Science Talk

Physics Nobel Laureate Steven Weinberg

Nobel physicist Steven Weinberg spoke to an audience of science journalists, and then to podcast host Steve Mirsky

Nobel physicist Steven Weinberg spoke to an audience of science journalists, and then to podcast host Steve Mirsky. Web sites related to this episode include and

Podcast Transcription

Steve: Welcome to Science Talk, the more or less weekly podcast of Scientific American, posted on November 15th, 2010. I'm Steve Mirsky. Steven Weinberg is a legend in physics. In 2009 Weinberg gave a talk to an audience of science writers at the annual meeting of the National Association of Science Writers in Austin, Texas. What you're going to hear now is a heavily edited version of that talk, which I've been holding all this time because in the current issue of Scientific American magazine we have an interview with Steven Weinberg, so we wanted to put the two together. Now he was a challenge to record, because he was holding both a laser pointer and a microphone and, like any great absent-minded professor, he would occasionally point with the microphone and speak into the laser pointer. Anyway, Scientific American Editor in Chief Mariette DiChristina introduced Weinberg.

DiChristina: He is the author of more than 300 articles on elementary particle physics and his research has been honored with many awards including the 1979 Nobel Prize in Physics and a National Medal of Science. His books include, for popular readers: The First Three Minutes, which I've read and I loved; Dreams of a Final Theory, Search for the Fundamental Laws of Nature. He also has just joined Scientific American for its Board of Advisors. And so, with that I welcome Steven Weinberg. Thank you very much. (applause)

Weinberg: In few months, [before] the end of the year we will begin operations at what will be the largest scientific instrument ever built, the Large Hadron Collider. It's 27 kilometers in circumference; it's run by the pan-European laboratory CERN. Inside this tube there is an evacuated series of magnets which bend a beam in fact two beams of protons that go round and round in opposite directions [being] accelerated by electromagnetic fields up to higher energies than have ever been achieved artificially here on Earth. There are two beams because if you fire a beam of high-energy particles into a stationary target, most of the energy just goes into producing the recoil of the target, which is not interesting, and so today increasingly these accelerators are designed to have two beams which collide head-on. In that way there is no net momentum, and all of the energy available goes into producing new matter. And that's the point. Again referring to the famous E = mc2, to produce a certain mass, m, you need a certain energy E. We believe there are exciting new particles to be discovered with masses so large that no previous accelerator had the energy available to produce them. As is very often the case with accelerators, there are some things that we think are likely to be discovered and we'll be very surprised if they're not. There are other things that may be discovered. We have a menu of possibilities. And then there's also the possibility that something is discovered that nobody anticipated. What we already know about the nature of matter and force is crystallized in what is called the standard model of elementary particles. It's a theory of all of the particles we observed, and with the exception of gravitation, all of the forces that act on them; it's a theory that was developed in the 1960s and 1970s and then through a series of experiments in the 1970s and 1980s, it became well established as part of the standard canon of scientific knowledge. All ordinary matter—atoms, molecules, people, stars, galaxies—are composed of just two types of quarks, and electrons. There are also neutrinos which are continually being emitted by stars [in the course] of the processes that produce their energy. In addition to these particles, there are heavier particles, which don't appear in ordinary matter because there's so heavy; they're unstable and they decay into the particle's I mentioned—electrons, neutrinos and the two lightest types of quarks. There are heavy quarks, in fact a total of six types of quarks, and the electrons have particles that are very similar except they're much heavier, called muons and tauons.

The forces between these particles are transmitted, first of all by photons which carry the electromagnetic force, and much heavier particles called W and Z, which transmit a related force, a very closely related force, called the weak nuclear force. There are also particles called "bluons," which transmit the strong nuclear force, which holds the quarks together inside the neutron and proton, which are inside atomic nuclei. All of these particles, in the simplest version of the standard model, these particles are all massless. That's what makes it an elegant theory. The symmetries do not allow masses and at the level of the equations of the theory, the symmetries are manifest. And when you look at the equations, you see that the W and the Z and the photon are appearing in exactly the same way.

Something intrudes to break that symmetry and gives some particles masses. It gives the W and Z their very large masses, almost 100 times the mass of the photon. It splits the electrons, which have some mass, from the muons and tauons, which are much heavier, and gives the quarks a variety of masses. That something we believe is another kind of particle called the Higgs particle. This was proposed as a mathematical possibility without reference to any particular theory of nature. This mathematical idea was brought into the theory of the weak and electromagnetic directions in the late '60s by myself and independently by Abdus Salaam. The particular particle being sought at the LHC is the one that first appeared in these papers in the late 1960s. That is something that is definitely expected to occur, and in a way it will be much more exciting if it isn't found than if it is. The LHC although certainly it would be ridiculous to say it was designed specifically to discover the Higgs particle, in its design that was one of the requirements, that it had to have enough energy to be able to produce this particle. It won't at first probably when it runs at reduced energy but eventually we expect that it will. Somewhat paradoxically the heavier it is the closer it is, up to the upper limit of where we expect it, the easier will be to discover, because it will have clearly visible decay modes. The Higgs, of course, being as heavy as it is will be unstable. No one will ever see a track of a Higgs particle. What we will see is its decay products and infer from that the fleeting presence of the Higgs particle. It has a variety of possible ways of decaying and the ones that are most visible and recognizable are only available if it is fairly heavy. If it is lighter, more of them will be produced but they'll be much harder to recognize. The large accelerator at Fermi Lab has already ruled out part of the range of relatively heavy Higgs. But it doesn't have the energy and luminosity to study the full range and probably the Higgs will be discovered at CERN. If Congress had not had the imbecility to cancel the Superconducting Super Collider, it would have been discovered long ago here in Texas.

