Trying to break a photon would create a quantum mess

When it comes to creating a mess, not even the world’s crumbliest cookie would compare to the theoretical destruction of a photon. Photons are fundamental particles of light. As fundamental particles, they cannot really be broken down into smaller bits. However, physicists have now shown what would happen in theory if you tried to crack a photon in half. You would not just end up with a second photon. Their calculations show that 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 that it was nonsense. Then he read it carefully and enjoyed the challenge. Faccio is a physicist at the University of Glasgow in Scotland who did not take part in the work. He noted, “The technique is legit,” and added, “The results look absolutely reasonable.”

The key to the analysis is that photons do not just act as pointlike particles. They also behave like extended waves. This dual nature got Johannes Skaar wondering about a specific scenario. Skaar is a physicist at the University of Oslo in Norway. He asked what would happen if you had a device fast enough to snip the wave of a single photon in half.

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 are just much more likely to create small numbers of them than huge swarms.

“This is a bit strange,” Skaar allows. But in the quantum realm, he adds, it is not actually that weird. Quantum physics is the science that describes how the very smallest things, such as photons, behave. 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 specific case, the mirror’s motion could provide the energy to spawn new light particles.