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It thus appears that, of all Galileian co-ordinate systems, one

(K) corresponding to a particular state of motion is physically

unique. This result was interpreted physically by regarding K as

at rest with respect to a hypothetical aether of space. On the

other hand, all coordinate systems K′ moving relatively to K were

to be regarded as in motion with respect to the aether. To this

motion of K′ against the aether ("aether-drift" relative to K′)

were attributed the more complicated laws which were supposed to

hold relative to K′. Strictly speaking, such an aether-drift

ought also to be assumed relative to the earth, and for a long

time the efforts of physicists were devoted to attempts to detect

the existence of an aether-drift at the earth's surface.