# Do photons have mass?

1. Oct 4, 2011

### Garvi

photon is a particle. then how come it does not have mass?

2. Oct 4, 2011

### 1MileCrash

The word "particle" doesn't imply mass. I couldn't think of a more vague word.

Furthermore, this is one of the reasons light is sometimes thought of a wave and sometimes thought of a particle. I'd rather it be a wave when talking about its lack of mass, and I'd rather it be a particle when talking about its momentum.

3. Oct 4, 2011

4. Oct 4, 2011

### sophiecentaur

Why should it have mass?
Look at what wiki has to say about the fundamental particles, how they interact and how the forces between particles can be accounted for. It's very involved.

But, personally, I think that referring to a photon as a particle is not helpful. There are many paradoxes involved if you want to look at is as a little bullet - like how big could it be? Would a photon of 200kHz radiation really be 1000 times 'bigger' than a photon of 200MHz radiation (bearing in mind that there would also have to be a thousand times as many 200kHz photons for the same power flow)?
I prefer to stick with an idea of a photon as being just a quantum of energy which only shows up when an em wave interacts with some system of charges, such as an atom, molecule or nucleus. That view doesn't actually run counter to the 'proofs' of it being a particle, such as the photoelectric effect and can save many sleepless nights, worrying about it.

5. Oct 5, 2011

### jishitha

Photons possess mass, when it is moving., and rest mass of photon is 0....

6. Oct 5, 2011

### DrDu

In superconductors, photons are massive.

7. Oct 5, 2011

### sophiecentaur

When is a photon not moving?

8. Oct 5, 2011

### DrDu

In the case of massive particles, the mass is the energy of the particle at momentum 0 (divided by c^2). If one extends this definition to the case of photons in vacuo one has to set the mass of the photon to 0 as E=c p and therefore vanishes at p=0.

9. Oct 5, 2011

### sophiecentaur

So when is a photon not moving? You haven't said. You have merely introduced a property of particles that do have mass. (A circular argument).
Measured mass of a photon, from wiki <1×10−18 eV/c2. Not 'measured' as zero because of the accuracy of the measurement that was possible - hence the upper limit is quoted.

The "non-zero mass" is referred to as "effective rest mass". That is hardly the same thing as mass. It's a bit like discussing semiconductor Holes. They are only a way of describing an observed conduction mechanism.

Last edited: Oct 5, 2011
10. Oct 5, 2011

### DrDu

I don't think this is circular. I just wanted to point out how the concept of (rest-) mass can be extended to objects which can't be at rest.

As to "effective rest mass" vs "true mass". Obviously it is something different to discuss propagation of light in vacuo vs in matter. But I don't think that there is a difference of principle. Electrons in vacuo aren't more fundamental than holes in solids. They both only describe an observed conduction mechanism, nothing more.

11. Oct 5, 2011

### sophiecentaur

That's a fair enough comment. I rather shot myself in the foot there!
I still think that there is a (quantitative) difference between the behaviour of a photon under most conditions ( i.e., it exhibits no mass pretty much all the time ) and the 'effective' behaviour of a photon under special fringe-quantum conditions in which a mass-like nature reveals itself.
But, as with the Higgs Boson and other frontier-bashing concepts, it may turn out to be very relevant to our better understanding and development of a TOE.
My problem is that such inconsistencies can be taken out of context by the 'less well informed' and applied to everyday situations where they are not relevant and can introduce even more confusion. That little word "effective" can so easily be confused with "it's really there" and could be used in arm waving explanations of such things as Light Pressure, where Momentum is the relevant quantity.

12. Oct 6, 2011

### joyever

so can we say light has mass?

13. Oct 6, 2011

### sophiecentaur

Can someone tell me of a time when a photon is not moving? If not, then what has "rest mass" got to do with this discussion?

The one quoted situation when a photon may 'exhibit the property of mass', still doesn't imply that the photon is ever stationary. So where does that take us?

Photons have momentum - they show it all the time. That doesn't imply mass.

14. Oct 6, 2011

### Khashishi

jigarbageha: I don't think your physics are wrong. Your disagreement is stemming from a non-conventional definition of the term "mass". "mass" when used by itself is ambiguous, but is conventionally taken to mean "rest mass", which is the energy left over when you've subtracted all the kinetic energy and removing any potential well. If you prefer to mean relativistic mass, you'd better spell it out, but the term isn't used much because it's redundant with the term energy.

15. Oct 6, 2011

### DrGreg

There are (at least) two competing definitions of "mass" within relativity. One definition is now used by almost all professional physicists; the other definition was used historically by physicists and is still used in some schools, and books aimed at the general public.

The old definition included an object's kinetic energy* as part of its mass (via E = mc2). Thus an object's "mass" (a.k.a. "relativistic mass") varied with velocity*; the object's mass when it was stationary* was called its "rest mass".

The new definition excludes the kinetic energy of the object and is therefore constant. In the old terminology it was "rest mass" and is now called just "mass".

It's unfortunate that both definitions are still in use, so if someone refers to "mass", you need to check whether they mean the old definition (="relativistic mass") or the new definition (="rest mass" or "invariant mass").

The equation relating mass (=rest mass) m to energy* E and momentum* p is$$E^2 = m^2 c^4 + |\mathbf{p}|^2 c^2$$This applies to particles with non-zero (rest) mass, but also applies to photons if you set m=0. This justifies saying that the mass of a photon is zero (in the new terminology), or that the rest mass of a photon is zero (in the old terminology) even though a photon can never be at rest. This is one case where it was less confusing to use the term "invariant mass" instead of "rest mass".

So, the answer to the original question "do photons have mass?" is "no" under the modern definition, but "yes" under the old definition which some people still use.

There is some more about this in the relativity forum's FAQ https://www.physicsforums.com/showthread.php?t=511175 [Broken].

____
*relative to some frame of reference

Last edited by a moderator: May 5, 2017
16. Oct 8, 2011

### juanrga

The concept of rest mass does not imply that a body is not moving. Electrons have rest mass but does not imply that are always at rest, neither that the concept of rest mass only applies when they are at rest.

Last edited: Oct 8, 2011
17. Oct 8, 2011

### Hootenanny

Staff Emeritus
Indeed this is only true, but that wasn't sophie's point. In order to define a non-zero rest mass for a particle, there must exist a fame in which the particle is stationary. There exists such a frame for an electron, but not the photon.

18. Oct 8, 2011

### sophiecentaur

Wouldn't a finite rest mass imply infinite mass at c? Is that not a clincher?

19. Oct 8, 2011

### juanrga

Rest mass is not the mass that a particle has only when is at rest.

The rest mass of a particle is also well-defined for frames in which the particle is not at rest. Rest mass is one of the properties that defines the particle (together with spin, charge..) and those properties are frame-independent.

From a particle point of view, the rest mass can be obtained from the mass operator in the energy-momentum space.

Maybe the term "rest" is at the root of the confusion. Just substitute the term "rest mass" by invariant mass or simply mass for avoiding it.

20. Oct 8, 2011

### sophiecentaur

Whatever sort of mass you are talking of, how would it not go infinite for a speed of c?

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