Martinus Veltman

The Development of Particle Physics

Category: Lectures

Date: 2 July 2008

Duration: 30 min

Quality: SD

Subtitles: EN

Martinus Veltman (2008) - The Development of Particle Physics

Particle physics mainly developed after World War II. It has its roots in the first half of the previous century, when it became clear that all matter is made up from atoms, and the atoms in turn were found to contain a nucleus surrounded by electrons

Lecture given by M. Veltman, Meetings of Nobel Laureates, Lindau, Germany This moment at a very special time because at CERN they will start up the latest machine for the investigations in particle of physics. So I thought it might be a good idea if we take a review of all of particle physics. You can do it, on top of it, one has the feeling, at least me I have, that we sort of get to the end, something has to happen, it seems we are getting to the, to the, we are running into a wall. So for this evening I thought it was a good idea to give you the history of accelerators. Accelerators are the main tool for particle physics. The underlying idea is this: you have the collision of two particles. At the moment of collision, you have an enormous concentration of energy. By the laws of Einstein E = mc2, you can then make particles whose mass is less than the amount of energy available. So it is a wonderful instrument to create particles of which you didn't know the existence. Of course, when people started with accelerators, they didn't really fully realize it, it came up as time went on, but you can see the long history of particle physics, which is the history of science and inventions, it’s something, yeah, well it's very interesting, I find it very interesting, so I decided that for this mixed audience, that is maybe the best thing I can do. So we start off - where do you start in history? Well you can of course start with Jesus Christ, although he did do very little science. I really wonder why in the Bible they say nothing about quarks and leptons, but anyway, but I've come back to what you might call the first accelerator and the first accelerator that you get in particle physics was simply made by making a high voltage and then have an electron cross that voltage and as it crosses the voltage it gets accelerated and you can do experiments with that. So the first thing that you have to do making an accelerator is making a high voltage and who started that? And that is very curious, of course, very few people know it. It started by somebody that I cannot get because this .*****. apparatus doesn't work. Oh my. I'll try something else. There. It started off with a guy called Nicholas Callan. He was an Irish priest who lived let's say around 1836, that's when he did his experiments, and he, he wanted to make high voltages, so he made a transformer that could make very, very high voltage. And always bigger and bigger. He had a big problem - how do you measure a high voltage? And his solution was simple. He took a number of students, and put them in a row and put the voltage on there. And from the way they jumped, he could deduce. But actually he ran into difficulties because in the college that he was, they noted that he was doing bad things to the students, so they forbid him to use any more students for these experiments. I think it's a pity, I think it’s a nice idea and so he started working with turkeys and that made his method a little bit more accurate. It's easier to measure with turkeys than students. And he, he made the, the first, the first really big transformer. You would be amazed how he used the primitive things that he was trying. He made up to 600,000 Volts. That's really something. So that was Nicholas Callan and he was succeeded by a German by the name of Rühmkorff, Hermann Rühmkorff. And he made this thing, it's called the Rühmkorff device. It is again a sort of a transformer and with it you can make voltages up to, well as I say, almost 1,000,000 Volts. And this thing became very important to the development of physics, because various laboratories would buy such an apparatus. One of them that who bought such an apparatus was Mr. Wilhelm Röntgen. Wilhelm Röntgen was from origin, he lived in Holland. But in Holland, when we have a person who’s good, we kick him out. And so that’s what we did with Röntgen, and he left for Germany where he became a professor. And then he discovered of course his Röntgen rays (X-rays) and he discovered them using these Rühmkorff things with the device that you see down there, where he had the electrons going from one side to the other and make a collision with that plate in the middle. And then at the moment of collision that created Röntgen rays. Now you know how Röntgen rays, how important they have been and are for humankind. Well Mr. Röntgen in the sense of Nobel Prizes, where you give it to the person who has done most for humankind I think was one the very best to have for the first Nobel Prize. Now that was Mr. Röntgen, so Rühmkorff and Röntgen. And the next thing that happened is that people started to investigate the discovery of Röntgen, the Röntgen rays. They didn’t know what they were, were they waves? Were they particles? No one knew. So that was the next race. This was in 1895 and this is the laboratory of Mr. Röntgen. It’s very different from CERN. You could still, you can see in the middle there the, the Rühmkorff and that was where Röntgen rays were discovered. And then following that, we got – here is Mr. Röntgen, still some details about Mr. Röntgen, and then what happened, first of all, two things happened. First of all, people tried to establish what was the nature of Röntgen rays and this was established basically by two Dutch physicists called Haga and Wind, in Groningen, and they established that Röntgen rays were waves. They actually made them bend, disperse on a slit. The slit’s like this so that it got very small. And it is a masterpiece because if you think of the wavelengths of Röntgen rays, to make the precision instrument is really a major achievement, but they did it, and they guessed, they got, they got it quite nearly right. They did their experiments at the end of the 1800s and so, come 1900, one knew more or less that Röntgen rays were at least waves. And then the next step was, what kind of waves? And very