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Confusion abt relation betwn time travel and speed of light

  1. Dec 3, 2012 #1
    i don't know in which section it belongs, so i am putting it here........

    according to various theories proposed by various researchers, time travel depends on the speed of light. as far as i have read/heard -
    i) anything traveling faster than the speed of light moves back in time.
    ii) anything traveling AT the speed of light remains stationary in time.

    now my question-
    if light is traveling at the speed of light (obviously), then why does it take time to travel?????

    it (light beam/ray etc) should be stationary in time and should reach the destination the moment it leaves the light source.
  2. jcsd
  3. Dec 3, 2012 #2


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    Science Advisor

    The backwards "solutions" are mostly nonsense. There is some sense in which it is true under GR, but it's far more complicated than that. Lets stay in realm of Special Relativity and stick to speeds up to speed of light.

    There are two concepts of time in relativity. There is time as a coordinate and there is proper time. You can think of proper time as aging of something. In other words, a clock you carry with you will measure your proper time. Also, the proper time and coordinate time agree for an object that's at rest in your chosen coordinate system.

    When we talk about time slowing down for moving objects, we are comparing time in different coordinate systems. Another way to look at it is that we compare proper time measured by two different observers.

    Now, if we happen to have a clock moving along with the beam of light, which will measure proper time for that beam of light, in any other frame of reference, that clock will appear to stand still. The proper time does not advance for a system that travels at the speed of light. Such a system does not age.

    The connection to time travel is pretty straight forward. If you wanted to see what the world is like thousands of years from now, all you have to do is get up to very near the speed of light. In that case, your proper time advances very little while proper time of the world around you advances a lot. Light speed is just the extreme case, where time for traveler would not advance at all.
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