Main article: Luminiferous aether
The basic idea of the æther as a physical
transmission medium is simple, and like all media,
if it exists, must have fundamental properties including a
pressure,
mass density, and
temperature. Further, if compressible, it will also exhibit a characteristic finite propagation speed,
c, at which all transfer of
momentum and
energy through it can be carried from one physical location to another. Compressibility also means that there will also be a distinct coefficient of
compressibility (and its inverse, a distinct
modulus), a characteristic
impedance, and the ability to create and sustain
wave activity. Any other properties, including
ponderable matter and the specific characteristics of waves are solely dependent upon specifics arising from these basics.
Further Einstein whose theory didn't require an ether revisited it:
Such a view, however, contradicts the continuum concept of space-time and fields and Einstein's statements in "Aether and the Theory of Relativity", May 5th, 1920:
"
More careful reflection teaches us, however, that the special theory of relativity does not compel us to deny ether." and "
To deny the ether is ultimately to assume that empty space has no physical qualities whatever".