How a new twist on quantum theory could solve its biggest mystery

The “wave function collapse” transforms vague clouds of quantum possibilities into the physical reality we know – but no one knows how. New experiments are finally revealing reality in the making

By Philip Ball

IN THE minuscule realm of atoms and particles, it looks as though things exist not so much as things at all, but as vague clouds of possibilities. They seem to be here, there and everywhere, or appear to be this and that all at once – until you look at them. Then the quantum haze is suddenly distilled into something definite and describable, a thing we recognise as “real”.

That much we know. The trouble is that quantum mechanics, the theory that describes this uncertain world, has been mostly silent about how the so-called “collapse” from fuzzy probabilities to solid certainties happens. Some physicists prefer to avoid the question altogether. Others suggest that we need to add something new to complete our understanding of how our familiar physical reality emerges from the quantum.

But what if the whole picture was there all along, and we just weren’t looking carefully enough? That’s the startling suggestion from recent experiments that have, for the first time, given us a glimpse inside collapse as it happens. Physicists are still coming to terms with what they have witnessed, and it is too early to say for certain what it all means. But already there are hints that the latest results could finally point the way towards the truth about how the world we know is conjured from the quantum realm.

Quantum theory enjoys exalted status in science because it describes the microscopic world with peerless accuracy. It was developed in the 1920s to explain why subatomic particles, such as electrons, seem to sometimes behave like waves, while light waves can show particle-like behaviour – …