Gerardus 't Hooft (2016) - How One Single Elementary Particle Can Make the Difference

As the other Laureates have expressed, I am also very thankful and highly honoured to be here in Lindau to talk to you, together also with all the other Laureates. So when you would be flying in an airplane high above the border between France and Switzerland you will see this beautiful landscape. You will see the Mont Blanc here somewhere. You will see a beautiful landscape. Here you see the lake of Geneva and the famous fountain at this spot. What you will not see from the airplane is this circle because it is a tunnel, about 100 metres at some places under the surface. What you definitely will not see in this tunnel is that there are particles moving in opposite directions at practically the speed of light. And experiments are being done to measure the effects that these particles will have on each other when they collide with the highest achievable energy per particle. You want to have particles collide in opposite directions against each other. That’s the most efficient way to fathom the smallest distance scales in the universe that we can put our hands on and that we can study. This machine is to be considered as the biggest microscope of the world that we can make today. And when you would be inside this machine, inside this tunnel, you will find this construction. These blue pipes that you see are magnets, each 15 metres long and 35 tons in weight. They generate the strongest magnetic fields that you can produce at this scale and this stability, using superconductivity again. And these magnetic fields are necessary to force particles to move in circles rather than in straight lines. And then they are electronically also being accelerated. This is one of the detectors that is made to measure what happens when particles collide. When particles collide at this energy, thousands of other particles are produced and they fly off in all directions. What you want to do is determine their identity, their masses, their velocities and so on, and in particular also the statistical distribution of how they are being produced. And you want to gain the maximum amount of information out of these. The lowest picture that you see here is the so-called ATLAS detector being built. Today you wouldn’t see it this clearly anymore because the whole thing is now filled with electronics and other apparatus to measure the properties of these particles. To just illustrate how big the whole thing is, there is someone standing here, if you look very carefully. So the rule of physics says that if you want to measure something accurately and you want to gain new territory, the apparatus in general has to be big. Like in astronomy the bigger your telescope the further you can see in the universe. Some people have the mistake, thinking that if you make something very small you can see very tiny things - that’s not the way it is. A fly has much worse eyes than we have. A fly cannot see another fly as clearly as we can because their eyes are so small. You need big eyes and the bigger the machine is the more accurately you can see. So one of the assignments, one of the first assignments that this machine was given was the assignment to find the Higgs particle. Now, of course, you can ask why is it that we really want such a particle? Is it true that this particle is responsible for the masses of the other particles? Why is it called the “God Particle”? Well, in this talk I want to claim that actually the story is a lot more complicated than all that. The Higgs particle is not a particle that creates mass or something like that. It has nothing to do with God of course. The actual story is much more interesting than the religious story, because I want to claim that the Higgs particle is a success, a prediction of theory. Other particles have been discovered by experimental observations - the J/psi particle was a beautiful experimental discovery. And only theoreticians could later say why they should have predicted it, but they hadn’t. And there are many examples of that, but the Higgs particle is the exception. It is really the outcome, the glorious outcome of theoretical investigations. So this talk is not about the sociology of science, for instance, how to choose one’s advisor or how to cheat on exams. No, I just have to claim I happened to be very lucky in my career. I was at the right place at the right moment and with the right advisor at the right moment. And all this in spite of my clumsiness in these matters. So this talk is about how to do good science. And I want to inspire all the people here and make them realise what good science really is. It is often not by having a great hunch or a great feeling, now we have to do it this way, now we have to do it that way. It is about being precise and making very accurate observations. If your observations, if your analysis is a bit more precise and a bit more imaginative than what your predecessors have done, you might be making an important discovery. And this holds for many examples in the world. For instance, if you just look up the history of science - just 400 metres or so from here, there is a marvellous library showing you all the works of Copernicus and Keppler. They were just more clever in their observations than their predecessors. And that was the beginning of marvellous discoveries of modern science. It happens over and over again. You can look at the heroes of the past like Newton or Maxwell, Einstein, Dirac. We all know the famous names of physics. What these people have done was they looked more carefully, more accurately. They had more imagination about what’s going on. And that’s