University of Oslo Physicists Model How Truncating Photons Creates New Particles

Johannes Skaar and the University of Oslo

The concept of “cutting” a photon seems like a contradiction in terms. Physicists generally think of light in terms of photons: massless, elementary particles “carrying” the forces of nature. As fundamental, elementary particles, photons cannot be divided into smaller pieces. However, because light also exhibits the dual nature of both a particle and a wave—a phenomenon famously demonstrated by British physicist Thomas Young in 1801 with the double-slit experiment—physicists have long understood that its spatial distribution is not confined to a single point. This duality led Johannes Skaar and his colleagues at the University of Oslo in Norway to a question that the team noted, Despite being a simple question, it appears that it has not been asked before.

The Physics of the “Truncated Photon”

Quantum Field Theory and the Mirror

The team’s mathematical model describes a scenario where a photon is traveling toward a mirror. As the front half of the light wave hits the mirror, it reflects; if the mirror is suddenly removed, the back half of the wave is free to pass through. Using quantum field theory, the researchers calculated how this setup would affect the dynamics of the photon’s waveform. They found that the act of removing the mirror acts as a tug on the quantum field that pulls energy from the nearby vacuum, spawning new light particles.

According to the paper, the results were not that the team had two photons, or even a photon and a vacuum. Instead, what they obtained was a complicated state involving photon numbers up to infinity. While the theoretical model suggests that an infinitely fast removal of the mirror would conjure an infinity of light particles out of thin air, the researchers acknowledge that infinite speed is impossible. However, even pulling the mirror away more slowly, Skaar notes, you end up with a possibility of several photons, or a bunch of photons. You are simply much more likely to create smaller numbers of them than huge swarms.

Measurement Paradoxes and Quantum States

Observer Perspective and Localized Measurements

Perhaps the most counterintuitive finding involves how the system behaves when observed. The researchers discovered that the state of the system depends heavily on the observer’s perspective. If you had a view of both sides of the mirror at once, you would witness the messy eruption of up to bajillions of photons. But, as Skaar notes, if you could see only one side of the mirror or the other, you’d see either a single photon or a vacuum.

The researchers found that if you tried to measure the states on either side of the split wave packet, you would get measurements that look exactly like a single-photon state on the left and a vacuum on the right. This suggests that the truncated photon creates a state that is locally equivalent to simple, known quantum states, even while the global reality of the system is significantly more chaotic. As Skaar describes it, That is really crazy.

Expert Reaction and Future Directions

Daniele Faccio and Gravitational Wave Catchers

Daniele Faccio, a physicist at the University of Glasgow in Scotland, admitted that his first-glance reaction to the study was: “nonsense.” However, he added, Then you read it, and I enjoyed it. The technique is legit.

Skaar hopes to probe the difference in perspectives more deeply in future work and explore what would happen if researchers tried to sever other types of fundamental particles that act like waves in quantum physics, such as electrons. While it is not immediately obvious what applications this research might have, Faccio suggests it might matter because there are funky things that people do with [photons] for sensing and measuring. Gravitational wave catchers offer one example where probing the nature of individual photons might be useful. For now, the truncated photon remains a mathematical exploration of how disturbing supposedly “empty” space can knock new photons loose, demonstrating that even at the quantum level, energy fed into a system can spawn new light particles.