Two thoughts particularly strike the reader of Sir Oliver Lodge's many writings (over 1100, according to the official bibliography). One is that much of the development of physics is a continuum, the history of which is largely unwritten; for ideas discussed by Lodge, sometimes on an almost casual basis, frequently turn up as 'new' years later in other contexts, and this is no doubt a much more general phenomenon. The second is Lodge's exceptional powers of qualitative thinking. This ability is not as common as we think, and is undervalued because we tend to write our history in terms of mathematical formalisms. Lodge was certainly a capable mathematician, but he tended to leave the creative thinking in this area to others, often giving the impression that he was merely illustrating their mathematical ideas when often he was ahead of them (he referred to himself as a 'light skirmisher'1.
Lodge's conception of physics cannot be understood without consideration of the ether. This is a greatly misunderstood concept, and our persistent misunderstanding of the concept has damaged our understanding of both the historical process and the nature of physics.
The ether is, of course, well-known as the 'medium' supposed to be necessary to transmit the electromagnetic waves of Maxwell's theory. However, in Lodge's work, even as early as 1882, it was always a somewhat abstract (nonmaterial) concept, and it became increasingly so over a period of twenty years at the end of the nineteenth century. Lodge's lecture, 'The Ether and its Functions', given at the London Institution on 28 December 1882,2 characterizes the ether as an absolutely continuous substance filling all space, vibrating as light, shearing into positive and negative electricity, constituting matter by its whirls, and transmitting by its continuity all the action and reaction of which this matter is capable; but it is clear from the context that this 'substance' is not a fully material one.
The history of the ether concept is a very involved one, and cannot be considered briefly without a good deal of distortion. Any full analysis of Lodge's contribution would need to take account of the work of his two closest associates, FitzGerald and Larmor, in addition to that of his more casual acquaintances, Poynting, Heaviside, Hertz, and Thomson, as well as the very significant contributions of his continental contemporaries, Poincaré and Lorentz. However, in approximate terms, Young, in proposing his wave theory of light, had supposed that objects moved freely through a stationary ether (1804); but this caused problems with aberration. Fresnel devised a more complicated theory in which the ether inside material bodies responded differently to the ether outside them (1818), while Stokes proposed that a moving object would drag the ether nearest to it and so change the speed of light (1845).
In their famous experiment of 1887, Michelson and Morley found no change in c due to Earth's motion through it. However, FitzGerald and Lorentz supposed, on the basis of electromagnetic theory, that an object such as a Michelson interferometer moving through the ether, but held together by electric forces, would contract by exactly the amount required to keep c constant. Stokes's theory was still a possible alternative, so, in 1891, Lodge built a machine to investigate ether drag. He would look for changes in the interference pattern produced by light beams sent along two different paths between two rapidly rotating metal discs. Like Michelson and Morley, he obtained uniformly negative results. The two experiments, when combined, implied that the velocity of light could not be used to detect the presence of an ether, a key aspect of the theory of relativity.
Lodge was President of the Physical Sciences Section at the British Association meeting in Cardiff in August 1891. His Presidential Address announced preliminary negative results on the ether drag experiment; but also contained other material relevant to the development of relativity theory; in particular, it alluded to the fourth dimension, with an early model of a 'world-line': 'events [said Lodge] may be in some sense existing always, both past and future, and it may be we who are arriving at them, not they which are happening', and he illustrated this 'possible fourth-dimensional aspect of time' using the analogy of a solid cut into thin sections.3 Interestingly, though 4-dimensional space-time is an important relativistic concept, in this case it had a 'spiritualist' aspect, an idea that goes back to the seventeenth century at least; and Lodge used his Presidential Address, at the same time, to stress the importance of the psychical research, which he had been carrying out in parallel to his work in physics since 1883.
Though the preliminary results of the ether drag experiment had been negative, Lodge carried on with it for several more years. The discussion in his 1892 paper on the experiment predicted the Sagnac effect, in which a beam of light is split by half-silvered mirrors, and the two beams are sent in opposite directions round a loop of mirrors, and made to interfere, as in his experiment. 4 For a stationary apparatus the beams of light will arrive at the detector at same time, but, if the apparatus is rotating, the beam travelling in the direction of rotation has further to travel, so the interference fringes will be shifted. Lodge did not observe the effect, though Sagnac did later. It is now applied as a significant correction to the GPS.
