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

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