The multiverse was proposed as a way to make sense of bizarre quantum behaviour

VICTOR de SCHWANBERG/SCIENCE PHOTO LIBRARY

A physics experiment published this year that claimed to measure a single photon in two places at once – and, in the process, discredit the idea of a multiverse – drew pushback from many sceptical physicists, but the scientists behind the demonstration stand by their claim.

In May, Holger Hofmann at Hiroshima University in Japan and his colleagues reported the results of a modified version of the famous double-slit experiment that showed individual photons being “delocalised”, or impossible to tie down in one place.

The original experiment, first performed in 1801, demonstrated that when light is shone through two thin slits onto a screen, it will produce a wave-like interference pattern. This pattern even remains when the photons are sent through one at a time, which many physicists take as evidence that even single photons behave like waves. But there was, and still is, much debate about what is really happening to the single photon and what it is that we are measuring.

When quantum particles are said to be wave-like, this tends to refer to a particle’s wave function, a mathematical description of all the possible places where a particle might be.

These possibilities are stacked on top of one another, making what is called a superposition, until they are measured. Most physicists argue that this measurement causes the wave function to collapse from a superposition into a single value.

One way to make sense of this is to suppose that a superposition of many possible universes exist on top of one another, with photons moving through different paths in each, and that photons from different universes can somehow interfere with each other. This is known as the “many-worlds” interpretation.

But Hofmann and his team say their experiment showed direct evidence of a photon travelling through both slits, which indicates that the wave function is not simply a mathematical tool but a guide to what is really happening. Therefore, it provides evidence against the idea of a multiverse.

However, many physicists took issue with the team’s approach, arguing that repeated statistical measurements can’t be used to infer the properties of particles. “I think you can’t make claims about a single photon with this,” Andrew Jordan at Chapman University in California told New Scientist in our original story.