During these years, Lorentz, Larmor, Poincaré, Thomson, Heaviside and others developed Maxwell's theory (sometimes in idiosyncratic ways) to effectively discover most of the individual formulae which now constitute the special theory of relativity (STR), while the work of Larmor and Lorentz led to a kind of abstract electron theory before the discovery of the equivalent material corpuscle by Thomson in 1897. Larmor's work was particularly interesting in being a kind of pre-quantum quantum theory. He had the concepts of discreteness and probability, but couched in a quasi-classical language. He also had positive and negative 'electrons' emerging simultaneously from the ether with opposite rotational strains (left- and right-handed). Lodge contributed to this emerging electron theory with qualitative ideas and some basic calculations. In March 1897, he calculated the electron's approximate size, or classical radius. On the basis of the ether theory, he claimed that atoms containing electrons were mostly empty space, and could be represented by planetary models (1902). While Thomson originally wanted to separate his 'corpuscle' (announced in April 1897) from the Larmor-Lorentz theory, FitzGerald quickly brought in the term 'electron', which he had already persuaded Larmor to use for his independent point-charges. The term had originally been used by FitzGerald's uncle, Johnstone Stoney, for the fundamental unit of charge.
While Lorentz, Poincaré, Larmor and others, had already produced many of the familiar 'relativistic' formulae as by-products of their more specialized models, Einstein's great advance in 1905 was to produce a kinematical theory that was not model-dependent, as theirs were. He didn't depend on the electron theory of matter, or rather a particular version of it that was already being superseded. Also, while the work of the electron theorists was really a precursor to quantum theory, Einstein saw that it was possible to do a classical approximation by privileging the idea of 'light' in a way that is bizarre if you analyse it from the subsequent view of quantum theory, but actually works in the special case he considered. STR is certainly not an obvious development of what went before, and it is a great disservice to Einstein's original turn of mind if we think that it is. The strange thing about STR is that Einstein uses the quantum process of light-signalling (almost certainly based on his own discovery in the same year of the light photon) as though it were classical! He creates concepts of simultaneity, light-signalling, and 'measurement' of a classical one-way 'speed of light', as though they actually have intrinsic meaning, and many people still think they have!
History and physics are also distorted if we fail to realise that Einstein's coup was succeeded by another, equally brilliant, when Minkowski, in 1907, linked space and time in a 4-vector formalism with an invariant space-time interval:
This is what we really mean when we talk about 'relativity', and it is what Einstein realised he had to use as the basis for his later general theory (GTR). Einstein's brilliant coup thus led on to the next great concept, but it was neither obvious nor strictly necessary; and the positions of such original and deep-thinking physicists as Lodge, Larmor and Lorentz are inexplicable if we don't take this into account. They were not being reactionary by defending the ether. They were saying that Einstein's theory needed a more fundamental explanation.
By 1912 there were effectively two competing explanations of the ether drift and ether drag experiments. They gave the same answers from different assumptions: the same length contraction, time dilation and mass increase, and absence of an ether effect relating to c. The Einstein-Minkowski approach finally won out, about 1915, because it was not model-dependent as the original Lorentz-Poincaré theory was. A few years later its ascendancy was sealed by the success of GTR (1919).
Despite this, Lodge's views on relativity never changed. According to his
reasoning, c must be constant in absolute space, and independent of
any motion of the source; but no experiment had yet shown, and no
terrestrially-based experiment could show, Einstein's further supposition
that c was independent of the motion of the observer. Relativity
required the equivalent status of motion of source or observer, but Lodge
believed that this was an unnecessary assumption:
'The doctrine
[of relativity] certainly explains the Michelson experiment, and my
experiment; nor has any experiment negatived it so far; and yet - well, it
hardly seems consistent with common sense. It seems to me that posterity
will formulate the doctrine a little differently.'
5
While Lodge was prepared to tolerate STR as just another way of saying the same thing as his favoured Lorentz-Poincaré alternative, in which ether remained equally undetectable by experiments on c, he didn't like Einstein's later theory at all because he thought that he was being bamboozled by mathematics, with relatively simple predictions being derived from an unnecessarily complicated apparatus. What he disliked most was the press's reporting of Eddington's 1919 eclipse expedition to measure the gravitational bending of light, the experiment which finally clinched the success of GTR. On 7 November, for example, The Times addressed its readers with the sensational headlines: 'Revolution in science. New Theory of the Universe. Newtonian Ideas Overthrown'. Lodge himself also seems to have been singled out and set up as a straw man, for The Times went on to say: 'It is interesting to recall that Sir Oliver Lodge, speaking at the Royal Institution last February, had also ventured on a prediction. He doubted if deflection would be observed, but was confident that if it did take place, it would follow the law of Newton and not that of Einstein.' And the article made a point of gratuitously recording that: 'At this stage Sir Oliver Lodge, whose contribution to the discussion had been eagerly expected, left the meeting.'
