Re: Followup: Lorentz v. Einstein on SR

From: Harry (harald.vanlintel_at_epfl.ch)
Date: 08/12/04 

Date: Thu, 12 Aug 2004 11:58:43 +0200

"Andrew Dabrowski" <dabrowsa@indiana.edu> wrote in message
news:cff0hp$1ll$1@hood.uits.indiana.edu...
> Martin Hogbin wrote:
> > "Andrew Dabrowski" <dabrowsa@indiana.edu> wrote in message
news:4119CEBC.3080109@indiana.edu...
> >
> >
> >>My main point in the original post was that the Maxwell equations (ME)
alone
> >>imply the contraction and time dilation of SR (special relativity),
> >
> >
> > That is not correct.
> >
> >
> >>once it is
> >>realized that ME are invariant under the Lorentz transformation (LT).
> >
> >
> > It is a _mathematical_ fact that Maxwell's equations are invariant
> > under Lorentz transformation. To get SR you need to postulate
> > either that the Lorentz transformations apply to the world in which
> > we live (which is more or less what Lorentz did) or that ME apply in
> > any inertial frame (which is pretty well what Einstein did).
> >
> > Martin Hogbin
> >
> >
>
> This is a subtle point which is easy to miss.
>
> Suppose a meter stick is lying on a table. Call the inertial frame of the
table
> K. The length of the ruler is determined by the molecular structure of
the rod,
> which is governed by the ME.

Where did you pick that up? Our equations don't govern nature, they merely
are part of our attempts to describe nature. That matters for logic, but
for calculations it doesn't matter.
Anyway, although I myself never calculated that, I think you are right that
the macro-molecular structure can be determined using Maxwell's and possibly
Newton's laws (does anyone here have a derivation of that?). Lorentz
expressed his regret that he didn't make that clear sooner, so that people
thought that the Lorentz-Fitzgerald contraction was purely "ad hoc".

http://philsci-archive.pitt.edu/archive/00000218/00/Origins_of_contraction.pdf

> Now accelerate it to some positive velocity (in the direction of its
length).
> Call the inertial frame of the moving ruler K'. The structure of the
ruler is
> still governed by ME, and because of the invariance of ME under LT, ME
holds in
> its usual form in the system K'. In K' the same ruler subject to the same
> equations will assume the same length, i.e. in K' the length of the moving
ruler
> is still 1 meter.

Indeed (but then, how could the length of a moving ruler compared to that of
a moving ruler be anything else than 1?).

> But 1 meter in K' is equivalent to less than a meter back in K. So
measuring
> the ruler in K shows that it has contracted.

Not exactly, as in a third system one may measure that it dilated! Different
interpretations exist:

- As expressed by Tom Roberts, the popular interpretation is that the ruler
in K' did not really contract, nothing happened to it (see my discussions
with him).
- According to Einstein, the ruler is contracted in K but is not contracted
in K' and reality is what one measures (Relativity of reality? Multiple
universes?).
- According to Lorentz and Poincare, something happened to the ruler due to
the change of speed, but we can't know what really happened; we can only
know that it seems contracted in K.

For me of these three interpretations, only the third one is logically and
scientifically sound, assuming one observer-independent reality. IMO it
makes sense not to unnecessarily dispose of the assumption that such a
reality exists.

> All this follows solely from the invariance of ME.

Many interpretations are given by physicists. Pragmatic physicists such as
Feynmann didn't bother about such questions, as they can't be answered with
measurements. Modern physics is about measurements.

Harald

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