Eli5: Can two photon particles hit each other? What happens if they did?
In: Physics
Great question.
Let’s start with what it means for two particles to “hit” each other – folks on reddit love to point out that you can never actually touch something, because your electrons repel the electrons of other atoms and keep contact from happening.
But that repelling *is* what it means for two things to “touch.” Fermions, a set of particles which includes protons, electrons and neutrons, cannot occupy the same space and state at the same time, but they can interact with each other via one or more of the fundamental forces (gravity, electromagnetism, and strong/weak atomic forces).
But photons are not fermions and they don’t obey that rule. They are bosons – not particles of matter, but a force-carrying particle. As such, they do not interact with each other directly, and cannot hit each other in the same way that fermions can.
However, high-energy photons (i.e. high-frequency/low wavelength) are *weird*. At that point the energy of the photon is so high that it can spontaneously convert into massive particles, including things like protons and electrons. When two photons of very high energy get close to each other, the matter that they’re continuously turning into and out of can interact. This can cause the light to scatter, or for matter to be temporarily created.
This interaction between high-energy photons accounts for why high-energy light tends to drop off rapidly as we look further away – it’s scattering each other and making it harder to reach earth.
Latest Answers