The Global Environment: Critical Issues for the Next Century - Henry W. Kendall Memorial Symposium at MIT (5/5)
WILCZEK: So I'd like to tell you today, as best I can in 45 minutes, in non-technical terms, the story of how scientists, with a very key contribution from Henry Kendall, as you'll see, have figured out some of nature's deepest and most beautiful secrets.
In 1931, after two decades or so of the most remarkable progress in physics in all of history, the model of atoms that we still use today emerged. This was the era of quantum mechanics and the model of atoms that arose was that atoms consist of a positively charged nucleus, which is very small, and which is where most of the mass of the particle of the atom is concentrated. Surrounded by a cloud of electrons which are all negatively charged and fill up a big space. And I've tried to crudely, in a cartoon fashion, indicate here that they're both waves and particles. But I won't even attempt to explain that.
Here. This is not drawn to scale, and the atom of course, is much smaller. In fact, the nucleus has a size of about 10 to the minus 13 centimeters, which is very, very small. Not only in absolute terms, but even compared to the size of atoms. Atoms are about 10,000 or 100,000 times larger than the nucleus inside.
Since for most purposes you can consider atoms as the indivisible units of matter, and certainly in chemistry they're so considered, it's very difficult to probe the inside of atoms using chemical or ordinary means. One needs to go to some extraordinary lengths to probe the inside of atoms. And it's, I think, a miracle, even in retrospect, that it could be done and figured out.
But in 1931, when the problem of understanding the inside of the atom, the nucleus, really took its modern shape, it was appropriate to draw it as this drawing here. As a black box. Then, the first breakthrough in understanding what this black box consisted of was made by one of the people in this picture, James Chadwick. You can probably guess which one it is. I'll give you a hint. It's the one who looks very satisfied with himself.
First, Chadwick discovered the elementary particle called the neutron. And with that discovery it became clear that the nucleus of atoms could be considered as made up out of two basic building blocks. Two more elementary building blocks, protons and neutrons. Protons had been known before, although not their essential nature, as the nuclei of hydrogen atoms. But single neutrons are unstable and had to be discovered many, many, many years later by Chadwick in 1931. '32, Maurice tells me.
It won't be essential for what follows. So at that time in 1931 or 1932, the problem of understanding protons and neutrons rose to the top of the physics agenda. Physicists had understood atoms, at least in principle. And the frontier was to understand this deep inside. This deeply mysterious black box of very small and seemingly inaccessible, the atomic nuclei.
It soon became clear that, essentially, new physical laws would be involved in understanding atomic nuclei. And in particular what held them together. The known forces at that time, the gravitational and the electric forces, clearly wouldn't do the job.
The electric forces, in fact, would tend to make the nuclei blow apart because like charges attract and-- I'm sorry, like charges repel. And one has lots of positively charged protons and the neutrons have no electric charge at all. And gravity is just much too weak acting on such small amounts of matter.
So, a basically new force was realized to be involved and got to be called the strong force because it's the strongest force in nature. It takes the strongest force in nature to oppose these-- make these particles, against the will of electromagnetism, congregate together in a very, very small volume.
So to figure out what these forces were, first people just collided protons and neutrons and tried to measure how they deflected each other. And from that infer what the forces were. And what resulted from those efforts over many years, by many physicists, was that the forces were extraordinarily complicated.
People were perhaps hoping that they'd find some simple analog of the 1 over r-squared forces that characterize gravity or electromagnetism. But they found instead complicated forces that seemed to depend on distance in a complicated way. Depended on how the particles were spinning, on the energy in a complicated way, and just didn't seem to be simple at all.
So they had the bright idea of trying to look inside the protons themselves, or somehow find substructure in the protons. And hoping, I guess, just to find out what's going on. But hoping, I suppose, to find simplicity in some smaller bits out of which protons were made. Just as the forces between atoms are very complicated but reflect the fact that in atoms you have many electrons interacting altogether and that gives you chemistry out of fundamentally simple interactions between individual electrons.
If the hope was for finding something simple by banging the protons and neutrons together, and finding simpler things coming out, the result was a nightmare. Because what you find when you collide protons or neutrons at high energy, is that more comes out than went in. So instead of things getting simpler, you find out this completely novel situation in physics that you get the same stuff plus more. And things get, instead of simpler, nightmarishly more complicated.
