Hello,

I've been confused about something about time dilation due to high velocity for a while, and how it relates to our perception of time, and also light travelling across the universe. I tried asking a physicist about this, but his answer was so brusque I can't be bothered asking him to clarify, so I thought some philosophers may be more patient.

If an object in space is travelling much faster than you, time will pass slower in the frame of reference of that object in comparison to yourself. It's often described in thought experiments - and classic science fiction treatments, such as Joe Haldeman's "The Forever War" - that on a star ship, say, travelling away from the solar system much faster than the solar system is itself, then returning, time would be perceived to pass at it's usual pace to any human passengers. But when they returned to the Solar system, they would find that a much greater length of time had passed for those who stayed close to our sun.

What has always troubled me is what would be conceived to happen if the star ship was travelling at the speed of light. Now this is where the physicist got shirty with me - he pointed out that no mass can travel at light speed according to einsteinian physics, so we can't imagine any observers then. This seems to show a lack of philosophic adventurism to me. I vaguely remember that the time dilation would increase exponentially as the speed of light was approached.

But this would indicate that the rest of the activity in the universe would simply be progressing at more and more extreme time-progressions - as if a image on a video tape were going faster and faster and faster. Eventually - assuming the universe has an end - it would simply progress by in a wink of an eye.

But even at the speed of light, the universe is so huge it takes untold ages to traverse - if this is even possible. The assumption of the original thought experiment is that time seems to pass normally on the space ship. So then travellers on it will just perceive time progressing normally, and can observe their progress across the universe. Slower than light objects will be perceived to alter with time in a sped-up fashion, but it is not that time will cease to progress for people travelling at the speed of light. Otherwise, it would have been observed to slow down for them as they travelled faster and faster initially.

I think that's enough to get across my initial befuddlement at this. Is it that the physicists are talking about something else when they refer to "time" here - which can be measured using isotope decay, for instance, in an objective manner, and talking about "time slowing down/speeding up relative to different observers" is just a short hand, and they aren't really talking about the perception of time (or this isn't what they have in mind).

Anything to help me to get clarity in this - or even be clear on what's worrying me, as I'm not sure, would be very much welcome. Thanks so much.