Re: Length is not an intrinsic property of an object
From: Paul Draper (pdraper_at_yahoo.com)
Date: 10/13/04
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Date: 13 Oct 2004 05:43:09 -0700
"Bill Hobba" <bhobba@rubbish.net.au> wrote in message news:<ePYad.23686$5O5.5303@news-server.bigpond.net.au>...
> "joseph levy" <josephlevy1@compuserve.com> wrote in message
> news:ckhjev$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.
Understood. That wasn't the point of the post. The point was to teach.
<standing on soapbox>
It's a shame that most of the original posts in this n.g. are from
crackpots, the confused, the misdirected, but most of the effort in
response is spent shooting holes in their misguided notions -- myself
included. It might be worth a little effort using the forum to teach
those same people some proper physics and the proper method of doing
science.
<getting off soapbox>
I think there are at least two ways to teach SR. One way is to say,
"This is the way it really is. It works, and it has experimental
predictive power, and experiment supports it, despite your
dumbfoundedness. Deal with it." Another way is to say, "See, here is
where your assumption is tripping you up. Let's isolate the
misconception, erase it, and start putting together the structure from
scratch."
Personally, I favor the latter. And I believe it can be done, in this
case, in a way that is accessible to nearly everybody -- at least the
misconception-uprooting part.
>
> > 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
I didn't say that a synchronization procedure in a given frame wasn't
possible. Far from it, and you point out several examples. What I was
getting to was the fact that, following a synchronization, relatively
moving observers would not agree whether two future events were
simultaneous. Hence, simultaneity is not an intrinsic relation between
two events.
>
> >
> > 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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