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 æther of space.  On the other hand, all coordinate systems K1 moving relatively to K were to be regarded as in motion with respect to the æther.  To this motion of K1 against the æther ("æther-drift " relative to K1) were attributed the more complicated laws which were supposed to hold relative to K1.  Strictly speaking, such an æther-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 æther-drift at the earth’s surface.

In one of the most notable of these attempts Michelson devised a method which appears as though it must be decisive.  Imagine two mirrors so arranged on a rigid body that the reflecting surfaces face each other.  A ray of light requires a perfectly definite time T to pass from one mirror to the other and back again, if the whole system be at rest with respect to the æther.  It is found by calculation, however, that a slightly different time T1 is required for this process, if the body, together with the mirrors, be moving relatively to the æther.  And yet another point:  it is shown by calculation that for a given velocity v with reference to the æther, this time T1 is different when the body is moving perpendicularly to the planes of the mirrors from that resulting when the motion is parallel to these planes.  Although the estimated difference between these two times is exceedingly small, Michelson and Morley performed an experiment involving interference in which this difference should have been clearly detectable.  But the experiment gave a negative result —­ a fact very perplexing to physicists.  Lorentz and FitzGerald rescued the theory from this difficulty by assuming that the motion of the body relative to the æther produces a contraction of the body in the direction of motion, the amount of contraction being just sufficient to compensate for the differeace in time mentioned above.  Comparison with the discussion in Section 11 shows that also from the standpoint of the theory of relativity this solution of the difficulty was the right one.  But on the basis of the theory of relativity the method of interpretation is incomparably more satisfactory.  According to this theory there is no such thing as a " specially favoured " (unique) co-ordinate system to occasion the introduction of the æther-idea, and hence there can be no æther-drift, nor any experiment with which to demonstrate it.  Here the contraction of moving bodies follows from the two fundamental principles of the theory, without the introduction of particular hypotheses ; and as the prime factor involved in this contraction we find, not the motion in itself, to which we cannot attach any meaning, but the motion with respect to the body of reference chosen in the particular case in point.  Thus for a co-ordinate system moving with the earth the mirror system of Michelson and Morley is not shortened, but it is shortened for a co-ordinate system which is at rest relatively to the sun.