Light Scattering: The Physics of Lightsabers

This formula describes the total effective cross-section of the so-called “Halpern dispersion”. It is named after Austrian physicist Otto Halpern, who first described this phenomenon in 1931. To understand it, one must leave classical physics behind and turn to quantum electrodynamics – the description of electromagnetism in terms of quantum mechanical fields. Light is no longer a classical wave here, but consists of optical quanta, the photons. In the context of quantum field theory, a photon can transform into a particle-antiparticle pair, which cancel each other out almost instantly and produce a photon again. Although these “quantum fluctuations” happen all the time, they play no role in the classical description of light. However, they do exist, and enable the “scattering” of light upon light – thus theoretically also a lightsaber.

Where do we notice the scattering of light?

In Halpern scattering, the particle-antiparticle pair of one photon can interact with another photon. The cross section described by the formula is indirectly proportional to the eighth power of the mass of the pairs of particles generated. That’s why you can find the symbol in the formula M_H, electron mass. Direct evidence of Halpern scattering was only achieved in 2015 at the particle accelerator of the European nuclear research center CERN. The charged lead atoms there were accelerated and allowed to interact with each other. This creates strong electromagnetic fields, and photons from being excited by these fields can scatter onto each other. A total of 13 processes can be detected. However, the effects of Halpern scattering can also be seen when looking out into space.

Many astronomical processes produce gamma radiation, that is, photons of high energy. As they travel through the universe, they can interact with photons from the extragalactic background light, which is an extremely faint, diffuse light that has emanated from all the stars and galaxies since the formation of the universe. High-energy gamma photons are more likely to be scattered by background light, lowering their energy. In other words, at very long distances, Halpern scattering makes the universe opaque to high-energy light.