That's something we expect. When I say expect I don't mean we're certain. All we know for sure is that there is a symmetry between photons, Ws and Zs, among the, it's the same symmetry among different types of quarks among electrons and neutrinos. When I say a symmetry, I mean if you write down the equations, and you perform certain mathematical transformations on the symbols of the equations, [that] have the effect of turning Ws, Zs and photons into each other, and electrons into neutrinos and quarks of different types into each other, then the equations do not change their form. We know that that symmetry is there, it's been very well verified. We know that symmetry is somehow broken by something. We say it's spontaneously broken, because it's not a failure of the symmetry in the equations, it's the fact that the symmetry is not satisfied in the solution of the equations. The simplest picture is this simple elementary Higgs particle, but there are other possibilities and one of them so-called Technicolor, which posits the existence of a super strong force, much stronger than the ordinary strong nuclear force. In that picture, there really isn't a Higgs, but you have a whole variety of other particles that are bound together by this extremely strong force. That's a possibility and there are some theorists, it was a possibility first suggested by Leonard Susskind and myself independently. I don't think it's likely that that's what going to be found, because it leads to problems.  There are observations that you could only understand by tinkering carefully with the theory; it sort of begins to look like [Ptolemaic] epicycles and I don't find it as attractive as the original simple picture, but that's a possibility. And we have to remain open to that possibility. That's why it's not a sure thing that the Higgs will be found, but it's highly likely.

Then there are other possibilities, only speculative, which we have no confidence about but which would be extremely exciting. One of them and I think the best motivated of all the other possibilities is called supersymmetry. It has roots in the Russian literature which no one was reading at that time. Supersymmetry connects all the known particles with particles that are much heavier, so that we can understand why they are unknown, but that have different spin. One of the things that's so attractive about it is that for years and years it was thought [to be] impossible, to have a symmetry that united particles with different spin. [There] was a theorem called the Coleman–Mandula theorem that seem to rule out this as a possibility that any such symmetry would conflict essentially with special relativity. And then it was realized that there was a technical exception that allowed for this and into that tiny little gap [Wess and Zurnino] went roaring and invented this essentially unique symmetry. I want to emphasize that the minds of physicists can think of all kinds of possibilities, and when we speculate endlessly the results are likely to be [not very interesting.] It's when there are physical principles that narrowly restrict our speculations so that new ideas can only take one or a very limited number of forms that we begin to think that we've discovered something that is an opportunity that nature probably didn't passed up and most of us have this feeling about supersymmetry.

Steve:           I spoke to Weinberg briefly after his talk.

Steve:           Could you clarify the expectation for the LHC regarding supersymmetry versus string theory.

Weinberg: Well, I think there is a good chance, by no means a certainty, that the LHC will discover [signs] of supersymmetry. And supersymmetry is something you expect in a variety of versions of string theory, so that if we discover supersymmetry that will give us some kind of clue about super string theory, but what that clue is I can't imagine. And super string theory doesn't necessarily require that supersymmetry would appear at the energies that can be reached with the LHC. So, if we don't discover supersymmetry at the LHC, I don't think we will have learned much about string theory.

Steve:           And you said that something would be very exciting if we don't find the Higgs.

Weinberg: If we don't find the Higgs that would be very exciting because it means that some other theory has to be invented. We have this alternative theory of so-called Technicolor, which I mentioned, that instead of a Higgs being an elementary particle that there are strong forces that produce the breakdown of the symmetry, that would, those strong forces would produce a whole zoo of other particles; not a Higgs but other things that could be found—so-called "technipions" and "techniquarks" and things like that. Or we might find that something else entirely; I mean, not finding a Higgs would force us to be inventive whereas finding a Higgs would just show that everything is just as we expected.

Steve:           And as you said if we just find the Higgs, it's just confirmation of [the] standard model.

Weinberg: Yeah, in the simplest version.

Steve:           You can find the video of the entire talk by Steven Weinberg. Just google "Steven Weinberg NASW 2009" and it should be the first thing that comes up. Well that's it for this episode. We'll be back very soon with more from the November issue including a look at why women outlive men. Meanwhile, get your science news at, where you can investigate our dark matter interactive feature called "Dark Worlds, a Journey to a Universe of Unseen Matter". For Science Talk, the podcast of Scientific American, I'm Steve Mirsky. Thanks for clicking on us.

Nobel physicist Steven Weinberg spoke to an audience of science journalists, and then to podcast host Steve Mirsky. Web sites related to this episode include and

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