quickly they got to the idea that Röntgen rays were also electromagnetic waves just like normal light. And that of course was the next step, and by the time that it was 1902 or so, people had convinced themselves that Röntgen rays are what we think today, electromagnetic waves and the experiments and so on. One of the greatest men that worked with the results of Röntgen was Mr. Becquerel, he wanted to understand Röntgen rays, he didn’t understand them either. So he started doing experiments and totally by accident, Becquerel was an expert on fluorescence, he’s had little to do with Röntgen rays, but he didn’t know it, but he thought it was the same thing. And so he started doing some experiments with materials that were highly fluorescent and happened to be materials that contained uranium, radioactive uranium. And by accident with the photographic plates that he used, he so discovered radioactivity. And that, of course, is really where particle physics kicked off. Mr. Becquerel discovered that a decay of radioactive material and the next step, people started to investigate what happened there. And that is how you find particles: beta particles, alpha particles, you name, gamma rays - all of these things come out of studying decays.That’s from Becquerel. But one of his most prominent successors was Rutherford. And Rutherford ran a laboratory in England and he, he started doing extensive investigations in this, in this subject. And it is in this laboratory, the Cavendish Lab, that the first real accelerator was developed. I should say that the device is a Rühmkorff is also an accelerator, but on the scale of particles, it is not good enough really to do real stuff. Now for this, let me once more tell you the idea of an accelerator. You generate a point, a head-on collision one way or the other and, in which you get a concentration of an enormous amount of energy, and there you can create, you wait, particles coming out whose mass is less than the available energy. Now the first particle that you can discover with a mass less than this available energy is the pion, and the pion has a mass of 130 MeV, that means you need 130,000,000 V to get that kind of an energy, and they weren’t that far yet. Actually, that was not the way it went. The first thing that happened is that, at that Rutherford laboratory, they started studying nuclei and those issues of one nucleus to the other in which you see constantly a change of energy to mass and mass to energy. And it is at the Cavendish Laboratory that such an accelerator was developed. It exists to, to this day, such an accelerator as was developed there. No, it doesn't work.) The first accelerator, real accelerator made, was the one that at the Rutherford Lab. It's called the Cockcroft-Walton accelerator. You build up a high tension by doing it piece-wise. You pile it on top of one another. This instrument has been very, has been very important. You still find it today at all the leading accelerators as the first step, and in fact you find it also in the old-fashioned televisions with the tube that need a high tension. They usually have some kind of a Cockcroft-Walton in them. So this Cockcroft-Walton machine could go up to a lot higher and actually it could, it is well known, you could go to a million volts as you see without. That's the one that you see here, it's on exhibition in England. And this accelerator can be used to do nuclear physics and it is in that context, remember in 1932, that for the first time we got a real verification of Einstein's Law E= mc2. No, it took as much as 17 years before that important equation was ever verified experimentally, and that was by Cockcroft and Walton. And so you, they started doing nuclear physics, not really particle physics yet, because the energy wasn't high enough yet to create a new particles that were waiting in the wings, so to say. So, they did nuclear physics and studied the structure of the nuclei. Another person that became important here is Mr. Van de Graaff, who was a professor in America. And he made a machine by carrying electricity around some cord, some sort of a transport system. And so he made a big voltage between two such globes as you see there. The voltage that you can make in this way is quite high and the fact Van de Graaffs are used to this day to do the physics of nuclei. There you can still find them at various places. They make energy up to 14 million volts, so you can see where we got from, from Mr.Callan who was happy to get 600,000 Volts, 0.6 million. We now got in all these years up to 14 million. It's still not high enough to make new particles. But it helped a lot in studying the structure of nuclei. Well, the next important step in particle physics was that one started to use cosmic rays to study those and thereby understanding the reactions of particles, and in fact discovering new particles, because in cosmic rays you can find particles with an energy that is absolutely high enough to make all kinds of things. And very instrumental in that were two ladies, a most remarkable couple that you could imagine. A lady called Marietta Blau in Vienna, who was Jewish, and she left in time in 1938, before being murdered. She was not murdered. And then there was Herta Wambacher, she was a real Nazi, and in fact during the war behaved that way as well. You can see this very unlikely couple was working together and they developed emulsions. Now emulsions are photographic plates that you can use to register cosmic rays and so you can study what cosmic rays do. And they made great advances in making emulsions in such a way that you could actually study them. There is a long story about them too, but I won’t go into it simply because I won't have the time. But these were the next people in this, in this sequence and they, they made for the beginning of the, of the use of cosmic rays to discover new particles. One of the first new particles discovered was the muon. The muon is 100 MeV, so that's way above what even the Van de Graaff could do. But in cosmic rays you saw them go coming along, muons and pions. And muons and pions are particles that were found in cosmic rays, at that time. So this was around World War II. Marietta Blau, by the way, it’s sort of sad to say, she first went to Mexico. Einstein helped her to come into America and