Editor's Summary
14 August 2008
Spooky action: setting a speed limit
Einstein called entanglement, a type of correlation found only in quantum physics, 'spooky action at a distance'. Experimental tests known as Bell inequalities have all but ruled out a classical explanation for such correlations, but the possibility remains that a first event could influence a second one, if the influence occurs faster than the speed of light. An impressive new experiment has been performed with a view to establishing a lower limit for the speed of such hypothetical influences. The experiment involved a Bell inequality test lasting more than 24 hours between two Swiss villages 18 km apart and approximately east–west oriented, with the source located precisely in the middle. Taking advantage of the Earth's rotation, the experiment allowed the determination of a lower bound for the speed of any such influence. The conclusion was that the minimal speed of hypothetical spooky action at a distance, under plausible assumptions for this experiment, is at least 10,000 times greater than the speed of light. The existence in nature of a real spooky action at a distance is therefore deemed implausible.
News and Views: Quantum mechanics: The speed of instantly
Pairs of quantum-mechanically entangled particles seem to know at once what is happening to each other. Experiments show that even if this signalling is not instantaneous, it must be really, really fast.
Terence G. Rudolph
doi:10.1038/454831a
Letter: Testing the speed of 'spooky action at a distance'
Daniel Salart, Augustin Baas, Cyril Branciard, Nicolas Gisin & Hugo Zbinden
doi:10.1038/nature07121
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