This is really e equals mc squared read backwards. One is accustomed to thinking of e equals mc squared as the obtaining of energy from mass. In a nuclear explosion, say, you bring together some nuclei, they lose a little bit of mass, and energy is released.
Here, we get to read the equation in the opposite way. We've put in a lot of energy, protons moving really fast to each other. And we can get out more mass than we started with.
So, a typical result when you collide protons at high energy might be that you collide two protons. And out comes not anything simpler, but four protons, an antiproton, and even new particles that were unknown before. Anti-Lambdas and Ks and many, many more.
And soon, one had extensive tables. The so-called Rosenfeld Tables of all the various unstable particles that could emerge from proton collisions in addition to protons themselves. So, this is-- well, particle physicists in those days used to carry around something called the Particle Data Book which was conveniently made in wallet-sized form and came with a magnifying glass. And it contained roughly 100 pages like this of data about all the different particles that were known.
Through a series of ingenious deductions and brilliant leaps, some order was brought to this chaos of particles. And it was realized that one could rationalize their properties in terms of a few, simpler configurations of hypothetical substructures. That is, all these different particles could be understood as being different realizations of a few basic configurations of a much smaller number of particles.
The smaller number of particles were called quarks. And the different kinds of particles that were observed were mesons, which consists of a quark, and you see this thing, which is a missing quark, is an antiquark. Or out of three quarks, which make a so-called baryon. Or three antiquarks. I didn't want to draw these complicated things three times. These are the antibaryons made out of three antiquarks.
And then there were three different kinds of quarks. Up, down, and strange. And by having the quarks move in different ways or be vibrating in different ways, one could account for the vast number of particles that were observed. Many people contributed to this picture, but perhaps the most numerous contributions were made by this fellow. That's Murray Gell-Man, and he'll play a role in a joke to come.
But, although this rationalized the vast proliferation of particles to a certain extent, there was a very big weakness in this modeling. No quarks. You had, sometimes, a quark and an antiquark. Sometimes three quarks. Sometimes three antiquarks. But never a single quark.
And certainly no theory of the forces. In fact, they'd have to be some very, very peculiar forces so that quarks couldn't get out even though they were in. And, of course I've simplified this story. This fact that the quarks weren't observed and that there was no theory of the forces that might hold them together made some people skeptical. And there were various competing ideas, in fact, and there was a lot of frustration.
This was the period when some physicists got involved in analogies with Buddhism and Hinduism and the Tao of physics, and the early days of string theory, and other things. In retrospect, it's clear that what was missing was the right tool. And this turned out to be the right tool to clarify the situation.
This is the two mile linear accelerator at SLAC. If you-- or it is SLAC. I'm sorry. At Stanford, or wherever it is. And if you notice the change in topography there, this is cleverly built over the San Andreas Fault. But it's accurately linear.
It's no longer over the San Andreas Fault, I'm told. Here it is with later editions and in color from the Web. And using this tool, which I'll explain a little bit in a moment, three heroes, Professor Taylor, who is in the audience today, Professor Friedman, who I believe is in the audience also, and Professor Kendall, who we've been remembering and honoring today. Along with many others. But they spearheaded the experiments that took the understanding of the strong interaction to a new level.
Basically, this new instrument is a very sophisticated form of microscope. You see, to study the structure of atoms, let alone atomic nuclei, using ordinary light is totally insufficient. Ordinary light has a wavelength which is much, much larger than the atom, let alone the nucleus.
And so when you try to see it, it's like, well, it can't be done. The resolution is just too poor. Those of us over a certain age will know what it means.
You need short wavelengths to resolve small structures. And to get sufficiently short wavelength photons, that translates into very high energies and very large momentum transfers. And the way to produce such photons is to accelerate electrons and allow them to collide with other charged particles.
Those collisions are mediated by so-called virtual photons whose energy and momentum and wavelength you can infer from how much the electron is deflected. And by sophisticated analysis of these scattering events, you can employ these photons as a precise tool to look inside the protons and neutrons.
And what Taylor, Friedman, Kendall, and their associates found, when they looked deep inside protons and neutrons, was that they were smaller things inside that seemed to have very simple structure where the charge was concentrated. So the protons and neutrons were not cloudy objects with a distribution of charge, but objects with little pellets of charge inside.
