Classic quantum experiment could conceal theory of everything

AN ICONIC physics experiment may be hiding more than we ever realised about the nature of reality. The classic “double-slit” experiment reveals the strange duality of the quantum world, but it may behave more strangely than we thought – and could challenge one of the most closely held assumptions of quantum mechanics.

Revisiting it could help unify quantum mechanics with the other pillar of theoretical physics – Einstein’s general relativity – a challenge that has so far proven intractable.

The double-slit experiment involves shining a light at two close-together slits placed in front of a screen.

Our classical view of the world suggests that photons of light should pass through one slit or the other, and thus create two parallel bands on the screen behind. But instead, the light spreads out into alternating bands of light and dark.

This interference pattern appears even if you send in one photon at a time, suggesting that rather than moving in a straight line, light behaves as both a wave and a particle at the same time. US physicist Richard Feynman said this experiment embodies the “central mystery” of the quantum world.

“Every student of quantum physics is taught how to calculate the interference pattern of the double-slit experiment,” says James Quach at the Barcelona Institute of Science and Technology in Spain.

To calculate the probability that a photon will arrive at some location on the screen, physicists use a principle called the Born rule. However, there is no fundamental reason why the Born rule should hold. It seems to work in all the situations we’ve tested, but no one knows why. Some have attempted to derive it from the “many worlds” interpretation of quantum mechanics, which proposes that all the possible states of a quantum system could exist in different, parallel universes – but such attempts have been inconclusive.

That makes the Born rule a good place to look for cracks in quantum theory. To unite quantum mechanics, which governs the universe on minute scales, and general relativity, which holds at immense scales, one of the theories must give way. If the Born rule falls over, it could clear a path to quantum gravity.

“If the Born rule is violated, then a fundamental axiom of quantum mechanics has been violated, and it should point to where one needs to go to find quantum gravitational theories,” says Quach.

Now, Quach has suggested a new way to test the Born rule. He started from another idea of Feynman’s: in order to calculate the probability of a particle reaching a certain place on the screen, you should account for all the possible paths it could take from the source to the screen, even ones that seem ridiculous. “This includes paths that go from here to the moon and back again,” says Quach.

Almost none of these paths should affect the photon’s final location, but there are some weird paths that could change the probabilities enough for us to measure the difference.

“If the Born rule is violated, it should point to how to find a theory of quantum gravity“

For instance, say there are three paths that a particle could take through the apparatus instead of the obvious two. The Born rule lets you calculate probabilities by considering interference between pairs of paths, but not between all three paths at once.

Quach shows that if you account for interference between all three paths, the probabilities will be different from what the Born rule predicts (arxiv.org/abs/1610.06401v1).

He suggests testing this with a double-slit experiment that allows for a third path, a wandering zigzag in which the particle goes through the left slit, over to the right slit, then heads towards the screen. If that third path interferes with the two more straightforward ones, the results should deviate from what the Born rule suggests.

Quach’s work is “extremely interesting and thought-provoking”, says Aninda Sinha at the Indian Institute of Science in Bangalore, a member of the team that first proposed exploring violations of the Born rule using winding, non-classical paths.

But he points out that Quach’s experiment could fail to capture other paths that might muddy the results.

The stakes are high. Finding violations of the Born rule could be the thin edge of the wedge that pries open the door to a more fundamental understanding of reality.

This article appeared in print under the headline “Double-slit jeopardy”