become reconciled to this negative result, for reasons now to be
discussed. The inherited prejudices about time and space did not allow
any doubt to arise as to the prime importance of the Galileian
transformation for changing over from one body of reference to another.
Now assuming that the Maxwell-Lorentz equations hold for a
reference-body _K_, we then find that they do not hold for a
reference-body _K _ moving uniformly with respect to _K_, if we assume
that the relations of the Galileian transformation exist between the
co-ordinates of _K_ and _K _. 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 ther 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 ther. To this
motion of _K _ against the ther ( ther-drift relative to _K _) were
attributed the more complicated laws which were supposed to hold
relative to _K _. Strictly speaking, such an ther-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
ther-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 ther. It is found by calculation, however, that a
slightly different time _T _ is required for this process, if the body,
together with the mirrors, be moving relatively to the ther. And yet
another point: it is shown by calculation that for a given velocity _v_
with reference to the ther, this time _T _ 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 ther produces a
contraction of the body in the direction of motion, the amount of
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