But Lodge was soon ready with his counter-attack. On 2 December, he took up Eddington's equation for light deflection:
He immediately sensed that 'gravitational redshift', which had been put forward as a further significant 'test' of GTR (and was essentially represented by the last term in the equation), was merely Newtonian in origin: 'The numerator is the squared velocity of free fall from infinity. And as a beam of light has really fallen from infinity, the expression at once assumes a common-sense aspect'.6 He even suspected that there was a common-sense way of deriving the full expression for the bending of light.
Two years later, he argued that the new 'quaternion spatial nomenclature' (or 4-D space-time) was more compact than the old Cartesian version with space separate from time; but, though Minkowski had ingeniously incorporated the two quantities into one equation, they still remained separate things.7 The space-time concept was not revolutionary, though it 'may possibly be found to have some metaphysical meaning'. The presence of the 'Maxwellian velocity' (c) in the ether theory had exactly the same effect of relating space and time. Einstein's work was a universal application of earlier results. It was, in fact, a 'fuller realisation' of the theory of the ether. This medium constituted the four-dimensional continuum or physical space-time of Einstein's theory. However, Einstein's theory did not employ the most 'ideal and direct manner', and it was 'unwise to load the new discoveries with an implication that the historical principles of geometry' had 'broken down or been detected as untrue'.
Apart from voicing criticisms, Lodge explored three new or nearly new ideas: gravitational lenses (1919); black holes (1921); and collapsed matter stars (1921). In a letter to Nature, dated 2 December 1919, he proposed that one could introduce 'the simple idea of refractivity, through a diminution of the velocity of light by a gravitational effect upon the ether's elastic or dielectric coefficient, employing the same factor as expressive of a refractive index'.8
Jupiter might act as 'gravitational lens', two stars either side of the planet being shifted relative to each other by 1/60 th of a second. If backed by a nebula or any luminous area, the light grazing the sun's rim all round would be brought to a focus at a position 17 times the distance of Neptune, while light from any larger circle would focus still further off in proportion to the area of the circle; from a uniformly luminous area there would be a focal line of constant brightness.
Then, in an address to the Students' Mathematics and Physics Society of
the University of Birmingham in 1921, he argued that:
a 'sufficiently massive and concentrated body would be able to retain light
and prevent its escaping', but the 'body' need not be a single star; it
could be a 'stellar system of exceedingly porous character'.
9
Versions of the classical concept of black hole had been put forward in the
eighteenth century by Michell and Laplace; and Anderson had recently
resurrected it. However, Lodge showed how it could apply to the whole range
of possible scenarios of interest today, and he also put forward the idea of
collapsed matter stars. 'For a body of density 1012, - which must
be the maximum possible density, as its particles would then be all jammed
together, - the radius need only be 400 kilometres. This is the size of the
most consolidated body. For anything smaller than that the effect would be
impossible.' 'If a mass like that of the sun (2.2 × 1033
grammes) could be concentrated into a globe about 3 kilometres in radius,
such a globe would have the properties above referred to; but concentration
to that extent is beyond the range of rational attention ... .'
However, a 'stellar system - say a super spiral nebula' 1015
times the mass of the sun - would not be 'utterly impossible'. 'What becomes
of the radiation poured into space by innumerable suns through incalculable
ages? Is it possible that some of it is trapped, without absorption, by
reservoirs of matter lurking in the depths of space, and held until they
burst into new stars?' He spoke of the conversion of radiation into
electrons with a velocity of intrinsic circulation of order c. 'On
this view the interior of an enormous stellar system could be the seat of
the generation of matter ... .'
Such forward thinking is typical of Lodge, and was generally based on good qualitative analysis, rather than random speculation. He also thought that the proton might be composite (which, of course, it is). It was just possible, he said, that 'the progress of discovery' will 'detach from the proton a positive charge more closely akin to the negative electron - in fact an image of it'. (In fact, a process of this type occurs in positive beta decay.) As a result of radioactivity: 'The formation of strange substances and unusual combinations may be expected and the composite nature even of the proton may yet be demonstrated by the emission of something fractional of extreme instability.' He was also one of the first to realise that, although STR didn't need the ether, GTR had to resurrect it. In November 1921 he reported: 'Eddington told me he had asked Einstein in Berlin recently, who said, 'No, I have no objection to the ether; my system is independent of the ether'.' 10 In fact, Einstein used the idea explicitly from about 1915, and even the word.
