Re: Length is not an intrinsic property of an object

From: Bill Hobba (bhobba_at_rubbish.net.au)
Date: 10/12/04 

Date: Tue, 12 Oct 2004 22:23:06 GMT

"joseph levy" <josephlevy1@compuserve.com> wrote in message
news:ckhjev$bal$1@ngspool-d02.news.aol.com...
>
> Paul Draper wrote:
>
> I've seen a number of threads where posters have difficulty with a
> basic concept having to do with length. What I hope to do here is to
> move the point of discussion from length to simultaneity, where it
> belongs.
>
> Length is defined by an agreed-on procedure. It is not an innate
> property of an object.

This is typical of a lot of stuff you see around here - basically semantic
philosophical quibbling without any real scientific content. However some
respected philosopher/physicists have mentioned they view reality as 'what
kicks back' - all the rest are human constructs eg concepts like length,
time etc see http://www.colorado.edu/philosophy/vstenger/Briefs/Real.htm.
Victor Stenger indeed holds the belief the other concepts are chosen
specifically to ensure our laws are coordinate independent. But as to the
scientific value of such musings - well that can be judged by the number of
new experiments such views suggest and their confirmation - a big fat zero.

> For an object at rest in the measurer's frame, it's easy.
> 1. Place a precalibrated ruler alongside the object.
> 2. Take note of the calibrated mark at one end of the object.
> 3. Take note of the calibrated mark at the other end of the object.
>
> For an object moving in the measurer's frame, though, it's a bit more
> tricky.
> 1. Hold up a precalibrated ruler parallel to the object's motion.
> 2. At a certain time mark, t0, take note of the calibrated mark at one
> end of the object.
> 3. At the VERY SAME time mark, t0, take note of the calibrated mark at
> the other end of the object. Why is this last step so important? A
> moment's thinking will tell you that if there is a delay between 2 and
> 3, the object will have moved, sliding along its path. Depending on
> which direction the object is going, you will end up with a number
> that is either longer or shorter than the object's length measured if
> it were at rest. So SIMULTANEITY is key to the definition of length of
> an object, especially in a frame where the object is moving with
> respect to the measurer.
>
> There is no other workable definition of length other than this
> operational one.
>
> Ah, there's the rub...
>
> For once we agree that simultaneity is crucial to the DEFINITION of
> length, then the real issue is that simultaneity is not something that
> two measurers moving with respect to each other will agree on. The
> "relativity" of length is directly attributable to the "relativity" of
> simultaneity.

But now we come to the other rub - the bit such folks who harp on about
simultaneity and OWLS and other such issues in SR choose not to mention - or
do not understand. SR deals with inertial frames that are by definition
isotropic. A frame that is isotropic allows many reasonable definitions of
simultaneity - all of them compatible with SR eg the Einstein sync
procedure, slow clock transport, firing bullets form a gun - the
possibilities seem endless and all lead to exactly the same conclusion - SR.

>
> Now, if folks want to talk about why simultaneity is not an inherent
> property of two events...

Now if folks what to talk about how reasonable definitions of simultaneity
in an inertial frame have nay problems.

Bill

>
> PD
>
> Answer to Paul Draper
>
> Thank you for your clear exposition of the relation between relativity
> of simultaneity and length contraction in the framework of the special
> theory of relativity. It removes all the ambiguities regarding length
> contraction which appears to be an observational but non real
> process.
> I completely agree. Now why two clocks whose time mark is t0
> don't tick simultaneously? because they are not synchronous.And
> why are they not synchronous ? because the synchronization procedures
> are arbitrary. Contrary to what is often claimed, the methods used to
> synchronize clocks are based on disputable assumptions, and this
> is so, as well for the Einstein-Poincaré procedure with light signals,
> as for the method of slow clock transport.
> (Note that in aether theories, length contraction is a real process and
> not a simple distortion of the measurement due to arbitrary
> synchronization.)
> For more complete information, if this queston interests you,
> you can consult my article"Synchronization procedures and light
> velocity" in my web site:
> http://levynewphysics.com
>
>
>
>
>
>

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