The Aether - What is it?Aether (sometimes colloquially shortened to ether) is an ancient concept dating back into antiquity. The spelling "Aether" is used to help avoid the inevitable confusion of its similarity to the chemical compound ether, thus the old saw, "Aether is the medium of space while ether is the gas that knocks you out". However, although it is technically incorrect, both spelling are currently used interchangeably.
Thus, in its most basic form, aether is nothing more than a distributed physical medium permeating the entire universe, endowing it [space] with measurable physical qualities. Einstein readily acknowledged this when, in 1920 (See "Sidelights on Relativity", A. Einstein, Dover Edition 1983 Page 23), he said:
"… space is endowed with physical qualities; in this sense, therefore, there exists an ether."
It is more important to point out here what aether is not required to be. Aether is not a preferred reference frame (it could be, but isn't required to be). The specific properties of the aether are not defined, as should be clear, from the above basic definition. To be a preferred reference frame, the aether and matter must be assigned certain characteristics, which is encompassed by specific models of aether. It is these specific variations and conceptual models that, once defined, can be evaluated and tested.
Contrary to popular myth, modern science isn't incompatible with, and does need, the aether concept. In fact, modern science could not be defined without incorporating some form of its definition into its foundation. Let's remember that the basic definition of aether is nothing more than a "physical medium permeating the entire universe, endowing it with measurable physical qualities". Currently, modern science uses the term "fields" or "fabric" instead of aether, since the term aether has become associated with a specific set of 19th century conceptual models considered to disproven and thus invalid.
Does the Michelson-Morley Experiment disprove Aether Theories?The answer is no. What the famous Michelson-Morley Experiment showed us is that the assumption that one could use the aether frame as a preferred frame of reference in which to measure such things as motion, is invalid. This told us more about the physical qualities of matter and electromagnetic energy, than about proof of the presence, of lack thereof, of an aether. It clearly demonstrated our conceived notions about matter (at that time) and its interactions with the aether were incorrect. There have been several distinct experiments designed specifically to define the interaction of matter/EM energy with the underlying medium. Some of the more famous are:
A light beam is split into two perpendicular component paths, A and B which are of equal length. Path A is in the direction of earth's motion and path B is at right angles to A. It was expected that, due to earth's motion, the time to travel path A would be less than the time to travel path B, and therefore, upon reconverging the light beams the frequencies would be out of phase and would of necessity exhibit an interference pattern. No such interference was noted.
A variation on the Michelson Morley experiment, a light beam is split into two perpendicular component paths, A and B and paths A and B are of different lengths. Again, no such interference was noted, ruling out length contraction alone as being an explanation for the null results of the Michelson Morley experiment.
A parallel plate capacitor was suspended from a single line, thus allowing the plate to rotate freely around the line. It was expected that the translational motion of the earth would result in a magnetic torque force on the charges resulting in the alignment of the plates parallel to the motion of the earth. No such torque force was discerned.
A light beam is split into to perpendicular component paths, A and B. Path A travels through a glass disk from south to east at 45?, from east to north at 135?, from north to west at 225?, and from west back to south at 315?. Path B travels through the disk in exactly the opposite fashion. The beams are then reconverged and an interference pattern is determined. The glass disk is then rotated on its axis. It is expected that a fringe proportional to both the angular rotation and the area enclosed by the light path would result. The results of this experiment were in complete accord with expectations and completely consistent with an aether medium.
A variation of Sagnac's experiment described above, except that this variant used the earth's rotational motion. The results measured were again consistent with expectations and an aether medium.
A balanced, freely rotating ring made from a highly dense material (such as tungsten) is centered around a compass type, inertial needle. The ring does not touch the needle. With this system in a vacuum, the ring is rotated at a high rate of speed. Since the ring is balanced and in no way touches the needle there should be no unbalanced forces acting on or through the needle and the needle would not be expected to move. However, if a physical medium pervades space, the rotation of the ring will result in a vortex forming at the center of rotation. Since the needle exists in this vortex the needle will experience rotational drag and assume the rotational speed of the ring. This was a result predicted by general relativity (dragging the inertial frame) and identified 1915.
A light beam is passed perpendicularly through a flowing water stream. Differences in the index of refraction was measured relative to stationary water. The resulting measurements were fully consistent with both relativity and the aether concept.
Aether Einstein Relativity
Is Aether compatible with Special Relativity?Yes, the general concept of aether is compatible with special relativity. However, as defined by its postulates, special relativity doesn't need to invoke a physical medium to describe the processes. Thus, as Einstein stated, an aether isn't necessary for the formal definition of SR, as formulated. H. A. Lorentz formulated an aether based version of special relativity, known as Lorentz Ether Theory [LET] which is mathematically equivalent to classical SR in all known predictions and observations.
However, to ever understand the processes that give rise to special relativity, rather that just accept the fact that it occurs, one will need to re-introduce the physical medium concept and quantify the interaction of matter/EM energy that give rise to SR's results.
Is Aether compatible with General Relativity?Yes, as acknowledged by Einstein himself in the article mentioned above (See question one). Some have argued that Einstein didn't really mean that there is a literal physical medium, only that the concept must be compatible, or that he said this out of deference to Lorentz (who was the honoree of the gathering at which this paper was presented). However, one can point to an April 1950 article published in Scientific American (Volume 182, No. 4) titled "On the Generalized Theory of Gravitation" where Einstein says much the same thing, almost word for word. On page 3 we find:
"According to general relativity, the concept of space detached from any physical content does not exist. The physical reality of space is represented by a field whose components are continuous functions of four independent variables - the coordinates of space and time."
Since these articles are 30 years removed from each other (1920 & 1950), it becomes very clear that Einstein's position on this matter was neither changed or was deferential to placate Lorentz. For further support regarding the answer to this question, also refer to the full text of an address given by Albert Einstein in 1920, at the University of Leiden.
It is now commonly accepted by most scientists that Einstein proved there could be no Aether, which does not clearly reflect his true position. Einstein tried to unite matter and the force of gravity with space and time (general relativity). He writes;
"When forced to summarize the general theory of relativity in one sentence: Time and space and gravitation have no separate existence from matter." (Albert Einstein)
Thus he rejected the particle conception of matter and tried to unite matter with Aether (Space) by representing matter as spherical fields in space-time.
"Since the theory of general relativity implies the representation of physical reality by a continuous field, the concept of particles or material points cannot play a fundamental part, nor can the concept of motion." (Einstein, 1954)
Ultimately Einstein failed to achieve his dream of a unified field theory for matter. The current paradigm assumes that particles generate fields that act on other particles. The Aether itself is largely ignored, as it is unnecessary in this formalised mathematical conception of particles and fields (electromagnetic waves are assumed to not require a medium).
Wikipedia summarises this attitude of modern physics to the Aether rather well.
"In the late 19th century the luminiferous aether (light-bearing aether) was invoked as the medium for the propagation of light, when it was discovered, from Maxwell's equations, that light is an electromagnetic wave. By analogy to mechanical waves, physicists assumed that electromagnetic waves required a medium for propagation, and hypothesized the aether. This idea of an aether has since been rejected by the vast majority of scientists."
1. http://www.SpaceandMotion.com/Physics-Albert-Einstein-Leiden-1920.htm - Albert Einstein: Leiden Lecture on Relativity and Aether, 1920