# Weightless light?

1. Jan 4, 2005

### ironbling

I find it hard to believe that light would be wieghtless. If it was it wouldn't interact with surrounding as it would have no dimensions. Therefore it would travel at infinite velocity, wouldn't diffract, reflect or refract. Also it wouldn't be affected by gravity as gravity needs a mass to act upon as that is how wieght is defined. Therefore if light is weightless it must also have no mass. Thus it follows that it wouldn't be affected by gravity at all for example the kind of gravity that tends to be found around black holes. Am I right or am i missing the point???

2. Jan 4, 2005

### dextercioby

You are...missing the whole point.Light is not weightless.It has mass,relativistic mass
$$m_{rel,photon}=\frac{h\nu}{c^{2}}$$
It carries energy and momentum.Therefore both in relativistic and quantum relativistic interactions it behaves just like any other particle,electron,proton,neutrino...
It doesn't have rest mass,that's true,but for physics this is not an inconvenient.Moreover,it's useful,because the lack of rest mass means going at "c" and allowed developing of the Relativity theories and QFT description of other fundamental particles.
Light doesn't have dimensions.As a wave,we can speak about wavelength,true,but as a particle,it's a point particle,it doesn't have legth,width and height.

Daniel.