Quantum uncertainty controls 'action at a distance'
By Mark Buchanan Two things Albert Einstein did not like about quantum theory were its inherent uncertainty and its assertion that particles can remain weirdly linked even when separated by great distances. The former he dismissed with the phrase “God does not play dice,” and the second he called “spooky action at a distance”. Now a pair of physicists says that these two strange effects are intimately linked – and that uncertainty itself limits how “connected” separate particles can be. When two distant particles that are quantum mechanically linked, or entangled, are measured, the results are more similar than predicted by classical physics. “Nature is non-local,” says Sandu Popescu of the University of Bristol. “This is arguably the most important lesson of quantum theory.” But these non-local links are not as influential as they might conceivably be. Popescu and colleague Daniel Rohrlich, now at Ben Gurion University in Israel, calculated 15 years ago that the laws of physics could allow even stronger coordination between distant systems, leading physicists to wonder why quantum theory doesn’t go as far as it could. Jonathan Oppenheim of the University of Cambridge in the UK and Stephanie Wehner of the National University of Singapore suggest they’ve found a clue. The secret, they suggest, lies in another famous property of the quantum world – its inherent uncertainty. In quantum theory, states of a quantum system can never be defined with precision. The uncertainty principle, for example, implies that any effort to measure the position of an electron entails giving up precise knowledge of its velocity, or vice versa. Using information theory, a core tool of computer science that quantifies how much information is contained in any structure, Oppenheim and Wehner studied how the amount of uncertainty in a theory should influence the possibilities it presents for nonlocal connections. Hypothetical theories containing no uncertainty, they found, could feature coordination between distant systems as strong as the limit calculated by Popescu and Rohrlich. “Quantum mechanics could be even spookier,” says Oppenheim. “But Heisenberg’s uncertainty principle gets in the way.” The results do not suggest what underlying physical mechanism would link uncertainty to non-locality. But because the results rest only the ideas of information theory, they should hold not only for quantum theory, but for any conceivable future theory as well, the authors say. “This is a very original and important idea,” says Popescu. But he cautions that it is probably not the final word on why quantum theory is not more non-local than it is. “This work doesn’t finally solve the problem,” he says. “But it goes in a very new direction”, adding to other possible explanations that physicists have explored in recent years. Journal reference: Science (vol 330, p 1072) More on these topics: