Steven Weinberg and Sheldon Glashow have a lot in common. They were in the same class at the Bronx High School of Science in the late 1940s; they both attended Cornell University; and they shared the Nobel Prize in Physics in 1979 (with Abdus Salam). But they differ markedly when it comes to string theory, which Weinberg feels holds out the only hope at the moment for leading theoretical physicists to a unified view of nature. Here, Weinberg explains his thoughts on the matter.
Note: For a definition of unfamiliar terms, see our glossary.
Four in one
NOVA: String theory's great claim to fame is that it unifies the four forces. Why is unification so important?
Weinberg: Unification is where it's at. The whole aim of fundamental physics is to see more and more of the world's phenomena in terms of fewer and fewer and simpler and simpler principles. And the way you do this is not by having one book on electromagnetism, and another book on the weak interactions, and so on, but to have just one book on all the forces of nature. A simpler description—that's what we're aiming at.
It isn't written in the stars that we're going to succeed, but we're not going to be satisfied with any theory of nature that isn't unified. These have been the greatest steps in the history of physics—realizing, for example, that the same laws that govern the planets in their motions govern the tides and the falling of fruit here on Earth. In the end we hope we will have a single theory that governs everything.
NOVA: Do you think a theory that encompasses all four forces can be found?
Weinberg: I believe that there is a simple theory that governs everything—the four forces we know about, perhaps other forces as well. I'm not sure that's true. It may be that nature is irreducibly messy. I'm sure that we should assume it's not, because otherwise we're never going to find a fundamental theory. But even so, we're not guaranteed that we'll find it. We may not be smart enough. Dogs are not smart enough to understand quantum mechanics. I'm not sure that people are smart enough to understand the whatever-it-is that unifies everything. I think we probably are, because of our ability to link our minds through language, but I'm not certain.
I think the greatest obstacle we may run into is the unwillingness of society to keep spending money on sending telescopes into orbit or building large particle accelerators. We may hit a brick wall as society decides that learning the fundamental laws of nature just isn't worth the money. We'll just have to see.
NOVA: How do you respond to skeptics who say there can never be such a thing as a final theory?
Weinberg: Sometimes you'll hear people say that surely there's no final theory because, after all, every time we've made a step toward unification or toward simplification we always find more and more complexity there. That just means we haven't found it yet. Physicists never thought they had the final theory.
In the late 19th century, some physicists said that physics was petering out, that about everything had been done that could be done. But they had a very narrow view of physics, which, for example, didn't include the task of explaining the phenomenon of chemistry. One of the things that happened in the 20th century is that we now know why chemical properties are what they are on the basis of fundamental physics. I don't think anyone ever really thought the work of physics was done, and we certainly don't think so now. [For more on a "final" theory, see A Theory of Everything?.]
NOVA: What do you think it would mean to science, to society, or even to religious beliefs to find this final theory?
Weinberg: I don't think that discovery of a final theory of physics is going to end science. It won't even end physics, because there are countless problems that will remain, in which the difficulty is not that we don't know the fundamental principles, but that we don't know how to apply them because the phenomena are too complicated.
“It was a time when graduate students would run through the halls of a physics building saying they had discovered another particle and it fit the theories.”
You see that in a small way with regard to the weather. We know everything there is to know about the fundamental principles that govern fluids like air and water, but try to predict whether it will rain in two weeks in a particular place. You can't do it; it's just too complicated. I don't think there are any fundamental principles of physics and chemistry that we don't understand that are standing in the way of understanding intelligence or consciousness. It's very complicated. And that kind of problem will just go on and on forever.
NOVA: But will a so-called final theory change the nature of physics?
Weinberg: The discovery of a final theory that unifies everything will end a certain kind of science—the kind of science that proceeds by endlessly asking why. Why does the moon go around the Earth? Well, gravity holds it. And why does gravity behave that way? Well, there's curvature of space and time. And why is that true? Well, who knows, it may be a string theory of some sort. That series of why, why, why questions, like an unpleasant child, will come to an end in a final theory and then we will know. We will know the book of rules that govern everything.
State of the science
NOVA: Today one of the criticisms of string theorists is that they don't talk to experimentalists. That wasn't always the case, was it?