the way they made their discoveries. In my time as a student, the happenings inside the atomic nucleus, in particular inside the subatomic particles, were one great mystery. There were lots of things known because many of our predecessors – of course, hundreds of Nobel Prizes had already been awarded in this field. So yes, many important observations had been made but there was still very many mysteries left. For example the weak force. We knew a lot about the weak force. We understood that there are certain conservation laws. If you make substitutions of this sort then you see that you have to substitute 2 particles at a time. And then the weak force seems to be a very universal kind of force. So there are conservation laws, conservation of electric charge. But also conservation of something you could call weak charge or something that made this force remind us of electricity and other forces that were understood much better than this particular force. So one of our problems was understand the weak force. Besides that there was also the so-called strong force. And that was another chapter in our history of subatomic physics - equally interesting by the way. But my talk will mainly be about the weak force and about the new particles that are being suggested by investigating the properties of this weak force. If you look very carefully you say, hey, it looks as if there are more particles involved. You have heard Professor Veltman’s talk yesterday. And he explained that one gets the strong impression that this weak force is mediated by another particle called the 'W particle', W for weak - although maybe Weinberg likes to think that the W is for Weinberg. But maybe it’s really the intermediate force-carrying particle, just like a photon. And it looks a little bit like a photon. And then we needed another particle which I already mentioned and which was very important, the so-called Higgs particle. And now when people say the Higgs particle produces mass then that is a sloppy way of phrasing things because it works very well in publicity statements. So if you want to impress the public you say, we are discovering the particle that makes mass. But it must be awful to think of that. And it also works very well when we make a research proposal: 'We want to find the origin of mass.' If you know what that means, please tell me. I don’t think the Higgs particle is responsible for mass. But it is strongly related to the way massive particles work. And its interactions can be seen to be strongly related to the mass terms that are in the creations of particles. There is something, however, that’s rarely mentioned when people talk about the Higgs particle. It’s not only responsible for mass, it is very strongly related to another property of particles, besides mass they have spin. Everything in the universe, whenever it moves freely in outer space, it rotates. Galaxies rotate, stars rotate, planets, asteroids - they all are tumbling around in some way. And so do the subatomic particles. Now, I have to apologise to all the tennis fans and soccer fans here that I only could find a rotating soccer ball, I tried to make it look like a tennis ball. But anyway, they both rotate and spin, as you know very well, and that is very important when you play tennis or when you play soccer for that matter. So the spinning particle is very, very essential. So a detailed description of the weak force revealed that subatomic particles indeed are rotating, just like a tennis ball which is affected by the fact that it rotates. Also the motion of a subatomic particle is affected by the way it rotates. So if you are clever enough you can figure out the rotating motion. What you use here is actually very simple, rotation is associated with a conservation law, the conservation of angle momentum. And we all know what angle momentum, in principle what that is and how that a kinematic property of things which is revealed in a way that objects move. So yes, by carefully studying the motion of particles you figure out that particles can actually rotate. So when you consider the weak force and you realise that actually the weak force takes place in 2 steps, then you find that the particles there can change their electric charge which means that indeed these particles are fundamentally different from electromagnetism. The electromagnetic force never changes the charge of a particle – it affects particles which have mass, but the mass itself remains the same. During the weak force, when you look carefully here, this could be a charged particle, this could be a neutral particle. So electric charge is transported from here to here. So this is a photon that actually transmits electric charge. It also transmits weak charges. These things, in some sense that you can only describe mathematically, these particles also carry weak charge. And that turned out to be very important because it looks as if this force, the amount of weak charge that you see in these particles, somehow seems to be the same in all the weak interactions. So there is something universal about weak charge, just as universal as you have electric charge. As you know, when you study atomic physics or even sub atomic physics, all particles have a very typical amount of charge, equal to the charge of the electron or the proton. The sign may be different, sometimes they are neutral, but the charge comes in very specific, well-defined units and we have something similar going on in the weak force. So far what we understood about the weak force. But there is more. Again, honoured by a Nobel Prize, was the discovery that there is something funny going on with the weak force when you look at left- and