Re: Androcles and Draper resume Einstein 1905

From: Timo Nieminen (uqtniemi_at_mailbox.uq.edu.au)
Date: 01/15/05 

Date: Sat, 15 Jan 2005 11:48:28 +1000

On Fri, 14 Jan 2005, Androcles wrote:

> "Gregory L. Hansen" <glhansen@steel.ucs.indiana.edu> wrote:

> > If I may *** in, I think Draper's point was that when K watches k
> > measure
> > the length, he won't agree that it was a measurement of the length!
> > And
> > vice versa. So when k gives K a measured length, K will say "Your
> > number
> > is smaller than mine, but I don't know what the hell it is. I watched
> > you
> > do it, and that's not how to measure a length."
> >
> That's fine, but k is carrying out an illegitimate procedure. I had
> previously
> agreed with Paul that the measurement can only be carried out when
> the rod to be measured and the measuring rod were relatively at rest.
> Observer K will not agree with observer k's guesstimate,
> as you point out, but observer k himself will be uncertain.

So, then you have agreed that only what is called in SR the "proper
length" is to be accepted as the length of an object.

The proper length being invariant in SR (and Galilean relativity as well),
the answer follows trivially that observers k and K agree that the proper
length of the rod is the same. 

-- 
Timo
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