Light is electromagnetic radiation; that is, it is a moving wave composed of oscillating electric fields and oscillating magnetic fields. This radiation allows for radio signals, cellphone signals, WiFi signals, microwave ovens, light visible to our eyes, x-rays, as well as the gamma radiation we encounter with nuclear weapons. That’s the easy answer…and an explanation that the typical incurious person will readily accept.
Since you seem curious, let me begin with this quick overview of electromagnetism: According to electromagnetic theory, the presence of an electrical field requires the existence of a charged particle. If the charge of an object flips (or even changes slightly) it’s polarity (positive to negative, negative to positive, half-negative to quarter-negative), a magnetic field of proportional frequency and strength will result. Electromagnetic radiation (AKA light) possesses both fields, and by understanding how an oscillating electric field produces a magnetic field, the nature of light should be relatively easy to understand.
If you’re wondering WHAT is oscillating to produce this electromagnetic phenomenon in light, we need to dive a bit deeper. Considering that all atoms contain positively-charged protons and negatively-charged electrons, it’s reasonable to ask if light is just the result of a negatively-charged electron turning into a positively-charged proton. After all, according to the Standard Model, the photon (AKA light) is believed to be the force mediator of the electromagnetic field; it is the particle which physically bonds an electron to a proton. According to this model, protons and electrons are essentially engaged in game of tennis where the photon unites them in a common cause. It would be nice if this anthropomorphic analogy held up under under all current theories of physics, but it does not.
According to Einstein’s Relativity, no object with mass can travel at the speed of light. Electrons and protons both possess mass, so it’s not possible that THEY are the oscillating particles in light that are traveling AT the speed of light. So if not electrons or protons, what is the source of charge for light’s electrical field? This is where it gets deep and purely theoretical. What does an object experience when it travels at the speed of light?
If Relativity is correct, the faster an object travels through space, the slower it travels through time (and visa versa). Think about that for a second because it’s potentially one of the most profound observations of reality you will ever encounter. If correct, an object which travels through space at space-time’s speed limit must necessarily not experience time. If an object does not experience time, it must be able to exist in all of space without any spatial constraints. Therefore, a single photon should be able to exist at any point of space in the universe without violating the currently-understood laws of space-time physics. For some reason, I want to believe that the size of the observable universe suggests it’s comprised of more than just a single photon acting omnipresntly, and that the structure of the universe is more likely the result of a composite fabric of bosonic strings (of which light is a member) perturbed by fluxes of internal energy (see Bosonic String Theory). However, it’s hard for me to fully fathom the abilities of a particle which does not experience time. Therefore, I cannot fully give you an answer to your question of “What is light?” It could potentially be everything; then again, it might be just what current physcisicist suggest it is: a funadamentally unexplainable phenomenon that must be accepted for how we believe it behaves.
