The question of whether quantum mechanics is correct could soon be settled by observing the sky — and there are already tantalizing hints that the theory could be wrong. Antony Valentini, a physicist at Imperial College, London, wanted to devise a test that could separate quantum mechanics from one of its closest rivals — a theory called bohmian mechanics. Despite being one of the most successful theories of physics, quantum mechanics creates several paradoxes that still make some physicists uncomfortable, says Valentini. So far it’s been impossible to pick apart quantum mechanics from bohmian mechanics — both predict the same outcomes for experiments with quantum particles in the lab. But Valentini thinks that the stalemate could be broken by analysing the cosmic microwave background — the relic radiation left behind after the Big Bang. The cosmic microwave background contains hot and cold temperature spots that were generated by quantum fluctuations in the early Universe and then amplified when the Universe expanded.
Using the principles of quantum mechanics, cosmologists have calculated how these spots should be distributed. However, Valentini’s calculations show that the hidden-variables theory might give a different answer. “Any violation of quantum mechanics in the early Universe would have a knock-on effect that we could see today,” says Valentini. Almost all measurements of the cosmic microwave background seem to fit well with the predictions of quantum mechanics, says Valentini. But intriguingly, a distortion that fits one of Valentini’s proposed signatures for a failure of quantum mechanics was recently detected by Amit Yadav and Ben Wandelt at the University of Illinois at Urbana-Champaign. That result has yet to be confirmed by independent analyses, but it is tantalizing, Valentini adds.