This interpretation of what they were finding was pioneered by this fellow. Professor Feynman and also by Professor Bjorken, who's in the audience. Feynman called the things inside protons partons. And he had a simple and bold theory that partons should be very, very simple and used them to analyze the results of these experiments successfully.
Feynman and Gell-Mann were great rivals. They were both professors at Caltech. And Gell-Mann hated partons. Mainly, he hated the word. Of course he didn't hate partons.
My first-- and I can tell a little personal story here. One of my first experiences in physics and my first encounter with Gell-Mann was in Aspen in the summer of 1973. And being a young and naive student, I made the mistake of mentioning that I was working on partons. And I was treated to an unbelievable tirade.
Partons? Partons? What are partons? Oh, you mean put-ons? Put-ons? Those things Feynman talks about? They're just quarks, you know. There's no such thing. So, Gell-Mann also has a license plate on that says "quarks", one of these vanity plates. So that very night, I cut out a piece of cardboard in the shape of a license plate. I wrote "partons" and stuck it on.
Unfortunately, I think the wind blew. I don't-- not sure if he ever saw it. Anyway. To be fair, Murray wasn't the only one who had trouble with partons.
Because the interpretation Feynman was using was assuming that these particles inside, whatever they were, had very, very simple, ideally simple properties. In fact, too damn simple. They couldn't be reconciled with other things we knew about the general principles of quantum mechanics. This is a little bit subtle, but please bear with me because it's really the heart of the story.
Why do I say these particles shouldn't be, couldn't be, too simple? And it is paradoxical that they seemed to be as simple as they were? Well, it goes back to a phenomenon that we learned about in elementary electromagnetism. And it's been known since the 19th century that if you have particles that charge particles in a medium, there's a phenomenon of screening.
So for instance, if you have positively charged particles. Put a positively charged particle inside a medium that contains polarizable molecules, the molecules will tend to deform. The negative parts of the molecule will move towards-- will be electrically attracted towards the positive test charge all over. And as a result, that charge will tend to be neutralized.
This is what we call screening. So the charge you see if you're far away in a medium is partially canceled by the medium itself. And you see a smaller charge, we say. Or conversely, if you go closer and closer and look finer and finer, you should see a bigger and bigger charge.
Well, what does that have to do with scattering of elementary particles, which after all don't seem to be in a medium? Well, actually, in our modern understanding of quantum theory, so-called empty space really is a medium. There are quantum fluctuations going on all the time where virtual particles and antiparticles live for a short time and then reannihilate.
So I can draw the same sort of picture as I draw in a medium. But now with the axes being space and time instead of just space. Just different directions in space. So if I have a positive test charge, which just sits at one place in space, but lives along what we call its world line, moving along in time. It will polarize the virtual particles and we can have exactly the same sort of screening effect.
The normal expectation is that in any kind of theory that obeys the laws of quantum mechanics, one should have this kind of screening effect. And therefore, as one moves closer and closer to elementary particles, especially if they're strongly interacting, there's a strong force among them as we know the strong force has to be, one should get big screening effects. That's certainly how it would be if the strong force were just a heftier version of ordinary electricity and magnetism.
But what Taylor, Friedman, and Kendall saw, and what Feynman and Bjorken were using, was no structure at short distances. So things weren't getting more complicated. One wasn't peeling away some screening to see a bigger and bigger charge at short distances. In fact, one was seeing something with ideally simple properties at short distances.
So instead of the implication, I mean the implication of screening, which seemed to be the general phenomenon one should expect in quantum field theory, is that the closer you look, the more complicated things should look. But the experiments showed a gift from heaven. The closer you looked, the simpler things behaved. So one wanted a theory with the opposite behavior of antiscreening.
Now, since the heuristic explanation I showed you of screening is so simple, I hope you won't find it implausible that it's very difficult to find theories that have the opposite behavior. However, we found that there are such theories. In fact, there's almost only-- in essence, there's only one such theory. So from this very, very general kind of consideration, one is led to almost a unique theory of what the strong interaction has to be.
And it turns out that the theory one is led to is a generalization of electromagnetism. It's now called quantum chromodynamics, or QCD. The essence of which is that unlike-- and it's a generalization of electromagnetism, but with a very important twist we'll come to in a moment. It's a generalization with three different kinds of charges.
They're called colors. Of course, they have nothing to do with ordinary color. They're just three different kinds of charge. They also have nothing to do with electric charge. But they're the things not that photons care about, but what gluons care about.
