Will We Have A Final Theory Of Everything?

By Steven Weinberg

The 20th century was quite a time for physicists. By the mid-1970s we had in hand the so-called standard model, a theory that accurately describes all the forces and particles we observe in our laboratories and provides a basis for understanding virtually everything else in physical science.

No, we don't actually understand everything--there are many things, from the turbulence of ocean currents to the folding of protein molecules, that cannot be understood without radical improvements in our methods of calculation. They will provide plenty of interesting continued employment for theorists and experimenters for the foreseeable future. But no new freestanding scientific principles are needed to understand these phenomena. The standard model provides all the fundamental principles we need.

There is one force, though, that is not covered by the standard model: the force of gravity. Einstein's general theory of relativity gives a good account of gravitation at ordinary distances, and if we like, we can tack it on to the standard model. But serious mathematical inconsistencies turn up when we try to apply it to particles separated by tiny distances--distances about 10 million billion times smaller than those probed in the most powerful particle accelerators.

Even apart from its problems in describing gravitation, however, the standard model in its present form has too many arbitrary features. Its equations contain too many constants of nature--such as the masses of the elementary particles and the strength of the fundamental units of electric charge--that are there for no other reason than that they seem to work. In writing these equations, physicists simply plugged in whatever values made the predictions of the theory agree with experimental results.

There are reasons to believe that these two problems are really the same problem. That is, we think that when we learn how to make a mathematically consistent theory that governs both gravitation and the forces already described by the standard model, all those seemingly arbitrary properties will turn out to be what they are because this is the only way that the theory can be mathematically consistent.

One clue that this should be true is a calculation showing that although the strengths of the various forces seem very different when measured in our laboratories, they would all be equal if they could be measured at tiny distances--distances close to those at which the above-mentioned inconsistencies begin to show up.

Theorists have even identified a candidate for a consistent unified theory of gravitation and all the other forces: superstring theory. In some versions, it proposes that what appear to us as particles are really stringlike loops that exist in a space-time with 10 dimensions. But we don't yet understand all the principles of this theory, and even if we did, we would not know how to use the theory to make predictions that we can test in the laboratory.

Such an understanding could be achieved tomorrow by some bright graduate student, or it could just as easily take another century or so. It may be accomplished by pure mathematical deduction from some fundamental new physical principle that just happens to occur to someone, but it is more likely to need the inspiration of new experimental discoveries.

We would like to be able to judge the correctness of a new fundamental theory by making measurements of what happens at scales 10 million billion times smaller than those probed in today's laboratories, but this may always be impossible. With any technology we can now imagine, measurements like those would take more than the economic resources of the whole human race.

Even without new experiments, it may be possible to judge a final theory by whether it explains all the apparently arbitrary aspects of the standard model. But there are explanations and explanations. We would not be satisfied with a theory that explains the standard model in terms of something complicated and arbitrary, in the way astronomers before Copernicus explained the motions of planets by piling epicycles upon epicycles.

To qualify as an explanation, a fundamental theory has to be simple--not necessarily a few short equations, but equations that are based on a simple physical principle, in the way that the equations of general relativity are based on the principle that gravitation is an effect of the curvature of space-time. And the theory also has to be compelling--it has to give us the feeling that it could scarcely be different from what it is.

When at last we have a simple, compelling, mathematically consistent theory of gravitation and other forces that explains all the apparently arbitrary features of the standard model, it will be a good bet that this theory really is final. Our description of nature has become increasingly simple. More and more is being explained by fewer and fewer fundamental principles. But simplicity can't increase without limit. It seems likely that the next major theory that we settle on will be so simple that no further simplification would be possible.

The discovery of a final theory is not going to help us cure cancer or understand consciousness, however. We probably already know all the fundamental physics we need for these tasks. The branch of science in which a final theory is likely to have its greatest impact is cosmology. We have pretty good confidence in the ability of the standard model to trace the present expansion of the universe back to about a billionth of a second after its start.

But when we try to understand what happened earlier than that, we run into the limitations of the model, especially its silence on the behavior of gravitation at very short distances. The final theory will let us answer the deepest questions of cosmology: Was there a beginning to the present expansion of the universe? What determined the conditions at the beginning? And is what we call our universe, the expanding cloud of matter and radiation extending billions of light-years in all directions, really all there is, or is it only one part of a much larger universe in which the expansion we see is just a local episode?

The discovery of a final theory could have a cultural influence as well, one comparable to what was felt at the birth of modern science. It has been said that the spread of the scientific spirit in the 17th and 18th centuries was one of the things that stopped the burning of witches. Learning how the universe is governed by the impersonal principles of a final theory may not end mankind's persistent superstitions, but at least it will leave them a little less room.

Steven Weinberg is a Nobel laureate in physics at the University of Texas. His books include Dreams of a Final Theory