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Topic: Preferred Frame Theory indistinguishable from SR
Replies: 204   Last Post: Jul 13, 2010 11:11 AM

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 Daryl McCullough Posts: 2,871 Registered: 1/29/05
Re: Preferred Frame Theory indistinguishable from SR
Posted: Jul 6, 2010 11:18 AM

harald says...

>The twin scenario was presented by Langevin in 1911 to show that
>physical acceleration is "absolute", even more so with SRT than with
>Newton's mechanics.

What does that mean? As I said, proper acceleration (as measured by
an accelerometer) is absolute, but coordinate acceleration is certainly
not.

>He argued that these absolute effects detect the ether (what you call
>a "preferred frame").

If that's what he argued, then he was wrong. The fact that acceleration
is measurable does not imply the existence of a preferred rest frame.

Here's an analogy: A flat Euclidean plane has no notion of a preferred
direction. Any direction is as good as any other. But it certainly has
a notion of a *change* of direction. If you draw a path on the Euclidean
plane, then you can unambiguously determine whether the line is
straight or curved, because a straight line connecting two points is
shorter than any curved line connecting the same two points. If you
measure the lengths of two curves, you can determine which one is
straight.

A rest frame in Einstein's spacetime is analogous to a direction
in Euclidean space. There is no preferred rest frame in spacetime
any more than is a preferred direction in the Euclidean plane.
But a *change* of rest frames is certainly detectable, in the
same way that a change in direction is detectable in the Euclidean
plane.

>However, Einstein (1916) considered that the PoR of SRT has an
>"epistemological defect", since it relates to a privileged group of
>"spaces" that cannot be observed. And what he could not observe, he
>called 'factitious'. In other words, he rediscovered Newtons' argument
>but he found it unacceptable. He preferred to go the opposite route
>and extended the PoR as follows:
>
>"The laws of physics must be of such a nature that they apply to
>systems of reference in any kind of motion".

>As a result, physical acceleration is, according to Einstein's GRT,
>*relative* - which is just the contrary of what Langevin argued based
>on his "twins" example of SRT.

As I said, proper acceleration is definitely *not* relative, but
coordinate acceleration trivially *is*. But proper acceleration is
measuring acceleration relative to *freefall*.

>It should not be surprising that this was not only very confusing for
>bystanders (who already hardly understood the difference between the
>two theories), but that it even looked like a contradiction

I would like to hear any coherent explanation of why it looks like
a contradiction. The bare statement "The laws of physics must be of
such a nature that they apply to systems of reference in any kind of
motion" is not a contradiction---on the contrary, it is nearly a
tautology. You can always write the laws of physics so that you
can use an arbitrary coordinate system.

To derive a paradox from the twin thought experiment, you
need to reason something like this:

1. There exists two coordinate systems, C1 and C2, such that
the path of the traveling twin, as described in C1, is the
same as the path of the stay-at-home twin, as described in C2.

2. Therefore, the predicted age of the traveling twin, computed
using C1, must be the same as the predicted age of the stay-at-home
twin, computed using C2.

I don't see how 2 follows from the general principle of relativity, as
expressed in the sentence "The laws of physics must be of such a nature,
blah, blah, blah." From the latter, it follows that one can use either
C1 or C2 to compute the ages of the two twins, but it *doesn't* imply
that the ages will be the same. To be able to conclude that, you need
to assume a very specific form for the laws of physics.

--
Daryl McCullough
Ithaca, NY

Date Subject Author
6/26/10 Koobee Wublee
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