Steven Weinberg came up with a good idea one day while driving his red Camaro. The paper he wrote, “A Model of Leptons,” was just two and a half pages long—including references and acknowledgments. When it came out in 1967, it was largely ignored. But it became one of the most quoted physics papers ever and helped to earn Weinberg the 1979 Nobel Prize, shared with Abdus Salam and Sheldon Glashow.
In those two and a half pages, Weinberg showed that two of the four forces of nature, electromagnetism and the weak nuclear force, which outwardly seem completely different, could be different aspects of a single unified set of “electroweak” forces. This theory predicted the existence of a new neutral particle among those that carry out the action of the weak force, known as the weak bosons. And he showed how the innate symmetry of the electroweak forces becomes hidden or, as physicists say, “spontaneously broken,” so that we perceive electromagnetism and the weak force as dissimilar. This symmetry-breaking process endows particles such as quarks with mass.
Weinberg also contributed to a theory of a third force of nature, the strong nuclear force. Together these theories form the prevailing explanation of the material world, the Standard Model of particle physics.
Since then, Weinberg has continued to plumb the depths of nature, proposing theories that go beyond the Standard Model and hold hope of creating a fully unified theory—one that includes not only electromagnetism and nuclear forces but also gravity. Weinberg did early work on the leading candidate for a unified theory, string theory. He also has written books for general readers, most recently Lake Views, a collection of essays. Scientific American asked physicist Amir D. Aczel of Boston University to speak with Weinberg about the prospects of these theories, now that the Large Hadron Collider (LHC), the mammoth particle accelerator at CERN near Geneva, is hunting for the Higgs and other particles.
Scientific American: The Large Hadron Collider has been running for six months now, and there is a lot of excitement about it. Some people have even compared its expected results with the quantum and relativity revolutions of the first third of the 20th century. What is your view?
Weinberg: I think that it is exciting. Conceivably, it could produce a revolution in our thinking about physics comparable to the great revolutions of the early 20th century, but there is no reason to expect that. A revolution like that would be through something completely unanticipated—and so I can’t anticipate it!
In the near term, we’re trying to take the next steps beyond the Standard Model and also get to the point where we can confidently say something about what was going on in the early universe. That’s going to take a while. Beyond that, we look forward to tying it all up—to having a theory that accounts for all particles and forces. We don’t know what it will look like.
I do think that when we have a really comprehensive understanding of nature at the most fundamental level, it will percolate out into society in general. It will probably be very mathematical, and it will be a long time before the general public understands it, just as it took a long time before even scientists understood Newton’s theory. Eventually, though, the Newtonian picture of the world had a profound influence on the way people in general thought about the world and human life. It had effects on economics, biology, politics and religion. I think something like that may happen if we come to a really comprehensive theory of nature.
I think that our picture of nature is getting more and more all-embracing, and things that previously seemed very puzzling, like the nature of the force that holds particles together inside the atom, are now understood perfectly well—only to be replaced by other mysteries, like why the particles in the Standard Model have the properties they have. And the process of explaining things that have seemed puzzling, while discovering new puzzles, will go on for a long time. It’s just a guess, but I think that we’ll get to the point where there are no puzzles of this sort. And that will be really quite a remarkable turning point in the intellectual history of the human race.
The Higgs particle is often described as the LHC’s first big target, assuming the Tevatron collider at Fermilab does not find it first. How dependent are the electroweak unification and the Standard Model on the Higgs particle?
I would say they’re completely dependent on the idea that there is a broken electroweak symmetry. But if you then ask why the symmetry is broken, that’s open to question. The symmetry-breaking mechanism that appears in [Salam’s and my] electroweak theory requires the existence of a new particle, which has come to be known as the Higgs particle. Our simple picture led to the prediction of the ratio of the masses of the weak bosons, which seems to work beautifully.
But there is also another possibility, that the symmetry is broken instead by new strong forces and that there is no Higgs particle. These new forces have to be very strong, stronger than the ordinary strong force. Lenny Susskind and I independently worked out a theory we agreed to call Technicolor. It would give the same predictions for the masses of the weak bosons as the original electroweak theory, but it has trouble explaining quark masses. Some theorists continue to work on Technicolor and believe it’s a viable theory. And it may be true. If it is, the LHC should find it. Those Technicolor forces lead to a whole zoo of new particles.
So even if the LHC doesn’t find the Higgs, it can find something that plays an equivalent role, like Technicolor. You can actually show that without any new particles at all, you get into mathematical inconsistencies.
Another principle that physicists hope to confirm at the LHC is supersymmetry, the idea that particles of force, like the weak bosons, and particles of matter, like electrons and quarks, are deeply related. Some physicists are as confident about supersymmetry as Einstein was about relativity—so compelling it must be true. Do you feel the same way?
No, I don’t. Special relativity fit in so well with what was already known theoretically and experimentally—with Maxwell’s theory of electricity and magnetism, with the fact that nobody could discover effects of the “ether” that people had thought existed. If I were fortunate enough to have invented special relativity in 1905, I would have felt, as Einstein did, that that theory just had to be right.
