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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>Further, if light has no mass, does it also have no energy? e=mc2
E=mc^2 only applies to objects at rest, light moves so it is not enough.
For moving objects the formulate is
e^(2=) (mc^(2)2) + (pc)^(2) where p is th momentum of the object.
If the mass m=0 then (mc^(2)2) =0 and the formula is rescue to e^(2=(pc)2) => e= pc
So energy is momentum times the speed of light for a massless object. Light has momentum so it has energy. There are many ways do demonstrate this, a practical example is light sails. That is sails that work by reflecting sunlight, it has practical application that has been demonstrated with experimental spacecraft [https://en.wikipedia.org/wiki/Solar_sail](https://en.wikipedia.org/wiki/Solar_sail) There are lots of other measurements of this too.
A probe with mass is it increases when speed increases and would be infinite at the speed of light. So even if a photon had a minuscule mass at rest it would have an infinite mass at the speed of light
We can just look at the momentum light imposes in a light source, it is clearly not infinite, It was a flashlight that would get accelerated to the speed of light in the other direction.
The speed of light is also the same of all observers regardless of how they move the [Michelson–Morley experiment](https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment) shows that.
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