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

I hope this thread doesn't degenerate into a shouting match, because it interests me a great deal, having played with acceleration within the SR formalism in graduate school, eventually leading up to my dissertation:

MF: Your brief analysis in this thread appears to me to contain a couple of conceptual mistakes, which I presume you avoided in your published articles and books on the subject. Trivially, no one believes x(t) = x0 + a t^2, even in the non-relativistic domain that's incorrect, lacking a factor of (1/2) in the second term. Secondly, remember that the formula is only valid for cases involving non-relativistic speeds and constant acceleration according to some observer, being found by twice integrating (with respect to t) the definition dv/dt = a, for constant a. Thirdly, with these integrations and corresponding differentiations w.r.t. time t, one must specify _whose_ time is being used. The logical way to deal with constant acceleration under SR is to consider constant _proper_ acceleration (the acceleration of the accelerating observer). This yields the hyperbolic motion that JD refers to, although I think he should more carefully include the adjective "proper" in order to avoid commonly occurring confusion. [*] It has been studied frequently over the years, I'd have to check my files at the office to verify, but I expect the Wolfgang Rindler treats it his classic text, "Essential Relativity", as do Christian Møller, Sears & Brehme, and others in various books and articles. JD seems to be laying out a nice reiteration of parts of this treatment and extending it in a logical way, although other possible ways come to mind as well.

Fun stuff! I will definitely read the things you both (JD & MF) have shared, although it may have to wait for Thanksgiving break, much to do before then!


Ken Caviness
Southern Adventist University

[* Constant proper acceleration is quite different from constant acceleration according to an inertial observer. All my relativity materials are at work, but if memory serves, for the 1-d case, (proper acceleration) = (gamma^3)(acceleration), where gamma = 1/sqrt(1-v^2/c^2) and both velocity and acceleration on the RHS are as measured by an inertial observer, proper acceleration (by definition) as measured by the accelerating observer, or by a momentarily co-moving inertial observer.]

-----Original Message-----
From: Phys-l [] On Behalf Of Moses Fayngold
Sent: Sunday, 2 November, 2014 12:13 PM
Subject: 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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