Preferred frame

In theoretical physics, a preferred or privileged frame is usually a special hypothetical frame of reference in which the laws of physics might appear to be identifiably different (simpler) from those in other frames.

In theories that apply the principle of relativity to inertial motion, physics is the same in all inertial frames, and is even the same in all frames under the general principle of relativity.

In theories that presume that light travels at a fixed speed relative to an unmodifiable and detectable luminiferous aether, a preferred frame would be a frame in which this aether would be stationary. In 1887, Michelson and Morley tried to identify the state of motion of the aether. To do so, they assumed Galilean relativity to be satisfied by clocks and rulers; that is, that the length of rulers and periods of clocks are invariant under any Galilean frame change. Under such an hypothesis, the aether should have been observed.

By comparing measurements made in different directions and looking for an effect due to the Earth's orbital speed, their experiment famously produced a null result. As a consequence, within Lorentz aether theory the Galilean transformation was replaced by the Lorentz transformation. However, in Lorentz aether theory the existence of an undetectable aether is assumed and the relativity principle holds. The theory was quickly replaced by special relativity, which gave similar formulas without the existence of an unobservable aether. All inertial frames are physically equivalent, in both theories. More precisely, provided that no phenomenon violates the principle of relativity of motion, there is no means to measure the velocity of an inertial observer with regard to a possible medium of propagation of quantum waves.

...
Wikipedia