right-handed objects. So I like to collect shells, particularly, when I notice that some shells rotate opposite than other shells. And this is the property of the species. What you see in this picture is that there is a right-handed object and a left-handed object. And they look very similar, as if maybe somewhere during the evolution nature made a little error and turned this one into that one and its properties remained nearly identical. Then they evolved a bit, so if you look carefully it’s not exactly the same but it’s nearly the same. We have a similar situation in the elementary particles. Some particles look very much like the mirror object but not quite, there are subtle differences. And these had already been studied and understood qualitatively reasonably well, but they also caused a problem. It was this problem that eventually led us to believe there must be such a thing as a Higgs particle. Because how can you understand that there is a left-right asymmetry among the forces of particles? Well, that actually wasn’t so easy. But what you can say is that when a particle moves very fast you can look at how spin goes in relation with the motion the particle moves. So if the particles moves very fast you can say does it spin to the left or does it spin to the right. Those are the 2 mirror opposites that you can have. And since particles do have spin this property called helicity is a very important property that you can measure and you can check how things behave. And then there was something very remarkable going on with the weak force. Those weak charges, as particles K, seem to depend on helicity. So left-rotating particles have weak charge, where right-rotating particles are often neutral. So this holds for neutrinos, for instance. Since neutrinos only feel the weak force, nothing else, you can’t even make right-handed rotating neutrinos. So for a long time they were thought to be totally absent. Only more recently in the advances of the standard model it was discovered that, yes, right rotating neutrinos do exist but they are very, very difficult to observe. But we see their effects. And now there is something marvellous, something very nice about electromagnetism. If you have a photon interacting with a particle here then the helicity stays the same. So if this is a left helicity particle then the thing that emerges after photon interacted keeps its left helicity. That holds for photons. It was found there is a similar conservation law for the weak force. And that is how the weak force can function: that helicity is conserved and the weak charge depends on helicity and the weak charge is conserved. Now, all this if fine but all this did create a problem. Helicity can be very well defined for particles without any mass. And that is because those particles then have to move with the speed of light. They can never stand still because if they stand still they have no mass, no energy, no nothing. The particle simply is part of the vacuum space itself. Such particles do not exist. So only massive particles can be put to rest. In fact, all particles with mass can always be caused to stay at rest. Now, what is the helicity of a particle that stays at rest? Well, since I can’t define in which direction it moves, I cannot define helicity. So if particles move very slowly, then helicity is an ambiguous property of particles. If you move faster than that particle, its helicity just flips around. So helicity is an ill-defined quantity for particles with mass. Now, here was our problem, all particles that are associated with the weak force carry some amount of mass, some more than others, but have mass. So helicity is not a well-defined concept. So now what do you do? And here was the other way in which the problem manifested itself. Veltman explained in his talk how he kept finding infinities when you have these closed loops in your diagrams. So here there is a closed loop. When you have a closed loop, your computation of what happens in these processes becomes a lot more complicated. And that’s where something can become infinite. And in fact all these diagrams did carry infinities in them and it seemed to be impossible to renormalise those. And Veltman then explained in his talk that, well, he figured out that these particles must interact in more complicated ways as well. So he stumbled upon the Yang-Mills theory, which was a theory, which was designed for pure elegance and pure abstract ways of thinking. That seemed to me more beautiful than other ways of describing particles which behave a little bit like this object. But that theory didn’t quite describe the weak force correctly because these intermediate particles carry mass again. And when they carry mass again they show problems with spin. Problems a little bit more complicated than what I said about these animals which have either helicity left or helicity right. But these particles also have a mass. And so helicity for them is also not a well-defined concept. And that also caused difficulties in formulating, how electromagnetism should be generalised to describe these particles. In other words there were difficulties in getting an accurate description of the weak force. And you would not have noticed these difficulties if you didn’t do accurate calculations. And that’s what Veltman could do very, very well. He was in fact one of the first to design a computer program for doing algebra on a computer. You have to remember this was the 1960s. So nobody had done algebra on a computer, computers were enormously complicated things. The big computers for which he had to travel to Amsterdam every time he wanted to do a calculation, those big computers