This may not be the best place to ask this, but I want to ask a serious scientific question. Does light have a physical property, by that I mean does it weigh anything ? If it does, then should it not be possible to exceed the speed of light ? Say a beam of light were being directed towards a black hole. The black hole would at some point act on this beam and with it's strong gravitational pull accelerate it - don't forget that this beam of light would have been travelling at a uniform speed. So, would gravity accelerate it or not ? This has been driving me mad for weeks.... anybody out there got any thoughts on this ?--
McSpotlight: Right. OK. Photons do not have mass at rest; they weigh nothing. However, energy and mass are related by Einstein's mass-energy relationship (the famous e=mc squared equation); this means that if you give a photon some energy, it will have a mass associated with it (c is the speed of light in a vacuum, by the way). The more energy you give it, the more mass it will have (and the faster it will move) until you reach very near to the speed of light c by which time the photon will have a near-infinite mass. Quite apart from all else, this means that accelerating any conventional material to light-speed would require infinite energy, which is why you can never travel at the speed of light.
Secondly, remember the definition of acceleration; it is a vector quantity, not a scalar one; by definition it has both magnitude and direction. So you are accelerating when going around a corner - even when travelling at a constant velocity.
Now, a black hole will bend a beam of light; this is predicted by the General Theory of Relativity and has (apparently) been observed in the Universe. In fact, it doesn't even have to be something as massive as a black hole; a star or even a planet will do it - the theory of relativity was originally validated by observing the way a star bent the light from another star that went close past it.
However, the light cannot go faster than c; this is an absolute speed limit for photons; to make them go faster would require greater-than-infinite energy; obviously absurd. So no, their actual velocity doesn't change. Yet since their direction will change, their acceleration will.
So, to answer your question, it's accelerated but doesn't change its speed. Hope that helps!
Rex, McSpotlight.
