In classical physics, cutting something in two yields two halves. In quantum mechanics, attempting to divide a single photon produces an infinity of photons that appear from the vacuum. This is the conclusion of a theoretical study published in Physical Review Letters — and it illustrates just how much the quantum world defies ordinary intuition.
What you will learn
A photon does not split — it multiplies
A photon is an elementary particle. By definition, it is indivisible. But Johannes Skaar and his team asked a different question: what happens if we nonetheless try?
To test this theoretically, the researchers imagined a photon traversing an optical shutter — an ultra-fast mirror capable of blocking a fraction of a light pulse. If the shutter is triggered in the middle of the photon’s passage, it intercepts part of the wave associated with the particle. The quantum equations describing the underlying electromagnetic field allow us to follow exactly what happens then.
The quantum vacuum, a perpetual bubbling
To understand the result, one must abandon the intuitive idea of the vacuum as total absence. In quantum mechanics, the space called “vacuum” is in reality continually traversed by fluctuations of the electromagnetic field — phantom oscillations that only wait for a perturbation to manifest.
That is exactly what the shutter does. By activating abruptly, it perturbs these fluctuations and forces them to materialize. Photons emerge spontaneously, not from an external energy source, but from the very fabric of quantum space.
The result is not a photon on one side and vacuum on the other. It is a superposition of states containing an infinity of photons simultaneously.
The deceptive appearance of normality
What makes this phenomenon particularly perplexing is what we observe when we examine the two regions separately, on either side of the shutter.
Each side, taken in isolation, seems perfectly ordinary: on one side, an indistinguishable state of a single photon; on the other, what looks like plain vacuum. It is only by considering the whole system — the quantum correlations between the two regions — that the true complexity becomes apparent.
This gap between local appearance and global reality is one of the most troubling hallmarks of quantum mechanics. The information about the system’s true state does not reside in either part separately — it is encoded in their relationship.
Fundamental questions about measurement and localization
These results raise deep questions for theoretical physics. If a local measurement reveals only a fraction of a system’s quantum reality, what does that imply for our ability to probe and characterize complex quantum states?
The question of the spatial localization of information — where the “state” of a quantum system actually “resides” — is at the heart of the conceptual challenges highlighted by this work.
Skaar and his colleagues plan to extend their investigations to multi-photon systems and to other elementary particles such as electrons. The challenge: determine whether this behavior is unique to the photon or constitutes a general property of quantum particles subjected to a sudden perturbation of their environment.
