Is a photon's mass 0 or 1.67*10^-27 kg?

Question

For example, when a photon hits a particle, that particle moves from an energy level of n = 1 to some higher energy level such as n = 7, I recall needing the mass of a photon, which is 1.67x10^-27kg?

But other times, I recall hearing a photon is mass-less, and can be created or destroyed now? Wuut?

Answer 1

I think you are confusing "proton" and "photon". I agree, the spellings are much too close.

Answer 2

A proton's mass = 1.67*10^-27 kg

A photon has zero rest mass because it travels at the speed of light c in a vacuum.

Answer 3

According to our current understanding of physics (and reality) a photon has exactly zero rest mass. There is no ambiguity here, and there *is* a definitive answer to your question.



The mass you cite is the mass of a proton or neutron (which have the same mass to the number of significant figures you give). That has *nothing* to do with whether a photon has a rest mass.



You are correct that some systems can absorb a photon and as a result, become "excited" to a higher energy state. That does not mean a photon has a nonzero rest mass.

Answer 4

You cannot stop a photon nor alter its speed.

Hence F = ma or F = mg both become meaningless.

A photon has momentum.

For most of the last century this means that it was accorded a mass according to p = mc

More recently it has been considered to be massless.

Because if you stop it there is nothing left.

It has zero rest mass.



Is mass gained when energy is added or is mass converted to energy then back to mass?



The trouble is that there are a great many other definitions that have to be altered to fit with this new interpretation.



Hence there can be no definitive answer to the question.