# How is photon not a particle?

1. Feb 7, 2012

### Jarfi

Photons are emitted by electrons, they have no charge, mass... um don't they? And really no force, but they carry energy I know, so they are packets of energy travelling trough space..

But how it is not a particle? all particles travel in wave form, and are waves, and light travles in wave form too... What makes it not be a particle? is it because it emits no force?

What makes light light?

2. Feb 7, 2012

### JaredJames

Light doesn't carry energy, it is energy.

A particle has (rest) mass. Mass cannot travel at the speed of light.

A photon behaves as both a particle and a wave.

3. Feb 7, 2012

### James_Harford

First of all, light is particles (photons), and they are the carrier of electromagnetic force, which is one of the four fundamental forces of nature. The "wave form" applies to the way in which light behaves when it is not busy being a photon. No joke! It is called "wave-particle duality" -- a term coined by Louis de Broglie, who received a Nobel prize for successfully predicting that all matter will be found to behave like waves under the right circumstances.

As to photons having no mass, this is a conceptual hazard everyone crashes into on first acquaintance. The mass of a photon is a calculated abstraction, an invariant never directly observed, whose value happens to be zero. This is very important because it is why photons can only move at one speed, c. But, in all other respects, photons behave like particles with with energy E=hf, momentum p=h/λ, and if you like, even a conserved (relativistic) mass, $m = E/c^2$, although I must warn you that relativistic mass is currently out of fashion.

Last edited: Feb 7, 2012
4. Feb 7, 2012

### JaredJames

Is the c not squared?

5. Feb 7, 2012

### James_Harford

It is now! Thanks!

6. Feb 7, 2012

### Jarfi

Thanks man, sums it out perfectly, beautiful. So light is just another particle:D

7. Feb 7, 2012

### Staff: Mentor

I don't quite agree with this. Energy is transferred by any wave, not just light. A water wave is not energy.

8. Feb 8, 2012

### Jarfi

Yeah, well everything is energy, so particles transfer energy too. I have just been confused with what light excactly is. People talk about light in a different way than normal particles, but because light has no mass, it can travel on lightspeed, but why doesn't it have mass if it has energy? I mean E=mc^2

9. Feb 8, 2012

### OJFord

I don't see what you're disagreeing with - JaredJames' statement doesn't involve other waves in any way at all, he speaks only of light waves.

10. Feb 8, 2012

### Nabeshin

Unfortunately, Einstein's publicist perhaps wasn't as thorough as he should have been. The full equation actually reads,
$$E^2=m^2c^4 + p^2c^2$$

When things are moving slow relative to the speed of light, momentum is very small, so the mass term dominates and we recover the familiar $E=mc^2$. However, for the photon we easily see that since m=0, $E=pc$, all of its energy is momentum.

I am with Drakkith, I don't think you should say that light IS energy. Consider the mechanical definition of energy -- the ability to do work. So light is the ability to do work? That doesn't make much sense. Light HAS the ability to do work, i.e. it has energy, but to identify the two is a mistake. What is a photon? A quantum excitation of a particular mode in the electromagnetic field. I really don't think one should (or can) go farther than that in describing what it really is.

11. Feb 8, 2012

### James_Harford

The problem is entirely one of semantics. A photon does have relativistic mass. Solve the above equation for m and you get the relativistic mass of a photon of energy E. Relativistic mass, like energy, is a conserved quantity, the total amount in the universe is a constant, albeit unknown, quantity. Anything with a total energy, E, has a (relativistic) mass $m = E / {c^2}$.

Because of $E = mc^2$, relativistic mass is regarded as being the same quantity as energy where $c^2$ is just a conversion constant between two sets of units. For this reason, many consider "relativistic mass" redundant with total energy, and banish the term from discussion. Seems extreme to me, but it is the current fashion.

So in what sense does a photon have no mass?

