What does an experiment that seems to contradict Einstein’s theory of relativity really mean?
IN 1887 physicists were feeling pretty smug about their subject. They thought they understood reality well, and that the future would just be one of ever more precise measurements. They could not have been more wrong. The next three decades turned physics on its head, with the discovery of electrons, atomic nuclei, radioactivity, quantum theory and the theory of relativity. But the grit in the pearl for all this was a strange observation made that year by two researchers called Albert Michelson and Edward Morley that the speed of light was constant, no matter how fast the observer was travelling.
Some physicists are wondering whether their subject has just had another Michelson-Morley moment. On September 23rd researchers at CERN, Europe’s main physics laboratory, announced that subatomic particles called neutrinos had apparently sped from the lab’s headquarters near Geneva, through the Earth’s crust, to an underground detector 730km (450 miles) away around 60-billionths of a second faster than light would take to cover the same distance (see article). The difference in speed is tiny, but the implications are huge.
As every schoolboy (and journalist with access to Wikipedia) knows, this flies in the face of special relativity, a theory devised by Albert Einstein precisely to explain the observation of Michelson and Morley. Special relativity, which physicists thought they had tested almost to destruction, and found not wanting, states that as objects speed up, time slows down. Time stops altogether on reaching the 299,792,458 metres per second at which light zaps through a vacuum. Go any faster and you would be moving backwards in time.
If CERN’s neutrinos really are travelling faster than light, it is therefore a big deal. Modern physicists, aware of the hubris of their 19th-century predecessors, have never thought their subject closed. But nor have they found a chink in the armour of relativity that they could use to prise the whole thing open. This would be such a chink. Their caution in the face of the result—the public statements that it is probably explained by experimental error, even though the researchers involved have been over their equipment with a fine-tooth comb—is understandable. No one wants to get egg on his face by having missed something obvious.
A theory of everything
If the result is true, though, it does change everything. In particular, the likely explanation is that the neutrinos are taking a short-cut through one of the extra dimensions which string theory postulates are hidden among the familiar four of length, breadth, height and time. Measured along this five-dimensional route, Einstein might still be right. (It would not so much be that he made a mistake as that he did not know the whole story.) Indeed, moving beyond four dimensions in this way would also allow physicists to try to integrate Einstein’s work with quantum theory, the other great breakthrough of 20th-century physics, but one which simply refuses to overlap with relativity. A unified theory of everything, including perhaps as many as 11 dimensions, would then beckon.
That is a lot to hang on a single, unconfirmed observation. But then, in 1887, no one could have foreseen the consequences of the Michelson-Morley experiment. If a glitch is found in CERN’s result, the whole thing will rapidly be swept under the carpet and forgotten. If there is no glitch, an astonishing future of understanding beckons.