Quantum mechanics flummoxes physicists again

If you ever want to get your head around the riddle that is quantum mechanics, look no further than the double-slit experiment. This shows, with perfect simplicity, how just watching a wave or a particle can change its behaviour. The idea is so unpalatable to physicists that they have spent decades trying to find new ways to test it. The latest such attempt, by physicists in Europe and Canada, used a three-slit version — but quantum mechanics won out again.

In the standard double-slit experiment, a wide screen is shielded from an electron gun by a wall containing two separated slits. If the electron gun is fired with one slit closed, a mound of electrons forms on the screen beyond the open slit, trailing off to the left and right — the sort of behaviour expected for particles. If the gun is fired when both slits are open, however, electrons stack along the screen in comb-like divisions. This illustrates the electrons interfering with each other — the hallmark of wave behaviour.

Such a crossover in behaviour — known as wave–particle duality — is perhaps not too hard to swallow. But quantum mechanics gets weirder. Slow down the gun so that just one electron at a time reaches the screen, and the interference pattern remains. Does each electron pass through both slits at once and interfere with itself? The obvious way to answer this question is to watch the slits as the gun fires, but as soon as you do this the interference pattern disappears.

It's as if the electrons know when they're being watched and decide to behave as particles again. According to Nobel laureate Richard Feynman, the phenomenon "has in it the heart of quantum mechanics. In reality, it contains the only mystery".

Mind the gaps


The new three-slit version of the experiment

[link to www.nature.com]