Now, if one just had three copies of electromagnetism, one would still have screening and one wouldn't have advanced at all in this problem. However, when you have three different colors, there's another class of things that can happen. Not only can you have photons that respond individually to the colored charges, but you also have the possibility of one color turning into another.
And there's a richer theory, which is what QCD is, in which one includes not only gluons, which sense the different charges, which would be analogous to the photon, but also other kinds of gluons which actually change one kind of color charge into another. So instead of having three photons, it turns out you have eight. And some quick-witted person should ask me later why they're not nine, but I will save that for a question.
Now, that difference, that possibility of one color changing into another, and these new types of gluons, change the qualitative nature of the theory quite substantially. Because now, unlike in the case of photons, which are electrically neutral, the gluons themselves carry the charges.
So for instance, this gluon which changes green charge into red charge itself has one unit of green charge, and minus one unit of red charge. So that when it does the changing the charges can be conserved. And the consequence of this, since that gluons respond to color charge, and at the same time carry color charge, is that gluons interact with each other.
Let me bring this down to earth a bit. Well, let me bring it closer to experience. Many of you have probably seen the movie Star Wars. The original one. And laser sword fights play a very prominent role in Star Wars. Well, in the real world, laser sword fights would be very dubious affairs.
Because photons don't interact with other photons. So these laser beams would just merrily pass right through one another. Of course, it's a futuristic movie, and perhaps those lasers are actually lazing color gluons, then it would work. With color gluons they would really clash against each-- in fact, they'd produce very impressive explosions, presumably.
But there's a big difference. So photons just go through each other, the gluons interact with each other. And it's much more intricate, then, to understand what happens when you have the possibility of screening. Because the gluons themselves can move around the charge, distribute it in different ways.
And if I had another hour, I could probably make it plausible to you that this phenomenon actually leads to antiscreening. But I'll have to leave it at that. It does. By calculation.
So, one has gluons that interact with each other, and then the interaction among the quarks, which is ultimately responsible for the existence of protons and mesons. And the structure of all the strongly interacting particles is supposed to be due. At its core, the elementary process that underlies it is simply the exchange of these color gluons. So it's sort of a vast, but recognizable, generalization of what we know about in electrodynamics.
Well, since this theory was formulated, it's been tested in many ways. One thing that's happened is we progressed from linear colliders to circular ones and higher energies. And at this large electron positron ring near Geneva, and at many other accelerators, one has been able to test the theory, which was first invented just to understand the Kendall et al. experiment to work out its implications for all kinds of other experiments.
And here you see what, to me, is the most beautiful graph in all of physics. Which indicates the data from a large number of different kinds of experiments. Each of these data-- many of these data points represent the results of hundreds of independent measurements and the antiscreening effect, which one can calculate in the theory, is the fact that this curve goes up this way.
So that at-- well, at higher energies, which correspond to shorter distances, the effective strength of the strong interaction is getting smaller and smaller. So this one is plotted in increasing energy, which is the same, it turns out, as smaller and smaller distances.
We have, I think, what's fair to call compelling evidence that this theory is the correct theory of the strong interaction. There's lots of data that agrees with it and nothing that contradicts it. It's had two-- it's had several remarkable consequences outside of the original problem of understanding the strong interaction.
One is that cosmology, the study of the very early moments of the Big Bang, gets easy. It used to be very mysterious, what would happen when you pressed matter closer and closer together, because we knew that protons interact in very complicated ways with each other.
And it seemed to be a very-- that was the most powerful force, and we had no idea what would happen under those circumstances. And in the Big Bang, in the earliest moments of the Big Bang, when it's concerned with the properties of matter at extraordinarily high temperatures and densities, and that's precisely what wasn't known.
But now we've learned that that's exactly when physics gets simple again. It's when the particles are close together or at high energies that the effective strength of the strong interaction, which was the difficult thing to treat, gets weak. Where you get inside those antiscreening clouds, the vast buildup of charge goes away, and you see the elementary charges which are smaller. So cosmology has gotten much easier than it used to be. And with remarkable results, including the possibility of formulating the so-called inflationary model of the universe due to Professor Guth here at MIT.
Another thing that's happened is that it's been possible to formulate much more concrete ideas about the unification of all the forces of nature. This is due to two effects. First of all, as I've alluded to several times before, this theory of the strong interaction very much resembles the theory of electromagnetism. There's also a theory of the weak interaction, which I haven't said anything about. It's another interaction which also resembles those two.
