Eli5: how come photon doesn’t have mass?
In: Physics
In classical mechanics, mass is the relation between a force and the acceleration it creates. If I push you and you back off 2 steps, then your mass is whatever value makes this relation true.
In relativity mass changes a bit, because you consider that mass is not just some number, but it is a form of energy. The famous equation from Einstein shows that: E=mc^(2) just means that the mass m is stored energy that depends on the speed (if you remember classical mechanics, think a bit of the potential energy of gravity, where the energy is “saved” in the body when it is in height and it is “freed” when it falls). Obviously this equation in reality is much more complex because energy doesn’t only come from mass but from movement as well (like the classical kinetic energy).
If you have that the amount of energy is fixed, then the only things you can change in the equation are the speed and the mass. Basically we say that light has no mass because it is the only way to balance the equation that will manage to give you that the speed is actually the speed of light.
You have to take this explanations with a grain of salt, since they are wrong for the sake of clarity. No matter how simple you make it, it will never be ELI5. Others here have explained the real stuff, but it is way out of the scope of the sub. For fuck’s sake, when would you ever tell a 5yo about the Higgs field.
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