Tuesday, March 08, 2005

Ether - a null result, or an anulled result

Anyone who has studied the history of science is familiar with the seminal tale from the heroic age of physics of the Michelson-Morley experiment to establish whether or not light travels through a mysterious medium that fills space -- the ether.  Every high school physics student has taken down the same notes for more than a century.

1880, US physicist Albert Michelson invents the Michelson light interferometer -- an instrument that can measure the velocity of a beam of light with great accuracy by splitting it through a half-silvered mirror and then re-combining the beams.  If the recombined beams interfere with reach other, causing visible fringes on a screen, then one of them must have been delayed.

1887, Michelson and fellow American scientist Edward Morley build an interferometer with greater accuracy than ever before and use their instrument in a crucial experiment to determine whether light travels through the ether, or merely through the vacuum of empty space. The two physicists set up their instrument to measure the speed of a beam of light travelling in the same direction as the earth through space, and also a beam that travels at right angles to the earth’s direction of travel. If the ether exists there should be a minute – but measurable – drag effect on a beam of light that will delay it and show up as ‘interference fringes’ in the interferometer.  The experiment shows a 'null result' -- no matter how the interferometer is orientated with respect to the earth's movement, there is no measurable ether drag.


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