# What is the mass of photon? [duplicate]

I'm sorry if this question is asked before, but I searched through the site and none satisfied me.

In most of the books I've come across, they just write "rest mass of photon is zero." But never talk about the relativistic Mass. Even in other answers on this site they have written exactly the same.

And once in my class there was some discussion on which I said that Mass of photon is zero, but my teacher corrected me, saying "Rest Mass of photon is 0".

So, what is the real Mass of photon? Or does there even exist something as relativistic Mass of photon?

I know the equation $$m\gamma$$ gives indeterminate form thus can't be used for photons. And I've no confusion on energy momentum relation which uses the rest mass.

## marked as duplicate by Qmechanic♦Aug 19 at 8:18

• – Allure Aug 19 at 8:05

The mass of the photon is zero. The end.

Relativistic mass is a hazardous concept, and many authors refuse to use it. It makes the increase in kinetic energy of an object with velocity appear to be connected with some change in the internal structure of the object. See also this question on SE.

• Well, the argument in your second paragraph relies on the assumption that weight is proportional to mass, which isn't necessarily the case in this situation. – David Z Aug 19 at 8:21
• Hmm I thought I put "uniform gravitational field" in there, will edit. – Allure Aug 19 at 8:27
• The weight of a relativistic object is not simply its rest mass times $g$: it does in fact change with velocity. See physics.stackexchange.com/q/63961/106502, for instance. In your argument, you are tacitly assuming that the gravitational field appears the same in both reference frames. It does not. So in the lab frame an external observer will observe your weight change, while in your frame you will also observe your weight change, but as a consequence of the changed gravitational field. – Chris Aug 19 at 11:00
• If SR became inconsistent because you moved a $\gamma$ from one side of an equation to the other, it would have been inconsistent beforehand. – WillO Aug 19 at 11:43
• @Chris that's new, editing the example out. – Allure Aug 19 at 11:52

The concept of relativistic mass has been abandoned as it is just $$E/c^2$$. We can all agree that $$E\neq 0$$ for a photon. The modern concept of mass is $$E/c^2$$ in the rest frame, or for the mathematically inclined, the limit to the rest frame. The experimental upper limit on photon mass is extremely small, see https://en.m.wikipedia.org/wiki/Photon#Experimental_checks_on_photon_mass. Btw I consider reference 36 a fundamental mistake.

• If you define mass to be $E/c^2$ in the rest frame, then the mass of a photon is undefined. – WillO Aug 19 at 11:53
• For a photon the limit to the rest frame should be used. – my2cts Aug 19 at 16:28