Our understanding of the speed of light and many other things is predicated on the fact that light has no mass. As we can’t weight it directly like on a scale I am wondering (outside of mathematics) how we can test and prove this theory? Is it possible that light does have mass, it is just very very very small?
Further, if light has no mass, does it also have no energy? e=mc2 means energy for something massless would be 0. We know light has energy, so how does this equation work?
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Light is considered to have a zero REST mass, but it’s never at rest. It does in fact have relative mass because it contains energy. This is why light sails work. A photon of light also has gravitation coupled with that relative mass as even momentum adds to the energy stress tensor in gravitational equations.
If i’m going to explain it like you are 5, then you have to understand that energy and mass are the same thing. Mass is basically made of energy. But we say that they are equivalent. If mass bends space to create gravity then so does energy. If mass has momentum then so does energy. There is energy in light, in fact photons are force carriers for electromagnetic energy, there for it does in fact have mass and gravitation. It’s just tiny in relationship to other massive objects.
When someone says a photon has no mass, they are not entirely accurate. A photon has zero rest mass, but a photon is never at rest, so it always has relative mass as related to the energy in the photon.
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