then at some point she went back to Austria in 1960. But at that time in Austria they were sufficiently backwards to say that they would be happy to have this lady in the faculty, but she wouldn’t get any salary. So ladies here in the audience, remember what has happened since then. At least that barrier went away. Well then there’s another invention called the klystron, which was invented in California at SLAC at Palo Alto. It was used in radar in World War II to make very high-frequency electromagnetic waves. And the point is with these high-frequency electromagnetic waves, you could have them in a standing wave in the cavity and the wave was such that the particle get into the cavity gets accelerated by that wave, by those electromagnetic fields. So the klystron by this became a tool to accelerate electrons. And the first big use of that, maybe there were - I don’t know about other use - there may have been, but the first big use of it was the SLAC monster, which was built in 1962 or so. I remember I was there, I made pictures of it and I wondered what was going on. They had this two-mile monster, as they called it, to accelerate electrons - one klystron after the other. So that made a big step and the monster, the Stanford Linear Accelerator, has made gigantic contributions to particle physics which I will probably mention later. Stupidly enough, I've never understood why they did, but they built this thing just across the San Andreas Fault. And when you go to SLAC, and you can see the Andreas fault because somewhere there’s a hedge and then at the moment of San Andreas Fault one side of the hedge moved and the other moved the other direction and stands like this. So who would put an accelerator on there? Someday they will wake up and one piece of the accelerator will be here and the other will be here. But they did anyway and they claim that they have made precautions and so on, so I hope. So far they didn't need to do anything, and in fact the machine has been closed down. A big step was taken by Mr. Lawrence in Berkeley and he got the first idea of making a circular accelerator, where you accelerate the particles at every turn. And you see here Mr. Lawrence with the first cyclotron. He could keep in his hand, see, and yet it could make already 80,000 Volts, equivalent to 80,000 Volts. It’s pretty incredible. And as time went on, these machines were bigger and bigger, but they have been the instruments of choice just before World War II to invest. The SLAC was after World War II, it started in 1960. The cyclotron started in, what is 1929. And up to World War II and even thereafter, it was the prime instrument to do investigations in the domain of particle physics. Here you see what happened. Lawrence started with his 80,000 Volts and eventually the one of the largest sort of the maximum you can have is the cyclotron at CERN. You see here the ring. That's where the particles circulate, it’s a big magnet and at CERN they go up to 1 million million (sic) volts, so 1 billion Volts you would say, 1000 MeV. So that's a, that’s a big step. That CERN accelerator was constructed around 1960 and it has done much useful physics. A big step forward was taken with the following: the cyclotron is a sort of a pancake and particles that get accelerated start in the middle. There's a magnetic field which keeps the particles in orbit, and as the energy goes up, the orbit becomes bigger and bigger. And so the, the maximum you can achieve is determined by the size of the magnets. If you accelerate any further, it won't stay in the magnetic field, but circle out. So the next invention was to, to keep the orbit stable but the magnetic field different. So what you did, you let particles go around and as they went faster and faster, you increased the magnetic fields to keep them in orbit. So that was the next idea, and the first implementation of that I think was the Cosmotron, which you see here, the Cosmotron in Brookhaven. That was in the beginning ‘60s. It was a great instrument for people from Columbia a nearby instrument to do experiments or university to experiments. And it goes on the principle that you have a ring, and there’s a variable magnetic field, and as you accelerate the particle every time they go around, you whip them with an electric field, and you make the magnetic field stronger so as to keep them in orbit. The difficulty with the Cosmotron was that as you got to higher energies, the beam got sort of diffused and the beam was became too diffused to do good experiments with it. So they, another idea was needed, and the idea that was needed and in fact found, was what's called strong focusing, invented by some Greek engineer, Christofilos, who published it and even patented it, but it was invented independently also at the Brookhaven. And this alternated, alternating gradient synchrotron have a magnetic field in such a way that it keep the beam tight. And then you could go to much higher energy. And this was, this, this machine was built at Brookhaven around 1959 and also at CERN. At CERN they were a little bit faster, they were half a year faster than the Brookhaven people, which advance they have not been able to profit from. In any case they make the, this, this proton synchrotron, as it was called. And it could reach an energy of 30,000 MeV. So we now go all the way from this monk, from this priest with his 0.6 million up to here, 30,000 million. That was the progress of some, almost a century. So the first machine at CERN was the PS. It had a radius of 200 metres. The radius determines to what energy you can accelerate the particles with the corresponding magnetic fields in that you can still control them. You accelerate them further, you can’t have strong enough magnetic fields to keep them inside. So the strength of magnetic field that you can reach and the size of the accelerator determine how much you can in actually, in actual fact accelerate the particles. So that was the Proton Synchrotron at CERN. One of the first experiments there was a neutrino experiment. We have an expert on this sitting here, Steinberger, he was part of the experiments both at CERN and Brookhaven. The CERN experiment went wrong for reasons that I will not elaborate, and the people in Brookhaven won the race there by the idea was to demonstrate the existence