B General Relativity & Ether: Clarifying a Contradiction?

[DanAil]

Hope this question can be quickly clarified:
There was a statement that the General Relativity can be interpreted by speaking of an ether whose state varies from point to point. Is this correct?!

 

[Nugatory]

There was a statement that...

A statement where? You will get better and more helpful answers if we know where you saw that statement, as it can be interpreted in several different ways, some more sensible than others.

 

[DanAil]

A statement where?

Good point! Here is for example a link with the short paragraph containing the statement:
https://einsteinpapers.press.princeton.edu/vol7-trans/91

 

[Dale]

General Relativity can be interpreted by speaking of an ether whose state varies from point to point

I think an aether without a rest frame is no aether. But authors can define terms how they like so I suppose someone who really wanted to could deliberately define the word “aether” to refer to something that would match that qualification. Many people, including myself, would reject such a definition.

 

[PeterDonis]

There was a statement that the General Relativity can be interpreted by speaking of an ether whose state varies from point to point.

Only if you are willling to use "ether" as a synonym for "the geometry of spacetime" and nothing else. In particular, as Einstein says in the reference you give, you must "be careful not to attribute to this ether any matterlike properties (e.g., a distinct velocity at each point)." Which defeats the whole purpose of using the term "ether" in the first place.

 

[Ibix]

Given that Einstein, in the last sentence at that link, says 'one has to be careful not to attribute to this "ether" any matterlike qualities', I think a couple of things stand out. First is the use of scare quotes around the word ether, suggesting to me that Einstein thinks that really it's not a particularly appropriate term.

Second, that it doesn't have any matterlike qualities suggests that he's referring to the Lorentz ether, which is a term usually seen in the context of special relativity. The Lorentz ether has no properties beyond its existence, serving only to pick out one inertial frame as the true reality while all others are in some sense not real. Of course, which frame is so blessed is not detectable, so most people simply ignore the whole concept on the basis of Occam's Razor. I don't see why you couldn't apply the concept to general relativity if you were so inclined - pick some foliation of spacetime and declare it the One True Foliation. Again the choice of which foliation is the One True one is undetectable, so is a free choice. And again, in the modern view Occam's Razor would urge you to reject the concept. This was probably not so clear cut in 1918.

 

[vanhees71]

Some people also say that the "ether" of modern physics is the vacuum of quantum field theories. I find this utterly misleading since the word "ether" (or rather "aether"?) is loaded with historical context. In the 19th century it was understood to be some medium whose vibrations are light in the same sense as sound are vibrations of the air. Now in the relativistic context the presence of a medium indeed breaks Lorentz invariance in the sense that there is a reference frame preferred by the physical situation, i.e., the (local) rest frame of that medium. Of course Lorentz invariance is not broken at all simply by the presence of the medium but there is a time-like four-vector field defined by the medium, which is its four-velocity (flow) field.

The vacuum of a QFT, however, is precisely NOT in any way distinguishing any inertial frame from any other by definition, because it's invariant under Poincare transformations. So it's good practice to call the QFT vacuum the vacuum and not "ether" or "aether".

The great achievement of Einstein's relativity (particularly the general one) is that it got rid of the fiction of an absolute space and an absolute time, introduced by Newton (who nevertheless stressed "hypotheses non fingo" ;-)). As already Leibniz stated, there's no observational way (even in Newtonian mechanics) to distinguish one inertial reference frame from any other, but of course Newton could never agree with that statement, because it came from Leibniz ;-)).

 

[DanAil]

Thank you all for the answers - good insights. It makes sense statements made over a century ago to be updated with the current understanding of the subject.

 

[Sagittarius A-Star]

Only if you are willling to use "ether" as a synonym for "the geometry of spacetime" and nothing else.

