What is the truth about photons and their mass?

[ImAnEngineer]

Everyone agrees a photon has no "rest mass". Everyone agrees a photon has "relativistic mass". What they disagree over is whether it has "mass", i.e. does "mass" mean "rest mass" or "relativistic mass"? This is just convention, but there's no convention that everyone agrees with. It seems most professional experts use "mass" to mean "rest mass" (and never refer to "relativistic mass"), but some don't.

Er, "relativistic mass". "Relativistic mass" and "total energy" are two names for the same thing (in different units). Why have two names when one will do?

Thanks for clearing it up. I get it now!

I agree with the science and understand the terminology here. Just curious whether the "originally" is historically correct - after all, isn't energy in special relativity a new thing? It only approximates the Newtonian one at low speeds, and I've heard it said that special relativity shows that "energy" is not conserved, but physicists like "energy" conservation so much, they redefined "energy" so that it is still conserved.

Interesting question, I would like to know that as well.

 

[ZikZak]

The problem with "Relativistic Mass" is how it is used and abused by high school and college science teachers, such as the one you are having this debate with.

After a semester of discussion about the Newtonian ideas of what mass is, defining it as the inertia of a particle in F=ma, and also its gravitational charge in F=Gmm/r^2, your teacher now smugly declares that photons have mass m=E/c^2, and so there. But this is exceedingly disingenuous. Yes, the photon has "relativistic mass" E/c^2, but what does that "mean" now? Clearly, it's not the m in F=ma NOR in F=Gmm/r^2 any longer. Not when you're looking up close at the photon. So what is it? Your teacher has just given you a meaningless and misleading bit of information. This situation could be resolved by your teacher actually sitting down and explaining to you the senses in which the energy of the photon actually behaves in ways we think of as "massive," such as carrying momentum (but not momentum mv) and generating gravity (but not Gmm/r^2). But instead, he decided (unfortunately, and like many teachers who lack deep understanding of the theory) to simply "wow" you with this cool thing, that photons have mass, which has had the predictable result of confusing you because you actually want to understand what he means by that.

Even worse would have been the standard tactic of defining for you the relativistic mass m=\gamma m_0, making you do multiple problems finding relativistic masses, and then declaring that this increase in mass is what prevents you from accelerating massive particles to c (since after all, the mass=inertia is increasing, right?). The implication being that the relativistic mass is the inertia of the particle. Wrong. If you accelerate a particle from rest with a constant force, measure its coordinate acceleration, and then find the mass using m=F/a, you do NOT find that the "mass" defined this way is the relativistic mass of the particle.

So this is a concept that not only already has a name ("total energy") and is not in need of a new one, but is a concept that is explicitly misused to confuse newcomers. That is why some of us object to its use at all.

 

[atyy]

http://books.google.com/books?id=ipY8onVQWhcC&printsec=frontcover#PPA83,M1

 

[ImAnEngineer]

...
So this is a concept that not only already has a name ("total energy") and is not in need of a new one, but is a concept that is explicitly misused to confuse newcomers. That is why some of us object to its use at all.

If one objects to use the concept of relativistic mass, then what would be said about the mass of a photon? Simply that the mass is zero (i.e. the rest mass)?

And if photons have energy which they do, and E=mc², then what could be said about m? It is certainly not the rest mass. Or would this equation not be used at all?

 

[ImAnEngineer]

http://books.google.com/books?id=udA0Gjq8vHIC&printsec=frontcover#PPA16,M1

What about it?

I do own Richard Feynman's lectures on physics and it includes special relativity.

 

[lightarrow]

If one objects to use the concept of relativistic mass, then what would be said about the mass of a photon? Simply that the mass is zero (i.e. the rest mass)?

Yes.

And if photons have energy which they do, and E=mc², then what could be said about m? It is certainly not the rest mass. Or would this equation not be used at all?

They already answered you. You can use equations if you know their meaning; in E = mc² what is E? Is the total energy? Is the rest energy only?
The answer is the second.
The correct equation for total energy, as already written by others is:

E² = (mc²)² + (cp)²

The other, simpler equation is just a specific case of this (that is, the case in which p = 0).

 

[ImAnEngineer]

Yes.
The correct equation for total energy, as already written by others is:
E² = (mc²)² + (cp)²
The other, simpler equation is just a specific case of this (that is, the case in which p = 0).

But photons do have momentum, so then it doesn't make sense to write E=mc² for photons at all?

Instead you could write, however:
m=\sqrt{\frac{E^2}{c^4}-\frac{p^2}{c^2}}

Then how would one that objects to use the concept of relativistic mass call this m?

 

[Count Iblis]

But photons do have momentum, so then it doesn't make sense to write E=mc² for photons at all?

Instead you could write, however:
m=\sqrt{\frac{E^2}{c^4}-\frac{p^2}{c^2}}

Then how would one that objects to use the concept of relativistic mass call this m?

This is called the invariant mass, or just mass.

 

[ImAnEngineer]

This is called the invariant mass, or just mass.

I really lost you guys. I understood that people who object to the use of relativistic mass use the rest mass instead (same thing as invariant mass?).

