Einstein's Ether: F. Why did Einstein Come Back to the Ether?
| Scientific Paper | |
|---|---|
| Title | Einstein's Ether: F. Why did Einstein Come Back to the Ether? |
| Read in full | Link to paper |
| Author(s) | Galina Granek |
| Keywords | Minkowski's absolute world, action-at-a-distance, Mach's ether |
| Published | 2001 |
| Journal | Apeiron |
| Volume | 8 |
| Number | 3 |
| No. of pages | 10 |
Read the full paper here
Abstract
According to conventional wisdom, Poincaré failed to derive a relativity theory mainly as a result of his stubborn adherence to the ether. In (1905) Einstein constructed a relativity theory that was based on the assertion that the ether was superfluous. In 1908 Minkowski formulated the theory of the "absolute world". The nineteenth century ether no longer existed. A new kind of ether (space-time) came into being. One could keep on maintaining the ether, and at the same time strip it of the notion of absolute rest. Einstein seemed to agree, and after 1916 he returned to the ether. In 1920 he combined Minkowski's absolute world concept and Mach's ideas on rotational movements: in order to cancel action-at-a-distance, the inertial interactions between matter and fixed stars should be mediated by a medium. Einstein called Mach's medium "Mach's ether". In this paper I demonstrate that Einstein's 1920 reasoning hardly differed from the one Poincaré had presented prior to 1905. Thus, whil Einstein was a hero because he did away with the ether, this situation lasted a few years only. This is not to underestimate the magnitude of Einstein's achievement, but to emphasize the limits of simplistic comparisons between Einstein and Poincaré.
Overview
This is the sixth and concluding instalment of Galina Granek's "Einstein's Ether" series in Apeiron, written from her Haifa dissertation on the historiography of relativity. It contains no physics in the sense of new derivations; it is a documentary argument about what Einstein said, when, and under whose influence. The target is the textbook contrast in which Poincaré is the figure who could not let go of the ether and therefore missed relativity, while Einstein is the figure who abolished it. Granek's claim is that the contrast will not survive the record: Einstein abolished the ether in 1905 and had reinstated it, under a new description, by 1916–1920, and the reasons he then gave for reinstating it are substantially the reasons Poincaré had already given in 1900.
The argument is assembled almost entirely from quotation. Granek's method is to lay Einstein's own 1916 letter to Lorentz and his 1920 Leiden lecture Äther und Relativitätstheorie beside Mach's 1893 Mechanics, Newton's 1693 letter to Bentley, Minkowski's 1908 "Raum und Zeit", and Poincaré's 1900 Paris congress address (reprinted in Science and Hypothesis, which Einstein is documented to have read before 1905), and let the overlap speak. Her conclusion is deliberately double-edged: Einstein's achievement is not diminished, but "simplistic comparisons between Einstein and Poincaré" have limits, and the ether-versus-no-ether axis is the wrong one on which to draw them.
The argument
Mach's problem: rotation and action at a distance
Granek opens with the difficulty that drives everything after. Mach had objected to Newton's absolute space but recognised that replacing it with relations to distant masses raises a problem of its own: nobody, he wrote, would believe that disturbing one body in an isolated system "will immediately cause a disturbance of the others as a consequence." Relative motion, Mach argued, "is determined by a medium in which K exists"; we "should have to picture to ourselves some other medium, filling, say, all space", of which we have "at present no adequate knowledge" — and such a medium would be "in every respect a more valuable acquisition than the forlorn idea of absolute space." So the Machian critique of absolute space does not eliminate the need for something besides observable bodies; it converts that need into a demand for a medium. Granek adds, in a long footnote, that Newton had felt the same pressure from the other side: gravity as innate action across a vacuum was "so great an absurdity" that no competent thinker could accept it, and in the Principia scholium Newton allowed that attraction might arise "from the action of the ether or of the air or of any medium whatever."
Minkowski's absolute world
The middle term of the argument is Minkowski. In the 1908 Cologne lecture the relativity postulate becomes invariance under the Lorentz group, which Minkowski calls "the postulate of the absolute world" (Postulat der absoluten Welt): what is given by phenomena is the four-dimensional world, with the split into space and time free within limits. Granek's reading is that this does not abolish an ether so much as replace one: "The Lorentzian nineteenth century ether, corresponding to the idea of absolute rest, no longer existed; however a new kind of ether (i.e. the space-time substratum) did exist." Minkowski's substratum simply lacks the idea of motion — and therefore one "could maintain the ether, and yet at the same time strip it of the notion of absolute motion."
