Physics & Math / Cosmology

Einstein didn’t care much about experiments. Of the three tests he proposed for general relativity, the first—that clocks should tick slower in a gravitational field—wasn’t satisfied until after his death. Early experiments tended to contradict the prediction. His second prediction, that light from distant stars would be deflected by the warped space-time around the sun, catapulted him to world fame in 1919, when observations of a solar eclipse seemed to confirm his prediction. But as historians have since shown, the 1919 measurements were equivocal at best.

The one unequivocal verification of Einstein’s theory during his lifetime was his explanation of a tiny anomaly in the orbit of Mercury. When he finally got that calculation to work, that was the only evidence he needed that space and time really were warped. “Nature had spoken to him,” wrote biographer Abraham Pais. “He had to be right.”

We now know he was. Just last year, for instance, radio signals transmitted from the Cassini spacecraft on its way to Saturn proved to be deflected by the sun by just the amount predicted by general relativity. Einstein would have been unimpressed. He believed his theory was correct because it was consistent, simple, and beautiful. “It is my conviction that pure mathematical construction enables us to discover the concepts and the laws connecting them, which give us the key to the understanding of the phenomena of Nature,” he declared in 1933.

For better and worse, this aspect of Einstein’s approach to physics is very much alive today: “Pure mathematical construction” is an industry that employs hundreds of workers. What they have been trying to construct is a unified theory of the four forces—gravity, electromagnetism, and the strong and weak nuclear forces. To build that, they first have to find a way of describing gravity, not in Einsteinian terms as a curvature of space-time but like the other three forces, as an exchange of force-bearing quanta. This quantum gravity theory would take over from general relativity in the extraordinarily tight quarters—the very core of a black hole, the very instant of the Big Bang—where relativity now predicts, absurdly, that space-time is infinitely curved. The final unified theory, in turn, would gratify most physicists’ deep belief that the four forces really were equivalent once, in the first nanosecond after the Big Bang. That idea “is so beautiful that it has to be that way,” explains physicist Clifford Will of Washington University. “That’s an Einsteinian point of view.”

Sure enough, there is as yet no experimental evidence to support this view—indeed, the improbability of ever reproducing black holes or big bangs in the laboratory has caused some researchers to wonder whether there will ever be any evidence. “Since the middle 1970s we’ve been in a situation in fundamental physics in which theory has run on, largely unchecked by experiments,” says Lee Smolin, a theoretical physicist at the Perimeter Institute near Toronto. (Smolin’s article on Einstein and his scientific legacy begins on page 36.) “And most of us think this has been very bad.”

But lately Smolin and others have been finding grounds for optimism: The reign of the theorists seems to be coming to an end. The several schools of quantum gravity, physicists have realized, are slowly reaching consensus that incremental violations of relativity might be detectable even outside a black hole, and without building a particle accelerator the size of the universe. For that reason, and also because experimenters like to poke at the marble facades of grand theories, Einstein’s ideas are being tested with increasing frequency these days and to extraordinary precision. What follows is a sampling of these experiments—just a few of the many now under way or on the drawing boards.

“I do not consider the main significance of the general theory of relativity to be the prediction of some tiny observable effects,” wrote Einstein in 1930. Nonetheless, it was tiny observable effects that eventually proved that in certain situations Newton’s physics needed to be replaced by Einstein’s. In the same way it will be tiny observable effects that will one day prove where the limits of Einstein’s genius lie. “People sometimes ask, ‘Why have you felt you should persist in this way to perform a test of Einstein?’ ” says Stanford University physicist Francis Everitt, one of the leaders of a satellite experiment called Gravity Probe B, which was launched in April. “Aren’t Einstein’s theories all established and confirmed? After all, it’s a hundred years ago that he developed his first theory of relativity. Don’t we already know it all? And the answer is no.” Some of the most important experiments under way may answer the questions that follow.