# Does light have mass?

1. Jun 19, 2008

### offtheleft

funny little topic a friend and i were on. i think it does.

2. Jun 19, 2008

### GRB 080319B

Last edited by a moderator: Apr 23, 2017
3. Jun 19, 2008

### pmb_phy

Yes. Light does have mass. But we need to qualify that by providing the definition of mass as I have just used it. The term mass (as used here) is defined as the m in p = mv. When defined as such this mass is more precisley know as inertial mass. There is another definition which is also used quite a lot. If you have a point particle and you measure the inertial energy to be E and the magnitude of the momentum to be p then the quantity m0 in the expression

E2 - (pc)2 = m02c4

is referred to as the proper mass of the particle. The proper mass is related to its inertial mass buy

$m = \gamma m_0$

When it is said that the mass of a photons is zero it is because the energy of a photon is related to its momentum by E = pc. Substituting this value into the expression above imples that m0 = 0.

So if you want a definitive answer to your question you have to first state what you mean by the term mass.

Pete

4. Jun 20, 2008

### ZapperZ

Staff Emeritus

Zz.

5. Jun 20, 2008

### pmb_phy

The FAQ asserts that the relativistic mass is defined as $m = \gamma m_0$ whereas relativistic mass is defined as the m in p = mv. This is an important difference.

Pete

6. Jun 20, 2008

### lightarrow

An interesting thing is that a beam of light has no mass (I'm only referring to "proper" or "invariant" mass, which is what is usually called simply "mass"), while light propagating simultaneously and isotropically in all directions DOES have mass!

7. Jun 20, 2008

### pmb_phy

There is a new relativity FAQ on the notion of relativistic mass. Its located at

http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html

There is also another FAQ on this topic at

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html

Pete

Last edited by a moderator: Apr 23, 2017