Weinberg: There was a marvelous period from, I'd say, the mid-'60s until the late '70s when theoretical physicists actually had something to say that experimentalists were interested in. Experimentalists made discoveries that theoretical physicists were interested in. Everything was converging toward a simple picture of the known particles and forces, a picture that eventually became known as the standard model. I think I gave it that name. And it was a time when graduate students would run through the halls of a physics building saying they had discovered another particle and it fit the theories, and it was all so exciting.
Since the late '70s, I'd say, particle physics has been in somewhat of a doldrums. Partly it's just the price we're paying for the great success we had in that wonderful time then. I think cosmology now, for example, is much more exciting than particle physics. The string theorists are trying to push ahead without much support from relevant experiments, because there aren't any relevant experiments that can be done at the kind of scales that the string theorists are interested in.
They're trying to take the next big step by pure mathematical reasoning, and it's extraordinarily difficult. I hope they succeed. I think they're doing the right thing in pursuing this, because right now string theory offers the only hope of a really unified view of nature. They have to pursue it, but the progress is glacially slow. I'd rather study continental drift in real time than be a string theorist today. But I admire them for trying, because they are our best hope of making a great step toward the next big unified theory.
Perhaps the next round of experiments with the big new accelerator that's coming on line in Europe, the Large Hadron Collider, will discover something just wonderful that gives us a kick in the pants and gets theory and experiment marching together again. We don't know. It's been a tough time.
NOVA: You mentioned the standard model. How does it account for matter and forces?
Weinberg: The standard model is a theory of fields. Fields are things that pervade all space. There's an electromagnetic field, there's a gravitational field, there's an electron field. Each quark has its own kind of field, and every kind of elementary particle we observe, whether it's a quark or an electron or a photon or whatever—it's just a bundle of energy and momentum of these fields. And these fields interact with one another in rather simple ways.
One of the hallmarks of the standard model is that the ways these fields can interact with one another—trade energy and momentum with one another—are very limited. Otherwise the theory's mathematical consistency breaks down. So the standard model is an extremely constrained theory. You're not allowed to invent anything you like. It describes everything we see in the laboratory. Aside from leaving gravity out, it's a complete theory of what we see in nature. But it's not an entirely satisfactory theory, because it has a number of arbitrary elements.
“If string theory unifies gravity with the other forces, then I think we’ll be within our rights to ring a bell and say, ‘Hey, this is it.’”
For example, there are a lot of numbers in this standard model that appear in the equations, and they just have to be put in to make the theory fit the observation. For example, the mass of the electron, the masses of the different quarks, the charge of the electron. If you ask, "Why are those numbers what they are? Why, for example, is the top quark, which is the heaviest known elementary particle, something like 300,000 times heavier than the electron?" The answer is, "We don't know. That's what fits experiment." That's not a very satisfactory picture.
So the standard model is not the end. We know that we have to do better than that. We will never be able to test string theory by observing the strings. That's a scale that I don't think we'll ever be able to reach; it's too small. But if string theory unifies gravity with the other forces, and correctly predicts the 18 mysterious numbers in the standard model, then I think we'll be within our rights to ring a bell and say, "Hey, this is it."
The beauty of string theory
NOVA: String theory makes some pretty bizarre predictions. How is it regarded by the general physics community?
Weinberg: I don't think anyone ever thought of string theory as a crackpot theory. The people who were working on it were working in the recognized traditions of elementary particle physics or fundamental theoretical physics. Even the ideas that seemed strangest, like the idea that there were extra dimensions, had a long history in physics. Einstein had flirted with the idea of a fifth dimension as a way of unifying electromagnetism with gravity.
But there has been a division among physicists, not so much as to whether or not string theory will ultimately be proved to be right or not, but as to whether it's worth working on something that's so far removed from experimental reality. I would say I'm awfully glad that not every theoretical physicist is working on string theory, and I'm awfully glad that some of them are.
NOVA: If string theory doesn't have testable predictions, is it science or is it philosophy?
Weinberg: Sometimes people say that string theory, because it's unrelated to any experiment, is no longer science, it's just a kind of a mysticism. I don't think that's right at all. I think that the string theorists are trying to accomplish something that will be recognized if it succeeds in unifying all the forces, but it will be experimentally verified as well. It won't be experimentally verified by finding the strings themselves—by seeing the one-dimensional little rips in space that we call strings—but it will be experimentally verified if it explains the things that are still mysterious about the physics we know about. It is just a part of ordinary science. Unfortunately, it's further removed from observation than most parts of science but not hopelessly removed from it.
