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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.

Qmechanic
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ParadigmShift
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2 Answers2

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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.

Allure
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    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 '19 at 08:21
  • Hmm I thought I put "uniform gravitational field" in there, will edit. – Allure Aug 19 '19 at 08:27
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    The weight of a relativistic object is not simply its rest mass times $g$: it does in fact change with velocity. See https://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 '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 '19 at 11:43
  • @Chris that's new, editing the example out. – Allure Aug 19 '19 at 11:52
  • The example was of course completely wrong, but my (possibly incorrect) understanding is that we edit for clarity, but use comments to correct errors. As for the remaining "It makes the increase...appear to be connected with", any concept will mislead you if you use it wrong. – WillO Aug 19 '19 at 12:10
  • @WillO You're mixing up a couple policies here. Edits by third parties are not supposed to correct errors when they change the meaning of the post, but instead leave a comment for the OP. The OP is then free to make these edits. – Chris Aug 19 '19 at 17:34
  • Sorry, this is bullshit. If you let an electron annihilate with a positron, all you get is two gamma photons, $511keV$ each. What happened to the mass? Did it disappear? No, it is carried away by the two photons. These photons also carry an impulse equal to that mass times their speed (which is $c$), and they can recreate an electron/positron pair if they happen to collide with a suitable partner photon. Btw, the "Controversy" section of the wikipedia page that you linked to has since been deleted... – cmaster - reinstate monica Dec 23 '20 at 10:48
  • @cmaster-reinstatemonica right, but the photons' mass is zero. We say mass was converted to energy. The controversy section of the Wikipedia page is still there, it's just been renamed to "Popular science and textbooks". If you want to see the page in the form it was when I linked it, you can always look through the history of the page, as well. – Allure Dec 23 '20 at 10:52
  • @Allure Yes, I've read that section. However, all I gather from reading it is that modern authors tend to fear the confusion that people have who have not yet had much training in relativity. They opt for directly describing more abstract variables like the momentum. However, once you get into the habit of realizing that energy and mass are equivalent, there is no confusion in the concept of relativistic mass. Instead, it's rather useful as the mass of any volume of space can only change by what mass crosses its surface. – cmaster - reinstate monica Dec 23 '20 at 10:58
  • @cmaster-reinstatemonica yes, but once you get to the point where you think of energy and mass are equivalent, you would not think of mass, but rather mass-energy. The mass of the photon is still zero, even if its mass-energy is not. – Allure Dec 23 '20 at 11:07
  • @Allure Mass-energy is just a newer term for relativistic mass. It may lead to less confusion, yet there is a whole ton of literature out there that talks about relativistic mass, so that this term still needs to be taught, imho. – cmaster - reinstate monica Dec 23 '20 at 11:18
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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.

my2cts
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