Published online 15 May 2008 | Nature | doi:10.1038/news.2008.829

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Observations of the cosmic microwave background might deal blow to theory.

The background patterns of space could help us focus on quantum problems. NASA / ESA / Hubble Heritage Team

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.

For instance, quantum theory uses probability to describe the properties of a particle. These properties obtain definitive values only when they are measured, which means that you cannot predict a particle's position or momentum, for instance, with certainty.

These premises troubled Albert Einstein. He believed that particles contain extra properties — or 'hidden variables' — that determine their behaviour completely. If only we knew what these hidden variables were, we could predict the fate of particles and the outcome of measurements with certainty. Bohmian mechanics is one of a suite of 'hidden variables' theories — many now discredited — formulated to tackle this problem.

Neck and neck

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.

“It’s far too early to say that this is definite evidence of a breakdown in quantum mechanics – but it is a possibility.” Antony Valentini

Imperial College, London

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 (see 'Deflating inflation?'). That result has yet to be confirmed by independent analyses, but it is tantalizing, Valentini adds.

“It’s far too early to say that this is definite evidence of a breakdown in quantum mechanics — but it is a possibility,” he says.

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Cosmic predictions

Hiranya Peiris, an expert on the cosmic microwave background at the University of Cambridge, UK, is impressed by the new work. “This is a pretty cool new idea,” she says. “Nobody has ever thought of using the cosmic microwave background to look into really fundamental quantum questions — cosmologists just assume that quantum mechanics is correct,” she says.

But Peiris adds that Valentini must now come up with more detailed predictions about the types of distortion that will arise in the cosmic microwave background to convince cosmologists that they are really caused by a breakdown of quantum mechanics. “He has thrown some really exciting ideas out there, but now he needs to do the nitty-gritty calculations,” she says.

Vlatko Vedral, a quantum physicist at the University of Leeds, UK, agrees that the cosmic microwave background will be a useful way to test quantum mechanics. But he adds that even if quantum mechanics is shown to break down in the early Universe, that doesn’t necessarily mean that the hidden-variables theory is correct.