6
' Subtle is the Lord...'
6a. The Michelson-Morley Experiment
Maxwell's article Ether, written for the ninth edition of the Encyclopedia Britan-
nica [Ml], begins with an enumeration of the 'high metaphysical ... [and] mun-
dane uses to be fulfilled by aethers' and with the barely veiled criticism that, even
for scientific purposes only, 'all space had been filled three or four times over with
aethers.' This contribution by Maxwell is an important document for numerous
reasons. To mention but three, it shows us that, like his contemporaries, Maxwell
was deeply convinced of the reality of some sort of aether: 'There can be no doubt
that the interplanetary and interstellar spaces are not empty but are occupied by
a material substance or body, which is certainly the largest, and probably the most
uniform, body of which we have any knowledge'; it tells us of an unsuccessful
attempt by Maxwell himself to perform a terrestrial optical experiment aimed at
detecting the influence of an aether drag on the earth's motion; and it informs us
of his opinion that effects of the second order in v/c (v = velocity of the earth
relative to the aether, c = velocity of light) are too small to be detectable. This
last comment was prompted by his observation that 'all methods ... by which it
is practicable to determine the velocity of light from terrestrial experiments
depend on the measurement of the time required for the double journey from
one station to the other and back again,' leading to an effect at most of
0((u/c)^2 ) = 0(10-^8 ).
However, Maxwell still hoped that first-order effects might be astronomically
observable. The example he gave was the determination of the velocity of light
from the eclipses of Jupiter's satellites when Jupiter is seen from the earth at
nearly opposite points of the ecliptic. If one defines the aether* in the sense of
Maxwell, or, which is the same thing, in the sense of Augustin Jean Fresnel—a
medium in a state of absolute rest relative to the fixed stars, in which light is
propagated and through which the earth moves as if it were transparent to it—
then one readily sees that the Jupiter effect, if it exists at all, is of first order in
the velocity of the solar system relative to this aether.
*For a review of aether theories and aether models, see especially [LI] and [SI]. Some speak of
aether, others of ether. I prefer the former. In quotations I follow the predilections of the original
authors, however.Ill