens of neutons. So 10 neutrons is the force on one kilogram. from gravity. So it's just it's a macroscopic force. It's a sort of force that you can feel and you know, for carrying a kilogram around all day.

You'll begin to notice it. And it's acting on these incredibly tiny particles. So the force of the whole earth acting on each of us is the same order as the force on those two little particles. When do you make of that? It's wow.

I mean, you're right, it's a wow thing. When you were saying, is it our conscious problem? I'm fascinated. If I was starting today, I don't think I'd be doing particle physics. I think I'd be fascinated by the nature of consciousness. And the question I keep thinking, you know, as a particle physicist, one atom is the same as another atom.

How many of them do you have to put together before they think that they're you? Before they become self-aware. When does the leap happen? Yeah. It was clearly a number that is smaller than 10 to the 34 or so, because that's what we're made of. It's clearly more than 10. but there must be some order of magnitude.

I don't even know how you start addressing that question. But that was the thing that my, when I read Bill Bryson's book, which was really his journey into understanding, and it started off with this thing that you've got these atoms moving around, and for a brief period of about a century, they think that they are you. And that was one of those mind-blowing,

that's the right metaphor. That moment, I thought, wow, it suddenly hit home, that. It was a great one. And it's troubled me ever since. I suppose I would be remiss being a particle physicist and not mentioning Schrodinger's cat having talked about quantum mechanics I thought you would or you wouldn't

I'm on that note It's this story that Schrodinger said that if you take a cat and you put it in a sealed box and then you have some radioactive source which has a 50-50 chance of decaying and if it decays then the cat will die, you don't know whether the cat is alive or dead until you open the box, and because it's an isolated system, then the cat is in this superposition of states,

as we say in quantum mechanics. For me, the reason why he was talking about it was the ridiculousness of quantum mechanics when you apply it to the macroscopic scale. So if you take a person, a person has consciousness. We have this collection of atoms which gives us a sense of self and awareness. So at what point does the cat not know that it's alive or dead at the same time?

It's this extrapolation from the quantum world, which works perfectly on those scales to our day-to-day experience. That's the rub, as it were, between the two. I think we're about to be interrupted, which is unfortunately, please. He's the producer. Coffee, please. Coffee, please. In our time with Melvin Bragg is produced by Simon Tillotson. Hello, I'm May Martin from Grownup Land, the podcast where each week, Bishika Ailey and Ned Cedric and I untangle the adult world's most complex issues with the help of programs that you can hear on BBC Radio 4.

Yeah, we only really deal with the big stuff. How close do we have to be for you to get a friendship tattoo with me? I could do it for you. I've done it with a needle and a bick pen. Wait, a needle and a bit... Were you in prison? No, I was at a dinner party and things got out of hand. I mean, that is an out-of-hand dinner.

But when a dinner party gets out of hand for me, we crack into their parents' port. That's grown-up land, and you can find it wherever you found this.