A controversial experiment threatened to kill the multiverse in 2025

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.

The claim that you can resolve fundamental issues of quantum mechanics by experiment was met with “lots of scepticism”, says Hofmann, in part because their experimental measurement technique was partially new, he says.

“We are stepping on several people’s feet,” says Hofmann. This is because most interpretations of quantum mechanics assume that the values you measure, and their mathematical interpretation, represent the real world. “The many-worlds interpretation is simply the most extreme manifestation of that assumption,” says Hofmann.

Instead, their work shows that the mathematical wave function isn’t real, and the only reality is what you can measure, says Hofmann. “Superpositions make it look as if the state is described by such hypothetical measurement outcomes, but the actual experimental evidence contradicts such overinterpretations,” he says.

While Hofmann and his team have struggled to get the paper published in a journal, they have been invited to speak about the work at other research groups, and are continuing to develop the work further. “I fully expected some pushback. In fact, it would hardly be worth doing this work if it was easy. Minds need to be changed, and that takes a lot of time,” says Hofmann.