Michelson-Morley experiment

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 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.