Re: Some comments about Lorentz contraction (Answer to ***)

From: Dirk Van de moortel (dirkvandemoortel_at_ThankS-NO-SperM.hotmail.com)
Date: 09/20/04 

Date: Mon, 20 Sep 2004 18:51:14 GMT

"joseph levy" <josephlevy1@compuserve.com> wrote in message news:cin4kq$cq9$1@ngspool-d02.news.aol.com...
> Dear Dirk
>
> Thank you very much for your answer. (see original message and answer
> below.) Actually, according to SRT the proper length of a rod does not
> vary.
> In order to measure the length of a moving rod AB, according to Einstein,
> one must measure the distance separating two clocks displaying the same
> reading placed in front of A and B during the travel of the rod.

Yes, provided these clocks are at rest and synchronized in the frame in
which the measurement is done.
If the rod is at rest in the primed frame (with proper length dx') and we
measure the length in the unprimed frame, then with the usual setup we
have
        dx' = gamma ( dx + v dt )
If the clocks on both ends show the same time, this means
        dt = 0
so
        dx = dx' / gamma

> If the clocks were exactly synchronized, the result of the measuring would
> be the proper length.

As you see above, it would be the proper length divided by gamma.

Dirk Vdm