How do we know light has no mass?

Question

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?

In: 42

Answer 1

If light had mass it’s speed in a vacuum would not be constant, which isn’t something borne out in any experiments. You could also slow it by removing momentum

Rather, like electrons, it’s speed would vary based on it’s energy or momentum. However the energy or momentum of light merely influences it’s wavelength but has no effect on it’s velocity(at least in a reasonable vacuum, that is. Solid materials are a different animal.)

You can observe this fact in that adding more thermal energy to a hot tungsten filament in an old fashioned incandescent bulb, changes the wavelength of light given off by it.

A further argument in the same vein is the recent confirmation of gravitational waves, as well as the observation that their speed is it least experimentally, the speed of light within observational rounding error (gravitational waves are damn hard to detect so the precision at which we can measure thrift speed is less thsn the precision we can measure light speed.) Which was predicted from general relativity but not experimentally demonstrated until the least 10 years. Specifically some identified gravitational waves events have been able to be pegged to observed cosmological events in distant galaxies like gamma ray bursts or radio bursts. The fact that visible light observations coincide very closely in time with gravitational wave observations puts a good limit on the size of the objects causing them. Such waves are caused by massive objects but do not have mass themselves. So many of their properties are analogous to light.