Michelson-Morley experiment

In 1887, the Michelson-Morley experiment, one of the most famous and
important experiments in the history of physics, was performed.

Physics theory of the late 19th century postulated that, just as water waves
and audible sound waves require media (water and air, respectively) in which
to move, so light waves required a medium (called the "luminiferous aether")
to travel across space. However, given that the speed of light is so great,
it took some thought to design an experiment that might detect the presence
and properties of this aether.

Albert Abraham Michelson and Edward Morley took the approach of measuring 
the relative speed at which the Earth was passing through the aether. They 
reasoned that, if the luminiferous aether was real, the Earth would at all 
times be moving through it like a plane through the air, and producing 
a detectable "aether wind".

The Earth travels a tremendous distance each year in its journey around the
sun, at a speed of over 100,000 km per hour, or around 30 km/second. In
addition, as the Earth moves around the Sun in its orbit, the direction of
the "wind" relative to the a fixed star's position as measured in an
Earth-based laboratory would also vary, making the effect easier to detect;
so the experiment would be carried out at various different times of year.
This would also help separate out any effects that might arise from the
"wind" caused by the motion of our Sun as it travels through space.

The effect of the aether wind on light waves would be like the effect of a
strong current in a river on a swimmer, moving at a constant speed, back and
forth between two points.

If the second point is directly up stream of the first point, the swimmer
will be slowed down by the current on the way from the first to the second
point; and similarly will be sped up on the return trip.

On the other hand, if the line between the starting and ending points is at
right angles to the current, then in both directions, the swimmer will have
to compensate by swimming at a slight angle to his desired goal.

The cumulative round trip effects of the current in the two different
orientations result slightly in favor of the swimmer travelling at right
angles to the current. Similarly, the effect of an "aether wind" would be
that it would take slightly longer for a beam of light to travel round-trip
in the direction parallel to the aether wind than it would for a beam of
light to travel the exact same round-trip distance at right angles to the wind.

The key word here is "slightly". Over a distance on the order of a few
meters, the difference between the time taken for the two round trips would
be exceedingly small - on the order of a millionth of a millionth of a
second. Fortunately, Michelson had already spent a great deal of time and
thought on the problem of measuring the speed of light, and had developed
several techniques for measuring differences of this type.

Michelson and Morley set up what has come to be known  [Interferometer.png]
as a Michelson interferometer. The Michelson-Morley
experiment was performed in the basement of a stone building close to
sea-level. A half-silvered mirror was used to split a beam of monochromatic
light into two beams travelling at right angles to each other. After leaving
the splitter, the beams were each reflected by a mirror, and recombined,
producing a pattern of constructive and destructive interference. Any slight
change in the amount of time the beams spent in transit would then be
observed as a change in the pattern of interference.

To account for possible imperfections in the construction of the equipment,
the entire device was placed on a rotating bed, so that it could be rotated
through the entire range of possible angles to the aether wind.

The most famous failed experiment

After all this thought and preparation, it is ironic that the experiment
became "the most famous failed experiment of all time". Instead of providing
insight to the properties of the aether, the experiment instead produced
none of the effects which would have been expected if the Earth's motion
produced an "aether wind"; instead the apparatus behaved as if there were no
wind at all - as if the Earth had no motion at all with reference to a medium.

This result was rather astounding, as it was unexplainable by the
then-current theory of wave propagation. Several explanations were given;
among them, that the experiment had some hidden flaw (this was apparently
Michelson's initial belief), or that the Earth's gravitational field somehow
"dragged" the aether around with it in such a way as to locally eliminate
the effect.

Ernst Mach was among the first physicists to suggest that the experiment
actually amounted to a disproof of the aether theory. Developments in
theoretical physics had already begun to provide an alternate theory,
Fitzgerald-Lorentz contraction, which explained the null result of the
experiment. The development of what later became Einstein's Special Theory
of Relativity provided a complete explanation which did not require an
aether, and was consistent with the results of the experiment.

Michelson was never quite convinced of the non-existence of the aether, and
performed several more accurate versions of the experiment until his death
in 1931. Morely was also not convinced of the non-existence of the aether,
and he went on to conduct experiments with Dayton Miller. In 1932 the
Kennedy-Thorndike experiment modified the Michelson-Morley experiment by
making the path lengths of the split beam unequal, which would have made the
Fitzgerald-Lorentz contraction hypothesis unable to explain the null result.

The Trouton Noble experiment is regarded as the electrostatic equivalent of
the Michelson-Morley optical experiment.