Einstein's return, 1916 and 1920
The documentary core. In the letter to Lorentz of 17 June 1916 Einstein concedes that "the general relativity theory admits of an ether hypothesis as does the special relativity theory", the difference being that the metric tensor as ether "is not that of a rigid body in an independent state of motion, but a state of motion which is a function of position determined through the metrical phenomena." At Leiden in 1920 the concession becomes public and positive: "To deny the ether is ultimately to assume that empty space has no physical qualities whatever. The fundamental facts of mechanics do not harmonize with this view." The reason given is the rotation of a freely hovering system — Newton's bucket — whose behaviour depends on a state of rotation not appertaining to the system in itself, so that "besides observable objects, another thing, which is not perceptible, must be looked upon as real."
Einstein then rejects Mach's own remedy: inertial resistance opposed to the relative acceleration of distant masses "presupposes action at a distance", which the modern physicist will not accept, "and so he comes back once more, if he follows Mach, to the ether, which has to serve as a medium for the effects of inertia." This medium Einstein calls Mach's ether, and marks its one decisive difference from the ethers of Newton, Fresnel and Lorentz: it "not only conditions the behaviour of inert masses, but is also conditioned in its state by them." The Leiden summary is quoted in full: "according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists ether", and "space without ether is unthinkable", though "the idea of motion may not be applied to it."
The Poincaré parallel
Granek closes by pairing this with Poincaré. Poincaré's 1900 argument was that an ether is required so that motions do not take place with respect to empty space — formally the same move Einstein makes in 1920. And she cites Kostro's report that Einstein by 1930 held space to be "a primary thing" from which matter is derived, "eating up matter" — against Poincaré's 1909 Lille lecture, where the electron's apparent mass is the inertia of the ether, the real mass negligible, so that "there was no matter, there were only holes in the ether." Her verdict: Einstein "returned to the 19th century concept of the ether, but stripped of it its most important characteristic: a medium in a state of absolute rest", and so "came extremely close to Poincaré's ideas after 1915."
Assessment
The paper's strength is that it does not need the reader to take anything on trust. The 1916 Lorentz letter and the 1920 Leiden lecture say what Granek says they say, and the dates and venues she gives check out — Minkowski at Cologne on 21 September 1908, Einstein at Leiden on 5 May 1920, Newton to Bentley in 1693, Poincaré at the 1900 Paris congress. Against the still-common textbook picture in which the ether was killed in 1905 and stayed dead, the corrective is worth having, and it is worth having from a historian of science rather than from a partisan of the ether: Granek is not arguing that there is an ether, only that the man credited with abolishing it spent thirty years saying there was one. The framing is also unusually fair to Einstein; the concluding move is explicitly not to deflate him but to point out that ether-adherence is a poor axis on which to rank him against Poincaré.
The difficulty is that the paper's central claim — that Einstein's 1920 reasoning "hardly differed" from Poincaré's before 1905 — is undercut by the very quotation Granek chooses to end on. Einstein's stated point of difference is that Mach's ether "is also conditioned in its state by" the masses it acts on. That back-reaction is not a detail. It is the content of the field equations, it is what makes the metric a dynamical variable rather than a background, and it is exactly what no pre-1905 ether, Poincaré's included, possessed. A medium with no state of motion, no parts that can be tracked through time, and a state determined by the matter distribution is not the nineteenth-century ether minus one property; the properties it retains — that it is real, that it mediates, that it has physical qualities — are the ones that are common to almost any field theory. Granek's own sentence "stripped of it its most important characteristic" concedes more than her thesis can afford.
The influence claim is weaker still. What Granek establishes is a similarity of argument plus opportunity (Einstein read Science and Hypothesis before 1905). What she does not produce is any document in which Einstein connects the 1920 argument to Poincaré. The argument he actually gives is Newton's bucket read through Mach, and he names Mach repeatedly and at length in the passages she quotes. Since Granek's own opening pages show that the bucket-plus-medium argument is already complete in Mach 1893 — and, in embryo, in Newton's own scholium — the inference from resemblance to inspiration has a common source available to it that requires no Poincaré at all. A parallel of conclusions is not evidence of transmission when both parties are demonstrably reading the same third author.
Two smaller points. Granek uses "absolute rest" and "absolute motion" interchangeably in the Minkowski section, which blurs what exactly Minkowski's substratum lacks. And she leaves the story at 1930, at the high-water mark of Einstein's space-as-primary rhetoric, without noting what followed: Einstein progressively abandoned Mach's principle as a constraint on general relativity, since the theory admits vacuum solutions, rotating-universe solutions, and asymptotically flat solutions in which inertia is plainly not determined by distant matter. The paper is therefore a sound and well-documented account of one episode — Einstein's ether talk of 1916–1920 — presented as evidence for a stronger thesis about influence and continuity that the episode alone does not carry.