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Length Contraction

  1. Sep 21, 2010 #1
    I'm having a hard time understanding length contraction. can anyone explain it to me in simple terms?
  2. jcsd
  3. Sep 21, 2010 #2
    just remember that the length of an object in one reference frame is the distance between the front and back at one simultaneous moment so relativity of simultaneity enters into it.
  4. Sep 22, 2010 #3
    Alright, I understand that. I have been researching Length Contraction and i have found that there is no real evidence to prove length contraction. is this correct?
    Also alot of places state that since time dilation is true then length contraction is true as well because they are concurrent. I don't understand this statement. How are these two phenomena concurrent?
  5. Sep 22, 2010 #4


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    Somehow you seem to have missed the point. Length contraction and time dilation are effects that are observed by someone moving relative to some clocks and rulers ( yours, for instance).

    So if I cruise by your spaceship in mine, I will see your clocks running slowly and your rulers appear shorter than mine. But nothing has actually happened to any clocks or rulers. Naturally, you will think the same about my clocks and rulers.
  6. Sep 22, 2010 #5
    I think length contraction was initially proven by the 1888 experiment of Michelson-Morley.

    Go https://www.physicsforums.com/blog.php?b=1959 [Broken] for an explanation by starthaus "MMX.pdf."

    Later backed up by Lorentz-?Fitzgerald?
    Last edited by a moderator: May 4, 2017
  7. Sep 22, 2010 #6
    Actually, if I got it right: if a train is 100m long at rest in the stationary frame, it will still be 100m long when it starts to move, when observed from the stationary frame. But the observer in the train will experience contracted lengths along the path of movement.
  8. Sep 22, 2010 #7
    No, MMX doesn't have anything to do with length contraction. Check https://www.physicsforums.com/showthread.php?t=229034". At this point there are no direct experimental proofs of length contraction.
    Last edited by a moderator: May 4, 2017
  9. Sep 22, 2010 #8
    The MMX has everything to do with length contraction. Consider an inertial apparatus has two orthogonal arms x and y that are equal length in the rest frame of the apparatus, such that a omnidirectional signal propagating from the origin reflects off mirrors at the ends of the arms and return simultaneously to the origin. In an inertial reference frame moving parallel to the x arm the simultaneous return of the signals to the origin can be explained if the moving observer measures the x arm to be shorter than the y arm, if we insist that the speed of light is the same for all observers and independent of the velocity of the source. Changing the clock rate of a clock co-moving with the origin cannot make the signals return simultaneously. The MMX result can alternatively be explained in terms of time dilation like this. The observer at rest with the apparatus can take the point of view that the clock of the moving observer is time dilated and out of sync and so this causes the moving observer to incorrectly measure the length of the x arm to be shorter.

    This is not quite right either if I understand you correctly. The observer in the stationary frame (the frame at rest with the track that does that does not accelerate) will measure the train to be shorter than 100m when it has velocity relative to the track. The observer onboard the train will still measure the length of the train to be 100m after it has accelerated and returned to inertial motion and after he has resynchronised all his clocks. The observer on the train after it has accelerated will measure objects at rest with the track to length contracted.
    Last edited by a moderator: Apr 25, 2017
  10. Sep 22, 2010 #9
    This is false, in the frame of an observer wrt which the Earth (with the lab and the MMX setup) are moving, length contraction is the correct explanation of the null result.
    In the frame of the lab, the isotropy of light speed is the explanation of the null result.
    Last edited by a moderator: Apr 25, 2017
  11. Sep 23, 2010 #10
    You've been educating me (stevmg) about this for a long time... I assume the false statement from psmitty is the statement that "MMX doesn't have anything to do with length contraction." (in this thread there have been too many negatives so that it becomes convoluted.)

    Of course textbook explanations and your blog on MMX referred to above establish this.

    Last edited by a moderator: Apr 25, 2017
  12. Sep 23, 2010 #11
    Sure it does, read my statement again.

    There is no DIRECT test for length contraction. This doesn't mean that MMX is not an INDIRECT proof of length contraction.

    My blog (and textbooks) show how length contraction explain the null outcome of MMX.
  13. Sep 23, 2010 #12
    starthaus -

    I was backing you... Your blog makes that point. My previous post must have been a quote without the
  14. Sep 23, 2010 #13
  15. Sep 23, 2010 #14

    Check it out (post 10) again - it looks right now but I am not used to going back before the last post to edit a prior post and the subsequent replies may be based on the "wrong" info in that prior post.

    Last edited: Sep 23, 2010
  16. Sep 23, 2010 #15
    yes, this makes sense
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