In 1905 A. Einstein banished the ether from physics in connection with the formulation of his Special Relativity Theory. This is very well known but less known is the fact that in 1916 he reintroduced the ether in connection with his General Relativity. He denominated it ?new ether? because, in opposition to the old one, the new one did not violate his Special and General Principle of Relativity. It didn't violate it because the new ether is not conceived as a privileged reference frame but it is considered as an ultra-referential primordial material reality which is not composed of points (or particles) and not divisible in parts and to which therefore the notions of motion and rest are not applicable. The purpose of this paper is to present a short outline of the history of Einstein's concepts on ether and to show which elements of the mathematical formalism of General Relativity were considered by Einstein as mathematical tools describing the relativistic ether, i.e. the ultra-referential space-time characterized with a certain kind of energy density. It will be indicated also that Einstein's intuitions and ideas concerning the ultra-referential space-time have to be investigated in the framework of Connes' non-commutative geometry, as the commutative geometries are not sufficient to do it. In Poland Michal Heller and his colleagues are trying to create an unification of General Relativity and Quantum Mechanics with the help of Connes' non-commutative geometry.
In the first part of this paper the author presents physical and philosophical reasons which, in 1905, finally impelled A. Einstein to deny the existence of the ether of 19th century physics, especially H. A. Lorentz's ether. Since Einstein, under the influence of P. Drude, identified Lorentz's ether with absolute space, the denial of Lorentz's ether meant, for Einstein, the denial of the existence of absolute space. The author also presents reasons of the same kind which impelled Einstein, when he was formulating the general relativity, to entirely deny the existence of physical space and time. This denial lasted from 1913 to 1916.
In the second part the author presents physical and philosophical reasons that impelled Einstein in 1916, to again recognise the real existence of space and time and to call them (connected in his theory into one space-time)" the new ether."
In 1905, as is well known A. Einstein began to deny the existence of an ether as it was conceived in 19th-century physics, in particular of Lorentz's ether, which was in the first place a privileged reference frame. He denied its existence because it violated his principle of relativity, according to which there is no privileged reference fram for the formulation of the laws of nature. Nevertheless, in 1916, after the definitive formulation of the general theory of relativity, Einstein proposed a completely new conception of the ether. In this conception, the new ether does not violate the principle of relativity because the space-time of the theory of relativity is conceived, in it, as a material medium sui generis that can in no way constitute a frame of reference.
As is well known, Einstein, having introduced the Special Theory of Relativity in 1905, proceeded to deny the existence of the 19th century liminiferous ether both in his technical papers as well as in his articles for the general public. This fact has occasioned that among scientists and subsequently among the general public he has acquired the reputation of being the destroyer of the ether concept in general.
Such an opinion is today propagated in textbooks, encyclopedias and scientific reviews. Therefore most physicists and philosophers are convinced that Einstein has removed the notion of the ether from physics forever. This opinion, however, is not precisely correct; because since 1916 the notion of the ether has found in Einstein's Relativity Theory a new and interesting application and development. Einstein himself emphasized this fact in the following words:
"This word ether has changed its meaning many times in the development of science... Its story, by no means finished, is continued by relativity theory." (Einstein and Infeld 1938).
The main aim of this paper is to present a historical outline of Einstein's ideas concerning the ether, especially the new relativistic ether (in the first part) and to discuss its physical meaning and physical properties (in the second part).
It will be shown that in the mathematical formalism of elementary particle physics, there exist (in a latent form) four types of constants that have the dimensions of action and are closely tied to the four fundamental interactions. It will also be shown that apart from Planck's natural units, there are four additional sets of units determined by universal constants. The physical meaning of the quanta of action and units under consideration (especially in the framework of the three-wave model of the elementary particle proposed by the author in 1978) will be discussed.