Talk:Einstein synchronisation

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If someone is going to create hr: (or sl:, ...) version: There's an article in Croation in the open access journal Prolegomeno on this subject:

• http://www.doaj.org/doaj?func=abstract&id=175679&toc=y

Pjacobi 09:06, 8 November 2006 (UTC)

[edit] Über die spezielle and die allgemeine Relativitätstheorie

I've read the book (in German) and I'm a bit confused which part of it supports this recent edit. To my best knowledge, and from a quick look into the online translation, in this book Einstein uses the midpoint-observer-method, which is of course completely equivalent to the back-and-forth-signal method. --Pjacobi 21:01, 9 November 2006 (UTC)

I've reverted for now. --Pjacobi 17:28, 12 November 2006 (UTC)

> [...] in this book Einstein uses the midpoint-observer-method

Sure. (As far as I can tell, Einstein didn't attach a name to the method; but observer M as "midpoint between" A and B plays a decisive role.)

> which is of course completely equivalent to the back-and-forth-signal method.

No -- these methods are inequivalent in several important ways (two of which had been indicated in the article):

1. They concern different notions.

The "midpoint-observer-method" itself is (merely) a method for deciding simultaneity of state pairs (and simultaneity is independent of appearance, labelling, or parametrization). In contrast, a determination about synchrony (i.e. both by the methods of 1905 and of 1917) requires the additional comparison of labels.

2. They have different requirements.

The synchrony definition of 1905 requires some particular real-valued parametrizations "τ" (separately for the states of A and of B), while neither the simultaneity definition of 1917, nor the corresponding definition of synchrony require such parametrization (at least overtly). (In other words: only the latter method allows for instance to decide whether two given Floral clocks are in synchrony, or not. In this respect, the latter method is certainly superior to the former.)

3. They have different results.

Considering two clocks, A and B, moving at the periphery of a rotating disk (within a flat space). Using the synchrony definition of 1905 they may find having been synchronous at least for some suitable parametrizations "τ_A" and "τ_B". However, using the definition of 1917 (along with the definition of "midpoint" based on projective geometry), they already fail to find any simultaneous state pairs (and consequently cannot find having been synchronous at all, regardless of any parametrizations or labellings), because there cannot be found any observer as "midpoint M between" A and B to begin with; i.e. no observer M satisfies the conditions that

- Observer A finds two successive "back-and-forth-signal" intervals to and from observer M the same as one "back-and-forth-signal" interval to and from observer B,

- Observer B finds two successive "back-and-forth-signal" intervals to and from observer M the same as one "back-and-forth-signal" interval to and from observer A, and

- Observer M finds same "back-and-forth-signal" intervals to and from observer A and to and from observer B.

I'm not certain whether Einstein after Dec. of 1916 devised even further definitions of how to measure synchrony or of simultaneity (or whether he even was the first to develop the "midpoint-observer-method"), or whether afterwards he may again have reverted to the earlier definition. But the progress Einstein had made since 1905 ought to be recognized in any case. Frank W ~@) R 23:38, 12 November 2006

I'm still rather unclear what you are arguing here. Can you be so kind to state specifically which sentences in "Über die spezielle and die allgemeine Relativitätstheorie" you have in mind? Or what secondary sources you are using? --Pjacobi 08:07, 13 November 2006 (UTC)

> [...] Can you be so kind to state specifically which sentences in "Über die spezielle and die allgemeine Relativitätstheorie" you have in mind?

Why - certainly I can. (I'll quote the relevant passages of course from the English translation, which is also avaliable online; namely from Sect. 8, i.e. the reference that had been listed in the article):

$ If the observer [...] placed at the midpoint M of the distance A B [...] perceives the two flashes of lightning at the same time, then they are simultaneous.

We agreed already that (in your words) "Einstein uses the midpoint-observer-method". However, I note and emphasized in (1.) above that this method in itself is (merely) a method of defining simultaneity, not (yet) of synchrony.

Moreover, I had indicated in (3.) above that the existence of a "midpoint between" two given observers is far from a trivial matter (nor is correspondingly the existence of a value of "distance between" two given observers, A and B). IIRC that's already sketched in Max Born's "Einstein's Theory of Relativity" (for instance; although there don't seem very many sources that even acknowledge Einstein's "use of the midpoint-observer-method" at all). Further on:

$ [...] that clocks [...] are set in such a manner that the positions of their pointers are simultaneously [...] the same.

This unarguably (?) concludes the synchronisation definition under consideration, i.e. what I'd consider "Einstein's synchronisation definition of 1917". I note in (2.) that "positions of pointers" may be compared and judged "same" vs. "not same" regardless of some particular parametrizations "τ_A" and "τ_B" which are explicitly required by "Einstein's synchronisation definition of 1905" as sketched in the original article. Consequently the method of 1917 may allow to determine whether or not certain Floral clocks were in synchrony, while AFAIU the 1905 method doesn't allow such a determination at all.

(Also, as an aside and a matter of stylistic clarity, I wouldn't necessarily use the word "same" in reference to distinct items; such as referring to the more or less similar appearances of two distinct clocks; and "display state" would seem not as pedestrian as "position of pointer" ...). Frank W ~@) R 21:22, 13 November 2006 (UTC)

Thanks for clarifying. I have to think about it. --Pjacobi 08:47, 14 November 2006 (UTC)

Fine, thanks for giving it more thought for now. Even better: have (eventually) all interested W:Readers recognize and think about the issue, elaborate or correct as needed, or find their thoughts expressed already. (For instance, the thought of having the listing of sources start with the primary ones; which happen to be in chronological order as well.) I'll restore the article accordingly. Frank W ~@) R 20:52, 15 November 2006 (UTC)

In case you are still watching this talk page: I've reading some more sources (rather randomly choosen, by using these readily available), and would judge that Einstein later (sometimes) uses the midpoint observer method is primarily rooted in its simplicity. It does in addition makes less assumptions and achivieves arguebly more, but only withing context of even more general test theories. The philosophical debate about "Einstein synchronisation" centers about the conventionality of choosing to 0,5. In this regard midpoint observer (or midpoint flash, which seems to be most popular introductory treatment) and the older back and forth method, are equivalent. In "midpont" methods α corresponds to the choice of the observer (flash) position.

Also there is an example of a widely used other synchronisation convention. The astronomical one which uses the backward lightcone of Earth (or an idealized "Earth" position) as surfaces of simultaneity. The 1987 Supernova is that one, whose light reached Earth in 1987.

Pjacobi 12:58, 4 December 2006 (UTC)