|1) No matter where you are and what sources of gravity are around,
when you see light go past you it will seem to travel at the usual
speed. That is, if you make some standard clock (say based on atomic
vibrations) and some standard ruler (say based on atomic sizes) the
light will go the same number of ruler spacing in a clock tick as it
would have in any other gravitational environement. |
If you try to extend that result to what would happen if your standard ruler and standard clock were moved somehwere else, odd things happen. For example, the clock gets out-of-synch with your clock. The amount of those weird effects does depend on gravity.
2) From the point of view of the object itself, it is still at rest (no matter what you think) and still has its usual mass. If from your point of view it is moving fast, than its mass according to you goes UP, not down. (Here by mass I mean the number you have to multiply its velocity by to get its momentum. Alternatively, we just say that the energy goes up.) That makes it harder to accelerate, and prevents it from ever reaching the speed of light.
So objects (like light) that do go the speed of light are not fast versions of ordinary objects with mass. They are a special class of objects, which would have no mass if they stood still, which travel only at the speed of light.
(republished on 07/23/06)