of two neutrinos, which they succeeded. So that was that big accelerator, and together with it get again to, to function, immediately came the discovery of another particle, namely the, the fact that there were two neutrinos, a muon neutrino and an electron neutrino. Well, of course it's never enough. Lots of experimentation and always these machines. But then came the next invention, which due to a most remarkable gentleman by the name of Bruno Touschek. Touschek was half a Jew. He come, came from Vienna. He worked with Heisenberg in Muenchen. And then World War II came and of course he was endangered, his life was in danger. He was actually not deported to any camp or something but he was kept in jail in Hamburg, in Germany. And he was visited there by a Norwegian physicist by the name of Wideroe. And Wideroe, Wideroe was a very inventive person, who invented all kinds of things concerning, concerning accelerators. I think he invented the Betatron or whatever. And then he discussed lots of things with Touschek. And Touschek’s miserable life, it really was miserable at that point. He got almost shot, he was shot by an SS officer and was left for dead, but it turned out he was still alive in the streets of Hamburg. He was always carrying around a big pack of books, so you can see this man there going, sort of typical for his life in the way I’ve of seen it. But after the war he went first to England and later to Italy. He became a big shot in Frascati. And his idea was there to make the first storage rings. And that was a wonderful invention. You see, up to then, you make particles, you accelerated them and by the time they got to full energy, you took them out and made them collide with stuff, with material, with target. And now Touschek had the idea of, it’s not just Touschek but I’m not to beside to think there was someone at Princeton by the name of O’Neill or something who also had the idea, most of them have two beams and have the two beams collide head-on. And even more, Touschek saw by my, saw by himself, if I have an electron ring and electrons goes this way, the positrons, which are easy to make actually, can be put in the same ring and will run the opposite way because this charge is opposite and the fields will work fairly properly to make them go against each other. By cleverly designing that machine, you can make the beams going and collide at certain points. The big question is, are these beams sufficiently concentrated so that actually you do have collisions of electrons with positrons. And the answer to that is yes, you can do that, but you have really to make these beams very, very tight and if you want to have a mental picture of it, imagine two needles that you will let collide point-to-point, it's sort of that way. So that's what, what Touschek did and so he built storage rings and that has been the next thing in accelerators. At CERN they did build those storage rings and they used 30 GeV protons to collide against the same kind of particles, 30 GeV protons. That happened there. But then to make a step to the present time, the machines that we have today are all storage rings colliders, particles run around, keep on running and occasionally they hit each other. And, prime example of that machine or the big examples of that big machine is first Tevatron at Fermi Lab which is still running today, has a diameter of 2 kilometres and then at CERN we have the large electron-positron collider, we have is starting in about 1970. And that goes in a, in an 8½ kilometre diameter ring. And now that machine has been stopped. We think we have done with it what can be asked from it and so instead in that same tunnel you now have the present machine, the Large Hadron Collider and that is at CERN in the same tunnel as LEP was and that’s still under started operating now. And the energy that is getting there - think of it, it’s 14 TeV and this collision, it is the energy you get is the same as when you would accelerate one particle to 14,000 TeV and a TeV is a thousand GeV which is thousand MeV and you hit and you can see where we have gone. I've summarized that at the end. We have climbed up from Rühmkorff is a 700,000 Volts, 0.7 MeV, to the cyclotron, 1000 MeV is one GeV. Then the CERN and the Brookhaven machine up to 7½ GeV in the collision point and now we are at 14,000 GeV and you can see that’s still an enormous step that we have taken and that the LHC is now going to explore and we can't wait to see what happens there. Is this the end? Well, there is some logical end here. It's very hard to make machines bigger than something which has a diameter of 8½ kilometres. You can put it in the, in the Boulevard Périphérique of Paris? What you do when you want to make a bigger machine? Make it around France? Make it in the planetary system? I mean that’s clearly non-sense. Though I think as magnitude goes, we are getting to the limit of the possible. There is still another alternative possibility, that’s using linear machines with klystrons and make them longer. Sort of practical is to go to about 30 kilometres. These are the machines that are in the drawing rooms at this time. People are making proposals for this type of machine. And I think that logically, I, I don't know if that will be the end, but it seems so. I should of course say that through all the time that I've been in particle physics, I've had that thought more than once, ‘we are reaching the end’. I thought so in ‘63 with the CERN machines. I didn't think of storage rings. So there are things happening that may, may make everything I say total nonsense. We don’t know, we may hope that we can go on, but for the time being with the way things are, I think we are sort of reaching as high as we can. The LHC is a miracle of working on the edge of technology. It’s using superconducting magnets to make magnetic fields that are the strongest that you can make and in fact like Carlo thought up the machine, he made them much stronger than they actually could be made. Right, Carlo? Hallo! So I leave you with this idea - is this the end? We do not know, of course we hope not. We still have many questions to ask from nature here, you see CERN and there. We ask ourselves if there is a future. We do not know. I'm sure it will last my life but that's about all I can say. Thank you.


