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Is it impossible to separate a distance of 1light hour in space?

  1. Oct 13, 2013 #1
    Is it impossible to separate a distance of "1light hour" in space?

    i was having a debate with a guy on a site and he argues it is impossible to separate such a distance in space.
    Suppose two observers A & B they are relatively at rest, A wish to do an experiment so A & B moves apart by sending laser signals, after a huge amount of time, they slows down their speed to get accurate measuremet, A continuously sends a laser beam to B and B does the same. A notices that a laser beam sent to B took exactly 2 hours to return, which means A is now exactly 1 light hour distant apart. B also does this, so B also concludes that he is exactly 1 light hour away from A. They does this until finally they became relatively at rest to eachother
    Or do they disagree?
    When A Calculates exact 1 light hour distance, won't that be 1lhr for B??
    His arguement was when one of the observers calculate his distance, the other won't get the same measurement because of different frames
    Is his argument correct?
    Is there really no way to separate a distance in space?
    Even i tried saying there is a 1lhr long nebula between these two observers, still he won't agree
     
    Last edited: Oct 13, 2013
  2. jcsd
  3. Oct 13, 2013 #2

    phinds

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    I see nothing wrong with your argument. You have specified a valid way to separate two observers by one light hour.
     
  4. Oct 13, 2013 #3

    mfb

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    It is important to be at rest relative to each other (you can check this via Doppler shifts, for example). If you are, you can measure the distance easily, and both will agree on the value. If you are moving slowly, the measurements will look a bit different, but you can still approach the final state of zero relative velocity.
     
  5. Oct 13, 2013 #4
    There has been a misunderstanding of the conditions of the experiment, the original question was:

    "Consider an object 'O' is travelling towards me at 90% speed of light which is placed at 1hr c away ....".

    The distance in the object's frame depends on when it is measured which wasn't specified.
     
  6. Oct 13, 2013 #5

    phinds

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    You seem to be addressing something from some other thread. What you quoted isn't here.
     
  7. Oct 13, 2013 #6
    The original post starts "i was having a debate with a guy on a site ...". I was that guy, the quote is from the other site (a Facebook group). The poster linked this thread in the original thread.
     
  8. Oct 13, 2013 #7

    mfb

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    Let's continue the discussion in this thread, two threads for the same topic are bad.
     
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