Does light have mass?

1. Oct 5, 2014

Young Seeker

I am new in this forum, and i'm just getting started. I am a very big admirer of astrophysics and i'm seeking answers and at the same time proposing new possibilities for some of the awesome mysteries of the universe. I'm young, so sometimes there is a great possibility tht i might add some stupid ideas, plz understand and forgive me on such occasions :)

Ok, so to my first question,

Does light have mass?

I have thought a lot abt it and i can't find a better view to see light as a unique, not yet identified substance, which can at some conditions exhibit mass, and at other conditions, massless

Please share ur views abt this............

2. Oct 5, 2014

Staff: Mentor

Last edited by a moderator: May 7, 2017
3. Oct 5, 2014

Drakkith

Staff Emeritus
Welcome to PF! First and foremost, I think you should understand that PF's primary goal is to help people understand physics as it is currently understood and practiced by mainstream professional scientists. Personal ideas and theories are not usually acceptable here because they typically run counter to the mainstream view and thus counter to PF's primary goal.

So while we can help you understand why light is massless according to current science, we cannot discuss any personal ideas you may have on it.

So to answer your question, light is massless when it exists as a freely propagating electromagnetic wave. I've heard of strange things happening when light travels in certain mediums, but as far as I know these phenomena are not described as normal light, but as an interaction between the EM wave and the medium giving rise to these effects. For the vast majority of cases you can consider light to be massless.

4. Oct 5, 2014

Young Seeker

ok, thanks for the reply, but can u tell me how can an objest, being massless interact with something that has mass? ( i'm talking about light getting reflected off a surface)

5. Oct 5, 2014

ZapperZ

Staff Emeritus
First of all, what rule is there that something must have mass for it to interact with something that has mass? Do you think an electric field has a mass? After all, that can interact with anything with a charge.

It is a good time here to re-examine your assumption.

Zz.

6. Oct 5, 2014

Orodruin

Staff Emeritus
Why would you think that they could not? Physics is a descriptive science where we introduce theories that describe what we can observe and we observe that light interacts with matter so naturally we create theories where this happens. We do not seek any deeper philosophical reason. Then you can of course ask why they interact in a given theory, in the case of light it is described by Maxwell's equations and the Lorentz force law.

7. Oct 5, 2014

Drakkith

Staff Emeritus
Sure. Light is an Electromagnetic wave. This means that light is a fluctuation within the EM field. What's fluctuating you may ask? The answer is something we call a "field vector". A field vector is a mathematical way of describing how a charged particle will interact with a field. In the case of the electric field, this field vector tells us which way the field is pointing and what the magnitude of the field is at this location. In other words, a field vector tells us which way a charged particle will move in response to the field and how much force is applied to the particle by the field.

Since an EM wave is desribed as a fluctuating field vector, this means that the wave exerts a force on a charged particle that alternates in direction and varies in strength as the wave passes over it the particle.

In other words, light is an EM wave of alternating force that acts on charged particles. No mass is required for the EM wave to interact with matter.

8. Oct 5, 2014

Staff: Mentor

Interactions have very little to do with whether or not something has mass, and more to do with whether or not it has charge. Photons are the carriers of the EM force so they interact with things that have EM charge. Gluons are the carriers of the strong force, so they interact with things that have color charge. And so forth.

9. Oct 5, 2014

Young Seeker

hmm, thank you very much guys, for your useful feedbacks :)

10. Oct 6, 2014

DrStupid

That depends on the specific conditions. Due to

$$E^2 = m^2 c^4 + p^2 c^2$$

light is massless for p=E/c (e.g. in the case of a single photon). Otherwise (e.g. in the case of a stationary radiation field) it has mass.

11. Oct 6, 2014

Staff: Mentor

Can you explain? This doesn't seem right.

12. Oct 6, 2014

DrStupid

Let's take a very simple example: Two photons with equal energy moving in opposite directions. This system has energy but no momentum and according to the equation above it's mass is m=E/c². Light is only massless if all photons are moving in the same direction. In any other case it has mass.

13. Oct 6, 2014

Matterwave

In what practical context would you ever consider this 2 body system together rather than simply treating the photons individually? This is not a bounded system, so I don't see how one could, for example, apply a force to the center of mass of this system.

14. Oct 6, 2014

Orodruin

Staff Emeritus
I would plot the event rate as a function of this "mass" in the ATLAS and CMS detectors, to find the Higgs. But the invariant mass of the two photons aside, this does not mean that the photons (or light) has mass.

15. Oct 6, 2014

Matterwave

You mean collide the two photons and hope they have enough energy to produce a Higgs? Seems unlikely even if the two photons were both gamma rays....and what if the two photons don't have enough CoM energy to produce a Higgs?

In addition, I have no idea what this has to do with the "mass" of the 2 photon system?

16. Oct 6, 2014

A.T.

Put the photons into an ideal mirror box.

17. Oct 6, 2014

Matterwave

So then you've created a bounded system. I have no problem with that and calling it the mass of the bounded system, box+photons. This is the same concept as a hydrogen particle in a higher energy state should have slightly more mass than a hydrogen particle in a lower energy state. Or that a hot box is slightly heavier than a cold, identical in every other way, box. However, I don't see any practical reason to consider two free photons traveling in different directions as one system (in particular, to consider the invariant mass of such a system), but I can of course be missing something!

18. Oct 6, 2014

Orodruin

Staff Emeritus
The Higgs was discovered mainly in the decay channel $H\to \gamma\gamma$. By observing the photon pairs and computing their invariant mass, an excess over background was found around 125 GeV. Of course, this is the mass of the Higgs, not the mass of the photons, but it does have physical significance even if the photons are not bound.

19. Oct 6, 2014

Staff: Mentor

Ah, ok. Was this sort of thing what you meant by a "stationary radiation field"? I ask because an actual radiation field doesn't have a description anything like this; what you've described is a highly idealized system.

20. Oct 6, 2014

Matterwave

You mean by computing the CoM energy and then setting $m\equiv E_{com}/c^2$?

EDIT: I guess I should have been clearer in this question. I meant, did they somehow find the invariant mass of the system of both photons (in one measurement), or did they find the Energy of each individual photon separately, but in the center of mass frame and simply do the above calculation?

In other words, were the 2 photons treated as a system, or treated separately, but in one inertial frame?

Last edited: Oct 6, 2014