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Question:
The physics students at Pocatello High school have five questions about light:
1. If light is a particle, how can it travel at the speed of light without becoming an infinite mass?
2. If light is a particle, when it leaves a flashlight why does not it have enough momentum to push the flashlight backwards?
3. Relativity states that o the observer, the mover's time always goes slower. Then how can one person's clock be ahead of the other's, considering either one would be the mover or observer?
4. If you could stand in a black hole and shine a light straight up, would it escape, turn around and come straight back down or orbit to the right or left?
5. If you could travel at speeds greater than the speed of light, would we travel back in time?



Replies:
Addressing your questions 1 & 2:
1) photons have zero rest mass so they can travel at the speed of light without acquiring infinite mass
2) photons DO carry momentum, so, indeed, they DO impart a backwards momentum to the flashlight... it is just extremely small!

John Hawley


Questions 3&4: 3) That is one of the fundamental paradoxes of relativity - your statement is correct in a sense - the problem is one embedded in your statement, that you are assuming that the mover and observer agree on which two points in time (at their particular moving locations) are simultaneous. Two points in time that the observer claims to be simultaneous, the mover would claim to occur at different times (unless the "points" were associated with exactly the same location in space). That is why space and time in relativity are melded into "space-time". Check out some library books on relativity! 4) As long as you pointed straight up, I believe the light would go up (spreading out under the influence of the strong gravity there) turn around and come back. I could be wrong though... The inside of a black hole is a rather speculative place to be... 5) In a sense yes - the actual act of travelling faster than light does not make you go back in time, you are just going forward in space very fast. But if it was possible to travel faster than light, then one could do it in a "frame of reference" moving with respect to the earth, and done properly that could mean going backwards in time relative to earth's time. Faster than light and backwards in time travel are essentially equivalent in special relativity.

Arthur Smith



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