# Do photons have mass?

1. ### RobertsMrtn

10
I have heard that photons do not have any mass which is why they travel at the speed of light.
However consider the following thought experiment.
You are box in space accelerating at 1 G.
From your perspective, you are in a gravitational field of 1G.
If you shine a light from one wall of the box to the other (across the gravitational field), it will hit the wall at a slightly lower point that it would if the box were traveling at constant velocity.
This is because the wall will have moved compared to where it would have been had the box been traveling at constant velocity.
From the perspective of the observer in the box, the light beam has been bent by the gravitational field.
The same will apply with a gravitational field created by the existence of an object with mass.
We can therefore say that the photon is attracted by an object with mass.
Does it not follow that the photon must its self have mass?

637
4. ### HallsofIvy

40,667
Staff Emeritus
"Fields" are not objects. They do not have paths to be bent.

5. ### wasi-uz-zaman

62
photons rest mass is zero , but what would be there mass at speed of light and different material light speed decreases does the mass of photon get decrease too?

6. ### pgardn

637
So if a charged object moves in a gravitational field...

By our definition of field, the fields produced by the moving charge (that does move through some curved path in space time) are not affected?

I know I should probably read something, but if the answer is painfully obvious like your first one, we enlighten a lowly classical poster quickly.

7. ### ZapperZ

30,155
Staff Emeritus
This makes no sense. Even if you want to content that there is something called "relativistic mass" for a photon, just look at the relativistic mass expression, which is

$$m = \gamma m_0$$

If $m_0 = 0$, what is m at ANY speed?

Zz.

8. ### RobertsMrtn

10
If the mass of a photon is zero, the proton drive idea for rocket propulsion would not work. (Newton's third law).

### Staff: Mentor

Newton's Third Law is basically about conservation of momentum. Even though photons don't have mass, they have momentum. If a photon reflects off something and changes direction, then the reflecting object must also change its momentum correspondingly.

10. ### DrewD

490
Newton's Laws were written down 300 years ago. While Newton thought that light was made of particles, the modern conception of a photon did not exist until a little over 100 years ago. Until then, the explanation of light was consistent with waves. Waves can carry momentum without mass.

Be careful when trying to combine the logic of two theories that may not be compatible.

11. ### RobertsMrtn

10
momentum

Momentum is mass times velocity. No mass, no momentum.

12. ### ZapperZ

30,155
Staff Emeritus
This is incorrect. Momentum can also be defined as hbar k. Look it up!

Zz.

13. ### DrewD

490
nope. I'm not interested in the conversation, so others will hopefully take over, but if you do not have a physics background (which this statement makes clear) you should ask questions instead of making statements like that. I will translate for you.

"In high school I was taught that momentum was mass times velocity. I don't understand how a photon can have momentum without having a rest mass. If it has no mass, shouldn't it have no momentum?"

14. ### RobertsMrtn

10

I do have a physics background. I do not however claim to know everything. I made the post to stimulate conversation on existing theories which may or may not be correct. One person behaving as if he has all the answers and disrespecting everyone else does not help.

15. ### No-where-man

184
But what fields are than, is there any definition of fields?
I mean you have energy fields and you force fields what's the key difference?

16. ### No-where-man

184
But do photons have some other mass? I forgot the term how you say it in physics.
I also read that Photons inside superconductors do develop a nonzero effective rest mass; as a result, electromagnetic forces become short-range inside superconductors.

17. ### ZapperZ

30,155
Staff Emeritus
This is now a completely different phenomena and such a discussion will confuse and derail this thread. If you wish to understand why photons have that effect in a superconductor, open another thread.

Zz.

18. ### ModusPwnd

Photons (plural) can have an invariant mass if their momentum sums to zero in your frame of reference. This system's invariant mass would not be equal to the sum of the photon's individual rest masses (which is zero).

There are lots of ways to look at mass, a "simple" concept that can get quite confusing.

### Staff: Mentor

Actually, the total momentum doesn't have to be zero. You can always calculate the invariant mass of a collection of particles using

$$mc^2 = \sqrt{E_{total}^2 - (|\vec p_{total}|c)^2}$$

For a given collection of particles (photons or otherwise) this gives the same result in any inertial reference frame. It's simply easier to calculate in the frame in which ##\vec p_{total} = 0##.

20. ### ModusPwnd

Then why do we say that a single photon has zero mass? I thought it was because there is no frame of reference with a photon having zero momentum. A collection of photons who's momentum cannot be zero in any frame of reference would still have an invariant mass? But a single photon would who's momentum cannot be zero would not? ugg, no wonder I failed out of physics... This stuff confuses me still, even having studied it for a bit. It seems to me if your collection of photons cannot be boosted to some frame with zero momentum then you will have a zero value for mc^2.

Sorry to cause any confusion, Ill shut up now since I dont really get it. :tongue: