Does light have mass?

farohar wrote:

:eek: It's the entire basis behind DeBroglie's theorem that won him a Nobel prize, no offence but who are we to believe, a forum poster of unknown credentials or someone who the physics community felt came up with something so significant that they gave him the most prestigious prize there is in the field?
All waves can be treated as particles with an associated momentum (and therefore a mass) and all particles can be treated as waves with an associated wavelength.

'Sometimes people like to say that the photon does have mass because a photon has energy E = hf where h is Planck's constant and f is the frequency of the photon. Energy, they say, is equivalent to mass according to Einstein's famous formula E = mc2. They also say that a photon has momentum, and momentum p is related to mass m by p = mv. What they are talking about is "relativistic mass", an old concept that can cause confusion. Relativistic mass is a measure of the energy E of a particle, which changes with velocity. By convention, relativistic mass is not usually called the mass of a particle in contemporary physics so, at least semantically, it is wrong to say the photon has mass in this way. But you can say that the photon has relativistic mass if you really want to. In modern terminology the mass of an object is its invariant mass, which is zero for a photon.
Sometimes people say "mass" when they mean "relativistic mass", mr but at other times they say "mass" when they mean "invariant mass", m0. These two meanings are not the same. The invariant mass of a particle is independent of its speed v, whereas relativistic mass increases with speed and tends to infinity as the speed approaches that of light, c. They can be defined as follows:
mr = E/c2
m0 = sqrt(E2/c4 - p2/c2)

where E is energy, p is momentum and c is the speed of light in a vacuum. The speed-dependent relation between the two is
mr = m0 /sqrt(1 - v2/c2)

Of the two, the definition of invariant mass is much preferred over the definition of relativistic mass. These days, when physicists talk about mass in their research, they always mean invariant mass. The symbol m for invariant mass is used without the subscript 0. Although the idea of relativistic mass is not wrong, it often leads to confusion, and is less useful in advanced applications such as quantum field theory and general relativity. Using the word "mass" unqualified to mean relativistic mass is wrong because the word on its own will usually be taken to mean invariant mass. For example, when physicists quote a value for "the mass of the electron" they mean its invariant mass.
At zero speed, the relativistic mass is equal to the invariant mass.'

Who are you going to believe, some French Physicist or a moderator on boards.ie? ;-)
Also, they were joking with you about me banning you.

The rest mass of a photon is zero(although a photon can never be at rest).
It doesn't get a mass greater than 0 by 'stopping'.
Light is always invariantly massless. At zero speed both the relativistic mass and invariant mass are zero.
So, light is not gaining mass by stopping, it has zero mass at zero speed. It can however gain a relativistic mass when it is moving.

off to the cinema, any more questions will be answered on the morrow.