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Velocity vs. Time

  1. Jan 25, 2010 #1
    This has been bothering me.

    As you will know, when something reaches the speed of light, its speed in time slows down. So I have been wondering, if we were to develope intersteller light speed travel and could move at the speed of light we would reach proximus Centuri in 4.3 years. But... time would proceed faster than the person going at light speed. So if it took four years for them, it would take millions of years for the people on earth. So what is the point, we would reach there faster if we were in the machine, but slower on earth.

    Explain why we would do this.
    And if I am right in saying this point.
  2. jcsd
  3. Jan 25, 2010 #2


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    Staff: Mentor

    If the ship were really capable of traveling close to the speed of light, as "seen" by an observer on Earth, then it would take about 4.3 years to reach Alpha Centauri according to the Earthbound observer. However, because of time dilation, it would take much less time according to the travelers on the ship.
  4. Jan 25, 2010 #3
    To elaborate on this, you should remember that those 4.3 lightyears are from earth's observance, not from the travelers.

    from the point of a traveler as V approaches C, the time taken to travel a distance approaches 0.
  5. Jan 26, 2010 #4
    How is that possible, when something goes at the speed of light, it would take four years in their perspective as it would take them four years as that is how fast they are going?

    But I thought that if someone went at the speed of light it would take them that long, but time would speed up around them. So as they gp a year, many thousands would go by for people going at normal speed.
  6. Jan 26, 2010 #5

    If something is moving the speed of light, from its prospective it would take 0 time to travel anywhere. a Photon is often considered all points in its path at the same time.

    Time does NOT speed up around them. Time slows down for them.

    I'm going to get so much hate for this. What I'm supposed to say is don't even imagine it because you're technically not allowed to travel the speed of light.

    lets say for a second I could

    I travel 1 light year away from earth at the speed of light. for earth 1 year passes. For me 0 time passes. I travel 1 light year back to earth. Now in total 2 years have passed on earth. for me 0 time has passed.

    edit: so if I'm 18 and my friend is 18, when I get back from my travel, he is now 20 years old. I am still 18. I feel as if no time has passed since i left. He has lived 2 years of his life.

    of course, this is just a thought experiment. matter cannot travel the speed of light.
    Last edited: Jan 26, 2010
  7. Jan 26, 2010 #6


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    Science Advisor

    Quite the opposite, actually. Consider a rocket moving close to the speed of light and some people left behind on Earth. The observers on Earth see time moving slowly for those on the rocket. Now, those aboard the rocket see the Earth moving away from them, and as such see time moving slowly for those on the Earth.

    So for your example:
    A) The people on the Earth record 1 year, and see only 1 day has passed on the rocket.
    B) The people on the rocket record 1 year, and see only 1 day has passed back on Earth.
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