But when m is calculated with the last equation I gave, it doesn't give zero which is the rest/invariant mass of a photon...

My guess is that people who object to relativistic mass just wouldn't use it (and express it in terms of energy instead) and have no other name for (relativistic) mass, is that correct?

 

[Count Iblis]

I really lost you guys. I understood that people who object to the use of relativistic mass use the rest mass instead (same thing as invariant mass?).

But when m is calculated with the last equation I gave, it doesn't give zero which is the rest/invariant mass of a photon...

My guess is that people who object to relativistic mass just wouldn't use it (and express it in terms of energy instead) and have no other name for (relativistic) mass, is that correct?

The energy of a photon is E = p c. So, you see that m = 0 for a photon.

 

[ZikZak]

If one objects to use the concept of relativistic mass, then what would be said about the mass of a photon? Simply that the mass is zero (i.e. the rest mass)?

One says that the mass of a photon is zero, because the mass of a body is its energy at rest, and photons are never at rest.

But photons do have momentum, so then it doesn't make sense to write E=mc² for photons at all?

No, it does not make sense to write that, because E=mc^2 is only generally true for bodies at rest. The correct general equation is E^2=mc^2+(pc)^2, from which you obtain the next expression:

Instead you could write, however:
m=\sqrt{\frac{E^2}{c^4}-\frac{p^2}{c^2}}

Then how would one that objects to use the concept of relativistic mass call this m?

But when m is calculated with the last equation I gave, it doesn't give zero which is the rest/invariant mass of a photon...

The m found this way is the invariant rest mass. For a photon, it is in fact zero.

 

[ZikZak]

http://books.google.com/books?id=ipY8onVQWhcC&printsec=frontcover#PPA83,M1

So... "If it's good enough for Feynman, then it should be good enough for anyone?" You appear to be arguing from authority.

 

[jnorman]

i think most people who object to "relativistic mass" tend to view systems in terms of total energy rather than invoking mass terminology at all. mathematically, it is much cleaner.

 

[ImAnEngineer]

...

The m found this way is the invariant rest mass. For a photon, it is in fact zero.

Ah, I see. If E² is substituted by (pc)² the result is zero, so it is consistent to call it just (non-relativistic) mass . I think everything is clear to me right now.

Thanks to everyone!

 

[delplace]

Recent experiments showed that photons are not wave and not particles. They are something different. From these results, photons could be a fragment of spac-time detached from the continuum. In that way their mass does not exist because space-time continuum has no mass. But it is clear that this subject often called "The nature of light" remains one of the greatest question of physics.

 

[Vanadium 50]

Recent experiments showed that photons are not wave and not particles.

Reference, please? Of course light has both wave and particle properties, but this has been known for more than a century.

From these results, photons could be a fragment of spac-time [sic] detached from the continuum.

Reference, please?

In that way their mass does not exist because space-time continuum has no mass.

Even if the premise were true, and the conclusion is true, I don't think this argument is valid.

 

[Vanadium 50]

I have no desire to continue this discussion via PM. I posted my question here, and would like the answers here. (Note however, that PF rules do impose some limitations on the kinds of references that can be posted)

 

[ImAnEngineer]

Why would you discuss it via PM? I was waiting for the answers here in this thread.

 

[delplace]

I have no desire to continue this discussion via PM. I posted my question here, and would like the answers here. (Note however, that PF rules do impose some limitations on the kinds of references that can be posted)

ok, I give you the reference in a well known journal reviewed by peers :

S. Gleyzes et al., Nature, 446, 297, 15 march 2007

Concerning the second point, the paper is in a review process, so I will wait a little bit

Best regards

FD

 

[delplace]

Why would you discuss it via PM? I was waiting for the answers here in this thread.

sorry but I was not aware that people don't like PM discussions. I give you the reference :

S; Gleyzes et al., Nature, 446,297, 15 march 2007

Best regards

FD

 

[ImAnEngineer]

sorry but I was not aware that people don't like PM discussions. I give you the reference :

S; Gleyzes et al., Nature, 446,297, 15 march 2007

Best regards

FD

Thank you

 

[Vanadium 50]

Recent experiments showed that photons are not wave and not particles.

The Gleyzes measurement shows no such thing - unless by "not wave and not particles" you mean "quantum mechanical objects with both wave and particle properties", which, as I mentioned before, is a century old - hardly recent.

 

[delplace]

Sorry but even if i confess that interpretation of this experiment is not easy, there is a strong debat around this question being more a vocabulary way of explaining things. If I try to be accurate : let me say : they can behave as both particle and wave but photons are not particles and not waves.

 

[ImAnEngineer]

Sorry but even if i confess that interpretation of this experiment is not easy, there is a strong debat around this question being more a vocabulary way of explaining things. If I try to be accurate : let me say : they can behave as both particle and wave but photons are not particles and not waves.

Then it's nothing new. It's been known for a long time that photons can behave as waves, as well as particles (and they can't be both at the same time, right?).

What I was more interested in, is that you posed that photons are a part of time-space detached from continuum (?) and therefore have no mass. Could you explain that a bit more?