# Special relativity and the Photon

#### discord73

If as an object's speed approaches the speed of light from an observer's point of view, its mass appears to increase, then shouldn't a photon appear to have an infinite mass?

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#### Nabeshin

Science Advisor
I think the crux of where this argument fails is that you're basing it on "approaching the speed of light". Obviously, photons never approach the speed of light, so the logic doesn't really apply to them. Basically, you have to treat photons different from all sub-light speed particles in that sense.

This is a crumby answer, I think, and someone else can probably come by and do much better, but that's my input for now.

#### dilletante

My understanding is that the mass does not increase, because photons have no rest mass to begin with. Photons do not approach the speed of light c because they do not accelerate -- they always travel at c when measured in a vacuum, from the moment of emission. They do have what is considered a relativistic mass defined as m=E/c^2 according to Einstein's formula, but that is a different kind of mass. It does however allow photons to interact with other mass gravitationally. Since their energy can vary according to wavelength but is not infinite, their relativistic mass is finite and calculable.

#### Doc Al

Mentor
If as an object's speed approaches the speed of light from an observer's point of view, its mass appears to increase, then shouldn't a photon appear to have an infinite mass?
The "relativistic" mass of an object with non-zero rest (invariant) mass increases with speed, but this doesn't apply to photons, which have zero rest mass.

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