Does Light Have Mass? Friend Debate

[offtheleft]

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

 

[GRB 080319B]

Search 'does light/photons have mass' on this website and you should find the answer in one of the multitude of threads on this question. Or you can go to this http://en.wikipedia.org/wiki/Photon#Physical_properties".

 

[pmb_phy]

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

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

 

[ZapperZ]

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

Please read the FAQ thread in the General Physics forum.

Zz.

 

[pmb_phy]

Please read the FAQ thread in the General Physics forum.

Zz.

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

 

[lightarrow]

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

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!

 

[pmb_phy]

Search 'does light/photons have mass' on this website and you should find the answer in one of the multitude of threads on this question. Or you can go to this http://en.wikipedia.org/wiki/Photon#Physical_properties".

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