Expert: Steve Nelson - 2/9/2008

Question
i had an argument with my physics teacher when doing a level physics in the 90's i failed and gave up on it.  but always thought i had won the argument,  or he was a bad teacher.  could you tell me which,

he told me that as you approach the speed of light time slows down, and if you could theoretically pass the speed of light (yes we all know you cant) time would move backwards.

he used the "moving away from a clock" example. and said "if you move away from a clock at close to the speed of light, the clock will slow down,  when you reach the speeds of light it will stop and as you pass it the clock will go backwards."

but i said to him "your just catching up with the photons of light, that doesn't mean your moving through time any more then i am when im looking at the light from a star, surely if i then turned around and when back towards the clock at the same speeds,the clock will move very fast, the time difference between what the clock says and how long ive been away should be equal.   

he couldn't answer me, but got rather annoyed,

can you help.

Answer
His argument is actually correct according to relativity.  Basically, as you accelerate to high speeds you change your inertial reference frame and thereby change where you measure a clock's starting time from and the rate at which it runs.  It's because the speeds involved are so far beyond our human experience that it's difficult to wrap your brain around, but it explains all the apparent "paradoxes" (I put that in quite because they're not really paradoxes at all when you think about them properly).  Catching up to those photons of light means that you're jumping to an inertial reference frame of the photons where "time=0" is different, which really does mean you'd move back in time.