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Travelling near the speed of light

  1. Sep 24, 2008 #1
    Hi I have some questions which I think should be fairly easy for many of you here.

    Basically I'm locked in a debate with a friend who believes that since Einsteins equivelence principle says we could be travelling at 9.8ms-2, then there's no way of knowing if we are or not!

    How would you know if you were travelling close to the speed of light? Without any external reference (ie you're locked inside a windowless spaceship perhaps)

    Would things inside your cockpit appear the same?

    I have an idea that there would be a kind of curtain of light, which spreads out as the source moves forward.

    Another example/problem might be two spaceship travelling alongside each other near the speed of light. would they be able to see one another, or, as I imagine it, would they be moving to fast to catch the light of their friends in the other ship?

    I'm not a physicist (as you can probably tell), but I know a bit of maths. Be gentle!


  2. jcsd
  3. Sep 24, 2008 #2


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    1. If you are traveling at constant velocity, no matter how fast, you would not notice anything different within your ship.

    2. Two ships traveling at the same constant velocity, would notice nothing different about the other than if they were both at rest.
  4. Sep 24, 2008 #3
    do you mean accelerating at 9.8ms-2?
  5. Sep 24, 2008 #4


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    Someone ought to point out that velocity is relative to some object or reference frame so the statement 'travelling close to the speed of light' is meaningless without specifying relative to what.

    Also it is not possible to distinguish regular motion from a 'state of rest' without reference to some outside frame.
  6. Sep 25, 2008 #5


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  7. Sep 25, 2008 #6
  8. Sep 26, 2008 #7
    so we could all be travelling in our universe close to the speed of light and nobody would know... interesting. assuming our universe is a closed box with no windows :wink:
  9. Sep 26, 2008 #8
    i mean us and our whole universe could be travelling at te speed of light
  10. Sep 26, 2008 #9


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    But that is a meaningless statement since speed must be relative to something else.
  11. Sep 29, 2008 #10
    for it to be observed, true.
  12. Sep 29, 2008 #11


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    Okay, what do YOU mean by "the whole universe traveling at the speed of light"?

    What do you mean by anything traveling at a given speed without it being relative to something else?
  13. Sep 30, 2008 #12
    Yes that is what I meant. Sorry if it wasn't clear.

    I think the point is that (as I understand it) relativity states that the speed of light will always be measured at a constant 299*10^6 m/s. This is made possible by changes in time. (Feel free to jump in if I'm wrong)

    Meaning those travelling within your own frame (ie the ships alongside each other) will observe each other "normally". My mistake came because I was thinking outside the frame I guess, where peculiar things would be observed.

    Thanks for your help anyway, it's hard work thinking in 4 dimensions! :cry:
  14. Oct 1, 2008 #13
    You need to go back and read redargon's post. He said "we could be travelling..." he did NOT say "we are travelling...". He only made an apocryphal statement; he did NOT postulate anything...

    And as far as we and our entire universe is concerned, the statement "we could be travelling near c" looses some of its incredulity once you throw in parallel universes. We could be travelling relative to other universes.

    I am NOT saying I believe we really are travelling near C... I just enjoy thinking out of the box sometimes for the sheer fun of wild imagination.
  15. Oct 2, 2008 #14


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    tct_college :
    You persist in using this meaningless phrase. There are probably millions of things in the universe travelling at near c velocities wrt to me right now. This means I am travelling at near c wrt to them. It makes no difference - I still feel as if I'm sitting in a chair, not moving.
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