Details of the experiment

Michelson and Morley had the clever idea of applying the 'timing the ferry' principle to light travelling through the imaginary ether. Their apparatus (now known as the Michelson interferometer) has already been described in Chapter 8, but is briefly described here for convenience.

A light beam is 'split' into two by a semi-silvered mirror. The two beams subsequently travel at right angles to each other. Each beam is reflected at a second mirror and returns to the semi-silvered mirror, where the beams recombine.

Suppose that the ether wind is blowing from left to right so that the light which travels via mirror 2 makes the round trip of ferry 2, downwind and against the wind, while the route via mirror 1 taken by the other half of the beam is a 'crosswind' route in both directions (Figure 15.4). If there is a time difference between the two journeys, there will be a phase difference between the two components of the recombined beam, and an interference pattern will be seen at the detector.

Michelson and Morley then rotated their apparatus slowly. They expected the phase difference to change as the crosswind route gradually became the downwind route. As the apparatus rotated, the fringes would then move across the null mark. They tried the experiment at different times during the day and night source

light from mirror 1

semi-silvered

mirror 2

recombined light

Figure 15.4 Michelson interferometer.

earth and sun move through the ether earth and sun move through the ether axes of the ether frame

Figure 15.5 The earth moves through the ether, which is stationary and provides an absolute frame of reference.

z and repeated on many occasions throughout the year. No fringe shift was ever observed, and thus the existence of the ether was not proven.

Note that any rotation should move the fringe pattern, so it is not necessary to know the direction of the ether wind to do the experiment.

The orbital velocity of the earth about the sun is 30 kms-1.

The sensitivity claimed in the original Michelson-Morley experiment ~ 7-10 kms-1.

The sensitivity of modern experiments (with X-rays) ~ 3 ms-1 (jogging speed).

If there is an ether, why is there no detectable ether wind?

Einstein asked why it should be necessary to fill space with integral numbers, with an imaginary ether, or with anything else, for that

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