Because there is a second definition of mass, called "rest mass". Relativistic mass varies with energy according to $E = mc^2$. That means the energy of motion, or kinictic energy K, of a moving object has its mass increase by $K / {c^2}$. Because $c^2$ is such a large number, the mass increase at a speed that is not an appreciable fraction of light speed is too small to measure. Still, since all fundamental particles of the same type have the exact same mass only when at rest, "rest mass" is an extremely important quantity to know.

It so happens that if we apply this calculation to a photon, we find a rest mass of 0, which has no meaning in the ordinary sense of the word. This is because, the phrase "Rest mass of a photon" is an oxymoron: there is absolutely no frame of reference in which a photon can be at rest. So folks omit the embarrassing qualifier, "rest", and just use the word, mass.

As a result, explaining this to newcomers without using the terms "relativistic mass" and "rest mass" becomes too convoluted to contemplate. But some attempt to do so. When that happens, remember this posting.

Last edited: Feb 8, 2012
12. Feb 8, 2012

### OJFord

That's not what I object to, in fact I would completely agree with the above - 'light has energy' is best.

It's just the 'carry' that I really don't like. It implies some sort of limb, or at least a force acting opposite the mass of the energy which it 'carries'.

13. Feb 8, 2012

### Staff: Mentor

This does not agree with the usual definition of energy, which is the ability to do work. To be fair there is a bit of disagreement with what energy "really is" to many people, but this does not affect the theories that use the normal definition, only the way people think about what energy is.

Waves are waves and must obey the same rules regardless. Energy can and is transferred by any wave, but the wave itself is not energy.

So you disagree with the statement "Electrons carry a negative charge"? C'mon, lets not get picky with simple words like this. It's much easier to say it carries the charge or the energy than to write a whole paragraph every time I post something to explain what energy or charge is and what is meant by saying it is "carried" with something.

14. Feb 8, 2012

### OJFord

Well, yeah.

"Electrons have negative charge"

As far as I am aware there's nothing scientific to say that the above isn't true (or in some way different from saying hey carry the charge), and it's no more difficult to say, and it makes more sense.

I guess I am fussy like that, but I just like it more.

15. Feb 8, 2012

### Staff: Mentor

Many physicists prefer to use the term invariant mass instead of "rest mass" for precisely this reason. Invariant mass is related to energy and momentum by

$$E^2 = (pc)^2 + (mc^2)^2$$

where m is the invariant mass. This equation works for all particles, whether massless or not, and for a particular particle, it holds in any inertial reference frame, using the same m, although E and p are different in in different frames. Hence the term "invariant mass".

For a photon (m = 0) the equation above becomes E = pc. Interestingly, this is exactly the relationship between energy and momentum for a classical electromagnetic wave that one can derive from classical electrodynamics. Maxwell's equations were already relativistically correct before Einstein came up with his relativity theory.

16. Feb 8, 2012

### Staff: Mentor

Then I recommend letting it go because it's simply not worth making a fuss over. I'm not arguing that light isn't energy simply because I like it more, but because there is a fundamental difference between things being made up of energy and having energy. There are plenty of things I'd prefer people say differently, so I do understand your situation though.

17. Feb 8, 2012

### ZapperZ

Staff Emeritus
We have had this type of discussion over and over again, and using the term "relativistic mass" for the photon not only does not make it any better, it ADDS to the confusion because the "m" in that equation is designated as the REST MASS. So how come for a photon, it is now suddenly a "relativistic mass"?

Furthermore, the usage of the term "relativistic mass" is no longer something that should be used[1]. There are plenty of indications that this term is misleading at best, and even Einstein in his later years have stopped using that term entirely[2].

The only term that has a clear definition that everyone do not argue about is the invariant mass. That is what we cite when we state the mass of all of these elementary particles that are inevitably created relativistically in particle colliders.

Zz.

[1] L.B. Okun Am. J. Phys. v.77, p.430 (2009).
[2] E. Hecht, Am. J. Phys. v.77, p.799 (2009).

18. Feb 8, 2012

### James_Harford

Agreed. Thanks for the references.