They're all different, but you can see a family resemblance among them. Since they are mathematically so similar, they all involve gluons which act on different kinds of charges. In fact, there are five different kinds of charges. If you put them all together, it's hard to resist the temptation to imagine that there's a unified theory where you have gluons that change all five different kinds of charges into one another.
The difficulty with that idea, at first sight, is that the strength of the different interactions seems very different. That's why one of them is called the weak interaction and another one is called the strong interaction. However, we've learned that the strength of the interaction you measure at some distance is not necessarily going to be its strength at another distance. And one has to do a calculation to extrapolate what the behavior is going to be when you get to the heart of the matter at very short distances.
So one can still hope, and test by calculation, that the different interactions which look so different at the distances we measure might turn out to be the same at much smaller distances. And it turns out, to a remarkable extent, that works out. One can extrapolate this very same calculation that leads to the measured change in the couplings where we can measure it to much shorter distances.
And then one finds that indeed, the strong, weak, and electromagnetic interactions do become unified. If, well, it works more or less if we include only the known particles. If we also include these supersymmetric particles, which are very popular, although not yet observed, then they meet perfectly.
So we're very encouraged that we're on the right track, both with the idea that the different forces get unified, and with the idea that one should have supersymmetry. As an unexpected bonus, even gravity seems to fall into place. So the story of the strong interaction has most unexpectedly had ramifications in cosmology and the unification of the forces of nature. But there are also challenges and opportunities that remain in the study of the strong interaction itself that I think are no less remarkable.
Let me first mention two slogans from John Wheeler, who is a physicist, a very famous physicist at Princeton. I guess we regard him as our patron saint similar to Viki Weisskopf at MIT. He's very good at coining catchy phrases, and two that I really like and apply to QCD are, "its from bits" and "mass without mass."
"Its from bits" is the idea that, ideally, the laws of nature should calculate the properties of matter, "its," starting only from purely logical or mathematical properties, "bits." And that's, to a remarkable extent, realized in QCD. I showed you a selection from the table of the vast number of particles that have been observed.
In QCD, the masses and properties of all those particles have to be explained from a theory that basically contains only two numbers. The number three, which is the number of colors. And the number two, which is the number of relevant quarks. That is the U in the D core.
So it really is "its from bits," one that's produced a vast amount of structure from purely logical and elementary numerical properties. Of course, to get from those simple numbers, the three and the two, to this table requires a lot of processing in between. And some of the world's most elaborate numerical simulations on massively parallel teraflop computers have been employed in that effort.
Another of Wheeler's slogans is "mass without mass." This refers to the idea that, again, it's e equals mc squared. Or rather, I should say, m equals e over c squared, that you can get mass starting with pure energy. Without putting any mass in.
That's exactly what happens in QCD and is actually what we believe. What all sane physicists now believe is true of the proton, for instance. Most of the mass of the proton comes not from the mass of the things that are inside, but from the energy in the fields that bind them together. The quarks and the gluons have very small or zero masses.
Implementing these slogans in detail to get from the ideal theory to concrete predictions about the world, is an ongoing challenge which is worthy and is receiving of a great deal of effort.
Another area in which we can hope to do much better, and we've made remarkable progress in the last couple of years, is in the study of very dense matter. It turns out, perhaps not surprisingly, but in detail it is surprising. That if you press quarks very close together, things simplify and one can start to actually solve the equations of QCD fairly simply. Whereas in general, they are very difficult to solve and require massive numerical work.
Well, this is extremely interesting work, and has led to, I think, a very beautiful result. Let me just try to give you the flavor of it by saying that the central prediction of this theory is that deep inside every neutron star, when the quarks get pressed really, really close together, they look like a diamond. They become a transparent, insulating material. So it's the diamond inside as big as The Ritz inside each neutron star.
So I think the quest to understand the strong interaction, which started as sort of just unpeeling the next layer in the onion of understanding the structure of matter, has been a truly remarkable success story in which Henry Kendall played a central role. It's a story that's not yet over, but I hope I've given you some flavor of what we've achieved so far. Thank you.
PROFESSOR: Thank you, Frank, for a beautiful talk. And I thank all the other speakers today, the panelists, and also you who are there, who are here to honor Henry. With that we'll close the meeting.