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Absolute Velocity

  1. Feb 16, 2014 #1
    In the spirit of this forum I should first announce:
    -generally, I have no idea what I'm talking about but look for you folks to enlighten me.

    If I am travelling at a significant fraction of the speed of light and I wish to know my current absolute velocity can I use the speed of light in different directions?

    If I'm moving at 50% the speed of light in some direction
    and I build a spherical shell with reflectors facing the center of the shell
    and I stand in the center of the shell and move with the shell with a stopwatch and shine a light at reflectors all around me in my shell
    1 - would i measure differences in the speed of light between different reflectors on the perimeter of the shell?
    2 - if that's possible couldn't I measure my absolute velocity and determine the direction I'm travelling?
  2. jcsd
  3. Feb 16, 2014 #2


    Staff: Mentor

    There's no such thing as absolute velocity; velocity is relative. (More precisely, any velocity less than the velocity of light is relative.)
  4. Feb 16, 2014 #3
    Thanks for the response - blows my mind!
  5. Feb 16, 2014 #4


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    Staff: Mentor

    The answer to #1 is "no", and therefore #2 is not possible.

    Any observer who is moving at a constant speed is entitled to think of himself as at rest and everyone else as moving in the opposite direction. This follows from Einstein's two postulates of special relativity, and is confirmed by an enormous amount of experimental evidence.
  6. Feb 16, 2014 #5
    Einstein's Theory of Relativity is based on two principles and one of them says "No matter how fast you are moving, the speed of light always remains the same for you i.e. c".
    For example, if you are travelling at the rate of 2*108 m/s, relative to earth, then essentially, the relative velocity between you and the light should be 1*108 m/s. But it doesn't happen, light always travel at c and you see the light waves passing you at the same speed as you are in rest.
    So, measuring the difference in velocity between you and the light waves (relative velocity between you and light) doesn't make any sense because it is always constant and is always equal to c.......
  7. Feb 22, 2014 #6


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    Ha! You are following the thinking of some great men. That is exactly what people thought. That is why the Michelson-Morley experiment, where they bounced light off reflectors in different directions, confused everyone by measuring the exact same time in all directions.
  8. Feb 23, 2014 #7


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    I made an animation to illustrate what would happen in a scenario like you describe:


    Now you will note that the light does not strike all the mirrors at the same time in the frame in which you are moving at 50%c but that is something that is outside your realm of measurement. If you could, then you would be able to measure your absolute velocity and determine the direction you are traveling.

    If you want to see more animations developing conceptually the Michelson-Morley experiment that FactChecker mentioned, check out this thread:

    "www.physicsforums.com/showthread.php?t=626807" [Broken]
    Last edited by a moderator: May 6, 2017
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