by Prof. Dayton C. Miller
(Cleveland Plain Dealer, 10 March 1940, "All
Feature Section" p.1 & 6)
The wave
theory of light received general acceptance in the 19th century, and this
required the hypothesis of a luminiferous ether, filling all space, even that
occupied by material bodies, through which bodies such as the sun and planets
move freely without disturbing the other.
Scientists sought to prove the existence of the ether by some direct
experiment, and in 1878 Maxwell of Cambridge University, England, first
proposed an optical experiment for this purpose.
It is assumed
that the ether as a whole is at rest, that light waves are propagated in the
free ether in any direction and always with the same velocity. The experiment
is based upon the argument that if the speed of light were determined by an
observer on the moving earth, the apparent speed would be different according
to whether the observer is moving in the line which the light is travelling or
at right angles to this line. The
detection of such an effect, would be accepted as proof of the existence of the
ether.
An
incomplete analogy is given by a power boat on a river, the speed of which
would be different when sailing down stream from the speed with the same power
when sailing across the stream. Not
only would the speed across stream be slower, but in order to reach a point
directly opposite the starting point, the boat would have to be headed somewhat
upstream.
The late
Prof. A.A. Michelson accepted the challenge of Maxwell's suggestion, and in
1881 devised the remarkable instrument called the interferometer, by means of
which it is possible to compare the speed of a beam of light which travels in
the line of motion of the earth in space, directly with the speed of a beam at
right angles to this motion, by detecting the difference in the two speeds. In the interferometer a beam of light is
literally split in two by a "half-silvered" mirror, and the two beams
may be made to travel paths at right angles to each other. At the end of the desired path, each beam is
reflected back upon itself and the two come together where they first
separated.
If the two
beams travel equal paths with equal speeds, the reunited beams of light will
blend with the waves of concordance. If, however, the speeds are different in
the two paths, the two beams come together with differences in the phases of
the waves, producing effects which are observed as the "interference
fringes".
The
interferometer enables one to detect exceedingly small differences in the
relative speeds of the light in the two paths, the measurements being made in
terms of the wave lengths of light.
Michelson's first apparatus proved inadequate as to sensitivity and
stability.
He came to
Case School of Applied Science in 1882 as the first professor of physics, and
became associated with Prof. Edward W. Morley of Western Reserve
University. Prof. Morley proposed
several important developments in the interferometer. The light path was increased in lengthy by multiple reflections,
and the optical parts were mounted on a stone block five feet square and one
foot thick. The stone was floated on a
large basin of mercury so that it could be easily rotated to bring one of the light paths of the interferometer
into line with the earth's movement in space.
With this
instrument, the famous "Michelson-Morley Ether-Drift Experiment" was
performed in Cleveland, in July 1887.
Again the results were inconclusive and the instrument was of
insufficient sensitivity for the delicate measurements.
Prof.
Michelson left Case School in 1889. In
1890 Dayton C. Miller entered the faculty and later became associated with
Prof. Morley in a repetition of the ether-drift experiment on a still larger
scale. They constructed an
interferometer about four times as sensitive as the one used in the earlier
experiment, having a light path of 214 feet, equal to about 112,000,000 wave
lengths.
Observations
were made with this apparatus in 1904.
The results were expressed as follows:
"If
the ether near the apparatus did not move with it, the difference in velocity
was less than 3.5 kilometers a second, unless the effect on the materials
annulled the effect sought. We desire to place the apparatus on a hill, covered
only with transparent covering, to see if an effect can be there
detected."
It was at
this time that Einstein became interested; and in November 1905 he published a
paper on "The Electrodynamics of Moving Bodies". This paper was the first of a long series of
papers and treatises by Einstein and others which has developed into the
present theory of relativity.
In the
first paper, Einstein makes the postulate that for an observer on the moving
earth the measured velocity of light must be constant regardless of the
direction of amount of earth's motion.
The whole theory was related to physical phenomena, largely on the
assumption that the ether-drift experiments of Michelson, Morley and Miller had
given a definite and exact null result.
The
deflection of light from the stars by the sun, as predicted by the theory of
relativity, was put to the test at the time of the solar eclipse of 1919. The results were widely accepted as
confirming the theory. This revived the
writer's interest in the ether-drift experiments, the interpretation of which
had never been acceptable to him.
The site of
the Mount Wilson Observatory, near Pasadena, Cal., at an elevation of about
6,000 feet, appeared to be a suitable place for further trials. An elaborate program of experimentation was
prepared.
When
observations are being made, the apparatus is kept in rotation on the mercury
float, and the observer must walk around in a circle about twenty feet in
diameter, keeping his eye at the moving eyepiece of the telescope attached to
the interferometer. The observer must
not touch the interferometer in any way and yet he must never lose sight of the
interference fringes which are seen only through the small aperture of the
eyepiece, about a quarter of an inch in diameter.
A set of
observations involve twenty turns of the apparatus and several hundred single
"readings" and occupies about eighteen minutes of time. The Mount Wilson series of observations
involves more than 200,000 individual interferometer readings, being more than
twice as many readings as had been made in all the ether-drift observations previous
to this time. This required that the
observer should walk in the dark, in a small circle, for a total distance of
190 miles.
In 1933 the
writer published the complete study of all the ether-drift observations with
conclusions which may be summarized as follows:
"A
relative motion of the earth and the ether should produce an effect which, as
observed in the interferometer, would vary both in magnitude and direction as
the earth rotates on its axis and as it revolves in its orbit; the effect
further depends upon the latitude of the station of observation. The actual observations indicate an ether
drift of ten kilometers per second which varies in a manner wholly consistent
with the theoretical requirements; however, the observed velocity of the drift
is smaller than had been expected, as though the ether through which the
interferometer is being carried by the earth's motion was not absolutely at rest."
"A
comparison of the effect of the orbital motion of the earth as observed in the
interferometer with its known value leads to the conclusion that the absolute
motion of the solar system in space has a velocity of 208 kilometers per
second. This cosmical motion is towards
an apex located in the southern constellation Dorado, the Sword Fish, in the midst
of the Great Magellanic Cloud of stars."
"In
order to account for the results here presented, it seems necessary to accept
the Lorentz-Fitzgerald theory of the contraction of matter moving through the
ether, or to postulate a viscous or dragged ether as proposed by Stokes."