Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Time slows at the speed of light

  1. May 26, 2008 #1
    I am not exactly sure how to present this question. Even thinking about it creates some confusion,so I will only try my best. My question is about special relativity. Since the first time I saw Carl Sagan's Cosmos series, I have been fascinated by this, but still don't totally get it. I can accept as fact that time slows down as you approach the speed of light, but actually understanding this presents a whole different problem.

    If for example time elapses differently for a stationary observer than it does for someone traveling at the speed of light, how can we determine how much time has elapsed? Suppose I am standing with a friend in an open field. My friend takes a round trip ride from earth to the sun at the speed of light (let's just pretend that he didn't get burnt to a crisp). Once he leaves my side in his imaginary spacecraft, I stand there waiting for him to return, which should only take about 16 minutes (round-trip). What will I notice when he returns? Is the amount of time that he was gone different for him than it was for me? For how long (stationary-earth-time) must he travel at the speed of light in order for there to be a visible difference in the aging process once we meet back up? If we as humans are accustomed to time passing at a very particular rate, what experience does the person traveling at the speed of light have? For example, he goes on a speed-of-light-journey let's say to Proxima Centauri. I sit around waiting for 8 1/2 years for him to get there and back. Does he feel as if he were only on a short journey? If so, according to his clock and his perception of elapsed time, how long did his journey last - for him? I hope I made my question clear. Any answers or additional info to help me understand this would be appreciated.
  2. jcsd
  3. May 26, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi Jasmin! Welcome to PF! :smile:
    Nothing … he can't get any younger, and you'll only be 16 minutes older! :smile:
    If he goes on an almost-speed-of-light-journey, then everything seems normal to him (apart of course from the immense g-forces at the start the turn-round and the finish!), and yes he feels as if he were only on a short journey. :smile:
    Depends how close he was to the speed of light … could even be less than a nano-second …

    The precise factor is √(1 - v²/c²), so the closer v is to c, the slower his time is. :smile:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook