Trying to break a photon would create a quantum mess

When it comes to making a mess, not even the world’s crumbliest cookie would compare to a photon.

Photons are fundamental particles of light. As fundamental particles, they cannot really be broken down into smaller bits. But physicists have now shown what would happen — in theory — if you tried to crack a photon in half.

You wouldn’t just end up with a second photon, their calculations show. Rather, up to an infinite number of new light particles would sprinkle out of thin air.

The researchers shared their findings in a paper that will be published in Physical Review Letters.

Daniele Faccio’s first reaction to the study was: nonsense. “Then you read it, and I enjoyed it,” he says. Faccio is a physicist at the University of Glasgow in Scotland who did not take part in the work. “The technique is legit,” he says. “The results look absolutely reasonable.”

Photon interrupted

The key to the analysis is that photons don’t just act as pointlike particles. They also behave like extended waves. That got Johannes Skaar wondering: What would happen if you had a device fast enough to snip the wave of a single photon in half?

Skaar is a physicist at the University of Oslo in Norway. His team modeled a scenario where a photon is traveling toward a mirror. The front half of the light wave hits the mirror first. It gets bounced back in the direction it came from. But then, the mirror is suddenly removed. The back half of the light wave is then free to pass through.

This, the math shows, would spew out a complex mix — or superposition — of different possible numbers of photons.

Removing the mirror infinitely fast would conjure an infinity of new light particles. Infinite speed is, of course, impossible. But even pulling the mirror away more slowly has striking effects. “You end up with a possibility of several photons, or a bunch of photons,” Skaar says. You’re just much more likely to create small numbers of them than huge swarms.

“This is a bit strange,” allows Skaar. But in the quantum realm, he adds, it’s not actually that weird.

Quantum physics is the science that describes how the very smallest things, such as photons, behave. And it has already shown that supposedly “empty” space is not truly empty. There is some structure to the fabric of “empty” space, and disturbing it is known to knock new photons loose.

In this case, the mirror’s motion could provide the energy to spawn new light particles.

Weirder and weirder

To Skaar, a possible cascade of new light particles isn’t the oddest outcome of the model. To him, the weirdest part is the math that describes what you’d see if you observed the system from different perspectives.

If you could view both sides of the mirror at once, you’d witness the messy eruption of up to bajillions of photons. But if you could only see one side of the mirror, you’d observe just one — or empty space.

“That is really crazy,” says Skaar. He hopes to probe what’s going on more deeply in future work. He also wants to explore what would happen if you tried to sever other types of fundamental particles. In quantum physics, particles such as electrons also act like waves. Attempting to snip them in half might lead to similarly bizarre results.

It’s not super clear what uses this research might have. “I’m going to speculate wildly here,” Faccio says. But “it might matter because there are funky things that people do with [photons] for sensing and measuring.” Gravitational-wave catchers, for example, play with light waves to detect ripples in the fabric of our universe.

Probing the nature of photons, Faccio says, may be helpful in fields that use such quantum sensors.