Theoretical physicists at the University of Oslo have modeled what happens when a single photon wave is cut in half by removing a mirror mid-reflection. According to a paper accepted by Physical Review Letters, this optical amputation pulls an infinite cascade of photons from the nearby vacuum.
Elementary particles like photons are fundamentally indivisible, meaning you cannot literally slice a single particle of light into two halves. But light travels as an extended wave packet with a spatial distribution, opening the door to a strange quantum shortcut. Led by theoretical physicist Johannes Skaar, researchers at the University of Oslo modeled a scenario where a photon travels toward a mirror, bounces its front half back, and then suddenly has the mirror yanked away while the back half continues forward.
University of Oslo Physicists Model the Truncated Photon
The study, titled Truncated Photon
and accepted for publication in Physical Review Letters, explores how an optical shutter—such as fast-moving mirrors that block or release light pulses—transforms a single photon into a superposition of modes traveling in opposite directions. Using quantum field theory, the team calculated the dynamics of the waveform resulting from this sudden truncation.
Rather than simply producing a second photon or leaving behind a vacuum, the mathematical model showed that cutting away a part of the photon conjures a complicated state involving photon numbers up to infinity. According to an accompanying Synopsis commentary, removing the mirror creates a sudden tug on the nearby quantum field, pulling out enough particles to form a sharp edge of increasingly more superposed photons.
“Despite being a simple question, it appears that it has not been asked before.”
University of Oslo researchers, via Gizmodo
What Observers See on Either Side of the Split Wave
The oddest outcome of the mathematical model involves what an observer would measure from different perspectives. If someone had a view of both sides of the mirror at once, they would witness the messy eruption of an infinite swarm of light particles. Yet, if an observer measured the states strictly to the left or right of the transition region, the results would look entirely normal.
Measurements on either side of the split wave packet would look “exactly like a single-photon state” on the left and a vacuum on the right, separated only by a very narrow transition region. Skaar described the counterintuitive setup to sciencenews.org, noting that that is really crazy.
“You end up with a possibility of several photons, or a bunch of photons.”
Johannes Skaar, University of Oslo, via Science News
Infinite speed is physically impossible, meaning an experimental setup would remove the mirror at finite speeds. Even with slower removal, Skaar explained that researchers are simply much more likely to create smaller numbers of photons rather than massive swarms.
Expert Reactions and Potential Quantum Sensing Applications
When Daniele Faccio, a physicist at the University of Glasgow in Scotland, first encountered the research, his initial reaction was skepticism. Then you read it, and I enjoyed it,
Faccio told sciencenews.org, adding that the calculation technique is entirely legitimate.
Faccio noted that researchers do funky things with photons for sensing and measuring, pointing to gravitational wave catchers as an example where probing the nature of individual photons might prove useful.
The Oslo team hopes to investigate the phenomenon further, including exploring what might happen if physicists tried to sever other types of fundamental particles that act like waves, such as electrons.
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