There are two kinds of ethers. First: the classical ether is extremely stiff medium where light waves are propagating, similarly like sound waves are propagating in a water. It must be hugely stiff, because the speed of light is depended on the stiffness and the speed of light is quite remarkable.
There is slight problem that if ether as stiff that it allows the speed for light to be 300 Mm/s, then how on Earth there can be inertial movement around the sun! And if ether does not interact with regular matter, then how come we can see the light that is pressure waves propagating through ether? Luckily this classical ether was refuted by Einstein and his (with little help from Planck) invention of quantum theory that pointed out that actually photons are quantum particles, not waves. And as they are particles, no ether as medium for light waves is required. Second: The other kind of Ether is Newton's fixed background or preferred frame of reference. Einstein developed this idea even further when he showed with general relativity that actually ether is not fixed, but the gravity can modify the geometry of absolute frame of reference. Einstein himself called correctly his general relativity as ether theory as it is based on a idea of an absolute frame of reference. In some other instances I have promoted Lorentz's theory of relativity. That is similar ether based kinematic theory as general relativity is for accelerating frame of references. That is, the kinematic motion in Lorentz's theory of relativity is always measured in respect of ether and if we choose Earth's gravity field as preferred frame of reference, then this interpretation agrees with every empirical observations so far. Although Lorentz's theory of relativity is ether theory, it has not been disproved and it happily agrees with M&M experiment and all the time dilation observations. Therefore this latter kind of ether, where ether or preferred frame of reference is Earth's gravity field, is not refuted. –Jouni
