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.

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