NOVA: Do you think that string theory could turn out to be just plain wrong?
Weinberg: I don't think it's ever happened that a theory that has the kind of mathematical appeal that string theory has has turned out to be entirely wrong. There have been theories that turned out to be right in a different context than the context for which they were invented. But I would find it hard to believe that that much elegance and mathematical beauty would simply be wasted. And in any case I don't see any alternative to string theory. I don't see any other way of bringing gravity into the same general theoretical framework as all the other forces of nature. Yes, it could be entirely wrong. I don't think it's likely at all. I think it's best to assume it's not and take it very seriously and work on it.
NOVA: What is beauty to a theoretical physicist?
Weinberg: It may seem wacky that a physicist looking at a theory says, "That's a beautiful theory," and therefore takes it seriously as a possible theory of nature. What does beauty have to do with it? I like to make an analogy with a horse breeder who looks at a horse and says, "That's a beautiful horse." While he or she may be expressing a purely aesthetic emotion, I think there's more to it than that. The horse breeder has seen lots of horses and from experience with horses knows that that's the kind of horse that wins races.
“The kind of beauty that we search for in physics is a large part of what attracts people to string theory.”
So it's an aesthetic sense that's been beaten into us by centuries of interaction with nature. We've learned that certain kinds of theories—the kind that win races—actually succeed in accounting for natural phenomena. The kind of beauty we look for is a kind of rigidity, a sense that the theory is the way it is because if you change anything in it, it would make no sense.
String theories in particular have gotten much more rigid as time has passed, which is good. You don't want a theory that accounts for any conceivable set of data; you want a theory that predicts that the data must be just so, because then you will have explained why the world is the way it is. That's a kind of beauty that you also see in works of art, perhaps in a sonata of Chopin, for example. You have the sense that a note has been struck wrong even if you've never heard the piece before. The kind of beauty that we search for in physics really does work as a guide, and it is a large part of what attracts people to string theory. And I'm betting that they're right.
Where string theory comes up short
NOVA: Do you think there is any sort of downside or danger for physics as a whole that string theory is the hot thing that everyone is going into these days?
Weinberg: It's really a pity that we have a generation of very bright theoretical physicists who work on string theory and who, because of that work, become really detached from experiment, don't follow experiment, don't really understand what's possible experimentally. In my generation you had to pay attention to experiment. The theories were developed through a process of give-and-take with the experimentalist. That's been lost. I don't see what to do about it. I don't think it does any good to deplore it. The string theorists are doing what they have to do. This is the only way they can make progress, and we just have to hope for a better time in the future when experiment and theory come back together again.
NOVA: What do you see as string theory's greatest failure?
Weinberg: A disappointing aspect of string theory is that it has so far failed to shed any light at all on what is probably the biggest outstanding problem in the physics of what we can actually see in nature—the failure to understand the energy of empty space, the so-called cosmological constant. If you try to calculate the energy in empty space, taking into account only fluctuations in fields of wavelengths where we understand the physics, you get an incredibly large energy, much too large to possibly fit what we know about the expansion of the universe. There must be some complicated cancellations that make the energy in empty space very small.
String theory provides not the slightest shred of insight as to why the energy of empty space is as low as it experimentally seems to be. And that's precisely the kind of thing that one would think string theory would be able to help with. I'd say that's the biggest disappointment so far, that in the one area where you might expect some kind of quantitative general idea to come out of string theory that might actually be useful, it has failed to provide it.
NOVA: How do you think that this era in theoretical physics, string theory in particular, will be remembered, say, 50 or 100 years from now?
Weinberg: I think 100 years from now this particular period will be remembered as a heroic age when theorists cut themselves temporarily free from their experimental underpinnings and tried and succeeded through pure theoretical reasoning to develop a unified theory of all the phenomena of nature. On the other hand, it may be remembered as a tragic failure in which physics took a wrong turn and ignored the most important evidence, which was there in front of their noses. We don't know. My guess is that it will be something like the former rather than the latter. But ask me 100 years from now, then I can tell you.
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