If light has mass, what is it?
This subject is debatable ... A way to simplify things to get a general understanding is : Light is made up of photos, and photos have no mass ! So light has no mass :)
It is a debatible point as to whether or not light actually does have mass. I argue that it does, and that when in motion, light has an effective mass of m=h*f/c^2 per photon. I agree that light has ZERO REST MASS..but light doesn't exist at rest. If you beg to differ with me.. nsider the following before thumbing me down: #1: What is mass? Make your definition as non-circular as you can. For instance...the answer is not the amount of matter in an object. #2: What does E=m*c^2 mean? Are you sure? #3: E=m*c^2 belongs to a principle called mass-energy equivalence. Please revise your answer to #2 based upon this term. If you still think otherwise, why isn't it called mass-energy destruction and conversion?
Light photons are indeed 'massless,' at least insofar as experimentally establishing an upper limit for photon rest mass. Although photons do not have (or are not believed at this time) to have rest mass, they DO have momentum, p = hν/c, which satisfies the energy momentum/mass equivalence relation called out by Einstein's theory of special relativity, which if you read the fine print, states that E^2 = p^2c^2 + m^2c^4. Simply put, the energy of a photon E = pc is already fully 'baked in' to special relativity. Light photons not only have no rest mass equivalence, they don't even need any in the first place! See ref below: Hope this clarifies. Enjoy.
Photons of light are said to have an infinitesimal *rest* mass, but photons are never at rest so more correctly it can be said that light has no mass.
