Sheldon Glashow has strong opinions about string theory. Like how it has failed in its primary goal of incorporating gravity into the standard model of elementary particles. How its inability to be experimentally tested makes it "permanently safe" from either proof or falsification. How it's very beautiful, but he doesn't understand it. Here, Glashow, the Arthur G.B. Metcalf Professor of Physics at Boston University and winner of the 1979 Nobel Prize in Physics (along with Steven Weinberg and Abdus Salam), lets fly on the theory that, regrettably from his perspective, has been attracting the brightest young stars in the physics firmament today.
Note: For a definition of unfamiliar terms, see our glossary.
Advent of an upstart
NOVA: In the '60s and '70s when there were tremendous breakthroughs in particle physics, how would you describe the relationship between theory and experiment?
Glashow: I was at the University of California in Berkeley from roughly 1963 to 1966 as a professor, and I remember clearly that the experimenters and the theorists were in very close contact. Luis Alvarez, who was a very distinguished and brilliant experimental physicist, would hold a meeting at his home on a more or less weekly basis to which he would invite his experimental group and a few of the local theorists, myself included. It was a very wonderful experience. Each week or every couple of weeks we would hear about the latest discoveries—there would always be one or two—and we were trying to help the experimenters interpret their data just as they were posing questions to us about what these strange effects they saw in the laboratory were. It was a very intimate relationship.
This intimacy continued and it continues today certainly at my university. But oddly there has been a new development, in which a new class of physicists is doing physics, undeniably physics, but physics of a sort that does not relate to anything experimental. This new class is interested in experiment from a cultural but not a scientific point of view, because they have focused on questions that experiment cannot address.
So this is a change. It's something that began to develop in the '80s, grew in the '90s, and today attracts many of the best and brightest physicists. It's called superstring theory and it is, so far as I can see, totally divorced from experiment or observation. If not totally divorced, pretty well divorced. They will deny that, these string theorists. They will say, "We predicted the existence of gravity." Well, I knew a lot about gravity before there were any string theorists, so I don't take that as a prediction.
NOVA: When you first heard of string theory, what did you think of it?
Glashow: String theory has had a long and wonderful history. It originated as a technique to try to understand the strong force. It was a calculational mechanism, a way of approaching a mathematical problem that was too difficult, and it was a promising way, but it was only a technique. It was a mathematical technique rather than a theory in itself. Later on this primitive string theory that existed in the 1970s became combined with the theory of supersymmetry—which is another development in particle physics, a very interesting development that may have experimental consequences or not—to form superstring theory, which is very elaborate and very mathematical and founded on the principle that we must form a quantum theory that describes all of the forces of nature, including the force of gravity.
Now, the force of gravity is known to be very weak, and that means that it involves a parameter of distance that is many, many powers of ten smaller than the size of the proton or the smallest sizes that we can or will investigate in the laboratory. Conversely, it means that quantum gravity becomes apparent at energies that are simply far beyond the reach of any particle accelerator that exists or any accelerator that is ever likely to exist.
“The string theorists have a theory that appears to be consistent and is very beautiful, and I don’t understand it.”
So the nature of the quest to form a theory of all of the forces of nature, including gravity, drives on to a domain of energies and distances that is inaccessible to the experiment. No experiment can ever check up what's going on at the distances that are being studied. No observation can relate to these tiny distances or high energies. All we can do is look at the distant consequences, 10 or 20 orders of magnitude removed from these effects.
The string theorists have a theory that appears to be consistent and is very beautiful, very complex, and I don't understand it. It gives a quantum theory of gravity that appears to be consistent but doesn't make any other predictions. That is to say, there ain't no experiment that could be done nor is there any observation that could be made that would say, "You guys are wrong." The theory is safe, permanently safe. I ask you, is that a theory of physics or a philosophy?
A dissenter's view
NOVA: Is it science then, if it's not testable?
Glashow: What the string theorists do is arguably physics. It deals with the physical world. They're attempting to make a consistent theory that explains the interactions we see among particles and gravity as well. That's certainly physics, but it's a kind of physics that is not yet testable. It does not make predictions that have anything to do with experiments that can be done in the laboratory or with observations that could be made in space or from telescopes.
NOVA: If it's not testable, how useful is it?
Glashow: It leads to many interesting ideas. It is important in mathematics. String theory has had an impact on modern mathematics. They may even have a practical impact some day, these things that string theorists do. One never knows, just as number theory, the most useless of the mathematical sciences, has given us cryptography and has given us a secure way to encode information. The string theorist may also produce something equally useful. May. So it is science, it is physics, it is mathematics. It does stimulate ideas in related fields.
But in and of itself, it has failed in its primary goal, which is to incorporate what we already know into a consistent theory that explains gravity as well. The new theory must incorporate the old theory and say something more. String theory has not succeeded in this fashion. String theory has said something more, but it does not incorporate the details of the structure that preceded it, that is to say the standard theory of elementary particles. Until it does that, it is not yet physics in a conventional form. It is a perhaps promising corner of physics that may some day say things about the world. But today they're saying things about string theory to one another.
NOVA: Is there any danger in this for physics in general?
Glashow: There is today a disconnect in the world of physics. Let me put it bluntly. There are physicists, and there are string theorists. Of course the string theorists are physicists, but the string theorists in general will not attend lectures on experimental physics. They will not be terribly concerned about the results of experiments. They will talk to one another.
“We don’t listen to them, and they don’t listen to us.”