Particle physics mainly developed after World War II. It has its roots in the first half of the previous century, when it became clear that all matter is made up from atoms, and the atoms in turn were found to contain a nucleus surrounded by electrons. The nuclei were found to be bound states of neutrons and protons, and together with the idea of the photon (introduced by Einstein in 1905) all could be understood in terms of a few particles, namely neutrons, protons, electrons and photons. That was the situation just before WW II.

During WW II and directly thereafter information on the particle structure of the Universe came mainly through the investigation of Cosmic rays. These Cosmic rays were discovered by Wulf (1909) through measurements on the top of the Eiffel tower and Hess (1911) through balloon flights. It took a long time before the nature of these cosmic rays became clear; just after WW II a new particle was discovered by Conversi, Piccioni and Pancini. This particle had a mass of 105.65 MeV (compare the mass of the electron, 0.511 MeV and the mass of the proton, 938.272 MeV). The development of photographic emulsions led in 1947 to the discovery of another particle, the charged pion (mass 139.57 MeV), by Perkins. In subsequent years yet more particles were discovered, notably the K-mesons and the "strange baryons" such as the Lambda (mass 1115,683 MeV). Gradually the phenomenology of all these particles developed, new quantum numbers were invented and classification schemes developed. At the same time, the development of new devices and methodsgreatly furthered the knowledge of elementary particles. The most important of these are the particle accelerators, the cyclotron and developments thereoff, and the detection instruments such as bubble chamber and spark chamber.

