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Re: [Phys-L] relativistic acceleration of an extended object

I reread carefully the previous correspondence. UNDER THE CONDITIONS THAT I TOOK CARE TO EXPLICITLY FORMULATE, I do not see any errors in my conclusions. If you admit that they may be correct "...perhaps in the non-relativistic limit... ", this is already killing for your statements since the non-relativistic limit is merely a part of relativistic domain. The high school relationship between given acceleration and resulting displacement (starting from rest) which I used, is universal, mass-independent, and applies at any, even superluminal, speed. The latter may be realized, e.g., by an appropriate light spot zipping across a screen (the mathematical structure of Relativity treats superluminal and subluminal motions on the same footing).

As to your article, I had explained why I could not read it. Reciprocally, you could read my article with much more detailed analysis of the whole situation than presented here, and under different possible conditions. As to your statement "Shape is determined by proper length..." , I can also refer to another relevant article, "Two Permanently Congruent Rods May Have Different Proper Lengths" in the arXiv.

I would appreciate if anyone could show where and why my arguments are wrong.

Moses Fayngold,


On Saturday, November 1, 2014 1:19 PM, John Denker <> wrote:

On 11/01/2014 09:51 AM, Moses Fayngold wrote:
But I have some comment on the basic statement in the text itself:
" order for the object to maintain its shape, different parts
will need to accelerate at different rates".

This statement is ambiguous. Its truth value depends on the chosen
reference frame (RF) and on definition of "shape".

If you would read the article, you would find the answers
to those questions. Acceleration means proper acceleration
at each point. Shape is determined by proper length, measured
along a contour of constant time. All observers agree that
the contour in question *is* a contour of constant time, so
there is no ambiguity whatsoever.

Let us define the
shape as an instantaneous configuration of the object in a given RF.
Then it is easy to see that in the initial rest frame of the object
(frame A), the statement is wrong.

It's not wrong.

Different acceleration rates for
different parts will surely distort the shape.

False. Read the article already.

In order to conserve
it in A, we need the same acceleration a for all parts.

False (except perhaps in the non-relativistic limit, which
is not what we are talking about here).

Would it kill ya to read the article?

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