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The Physics of Light and Time Travel

Below is a question which I have posed to my good friend's brother who is an astrophysicist. I will post his reply once he gets back to me...

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I'm going to send this to you now before I forget again. If you could ask Adam for his thoughts on this it would be greatly appreciated!

My question stems from an episode of "Into the Universe with Stephen Hawking" that I saw about a month ago which covered the subject "Is Time Travel Possible?". I will try my best to word the question in a way that makes sense but for me the concept which I sort of understand in my brain doesn't quite make sense when I try to articulate it...

First to summarize ... according to Stephen Hawking it could be possible to travel into the future, but not into the past. The way I see it Stephen Hawking is amazing with what he knows and I can't imagine how much he had to "dumb" down his knowledge just to make the average person watching the Discovery Channel grasp his concepts. Also, I assume that all of his findings and theories are rooted in physical time/space (the way we experience reality with the 5 senses), and it probably strays away from other forms of theoretical physics which might focus on "higher dimensional realities" as a possibility where time would cease to exist altogether.

Anyways ... He was explaining how time slows down for the traveler as we get closer and closer to the speed of light. So for example, if we were to somehow accelerate to 99.9% the speed of light (it is apparently impossible to reach or exceed the exact speed of light), for every day which we traveled at that speed an entire year would elapse back on Earth. So traveling for 10 days at that speed would cause us to travel 10 years into the future on Earth. The only catch, according to him, is the fact that we would not be able to go back to our original time so it is basically a one-way ticket into the future.

So after the show I got to thinking about the light from stars. If the distance from stars to us is measured in terms of light years (i.e. the amount of time it takes for the light from that star to reach us in years), then a star which is 4 light years away from us would theoretically take just over 4 years to reach if we were traveling at the maximum velocity of 99.99999...% the speed of light. That would be 1,461 days of space travel. Going back to the illustration in the previous paragraph that also means that 1,461 years would elapse on Earth while we were en route to the star (but it would only be about 4 years for the traveler). Am I mistaken?

Taking that one step further ... since the light from stars travels towards us at the speed of light (go figure) ... What are we really seeing when we look at the night sky?? We must be looking into the past quite literally ... But my main question is How far into the past are we seeing when we are looking at a star? Does the same principle apply to light itself, when it is traveling at such a speed, is time compressed for it relative to our perception of time? So in other words, is the light which we see from a star 4 light years away only 4 years old, or is it more like 1,461 year old light (for our perception of time) because of how the speed of light bends time??

Does this makes sense at all? Would appreciate any insight about this because it's not making complete sense to me...

Thanks !

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I look forward to receiving his answer. Please note that I have not formally studied physics and I'm just approaching things as a layperson. My question might be very elementary for someone who has actually studied physics (in fact maybe it is even addressed for a first year physics student), but I found the topic to be fascinating...

Saturday, August 14 2010 3:29:48 PM in Science

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