Here’s what would happen if you tried to break a photon in half

If you think breaking a Nature Valley granola bar makes a mess, try a photon.

As fundamental particles, photons can’t really be cleaved into smaller pieces. But if you tried cracking a light particle in half, physicists calculate that you wouldn’t just end up with a second photon. Rather, up to an infinite number of new photons would crumble out, the researchers report in a paper accepted to Physical Review Letters.

Daniele Faccio’s first-glance reaction to the study was: nonsense. “Then you read it, and I enjoyed it,” says Faccio, a physicist at the University of Glasgow in Scotland. “The technique is legit.”

The analysis hinges on the fact that photons aren’t just pointlike particles. They also act as extended waves. That got theoretical physicist Johannes Skaar and his colleagues at the University of Oslo in Norway wondering: What would happen if you had a device fast enough to snip the wave of a single photon in half?

Using quantum equations, Skaar and his colleagues modeled a scenario in which a photon is traveling toward a mirror. The front half of the light wave hits the mirror first and gets bounced back in the direction it came from. But suddenly the mirror is removed, and the back half of the light wave is free to pass through.

This would spew out a complex mix, or superposition, of possibilities with different numbers of photons, the math shows. Removing the mirror infinitely fast would conjure an infinity of light particles out of thin air. Infinite speed is of course impossible. But even pulling the mirror away more slowly, Skaar says, “you end up with a possibility of several photons, or a bunch of photons.” You’re just much more likely to create smaller numbers of them than huge swarms.

“This is a bit strange,” allows Skaar. But by quantum standards, it’s not actually that weird, he says. Physicists knew that disturbing supposedly “empty” space — known as a vacuum — could knock new photons loose. In this case, the energy fed into the system by moving the mirror could spawn new light particles.

To Skaar, the oddest outcome of the mathematical model is what you’d see if you observed the system from different perspectives. If you had a view of both sides of the mirror at once, you’d witness the messy eruption of up to bajillions of photons. But if you could see only one side of the mirror or the other, you’d see either a single photon or a vacuum.

“That is really crazy,” says Skaar. He hopes to probe that difference more deeply in future work — and explore what would happen if you tried to sever other types of fundamental particles that act like waves in quantum physics, such as electrons.

It’s not immediately obvious what applications 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 offer one example. Probing the nature of individual photons, Faccio says, might be useful in fields that use such quantum sensors.