Invisible hand, and a quick one at that

Any alternative to quantum weirdness would require faster-than-light communication.

It’s another bad day for Einstein. He either has to give up relativity or embrace quantum mechanics.

Reality seems governed by the kind of randomness that Einstein loathed and that quantum theory is rooted in. But any alternative explanation would have to allow information to travel at least 10,000 times faster than light, physicists have now shown in the most stringent such test to date.

Nicolas Gisin and his team at the University of Geneva sent pairs of photons traveling separately along optical fibers. Without weirdness of quantum mechanics, the photons’ behavior could only be explained if photons separated by 18 kilometers could influence each other virtually instantaneously. That would be a blatant violation of the solidly tested principle that nothing travels faster than light — part of Einstein’s theory of relativity.

The team sent the photons along two optical fibers, from Geneva to the nearby towns of Jussy and Satigny, they describe in the Aug. 14 Nature. The photons were generated in a state of quantum uncertainty, so that their departure times would be slightly fuzzy.

When a photon arrived at its destination, it was detected. This detection gave the photons’ travel times precise values, erasing the uncertainty. The travel times showed small, random variations from one photon to the next, Gisin explains. Quantum theory predicts that in this type of situation, it is impossible to predict the exact travel time in advance, and that, in fact, even the photons themselves don’t know what the travel time is.

The physicists also created the photons in such a way that the destiny of each photon sent to Jussy was linked by quantum entanglement to the destiny of a photon sent to Satigny. Quantum entagled particles form one system, rather than separate systems with independent properties.

In this case, the travel times were correlated, and once a photon was detected, the travel time of its twin ceased to be undefined. Once measured, the second photon’s travel time turned out to be identical to that of its entangled twin.

But, to a quantum mechanics skeptic, it’s as if one photon let the other know what value to pick. For one photon’s choice to affect the other’s, information would have to travel the 18 kilometers separating the two towns in virtually no time. The team couldn’t prove that information traveled instantaneously. But because their experimental errors were limited to time differences of less than one-third of a billionth of a second, they could prove that — if one photon influenced the other — the information must have traveled at least 10,000 times faster than light.


n principle, a new theory could replace quantum mechanics and propose that the choices that seem random to an observer in Satigny really are influenced by events in Jussy, and vice versa. But experiments such as Gisin’s show that such a theory requires instantaneous, or at least incredibly fast, communication. “It sounds so extreme that most physicists would agree that it’s implausible,” Gisin quips.

Valerio Scarani of the National University of Singapore agrees. Insisting on looking for an intuitive explanation — one that appeals to common sense rather than to quantum weirdness — forces a person toward conclusions that are “frankly, a bit absurd,” he says.

The only plausible explanation left, Gisin concludes, is the one accepted by most physicists: No information is exchanged between distant photons. Unfortunately, that defies common sense, too, Gisin admits. “Nature seems to produce random events that manifest themselves at several locations at once,” he says.

Gisin and others before him had already found similar speed requirements as those in the new Nature paper. But each of those previous experiments was done in one particular frame of reference in space and time. So those experiments left open the possibility that, in a different frame of reference, information could travel at more reasonable speeds. The new Geneva experiment was the first one to rule out all frames of reference, by performing measurements around the clock and exploiting Earth’s rotation.

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