# Do photons have relativistic mass?

I am conducting research on photons and was wondering if they have relativistic mass. I already know that they they have zero rest mass. Any answers are welcome!

• Did you look at the relativistic energy-momentum relationship?en.wikipedia.org/wiki/Energy%E2%80%93momentum_relation – CuriousOne Oct 11 '14 at 19:40
• Duplicate: directly answered here - physics.stackexchange.com/q/34067 – Rob Jeffries Oct 11 '14 at 19:46
• @RobJeffries true that the answer to this question is given in that question, but they're not quite asking the same thing, I think, so I'd hesitate to call it a duplicate. – David Z Oct 11 '14 at 19:48
• If you are researching photons, I recommend this article by Nobel laureate Willis Lamb. It discusses the history of the photon concept, and the misconceptions around the concept. (Like Lamb, I avoid the use of the word as much as possible.) – garyp Oct 12 '14 at 0:39
• The relevance to my comment (not answer) is that the OP is conducting research on photons, and Lamb's article provides a history of the concept. There's nothing kooky about his Anit-photon paper. As for your dismissal of his analysis of the photoelectric effect: not so fast. I don't know if Lamb was a kook or not, but in the two cases considered here, he is not. But I'm with you on one thing. It's easy to come up with a list of Nobel laureates who are kooks. – garyp Oct 14 '14 at 12:37

If you did want to assign a photon a relativistic mass, there is no other parameter that could determine the mass besides its energy $E$, and based on units the mass would then have to be of the form $m=kE/c^2$, where $k$ is a unitless constant. Probably $k=1$, since we can take $p=mv$ as the definition of the relativistic mass, and $p=E/c$ for a photon.