It is assumed (though actually not yet proved) that the "rest mass" of a photon is 0. In special relativity the total energy of a particle is given by:
E^2 = p^2c^2 + m(rest)^2c^4
For a tyical particle with mass, if its momentun is 0 (ie if p = 0) then this becomes the familiar (though a bit misunderstood) equation E=mc^2. But for a photon, which is assumed to have rest mass = 0, it becomes E=pc. Hence photons can have momentum and energy, even though they don't have rest mass. And since they don't have rest mass they travel at the speed of light.
As for why photons are effected by gravity if they don't have mass - it's due to the effects of the curvature of space-time by an object with mass. When a photon travels from point A to point B it takes the shortest path, which in our normal everyday experience is a straight line. But when a massive star or black hole causes significant curvature of space-time that shortest path is not a straight line, and we see that the light path is bent.
It's also a consequence of Einstein's principle of the equivalence of gravity and acceleration. He maintained that if you are in a closed room (like an elevator car) and you feel a force pushing you downward it's impossible to tell whether that force is due to gravity (ie your weight) or due to an upward acceleration. So when you feel your weight pressing through your feet onto the floor it's impossible to say whether the elevator car is stationary on earth or is in space and accelerating at 9.8m/s^2. Now suppose you are on an elevator accelerating upwards and consider what happens to a light beam that enters the car horizontally (perpendicular to your acceleration) through a small window in the side of the car - you would perceive that the light beam comes through the car and appears to curve downward slightly before hitting the opposite wall, due to the increase in the car's velocity during the time it takes the photon to traverse the car. If you believe Einstein's equivalence principe, this means that if the car is stationary but in a gravity field a beam of light entering through the window would also appear to curve downward. Hence light beams curve in gravitational fields. Note that this has nothing to do with classical physics, objects with mass, or the arc that a thrown object takes due to forces as defined by Newton's law of gravity (i.e. F = GMm/d^2).
*
Last edited by ChipB; Jan 4th 2013 at 12:26 PM.
* |