The Year in Science: Physics 1997

Score One (More) for the Spooks

By Robert Pool|Thursday, January 01, 1998
RELATED TAGS: SUBATOMIC PARTICLES
Einstein would not have been amused. Not only did researchers demonstrate last May a phenomenon that the Great One once disparaged as spooky action at a distance, but they proved it happens even at great distances. Worse, they performed the experiment in Switzerland, not far from the patent office where Einstein worked in 1905—the year he explained the quantum nature of light, which laid the foundation for quantum mechanics, which he later found so maddeningly spooky.

The spooky action in question involves a voodoolike link between two particles such that a measurement carried out on one has an instantaneous effect on the other, though it be far away—nearly seven miles away, in the experiment done by physicist Nicolas Gisin’s team at the University of Geneva. Gisin and his colleagues borrowed fiber-optic phone lines running from Geneva to two nearby villages. In Geneva, they shone photons into a potassium-niobate crystal, which split each photon into a pair of less energetic photons traveling in opposite directions—one north toward Bellevue and the other southwest to Bernex. At these two destinations, nearly seven miles apart, each photon was fed into a detector.

Common sense would suggest that nothing done to the photon in Bellevue could affect the photon in Bernex, or vice versa, but quantum mechanics never had much to do with common sense. For starters, the uncertainty principle says that Gisin cannot simultaneously know both the energy of a photon and the time it left the crystal in Geneva, at least not precisely. Furthermore, quantum mechanics insists that the photons don’t have precise properties until they are measured. To show what he saw as the absurdity of the claim, Einstein proposed a simple thought experiment in 1935, and this became the basis for Gisin’s complicated real one.

Einstein believed that the uncertainty principle was just a measurement problem, not a reality problem. His idea, in terms of the Geneva experiment, was that you could learn the energy of one photon by measuring the energy of the other one far away; by the same token, you could learn a photon’s arrival time by measuring that of its distant counterpart. After all, the two photons had to leave Geneva at the same time, and although their energies might not be equal, they have to add up to the energy of the parent photon. Assuming that these measurements could be made, and that they added up in this commonsense way, Einstein would be correct, and reality would be independent of measurement. Or you’d be forced to argue that the Bellevue measurement instantaneously and spookily changes the reality of the photon at Bernex, which to Einstein was an absurd suggestion. The mind game itself was proof enough for Einstein, but in 1964 physicist John Bell turned it into a testable hypothesis. He came up with an equation, called Bell’s inequality, that boiled the question down to a set of measurements of many photons hitting detectors. If energy and arrival time were absolute values, as Einstein believed, then these measurements would be true to Bell’s inequality. If, on the other hand, quantum mechanics was valid after all, and the precise energy and arrival time of a photon did not exist until they were measured, the measurements would violate Bell’s inequality.

In Gisin’s experiment, alas, Einstein and common sense were the losers. It’s as if he had flipped a coin at Bellevue, Gisin says, while his colleague had flipped one at Bernex, and each time he grabbed his coin out of the air and saw it was heads up, his colleague’s coin had simultaneously stopped spinning and landed heads up as well. And this happened thousands of times in a row. It is a very strange prediction, Gisin says, and because it is so bizarre, it deserved to be tested.

In fact, it had already been tested many times, most notably in 1981 when physicist Alain Aspect from the University of Paris first dazzled his peers by demonstrating the phenomenon. But Aspect separated his photons by only a few meters, and since then some physicists who share Einstein’s reluctance to abandon common sense had speculated that the spooky effect might decline with distance. We have now done it in the lab, and we have done it at 10 kilometers, and we found no significant differences, Gisin says. Common sense, at least in the quantum world, would seem to be a dead horse—but Gisin is planning one more crack at the corpse. He wants to set up a test at an even farther distance—perhaps the 60 miles that separate Geneva and Bern, the site of the patent office where Einstein worked. He even knows when he wants to do it: in 2005, the centennial of Einstein’s pioneering paper.
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