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If you're traveling at the speed of light, then time stops

  1. Nov 5, 2014 #1
    I think my understanding of STR might be wrong. I was watching a documentary where they said that if you're traveling at the speed of light then time stops. Now let's say I'm traveling through intergalatic space from the Milky Way to the Andromeda at 99.9% the speed of light. I also have my laptop and I am picking up signals about the events that are occurring in both galaxies. I can understand that those events that are occurring now in the Milky Way are events that I will never learn about. Any signal emitted to me from the Milky Way travels at the speed of light or less and since I am traveling at 99.9% the speed of light that signal will never reach me just as a car one mile behind me going 30 miles will never reach me if I am also going 30 miles an hour. I do not understand however why time should stop with respect to those events that take place in the Andromeda Galaxy. I am traveling towards it and am capable of absorbing bosons coming from that region of space. So why should the time in the Andromeda Galaxy stop?
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  3. Nov 5, 2014 #2


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    This is common nonsense in pop-sci. You cannot travel at the speed of light and there IS no frame of reference that is co-moving with light so saying that time stops is not meaningful. There is a FAQ entry about this on this forum but I can't seem to find it at the moment.

    EDIT: here's one: https://www.physicsforums.com/threads/rest-frame-of-a-photon.511170/
    Last edited: Nov 5, 2014
  4. Nov 5, 2014 #3


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    Didn't they also say that you couldn't actually travel at the speed of light for some reason like it would take an infinite amount of energy or some such thing? I'm wondering because next you say in your example that you aren't traveling at the speed of light but just 99.9% the speed of light.

    Why are you saying that? Aren't those signals actually some form of light signals, like radio signals? Wouldn't they travel at exactly the speed of light and eventually get to you since you are going slightly slower than the speed of light?

    Those signals are traveling from the Milky Way at the speed of light, not less, so they will always be able to catch up to you. Your analogy with the cars should be that you are traveling at 29.9 mph and a car behind you going 30 mph will eventually reach you.

    I think maybe you are thinking that it's your perception of events stopping that is the same as time stopping and if you were traveling at the same speed as the signals, then those coming from behind you would appear to stop while those coming from in front of you would appear to be going at very high speed. Is that right?

    But since you cannot travel at the speed of light, you will see those events coming from behind you in slow motion and the ones coming from in front of you in high speed. Does that resolve your problem?
  5. Nov 5, 2014 #4
    That's what I needed to know. This clears up my confusion and this makes sense. Thank you for helping me out.
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