had much less computation power than your telephone has today. But he had to go to Amsterdam with whole stacks of punch cards which he had to feed this computer. And then he had to wait for a day and then the computer came with his answer. So that was a very difficult way of doing things. But only if you did thing so accurately, you found that there were difficulties in this theory. So what was the cause of this difficulty, and how could you resolve the difficulty? And then Veltman explained in his talk, "Out of thin air came a particle without spin and that particle was the Higgs particle and it solved my problem." Well, I want to explain here that it wasn’t quite out of thin air. It was very accurate and logical reasoning that was needed to understand how this Higgs particle could resolve the remaining difficulty, the difficulties with the spins of all these particles. And the resolution actually already existed, but had not yet gained much attention. And these two gentlemen, Peter Higgs and François Englert, for their discovery in 1964, around that time – their discovery gained the attention of the Nobel Committee, but only much, much later. In the early days what they discovered seemed to be totally esoteric. But they found a procedure that is called 'spontaneous symmetry breaking'. There is a third name here, Robert Brout. But it took so long for this Higgs particle to be finally observed, when these two gentlemen gained the Nobel Prize together. It was in particular Englert, but I think also Higgs, who both expressed their deep sorrow for the fact that Robert Brout had deceased just a few years earlier. So he couldn’t feel the joy of his work finally being recognised in Stockholm. But that’s the way it is, Stockholm doesn’t give any post-mortem prizes. You wouldn’t be enjoying that so much anyway. So this is sad, but Englert expressed in his, when he accepted the Nobel Prize, he said, so they left the space open for Robert Brout who should have shared this prize." They proposed something called 'spontaneous symmetry breaking'. And what spontaneous symmetry breaking means is - well, spontaneous symmetry breaking changes everything in the behaviour that particles have. And for instance it adds mass to particles. So it’s not so much the Higgs particle itself, it’s the whole phenomenon of spontaneous symmetry breaking that can add a mass term to the equations of particles. So spontaneous symmetry breaking can be very easily explained, particularly if you come from Mexico and you wear a Mexican hat. Then this Mexican hat has a perfect rotational symmetry. So if you put a tennis ball right on top of the hat, it will stay there forever. Well, maybe if you are a theoretical physicist you will say, it stays there because it can’t choose any direction to fall into. But any experimental physicists will say no, no it will fall in the rim. But when the particle is in the rim, the theory gains a new degree of freedom, that it can go around in the rim. But this particle then takes the place of the field of some new degree of freedom in nature. A degree of freedom meaning that you describe a new particle. As long as a particle was sitting upstairs it had 4 degrees of freedom, it could rotate in 4 different directions of which only 2 are being displayed in this picture. But once it sits in the rim 3 of the 4 directions become indistinguishable. So they do not correspond to particles. Only 1 component does correspond to a particle and that is when the particle moves in the radial direction because the total combination of all fields remains constant when you transform it. And that combination of field corresponds to a particle. That was going to be the Higgs particle. So yes, this requires some advanced quantum field theoretical reasoning that, I presume, many of you will not follow in detail. All I wanted to say with this picture is that as a phenomenon called 'symmetry breaking' - And the outcome of that is that there is a sufficient amount of asymmetry which also affects the weak charge. The weak charge can now be communicated to the vacuum, so that it’s no longer exactly preserved. And this way we could figure out by carefully looking at the equation of the theory that this way you change everything. And you make the conservation of weak charge still a useful property that you can apply to see how the infinities disappear from the Feynman diagrams. So also the infinities, as Veltman explained, had to do with what happens at very small distances, if you look very, very close when particles come very, very close. When particles come very, very close the symmetry breaking disappears. So now you can use the perfect symmetry of the weak force to remove all the infinities that you encounter. So it wasn’t completely an accident, it was a fundamental physical property of symmetry breaking that would now resolve the difficulties. So should you call this a God Particle? Well, the word 'God Particle' comes from a physicist in an adjacent topic of physics who called this the, well, expletive deleted particle. And so then a journalist - well the journal in which he said this refused to print that word that he used, but they said, well, let’s call it just a God Particle. And then since God must be responsible for creating mass, this was picked up by the public and now, of course, adding mysticism to the whole story. And my goal is to remove the mysticism again, nothing mysterious about what we are doing. So this particle eventually provided for the last link in the standard model. This is my way of depicting the standard model as a box of toys because we love to play with these things. But they are the fundamental constituents of