If you have ever travelled by air you will have seen that on the noses of most airliners there are two or more small tubes facing forward, like forward-firing peashooters.
These are Pitot tubes and are used to measure the aircraft’s air speed: the speed with which the plane is moving through the air. In 1887 Albert Michelson and Edward Morley set out to measure the Earth’s “aether speed,” the speed the Earth is moving through the aether.
Space is very nearly empty, and it puzzled scientists for a long time as to how light manages to pass through it. They knew that sound waves are carried by the air and water waves by water; so what carries light waves through space? No suitable material could be found, so the idea grew that the universe is filled with “aether”, an otherwise undetectable substance that vibrates and carries light waves. Michelson and Morley set to measure the Earth’s aether speed.
The Earth is moving round the Sun at around 110,000 kilometres an hour. If we look at a star lying ahead of the Earth, its light should reach us at the speed with which the light moves through the aether plus the speed with which the Earth is moving in the opposite direction. If we look at a star behind us, its light should be reaching us at the speed the light travels through the aether minus 110,000 km/h. If the Solar System and our galaxy are also moving through the aether, there would be some other speed differences depending upon the direction in which we are looking. So Michelson and Morley set out to measure the differences in the speed light passed through their laboratory for different directions. The result they got was rather sensational and led to their experiment becoming famous, because they got no result. There was no difference between their measurements, no matter which directions they looked. It was deemed very unlikely the aether was moving along with the Earth. This would make us rather special in the universe. The other possibility was that the idea of the aether was wrong, and that it does not exist. However, part of the explanation was already out there. In 1865 James Clerk Maxwell showed that electromagnetic waves, such as light or radio waves, need no medium to carry them. The aether was redundant.
However, that left another problem. If you throw an apple core out of the window of a car moving at 80 km/h and that core hits the windshield of a car moving in the opposite direction at 80 km/h, that apple core will hit at a relative speed of 160km/h, which is just one of the many reasons you should not try this experiment. So if light is not carried by something, measurements of the speed of light should depend on the speed and direction of the thing producing it and the speed and direction of the laboratory making the measurement. However, as Michelson and Morley found, that is not the case either. There was no difference in the measurements of the speed of light no matter how the light source or laboratory were moving.
Albert Einstein explained all this by suggesting that space and time are not absolute things that everything in the universe experiences identically. Everyone has their own experience of space and time depending on their environment and how they are moving. This distortion of space and time makes light always seem to be moving at exactly the same speed, for all observers, at all locations in the universe. Nobody is unique. Moreover, the more experiments we do, on Earth or in space, the more we confirm what Einstein suggested. On 22 September the Sun will cross the equator heading south. It will rise almost exactly in the east, set almost exactly in the west and the daylight and night time hours will be the same: 12 hours each. This is the autumn equinox.
Saturn lies low in the southwest during the evening. Venus and Mars are low in the east before dawn, with Jupiter low in the dawn glow. The Moon will be Full – and eclipsed – on the 27th.
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory, Penticton.