Einstein formulated this in a letter to Lorentz from June 17th, 1916, in the following way:

$$g_{\mu\nu} = \text{ether}$$

Source:
https://einsteinpapers.press.princeton.edu/vol8-trans/250?highlightText=ether

 

[PeterDonis]

Einstein formulated this in a letter to Lorentz from June 17th, 1916, in the following way

Yes. That just underscores what I said.

 

[PeroK]

Einstein formulated this in a letter to Lorentz from June 17th, 1916, in the following way:Source:
https://einsteinpapers.press.princeton.edu/vol8-trans/250?highlightText=ether

That is a quotation out of context. The paragraph is hypothetical.

I grant you that the general theory of relativity is closer to the ether hypothesis than the special theory of relativity. But this new ether theory would not violate the relativity principle any more. For the state of this guv = ether would not be that of a rigid body in an independent state of motion.

 

[pbuk]

Einstein formulated this in a letter to Lorentz from June 17th, 1916, in the following way:
Source:
https://einsteinpapers.press.princeton.edu/vol8-trans/250?highlightText=ether

Einstein wrote to Lorentz in German (the native language of them both). The German word for the chemical substance "ether" is "äther" which is transliterated into the English alphabet as aether and normally pronunced in English the same as "ether" but in German is more like "aiter" (whereas if "ether" were a German word it would sound more like "ehter").

Corrections gratefully received from @fresh_42 or any other native German speakers (and I learned my German in Bavaria anyway )

 

[Sagittarius A-Star]

Einstein wrote to Lorentz in German (the native language of them both). The German word for the chemical substance "ether" is "äther" which is transliterated into the English alphabet as aether and normally pronunced in English the same as "ether" but in German is more like "aiter" (whereas if "ether" were a German word it would sound more like "ehter").

Corrections gratefully received from @fresh_42 or any other native German speakers (and I learned my German in Bavaria anyway )

The German word has several meanings, dependent on context.

Source:
https://de.wikipedia.org/wiki/Äther

 

[fresh_42]

Äther is pronounced 'a:ter' with a long 'a' as in 'mad'.

There is a nice anecdote about it.

A surgeon who only cared about his specialty read the headline of an article in a magazine after breakfast:

Einstein abolishes the ether!
​

Shaking his head, the doctor closed the journal: "A small appendicitis would probably be enough to convince this gentleman of the necessity and usefulness of the ether!"

 

[pbuk]

Äther is pronounced 'a:ter' with a long 'a' as in 'mad'.

Are you sure? The 'a' in (English) 'mad' has a similar sound to the (German) 'kann': I think 'ä' is more like (English) 'hair'.

 

[fresh_42]

Are you sure? The 'a' in (English) 'mad' has a similar sound to the (German) 'kann': I think 'ä' is more like (English) 'hair'.

'kann' is, well, I cannot type that word in here: '#unt' without the 't' at the end, of course.

I think only in Scotland is 'mad' pronounced as a German 'a' as the 'u' in undo. I learned it as æ (bad, land, sand). 'Mud' has a German 'a' sound.

 

[Sagittarius A-Star]

Are you sure? The 'a' in (English) 'mad' has a similar sound to the (German) 'kann': I think 'ä' is more like (English) 'hair'.

Here you can click on an audio recording for pronunciation of "Äther":
https://dict.leo.org/englisch-deutsch/äther

 

[pbuk]

'kann' is, well, I cannot type that word in here: '#unt' without the 't' at the end, of course.

Only by a native: think of kann pronounced by an Englishman! Actually I don't think there is any German word with an 'a' as in 'mad'.

I think German long 'a' (Bad, Rad), is closest to English car, father or palm.

 

[fresh_42]

Only by a native: think of kann pronounced by an Englishman! Actually I don't think there is any German word with an 'a' as in 'mad'.

I think German long 'a' (Bad, Rad), is closest to English car, father or palm.

Yes. But 'ä' (sad) and 'a' (car) are different.

The Scottish actually pronounce many vowels as in German. (Not that this helps to understand them.)