matter. But now this one particle with this perfect round symmetry, the Higgs particle, was now the one missing link in the standard model. It has now finally been observed and measured in the LHC. So yes, in 2012 this particle was actually announced as being discovered by the LHC measurements. How does it go in practice? Well, it’s very difficult. If you collide particles with the energy I just explained, the maximum amount of energy that you can reach is about 7 teraelectronvolts per particle. At that energy you can finally make this Higgs visible. But even at that energy only one out of a trillion collisions produces a Higgs particle, a physical Higgs particle that you can measure. And it is produced in a number of different ways. You can have 2 gluons coming out, making a triangle diagram and out comes a Higgs particle. You can have 2 gluons produce different ways of Higgs particles with a top and an antitop particle produced and so that the quarks can come together and produce indirectly a Higgs particle. And the Higgs particles in turn can decay in many different ways. So they check all the decay processes that can occur and then identify the possibility of a particle being produced. What does this all lead to? Well, you have here a curve. And the green curve here is the curve if you assume no Higgs present anywhere in this domain. And the little bump here occurs when certain outcoming particles have a joint energy of a little bit over, well, somewhere around 125 GeV, giga-electronvolts. At that point a small peak appears. Now, it’s the problem of experimentalists to find out how significant is this peak statistically. And they find at some point it went more than 5 sigmas, 5 standard deviations significance. And this turned out to have exactly the properties of the particle. This is the discovery. But they found in other channels also a bump at exactly the same mass value, about 126. Later they found the more precise number, now they have more accurate measurements: I don’t know how many minutes do I have left? About one. About one. (Laughter) So I will very briefly mention about present investigations because this Higgs particle was, I think it was a glorious result and we are all extremely happy that the theory was finally vindicated. We now understood of this all more, better than ever before. So what are we doing now? Well, one of the major difficulties in our field of science, the fundamental properties of matter, is the occurrence of black holes. Black holes have already been mentioned several times here. But it’s the idea that space and time get curved. I have here 2 regions of the universe, here and here. The green and the brown region are 2 parts of the universe. But the black hole occurs, it occurs necessarily in accordance with Einstein’s equation. So it put too much mass together. And then this thing will happen. There will be what we call a wormhole that connects 2 regions of the universe. It sounds very mysterious, so mysterious that still some people doubt that it actually occurs. But if you do the calculations, again precisely enough, there is no doubt that black holes can be made. And in fact they have been observed by some of us in many different ways. So there’s no doubt about the existence of black holes. But the difficulty only comes when you try to apply quantum mechanics to this system. If you say, well, the black hole should, as a whole thing, also obey the laws of quantum physics, of the tiniest elements of matter. Now, black holes normally are very, very large and not very tiny and being large you don’t notice any quantum effects. But if you do, and this was sort of latest discovery and I think it’s very important if you understand what it really means, there is something very strange. If you have here again a black hole like this and you would enter at this side of the black hole you would re-emerge at the other side. And people had never realised that this is a prediction of quantum theory. That if you come in one side you re-emerge at the other. But you re-emerge time-inverted, parity-inverted, charge-inverted. So in a very strange way you re-emerge there. You would say it can’t be possible. It isn’t because there is a little problem here. You’d only re-emerge if inside you could go faster than the speed of light. You can’t and therefore you don’t really re-emerge. But the quantum oscillations do re-emerge. The quantum oscillations here are correlated to the quantum correlations here, or entangled, as you say today. A marvellous new discovery. And I think this is going to tell us something about the structure of space and time in ways that we only will understand in the future. In other words, there is lots of things to be done in our field in the future. And the chair is warning me that my time is over. Thank you very much.

Gerardus 't Hooft (2016)

How One Single Elementary Particle Can Make the Difference

Gerardus 't Hooft (2016)

How One Single Elementary Particle Can Make the Difference

Abstract

Billions of Euros have been spent recently just to find out more about the tiniest structures in the sub-atomic world. One of the main tasks of a new particle accelerator, the LHC, was to observe a new particle species, the Higgs particle. Exactly this particle was one of the main themes of my thesis in 1972. This particle would have to be quite different from anything known at the time, but without such a particle our theories about the electro-weak forces would not make sense. We experienced it as a triumph when finally the detection of this particle, exactly with the expected properties, was announced in July 2012.

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