At Harvard today there's a very strong group of string theorists upstairs on the fourth floor of the Jefferson Laboratory. Each week there are visitors from around the world giving lectures. I've occasionally attempted to attend these lectures. I can't understand the titles, and I can't understand the lectures, and it's not just me. I think most theoretical physicists who are not themselves string theorists could not possibly follow these lectures. In other words, we don't listen to them, and they don't listen to us. We can't understand them, and what we do is not of any direct interest to them.
It is a new discipline. Unfortunately, many of us have nothing in common with them, and many of them have nothing in common with us, except intellectually. Just as there's a biology department that I respect and understand a little bit, there's a philosophy department that I respect and understand a little bit, so there's a string theory structure. That's a problem, I think, in physics.
NOVA: Why is it a problem?
Glashow: The physics and astronomy enterprises in this country spend a great deal of money to do experiments on Earth and in the heavens. There are orbiting observatories, there are laboratories deep underground, there are accelerators in many countries, and these guys produce a lot of data in order to lead us to construct a better and more useful theory of nature. And it turns out that the best and the brightest young theorists, instead of being concerned about the experimental enterprise, are going off among themselves and doing their thing with the doors closed. Because no one else is interested in coming, they're all making these secret signs to one another and putting incomprehensible formulas together that to them are, of course, central and simple and predictive and whatnot but to us are a little bit irrelevant.
Now, what happens if there are suddenly some major experimental discoveries? There is a big accelerator, the Large Hadron Collider, which is scheduled to be completed in another five years or so. That should make lots of discoveries. Who will be interested in trying to fit these discoveries into the theory? It will be people like me, except we may be dead by then or if not we'll be rather old. Or it will be the young theoretical physicists, but the young theoretical physicists are doing string theory, and they ain't interested in the results of the experiments. Not now, and not then. So who's going to be there to continue the role of building a better theory of particle physics? That's why it's a problem. [For more on the results of experiments at particle accelerators, see Smashing Pictures.]
NOVA: What would string theory need to do to make a believer out of you?
Glashow: Well, you must understand that I don't understand string theory, so I can't describe its inner nature to any extent. But I could imagine that string theory would succeed in encompassing the standard model. It might then answer any number of outstanding questions. Why is the muon, some dumb particle, 200 times heavier than the electron? Why is the proton about 2,000 times heavier than the electron? Why is the electric charge of the electron what it is? Why are there six quarks in nature? Why not seven or eleven or five? There are many, many "why" questions. Also a number of 'how' questions. What is the mechanism that causes the weak interactions to be weak and the electromagnetic interactions not weak?
All kinds of questions remain. Many have to do with cosmology. How did the universe originate? How did the galaxies become distributed in space like the suds in the kitchen sink, as one of my colleagues has described it? Why is the cosmological constant apparently very tiny but non-zero and has a peculiar value that leads the universe to expand more rapidly?
These are some of the questions. I can give you a list of 30 or 40 questions. If they answer three or four of them, I get interested in string theory. They're answering a bunch of questions, but their questions lie completely within string theory, which has nothing to do with experiment. I want to see the questions answered that puzzle me, that affect me, like why are there six quarks?
Towards a unified field theory
NOVA: If the '60s and '70s constituted the era of particle physics, how will this current era be remembered?
Glashow: I think the late 1990s and the beginning of this 21st century will be remembered as the coming together of the large and the small, the convergence of cosmology and particle physics. The two sciences, the two least useful sciences, one dealing with the smallest things in the universe, the other with the biggest things in the universe and the universe itself, are coming together as they have been for many years but more so today.
The discoveries are astonishing. We've discovered not only that the universe is expanding—we've known that for a long, long time—but we've learned that that expansion is speeding up. Everyone thought that the expansion was slowing down. Everyone believed that there was a deceleration of the universe. Well, there isn't; there is an acceleration, quite a surprise.
“I simply can’t imagine why any sane person would imagine, discuss, or mention, except insultingly, the concept of a theory of everything. It's a stupidity.”
We've also been focusing in my field of particle physics on the particles we see about us and the particles we can make at the laboratory. We talk about quarks, we talk about electrons and particles like electrons. We have a list, a Periodic Table if you will, of the particles in our best theory.
What the astronomers have discovered is that most of the matter in the universe is not on our list. It's something else. And we haven't the foggiest idea of what it is. It may be some kind of supersymmetric particle. It might be black holes. It might be particles with funny names. But we don't really know what 99 percent of the matter in the universe is made of. So there's a coming together here, a big grand coming together. Those of us who study the birth of the universe and those of us who study physics at high-energy accelerators are doing the same thing. We physicists and cosmologists are brothers. We're twins, and we are trying to answer the same fundamental questions about the universe.
NOVA: People often talk about a "theory of everything." What do you think of that term? What would it mean to have such a thing?
Glashow: I think the concept is foolish. I don't have the hubris to imagine a theory of everything. I think that we scientists are seeking an understanding of the natural world. We come in various types—chemists and physicists and biologists and such—and we all have the same goal. We are making progress. The theories we have today of life and chemistry and physics are much better than they were ten years ago. And ten years from now they will be better still.
I don't know what it means to understand the process of nature perfectly. I don't know what a theory of everything could be. Is it a series of formulas? How could that be? It doesn't make any sense. So I would have to say that I simply can't imagine why any sane person would imagine, discuss, or mention, except insultingly, the concept of a theory of everything. It's a stupidity. And I believe that my string theorist friends would be among the least likely people to describe their work as a search for a theory of everything.
What we all want is a better theory of the universe, to understand our physical world in greater depth than we presently do. That's why, although I occasionally pick on the work string theorists do, I describe them as physicists. They are interested in the same problems that I am. They're approaching those problems in different ways, ways that they regard as somewhat more productive than I do. But they're not searching for a theory of everything. They're just trying to create better theories.
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