I don’t have that feeling about supersymmetry. It has a number of minor successes. It improves the prediction for a crucial parameter of the Standard Model. It provides a natural candidate for dark matter particles [see “Dark Worlds,” by Jonathan Feng and Mark Trodden]. It has a beautiful feature—that it’s the only conceivable symmetry that could unify particles like weak bosons with particles like electrons. But none of that is impressive enough to convince you that it has to be right.
You’ve worked on the anthropic principle—the idea that aspects of our universe have no deeper explanation other than that we live in a peculiarly habitable piece of a larger domain. In particular, you’ve argued that the anthropic principle is our best explanation for the density of dark energy, the mysterious stuff that is causing the expansion of the universe to accelerate. Can you tell us about it?
We speculate a lot about things we see as fundamental, like the masses of the particles, the different varieties of forces, the fact that we live in three space dimensions and one time dimension. But maybe all this is not fundamental but environmental. The universe may be much more extensive than we’ve imagined, with much more than just the big bang that we see around us. There may be different parts of the universe—where “parts” could mean various things—that have very different properties and in which what we normally call the laws of nature may be different and even the dimensionalities of space and time are different. There has to be some underlying law that describes the whole thing, but we may be much further from it than we now imagine.
When I first wrote about this in 1987—and this is still true—I was pretty open-minded about the various ways in which one could imagine that the universe had different parts, with properties like the density of dark energy varying from one part to another. One way is Andrei Linde’s chaotic inflation, in which there are many big bangs, occurring episodically here and there, each having different values of things like the density of dark energy.
As Stephen Hawking has described [see “The (Elusive) Theory of Everything,” by Stephen Hawking and Leonard Mlodinow; Scientific American, October], the universe may be in a quantum-mechanical superposition of different states, like Schrödinger’s famous cat. Just as it is possible for the cat to be in two states at the same time, in one of which he’s alive, in the other of which he is dead, so may the universe. In the state in which the cat is alive, the cat knows he’s alive, and in the other state he doesn’t know anything. In the same way, there are states of the universe where there are scientists exploring what looks to them like the whole universe, and there are other states where perhaps the universe is too small or goes through its history too rapidly, and there are no scientists and no one to notice what it’s like.
Anthropic arguments predict that the dark energy density will be small enough to allow galaxies to form, but not much smaller, because universes in which it is much smaller are rare. Through a calculation I did in 1998 with two astrophysicists at the University of Texas at Austin, Hugo Martel and Paul R. Shapiro, we came to the conclusion that any dark energy had to be big enough to be discovered pretty soon. Soon after, astronomers discovered it.
You bridge two different communities of physicists: those who do cosmology and general relativity and those who do particle physics and quantum theory. Do you think your dual expertise helps you see how to unify these two areas?
I don’t see a direction of unification yet. I certainly would like to. I have ideas about possible paths to unification that come out of experience in elementary particle physics. But whether those ideas have anything to do with the real world, it’s much too early to say.
String theory is often supposed to be the only way of dealing with infinities in the quantum theory of gravitation, but there is an alternative that’s based on quantum field theories of the same general sort as used in the Standard Model, and that I call asymptotic safety. The strength of forces goes to a finite value at high energy. They are prevented from—safe from—going to infinity.
For a long time the idea went nowhere because it’s hard to show that theories are or are not asymptotically safe. I did some preliminary calculations, which I thought were encouraging, but it got too hard, and I worked on other things. Then, starting a little before 2000, the subject was picked up by a number of people in Europe, who verified asymptotic safety in various approximations and showed that they are mathematically as well defined as the Standard Model.
How is this approach different from string theory?
It’s the opposite of string theory. In string theory you give up on the standard quantum field theory, and you invent something really new. String theory is a big step in a new direction. Asymptotic safety says that good old quantum field theory, of the kind we’ve been working with for 60 or 70 years, is all you need.
I’m not going to make a big pitch that asymptotic safety is the way to go. If it turned out that the truth is string theory, I wouldn’t be surprised. It’s beautiful mathematically, and it may really be the right answer. Asymptotic safety is just a possibility that is also worth exploring seriously.
So far neither approach has led to any great breakthrough, such as calculating the mathematical parameters of the Standard Model, the numbers that the model takes as a given, with no real explanation. That would be the real test—for instance, that you understand why particle masses have the ratios they have. Looking at these masses has been a bit like looking at documents in an ancient script like Linear A. We have all this text, but we don’t know what it’s telling us.
How do you find time to write on things other than physics?
I love physics—I really wouldn’t want to go back in time and choose any other career than the one I’ve chosen. But it’s a rather cold and lonely profession, especially for a theorist like me who doesn’t work much in collaborations. The work I do has nothing to do with human affairs; human interests and emotions don’t enter into it. It can only be understood by a limited number of fellow professionals.
To get out of the ivory tower, I like to think about other things and write about them. Also, like most scientists, I am keenly aware our work is supported by the public and that if we don’t try to explain to the public what we’re doing and what we hope to do, it’s hard to make a case that we deserve their support.