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.”
