Einstein's theory of gravity — general relativity — transcended Newton's by offering deeper insights. It accounted naturally, in a way that Newton didn't, for why everything falls at the same speed, and why the force obeys an inverse square law. The theory dates from 1916, and was famously corroborated by the measured deflection of starlight during eclipses, and by the anomalies in Mercury's orbit. But it took more than 50 years before there were any tests that could measure the distinctive effects of the theory with better than 10 percent accuracy. In the 1960s and 1970s , there was serious interest in tests that could decide between general relativity and alternative theories that were still in the running. But now these tests have improved so much, and yielded such comprehensive and precise support for Einstein, that it would require very compelling evicence indeed to shake our belief that general relativity is the correct "classical" theory of gravity,
New and different experiments are nonetheless currently being planned. But the expectation that they'll corroborate the theory is no so strong that we'd demand a high burden of proof before accepting a contrary result, For instance, NASA plans to launch an ultra-precise gyroscope ("Gravity Probe B") to measure tiny precession effects. If the results confirm Einstein, nobody will be surprised nor excited — though they would have been if the experiment had flown 30 years ago, when it was first devised. On the other hand, if this very technically-challenging experiment revealed seeming discrepancies, I suspect that most scientists would suspend judgment until it had been corroborated. So the most exciting result of Gravity Probe B would be a request to NASA for another vast sum, in order repeat it.
But Einstein himself raised other deep questions that are likely to attract more interest in the 21st century than they ever did in the 20th. He spent his last 30 years in a vain (and, as we now recognize, premature) quest for a unified theory. Will such a theory — reconciling gravity with the quantum principle, and transforming our conception of space and time — be achieved in coming decades? And, if it is, what answer will it offer to an another of Einstein's questions: "Did God have any choice in the creation of the world?" Is our universe — and the physical laws that govern it — the unique outcome of a fundamental theory, or are the underlying laws more "permissive", in the sense that they could allow other very different universes as well?
MARTIN REES is Royal Society Professor at Cambridge University and a leading researcher in astrophysics and cosmology. His books include Before the Beginning, Gravity's Fatal Attraction and (most recently) Just Six Numbers.