In the beginning sixties Gell-Mann and Zweig came up with the idea of elementary constituents called quarks. These quarks did have unusual properties, the main one being that they did have non-integer charge, in contrast to all particles known at the time that did have integer charge (such as the electron and muon with a charge of -1). For this reason the quarks were not immediately accepted by the community. In addition, as we know now, they can only occur in certain bound states such that the charge of these bound states is integer. Thus the quarks by themselves are confined to bound states. The reason for this confinement became clear much later, around 1972.

The theory of the forces seen to be active between these particles is quantum field theory (QFT), a theory of such complexity that its development stretched over many years. Around 1930 Dirac, Heisenberg and Pauli formulated the foundations of QFT, but it was soon discovered that the theory as known then was very defective, giving rise to infinite answers to well defined physical processes. Fermi was the first to apply QFT to weak interactions, notably neutron decay. The theory developed by Fermi was a perturbation theory, with answers given in terms of a power series development with respect to some small constant, the coupling constant. The lowest order approximation of Fermi's theory was quite successful, but any attempt to go beyond the lowest order met with failure. In any case, Fermi's theory involving the then hypothetical neutrino postuled by Pauli, was successful enough to cement acceptance of that particle.

A breakthrough was due to Kramers, who already before WW II discovered that QFT implied certain corrections to the atomic spectra. Experiments by Lamb actually measured such corrections (Lamb shift), and Kramers ideas found acceptance by the community. In addition, Kramers introduced the idea of renormalization, a procedure whereby the infinities of QFT were localized, and where outside these isolated parts perfectly precise calculations could be done. Feynman, Schwinger and others took up these ideas and developed the QFT of electromagnetic interactions, allowing very precise calculations of the Lamb shift and other corrections, commonly called today radiative corrections. These developments, including very successful experimental confirmations, took place around 1948.

The development of QFT of the weak interactions was very difficult and lasted till aout 1971. A new idea, the interplay of forces arranged in a very careful manner such as to avoid the occurrence of infinities, was developed. This is known under the name of gauge theories. In such a theory there is a multitude of forces and particles such that all irrepairable bad features cancel out. Thus the theory thereby predicted the existence of certain new particles, necessary to complete the complex structure of balancing infinities. The actual discovery of these particles, notably the Z0 and the charmed quark, topped by the discovery of the top quark in 1995, has firmly established the gauge theory of weak interactions.

The strong interactions, the forces responsible for the interactions between quarks and notably supposedly responsible for quark confinement, profited from the development of gauge theories. In the wake of the gauge theory of weak interactions also a gauge theory of strong interactions was formulated and investigated. An important step was taken with the establishment of asymptotic freedom for the gauge theory of strong interactions. By 1980 the Standard Model of Weak, em and strong interactions was settled; the Higgs sector of that model remains still to be tested, which hopefully will be done at least partially using the new machine L(arge) H(adron) C(ollider) at CERN, planned to start operation in 2008.