In Greek mythology, cutting off one head of Lerna’s Hydra only leads to two more growing in its place. A similar paradox occurs in the quantum world with photons: attempting to “cut” a particle of light generates an infinite number of new light particles.

Some particles are classified as elementary, meaning they cannot be divided into smaller constituents. For example, although a proton can be split into three quarks, each quark remains indivisible. But what occurs when one attempts to divide elementary particles?

Johannes Skaar, a professor at the University of Oslo, Norway, examines the phenomenon of a photon interacting with a mirror capable of this type of cutting.

Light, fundamentally a quantum entity, consists of photons or can be viewed as an electromagnetic wave. Thus, photons aren’t entirely localized like solid objects; they have extensions, or “tails,” across space. In this scenario, if the mirror moves rapidly enough, it can reflect some photons, effectively “trimming” the photon’s tail.

Leveraging quantum equations governing electromagnetic fields, researchers concluded that this truncation results in a unique quantum light state—one that is a superposition of infinitely many photons. In the quantum realm, empty space is teeming with fluctuating quantum fields, such as electromagnetic fields. By manipulating these fields, mirrors can excite these fluctuations, permitting the emergence of new particles.

“Rapidly altering mirrors or shutters disturbs the vacuum, conjuring photons from the void,” explains Samuel Brownstein from the University of York, UK. However, local observations reveal that this superposition state appears indistinguishable from a single photon on one side of the mirror and a vacuum on the other. This demonstrates how the concept of observation varies drastically in the quantum realm compared to our daily experiences, illustrating that “a highly complex object can seem utterly simple” in quantum theory.

Wolf Leonhardt from the Weizmann Institute of Science, Israel, has conducted experiments confirming that utilizing a sufficiently fast shutter in vacuum does indeed generate photons. However, practical experimentation of this new concept may pose challenges. Although advancements in ultrafast light manipulation are emerging, the shutters described in the study exceed current laboratory capabilities. Leonhardt emphasizes the necessity for further exploration into quantum vacuum phenomena, which could refine or alter the existing quantum field theory of electromagnetism.

Alongside addressing the locality issues in quantum theory, which relate to broader inquiries regarding causality in quantum particle experiments, Skaar and his team aim to expand their focus to include multiple photons and electrons simultaneously.