Why is the rest mass of photons 0?

Naty1

I'm guessing, but do not know, the replacement of the Maxwell Lagragian reflects an ever so tiny mass, hence slightly slower interaction via the EM field, or equivalently, a slightly slower photon. A slower photon would also curve a bit more in a gravitational field.

The discovery that light has a finite speed ended the concept of Newton's universal time and clearly defined cosmic distances...they become observer dependent. A related understanding is that Coulomb's law holds rigorously only if the electric charges are at rest with respect to each other.

Naty1

Everything has energy, therefore everything has relativistic momentum via mass-energy equivalence. It's an underlying symmetry, just like velocity changes distance and time....there are many subtlies our senses are not tuned to detect. Math sometimes helps pluck out some of the ambiguity

Yor_on

Thanks for that Naty.
But I still don't get it:)
You say "everything has relativistic momentum via mass-energy equivalence"
But what kind of 'mass-energy' conversions are you referring to?
The photon has no mass?

edguy99

Perhaps this visualization would be of some interest:

1/ Think of the photon of red light - 1.9 Evolts, 656 Nanometer wavelength, travelling at the speed of light 300picometers/attosecond. Pretend the photon is a single crest of this wave - 328 Nanometers long but very thin and pointed in some direction like a "light ray or packet or vector".

2/ Think of the electron - 511000 Evolts, Classical (lorentz) radius of 3 Femtometers, relatively stationary (1 or 2 picometers/attosecond) - known to spin.

Important observation: The classical electron radius is roughly the size the electron would need to have for its mass to be completely due to its electrostatic potential energy. The visuallization here is the photon (if you think of a photon as a very thin line 328 Nanometers in length) simply wrapping up around the electron and "disappears". Its forward momentum is transferred to the electron and and the electron moves a bit if it can.

Yor_on

Yes I think I see what you mean.
You want to see it as wave packets being in different states.
But what is invariant mass in that view?

And before you say something:)
Pinch yourself to see if you exist.
That's what's confusing me.

Not that we exist, but how to explain it as waves?