Does light have mass?

farohar wrote:

Well thought out Tar (and I know they were joking, as I said I've only ever once seen a Boards.ie mod behave that childishly), but since relativity tells us that mass increases by a factor related to your current speed relative to the speed of light even if this results in the mass we see for photons they must have an initial, at zero momentum, mass in order for this multiplied mass to be a non-zero value.
Also since mass is a measure of the difficulty in changing a thing's velocity, if photons had no mass then this would either mean that:
any force regardless of how small could change the velocity of a photon, rendering the "constant" (since light can be pulled into a black hole it is a reasonable assumption that light heading towards it can in fact travel faster than C) C non-existant
or that nothing could infact effect the velocity of a photon, which would mean that light would travel at the same speed in all materials regardless and more to the point could never stop or be absorbed by electrons and molecules.
Anyway, have to get back to work, will be back later, nice to have a genuinely thought inspiring thread in AH.

Hello Farohar, you are making a few mistakes at the fundamentals so I will go over these. Photons are massless particles, they must always travel at the speed of light. Here is a simple rule to remember, massless particles must travel at light velocity and massive particles can't travel at light velocity.
earlier you mentioned that you thought that photons have mass because of the famour law regarding gravity. However, this is in err, gravity can affect massless things too.
You may ask how...

All particles, including photons, move along geodesics in general relativity and the path they follow is independent of their mass. Also, light has energy and momentum which couples to gravity. The energy-momentum 4-vector of a particle, rather than its mass, is the gravitational analogue of electric charge. (The corresponding analogue of electric current is the energy-momentum stress tensor which appears in the gravitational field equations of general relativity.) A massless particle can have energy E and momentum p because mass is related to these by the equation m2 = E2/c4 - p2/c2, which is zero for a photon because E = pc for massless radiation. The energy and momentum of light also generates curvature of spacetime, so general relativity predicts that light will attract objects gravitationally.

You also appear to think that light can travel faster than the speed of light in a vacuum(C), this is not a reasonable assumption, and is in fact impossible, according to the general theory of relativity.
A black hole can 'trap' light but the light still always travels at constant velocity(light is not getting sucked into a black hole faster). To understand this you need to know that the black hole curves spacetime back on itself, so that all paths in the interior of the black hole lead back to the singularity at the center, no matter which direction you go.
It is not that light is travelling at a speed faster than C, but that it is always travelling at a speed C in a place that leads back in on itself.
A simple analogy, no matter what direction you go on earth in a 'straight line'("geodesic"), you never escape the Earth but instead return to the same point. Extend that analogy to the four dimensions of spacetime and you have a rough explanation for why light travels at light speed, but cannot escape the interior of a black hole.
To be "sucked" into a black hole, one has to cross inside the Schwarzschild radius. At this radius, you have to go faster than teh speed of light to escape, which is impossible. the analogy above gives another reason why light can't escape. The Schwarzschild radius can be calculated using the equation for escape speed.
vesc = (2GM/R)1/2

For photons, or objects with no mass, we can substitute C(the speed of light in a vacuum) for Vesc and find the Schwarzschild radius, R, to be
R = 2GM/C2

If the Sun was replaced with a black hole that had the same mass as the Sun, the Schwarzschild radius would be 3 km (compared to the Sun's radius of nearly 700,000 km). Hence the Earth would have to get very close to get sucked into a black hole at the center of our solar system.



To help understand why a photon is massless, read:
http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photonMass.html