See Inside A Matter of Time

A Hole at the Heart of Physics [Preview]

Physicists can't seem to find the time--literally. Can philosophers help?

In Brief


  • Physicists have turned to philosophers for help with merging quantum mechanics and relativity, which treat space and time in two very different ways, into a single theory of quantum gravity.
  • By revisiting a thought experiment created by Albert Einstein, philosophers have found that general relativity has puzzling implications for the question of whether space and time are entities in their own right or merely artificial devices to describe the relations between physical objects.
  • Philosophers are also helping to identify inconsistencies in physicists' theories by, for example, pointing out that attempts to explain the arrow of time--the asymmetry of past and future--often rely on circular reasoning.


FOR MOST PEOPLE, THE GREAT MYSTERY OF TIME IS THAT THERE NEVER seems to be enough of it. If it is any consolation, physicists are having much the same problem. The laws of physics contain a time variable, but it fails to capture key aspects of time as we live it—notably, the distinction between past and future. And as researchers try to formulate more fundamental laws, the little t evaporates altogether. Stymied, many physicists have sought help from an unfamiliar source: philosophers.

From philosophers? To most physicists, that sounds rather quaint. The closest some get to philosophy is a late-night conversation over dark beer. Even those who have read serious philosophy generally doubt its usefulness; after a dozen pages of Immanuel Kant, philosophy begins to seem like the unintelligible in pursuit of the undeterminable. “To tell you the truth, I think most of my colleagues are terrified of talking to philosophers—like being caught coming out of a pornographic cinema,” says physicist Max Tegmark, now at the Massachusetts Institute of Technology.

It was not always so. Philosophers played a crucial role in past scientific revolutions, including the development of quantum mechanics and relativity in the early 20th century. Today a new revolution is under way, as physicists struggle to merge those two theories into a theory of quantum gravity—a theory that will have to reconcile two vastly different conceptions of space and time. Carlo Rovelli of the University of the Mediterranean in Marseille, France, a leader in this effort, observes that “the contributions of philosophers to the new understanding of space and time in quantum gravity will be very important.”

Two examples illustrate how physicists and philosophers have been pooling their resources. The first concerns the “problem of frozen time,” also known simply as the “problem of time.” It arises when theorists try to turn Einstein's general theory of relativity into a quantum theory using a procedure called canonical quantization. The procedure worked brilliantly when applied to the theory of electromagnetism, but in the case of relativity, it produces an equation—the Wheeler-DeWitt equation—without a time variable. Taken literally, the equation predicts that the universe should be frozen in time, in direct contradiction with what we see.

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