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Can you actually measure an object's velocity?

  1. Feb 7, 2008 #1
    Since everything in the universe is moving relative to the other, how can one measure an absolute velocity? Therefore, how can the speed of light be absolute?
     
  2. jcsd
  3. Feb 7, 2008 #2
    We can't and we don't

    Light has the exceptional property that its speed (in vacuum) is independent of the speed of both source and receptor, this is why we always measure it to be c.
     
  4. Feb 7, 2008 #3

    chroot

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    The universe has only one velocity that all observers will agree upon, and it happens to be c. If you ask "why is it that way?", we cannot give you an answer.

    - Warren
     
  5. Feb 7, 2008 #4
    What if there is a tunnel that lowers the speed of light, observers would observe 2 different "constants".
     
  6. Feb 7, 2008 #5

    chroot

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    The speed of light cannot be lowered. All observers always agree on the speed of light.

    - Warren
     
  7. Feb 7, 2008 #6
    Ah, but it CAN.
     
  8. Feb 7, 2008 #7

    chroot

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    No, it can't. Please provide evidence, or retract the claim.

    - Warren
     
  9. Feb 7, 2008 #8
  10. Feb 7, 2008 #9
  11. Feb 7, 2008 #10

    chroot

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    When people refer to "the speed of light," they mean the speed of light in vacuum, which never changes. The decrease in propagation velocity in materials is due to frequent absorption and re-emission. In between those absorptions, the photons still travel at c.

    - Warren
     
  12. Feb 7, 2008 #11
    Admittedly, I did not notice. However, there still is the problem that if over 50% of the universe can not be converted into a vacuum then the speed of light is not officially a constant.
     
    Last edited: Feb 7, 2008
  13. Feb 7, 2008 #12
    Now you do, try to remember it next time you post such stuff.

    You are writing pure nonsense.
     
  14. Feb 7, 2008 #13
    True, but it sounded good at the moment so I posted. :D
     
  15. Feb 7, 2008 #14
    I would be suspicious of that article. There's a lot I don't understand, but the author made a big deal out of the variability of G (among other constants) without referencing any of the people who have so long worked on that (e.g. George Gillies). That would be like an article on QED that leaves out Schwinger - you have to ask why?
     
  16. Feb 7, 2008 #15

    pervect

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    Agreed. The question has been asked, good answers have been given (and unfortunately mostly ignored by the OP). I'm locking the thread, it serves no further purpose.

    If people are having trouble sorting the wheat from the chaff, I will point out that answers given by science advisors and staff are generally more reliable than answers given by non-science advisors. However, answers given by SA's and staff should not be taken as gospel, and we definitely encourage people to read standard textbooks and peer reviewed papers. We do not encourage "making it up as you go along" sorts of answers, though, that's one reason I'm locking the thread, we've seen too much of that here.

    The internet presents some special problems with repsect to relativity. There are some fine resources on the topic in the WWW, there are also many incorrect and downwright wrong websites as well.
     
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