But Lodge and Larmor, in particular, still felt that something important was missing. Relativity didn't answer the fundamental question. It avoided the problem of the ether rather than tackling it. There were serious unanswered questions, which could only be answered by truly understanding the ether. Special relativity had made that more difficult by giving the impression that the ether had been disproved. Lodge couldn't see why light should be privileged as a source of 'information'. We must distrust, he said, the 'popular methods of explanation' for the 'Larmor-Lorentz transformations', in which light is thought of as 'bringing information about events', thus 'giving us rather confused information about what happens to railway trains and embankments'. 11 They inevitably led 'one to ask what light has to do with it; why sound or a messenger-boy should not be used instead; and absurdities of that sort'. Light was 'not of fundamental importance as the unique and only messenger', but rather as the means of measuring experimentally the fundamental constitutional velocity of the ether.
In rejecting light, however, Lodge had to think of another way of getting a handle on the ether. The only other source of information was the theory of matter. Again and again he returned to the necessity of the positive electron. In 1922, he wrote: 'According to Larmor's theory the positive and the negative electrons can only differ, or at least must chiefly differ, in one being the mirror-image of the other. One for example might be a concentrated locked right-handed screw twist in the Ether while the other would be a left-handed contortion of the same kind, simultaneously and inevitably produced, and contorted with its fellow by transferable lines of force.'12 And he continued: 'Why negative electricity should differ from positive so greatly, or in any respect save in sign, is not at all clear; and it is difficult to understand how one of these entities can have been constructed out of the ether, without the simultaneous production of its opposite partner.' As late as October 1929, he commented in his review of Larmor's papers that 'the author's dissatisfaction with the concealment or sophistication of the positive electron is manifest'.13 However, a new theory had emerged which might provide the answer: 'the names to conjure with' were now Schrödinger, Heisenberg and Dirac, and the new wave or quantum mechanics would be 'the beginning of a comprehensive theory of the ether'. Larmor, he said, sees 'Maxwell in Dirac'.
Lodge and Larmor were absolutely right! The key to understanding the true meaning of STR is to look at the Dirac equation, to which STR is only a classical approximation, and which is inconceivable without the positive electron. It is significant, here, that space and time are not a 4-vector, as each is preceded in the equation by a different operator (or gamma matrix). Quantum Dirac supersedes classical Einstein-Minkowski! The thing that the Einstein-Minkowski formalism leaves out of the equation is the proper time T and, hence, causality:
We are told that the space-time combination is an invariant, but not what this invariant is, or why it's an invariant. Proper time and causality are added in STR as a 'common-sense' extra. But there is nothing common-sense about it at all. Proper time occupies the position that rest mass does in the energy-momentum relation. Causality has a very specific origin in quantum mechanics, which is intimately connected with the idea of the vacuum and nonlocality. And even the rest mass has a vacuum origin in the Higgs mechanism. Einstein was able to dispense with the 'ether' (vacuum) because he left the 'ether' term out of his equation!
Of course, in the historical context, he was right to do so, and his action ultimately makes it possible to identify the term which is most significant in a vacuum context. However, Lodge and Larmor were also right to insist that something was missing and that it would be explained by the theory of Dirac. Dirac's concept of a filled vacuum, in particular, explained the + / - electron in almost identical terms to the ones they had used. Nowadays we use the word 'vacuum' to represent the concept that Lodge and his contemporaries called 'ether'. It is an expression of the nonlocality inherent in quantum mechanics (and anathema to Einstein). It is a kind of expression meaning 'the rest of the universe'. We can't define a fermion without defining its vacuum. Lodge had been moving in this direction from the start.
Lodge's ether was always a more subtle concept than many people have realized: 'Objections to the ether are really objections to the nineteenth century conception based in terms of mechanical models. No such ether exists ... .' 14 'I have abandoned the old material ether of Lord Kelvin and the nineteenth century in favour of some hydrodynamic or other perfect mechanism at present unknown.' 15 The fact that mass is purely electromagnetic in origin, he said, must mean that all energy, including mc2, is due to space. Lodge believed that Einstein, in his later work, fundamentally agreed with him. The two men met in Oxford in June 1933. According to Lodge's notes of their conversation, Einstein said that he had gone through three stages with respect to the ether: first, a belief in the old dynamical theory; second, total disbelief; and finally, a belief that the ether is responsible for everything, though a disbelief that it has motion. 16
Scientific concepts seldom emerge in the clear-cut way that we like to present them, and, though the concept of 'relativity' is predominantly associated with Einstein, many other physicists played a part in shaping the theory. Lodge's contributions, though little understood today, were among the most significant - from early theoretical ideas, like worldlines and the Sagnac effect, through the experimental disproof of ether drag, to the brilliant conjectures concerning gravitational lenses, black holes and neutron stars of his later years. Not least among his contributions is the critical attitude he brought to the foundations of both STR and GTR, and his partial realization, along with Larmor, that the resolution of these difficulties required a deeper understanding in areas that we would now describe as quantum mechanics and particle physics. Some of these difficulties still remain to be resolved today.