Can Photons Cast Shadows? Light–Light Interactions Explained

Video Summary

In this MinutePhysics explanation, the possibility of photons casting shadows is explored. At first, photons are thought not to interact with light itself, but there are three indirect photon interactions that can cause photon–photon encounters: an electron mediated collision, gravity driven deflection, and high energy pair production leading to legitimate photon scattering. Each route is extremely rare, so a photon shadow is not observed in ordinary experiments. However, in the vastness of space, ultra high energy gamma photons traveling through space will increasingly encounter the cosmic microwave background photons, creating a real shadowing effect over cosmic distances. This video provides a clear, accessible overview of why shadows from light are not common and how the universe does lens and shadow light in surprising ways.

• Indirect photon interactions exist via electrons, gravity, or pair production
• Lab photon shadows are exceedingly hard to observe due to tiny interaction probabilities
• In space, background photons from the cosmic microwave background can shadow ultra high energy photons

Can Photons Cast Shadows Prologue

The video begins by posing a simple question: can light, made of photons, shadow other light? At first glance, the answer seems no because photons are electromagnetic waves that pass through each other. Yet physics allows for three indirect ways photons can interact with other photons, each with a different physical mechanism and a different practical consequence for shadows.

Three Indirect Photon Interactions

The presenter outlines three indirect photon–photon interaction channels. The first involves a photon colliding with an electron, and that electron then colliding with a second photon. While this path technically redirects photons, it requires a precise confluence of events, making it unlikely to produce a consistent shadow by itself.

The second channel relies on gravity. Photons carry energy and momentum, so in principle they can gravitationally attract one another, bending each other’s paths just as mass curves spacetime. However, the gravitational effect produced by a single photon is astoundingly small, far too weak to create any noticeable shadow.

The third channel becomes significant only at extremely high energies. High energy photons can spontaneously turn into particle–antiparticle pairs, such as electrons and positrons, and those charged particles can deflect other photons. This process, known as photon–photon scattering via quantum electrodynamics, allows solitary high energy photons to interact with each other in a meaningful way, but it remains extremely rare in practice.

What Kind of Shadow Do We See

Because direct photon–photon interactions are scarce, photons generally do not cast shadows in ordinary experiments. Even with meticulous, high powered laser setups, researchers struggle to observe any photon–photon scattering in the lab. The video emphasizes that, for shadow formation, such interactions do not provide a practical mechanism in typical conditions.

There is, however, one real, observable shadowing mechanism for photons in the cosmos. The universe is permeated by a bath of low energy photons—specifically those from the cosmic microwave background. Ultra high energy gamma ray photons journey through space and inevitably collide with these background photons. Over astronomical distances, these interactions accumulate, effectively shadowing the most energetic photons from distant sources. In this sense, you are literally shadowing ultra high energy gamma photons with the light from the Big Bang itself via the cosmic microwave background.

Bottom Line

Photons do not shadow light in everyday experiments, but the vastness of space provides a practical shadowing mechanism through light–light interactions with the cosmic microwave background. The video concludes with an accessible synthesis of how light can interact with light, the rarity of such events, and the big picture implication for our view of high energy photons in the universe.