Why is the relativistic mass of a photon not infinitely large?

Answer

Dear Jos,

As far as we know, photons have zero rest mass. Experimentally we cannot show that any given parameter is exactly zero. However, we can measure at the highest available resolution. That resolution gets better and better over time. So far, the assumption that the photon’s rest mass is zero still holds. (For more details on how this is measured and what the current upper limit is, I refer you to the English Wikipedia page.)

Since the concept of relativistic mass used in old textbooks is obsolete, I will talk about relativistic energy in my answer, where you could ask the analogous question.

• At any rest mass value strictly greater than zero, the energy approaches infinity as the velocity approaches c. The answer to the question “Will man ever be able to travel at the speed of light?” is for that reason: no.
• However, for the photon we need rest mass m₀ = 0 in the formula for relativistic energy, which remains finite. And the speed cannot then only be, but must be equal to the maximum speed according to the special theory of relativity: c, the ‘speed of light’ (the speed of photons in vacuum).

So the answer to your question is: because a photon has zero rest mass.

Regards,
Sylvia Wenmackers