In the context of 17th century science, two rival theories describing the nature of light were proposed, a wave theory and a corpuscular theory as proposed by Christian Huygens and Isaac Newton respectively, which might be seen as the start of the wave-particle debate. However, despite Newton’s support for the particle-like nature of light, he still assumed a physical aether existed, which by the 19th century had developed in support of the propagation of electromagnetic waves. In his 1864 theory of electromagnetism, James Maxwell had initially attempted to explain the phenomena by means of mechanical action through a medium occupying the space between two points. In 1870, William Clifford presented a paper entitled ‘On the Space-Theory of Matter’ to the Cambridge Philosophical Society. In this paper, Clifford forwarded the idea that matter is nothing but ripples, hills and bumps of space curved in higher dimensions, where the motion of matter is nothing more than variations in that curvature. In this context, it has been suggested that he anticipated some aspects of Einstein’s general theory of relativity by some 45 years, while recognising that this work was in-turn influenced by the earlier work of Bernhard Riemann, whose developments in mathematics helped described physical space in terms of a new geometry. The link between Riemann and Clifford’s ripples, hills and bumps of space might be seen in Clifford’s own words:
The axioms of plane geometry are true within the limits of experiment on the surface of a sheet of paper, and yet we know that the sheet is really covered with a number of small ridges and furrows, upon which the total curvature not being zero, these axioms are not true.
However, the Michelson-Morley experiment, carried out in 1887, to detect the motion of Earth through this ether unexpectedly failed to provide any evidence of the aether’s physical existence. In an attempt to explain the failure of the Michelson-Morley experiment, Lorentz and FitzGerald forwarded the Lorentz ether theory as a possible solution as to why the motion of an absolute aether could not be detected. However, the subsequent publication and acceptance of Einstein's 1905 special theory of relativity led most physicists to conclude that the notion of a luminiferous aether was not required, although it did not necessarily disprove its possible existence. However, in many respects, the earlier work associated with the Lorentz transformations simply became subsumed within Einstein’s emerging relativistic theory, the validity of which is widely accepted to this day. This said, while it is often assumed that relativity negates the idea of space as a wave propagation media, Einstein himself was often more circumspect on this issue. For example, earlier in 1895, he is quoted as saying:
The velocity of a wave is proportional to the square root of the elastic forces which cause [its] propagation, and inversely proportional to the mass of the aether moved by these forces.
Of course, it may be argued that Einstein was simply using the word ‘aether’ in this early quote to denote the gravitational field within general relativity, which was still some 20 years in the future. However, even in 1920, Einstein still appears to be treading a cautionary path as per the following quote:
We may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an Aether. According to the general theory of relativity, space without Aether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this Aether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it.
While we might now question the term ‘ponderable media’ we might also question how space is subject to curvature and expansion within the modern understanding of relativity and cosmology without space having some form of physical structure and, if so, why this structure cannot support the notion of a wave. Of course, relativity was not the only paradigm shifting theory of the 20th century, for in 1924, Louis de Broglie forwarded the idea of matter waves, which later developed into his ‘pilot wave model’ in which he states:
Any particle, even isolated, has to be imagined as in continuous ‘energetic contact’ with a hidden medium. If a hidden sub-quantum medium is assumed, knowledge of its nature would seem desirable. It certainly is of quite complex character. It could not serve as a universal reference medium, as this would be contrary to relativity theory.
Within the developing scope of quantum theory, spacetime is often described as being ‘ non-empty’ on the quantum scale, where energy fluctuations can create particle pairs, which can appear and disappear in equally small periods of time. Of course, it might be argued that if these particles are transitory, then their physical mass cannot be used as a fundamental construct of the physical universe. As quantum theory continued to develop, Paul Dirac would forward the idea that the quantum vacuum was a form of a ‘particulate aether’, although his aether hypothesis was said to mainly reflect a dissatisfaction with the developing idea of ‘quantum electrodynamics’. In Dirac’s words:
Physical knowledge has advanced much since 1905, notably by the arrival of quantum mechanics, and the situation [about the scientific plausibility of Aether] has again changed. If one examines the question in the light of present-day knowledge, one finds that the Aether is no longer ruled out by relativity, and good reasons can now be advanced for postulating an Aether. We have now the velocity at all points of space-time, playing a fundamental part in electrodynamics. It is natural to regard it as the velocity of some real physical thing. Thus, with the new theory of electrodynamics [vacuum filled with virtual particles] we are rather forced to have an Aether.
Later, Robert Laughlin, a Nobel Laureate in Physics, made the following reflective comment:
It is ironic that Einstein's most creative work, the general theory of relativity, should boil down to conceptualizing space as a medium when his original premise [in special relativity] was that no such medium existed. The word 'ether' has extremely negative connotations in theoretical physics because of its apparent opposition to relativity. This is unfortunate because, stripped of these connotations, it rather nicely captures the way most physicists actually think about the vacuum. Relativity actually says nothing about the existence or nonexistence of matter pervading the universe, only that any such matter must have relativistic symmetry. It turns out that such matter exists. About the time relativity was becoming accepted, studies of radioactivity began showing that the empty vacuum of space had spectroscopic structure similar to that of ordinary quantum solids and fluids. Subsequent studies with large particle accelerators have now led us to understand that space is more like a piece of window glass than ideal Newtonian emptiness. It is filled with 'stuff' that is normally transparent but can be made visible by hitting it sufficiently hard to knock out a part. The modern concept of the vacuum of space, confirmed every day by experiment, is a relativistic ether. But we do not call it this because it is taboo.
At this point, we might jump ahead to 1986, when John Bell suggested that an aether theory might help resolve the EPR paradox by allowing a reference frame in which signals go faster than light. In this context, he suggested that Lorentz contraction is perfectly coherent and could produce an aether theory perfectly consistent with the Michelson-Morley experiment. Bell went on to say that the idea of an aether was wrongly rejected on the somewhat philosophical argument, i.e.
That what is unobservable does not exist.
In this respect, some have argued that Einstein adopted his non-aether position primarily on the basis that it was simpler and more elegant without actually refuting the possibility of the existence of an aether. Later, along our developing timeline, Gerard 't Hooft, the 1999 Nobel Prize winner for physics, made another reflective comment:
We should not forget that quantum mechanics does not really describe what kind of dynamical phenomena are actually going on, but rather gives us probabilistic results. To me, it seems extremely plausible that any reasonable theory for the dynamics at the Planck scale would lead to processes that are so complicated to describe, that one should expect apparently stochastic fluctuations in any approximation theory describing the effects of all of this at much larger scales. It seems quite reasonable first to try a classical, deterministic theory for the Planck domain. One might speculate then that what we call quantum mechanics today, may be nothing else than an ingenious technique to handle this dynamic statistically.
So while the mainstream position may continue to reject the idea of space being a possible wave media, historical comments suggest that the idea has never really gone away or more importantly categorically disproved. Of course, despite this backdrop of historical questioning of the current accepted models of science, the dominance of the mainstream position suggests that any WSE model not only lacks credibility, but is obviously wrong. However, while recognising the weight of authority in opposition, it has not stopped some people from forwarding a number of variant models that attempt to describe some form of WSE model, which by inference, must position them outside mainstream acceptance, such that their work is invariably rejected by the peer review system.