# Photon mass?

1. Oct 26, 2005

### silici

hello its me again

Do photons have a mass if they are in movement ?

i know that they have no mass if they not in movement.

Could you please give me an explanation which i do understand?

thank you
silici

2. Oct 26, 2005

### Jimmy Snyder

There is a rule on this forum that says we're not allowed to discuss any theory that is less than 90, or more than 100 years old. I know a 100 year old theory that says photons in a vacuum are always in movement. Your statement about what happens if they are not in movement doesn't fit in with that theory and thus is forbidden by the rule. I hope that the rule will not be enforced in your case, but I have no influence in the matter.

As for your question, photons have no rest mass. How could they, they are never at rest. However, they do have energy and according to theory this energy is equivalent to mass. That does not mean that it IS mass, only that it acts like mass in some effects.

3. Oct 26, 2005

### pmb_phy

Let me clarify the term "mass" as I will use it below.

The term "mass" refers to "inertial mass" where "inertial mass" is defined as the quantity m such that, for a system of particles which interact oinly by contact forces, the quantity mv is a conserved quantity when observed from an inertial frame of reference. For details on this please see - http://www.geocities.com/physics_world/sr/inertial_mass.htm

If the particle is a tardyon then it moves at speeds v < c. The mass of a tardyon is a function of speed, i.e. m = m(v).

The proper mass of a particle is an interinsic property of a particle and, for a tardyon, has the value m0 = m(0). A photon is kind of particle known as a Luxon. Luxons can only travel at the speed of light. For a photon m0 = 0.

Pete

4. Oct 28, 2005

### Paulanddiw

5. Oct 28, 2005

### Paulanddiw

Apologies. I'm pretty clumsy with LaTex. The LaTex line should have been:
...S' with speed [ltex]2v_x[/tex] instead of [ltex]v_x[/tex]

6. Oct 29, 2005

### ZapperZ

Staff Emeritus
There are no photons that are "not in movement". So that should allow you to answer your own question.

Zz.

7. Oct 29, 2005

### QueenFisher

i have read elsewhere that photons have mass, but they must have energy cos they can move as light, but i thought energy and mass were interchangeable? and what about E=mc^2? if c^2 is a constant, surely energy and mass must have some relationship?

i'm probably showing my general ignorance of particle physics here, but it's been puzzling me for a while.

if anyone does reply, please make it understandable to a 17 year old who's not really that good at physics!

8. Oct 29, 2005

### pmb_phy

Yes. I did write that web page. I'mn in the process of reworking my entire website. The review process will entail taking comments like this and making sure that they are reviewed. I will print the page out, along with your remarks, and place them in a folder I have created for questionabl web pages. Thank you for pointing out your concerns. I'm in a lot of pain which is a remnant of back surgery I had last month (I hear that i can take many months to really recover from this surgery). Drugs (heavy narcotics) are helping me to some extent but my attention span has be drastically reduced due to it (pain/drugs). Even today the inssurance company people are being jerks so until Monday I have to live on about 1/4th the medication that I should be on. This means that I can sit in my apartment and not cry from the pain. :)

Please note that I have printed out the webpage and have attached your comments. It is now in my "Check/Fix" folder. Thank you for mentioning this. It may be some time until I get around to it.

Pete

Last edited: Oct 29, 2005
9. Oct 29, 2005

### Paulanddiw

Get well soon, pmb_phy

10. Oct 29, 2005

### pmb_phy

"Relativistic mass," yes. "Proper mass," no.