A) A Photon has no mass. B) A implies “It can’t rest”. C) Therefore it doesn’t have rest mass. D) Photon rest mass is zero.

Assertion and reasoning; A is correct. B is correct and follows from A. C is correct, it does follow from B. But D is incorrect it does not follow from A, B, or C. Its erroneous, a sloppy language that has been thought to be correct for ages now.

The trick is to realize there is no property called mass of photon — at-least in the same sense as it is for other particles with mass, therefore no rest mass. To say rest mass is zero is a special value of mass or rest mass. It just doesn’t have rest mass, as it neither has mass, nor rest, which are equivalent formulations, one leads to other. But A, B or C do not lead to D. They are not equivalent way of saying each other. They invalidate each other actually.


When I made a presentation on why we shouldn’t call photons mass a “zero rest mass” I was thinking we make such mistakes only informally until just couple minutes ago the above texts flashed across my rest seeking eyes. (– or check this one, there are also some other articles that talk about this idea, after I realized my mistake years ago, but perhaps they are in terse form, and will be revised upon availability of time and a priority)

This is at-least the 4th mistake I have spotted in text books in recent years. Other mistakes I realized as my own mistakes or of others in discussions.

I have made this terrible mistake of calling photon mass as a rest mass just like the authors of this text book. Photon just doesn’t have mass.

(Its fine to say photon has zero mass, but not fine to say photon has zero rest mass, just don’t use the rest term, it causes photon to smirk at ya and say look you don’t understand me, okay, so just bugger off)

Because it doesn’t have mass (from which it does follow, it has zero mass) photon flies off always at same speed, hence no rest frames. v=0 is not possible for waves called light. v=0 is possible for particles with mass. A mass-less particle is an extended region as its a wave and one cant wish a velocity for it as one can for particles with mass. Zero velocity for such extended regions with other conditions is out of consistency.

One should also remember E=mc-square is not valid for photons. Its valid for particles with mass only hence for processes involving photons and other particles but thats because particles other then photons have mass (and energy) but photons have energy as well (but no mass) and this energy is consumed or released by other particles (such as pions or electrons). All in all due to energy conservation E=mc-square is valid, but not for photons alone.

This book is one of my most favorite book at junior coll level, a brilliant piece otherwise. Written by highly qualified professors from Ivy leagues and the likes, but such errors have persisted from ages. This book was ” originally ” written in 1970s — A decade before I saw light of the day for the first time_ first used by me, 20 years ago, I was 17, when I was yet to be qualified to comment on Relativity.

Its a widely used textbook and admittedly one of the best and most brilliant textbook at this level. University Physics originally by Young et al.

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