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Shattering the `ether' theory

WHEN Isaac Newton speculated about an all-pervading ether, he didn't realize how futile it was. It would be another couple of centuries before the most important ``unsuccessful'' experiment in history would show there is no such thing. First carried out in April 1887 at what is now Case Western Reserve University, that experiment is the other important science anniversary celebrated this year.

Two Case professors - Albert A. Michelson (physics) and Edward W. Morley (chemistry) - tried to demonstrate Earth's motion through the ether. Instead, they shattered the contemporary scientific mind-set by discovering that the speed of light relative to Earth is unaffected by the planet's motion.

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It wasn't supposed to work out that way. Physicists were confident that an invisible, unfelt substance called ``the ether'' filled the universe. It was the medium that carried light waves. Thus, just as the speed of ocean waves relative to a ship depends on how the ship moves relative to the waves, so too the speed of light measured in a laboratory should depend on our planet's motion through the ether.

Michelson and Morley made highly precise measurements of light speed at various directions to Earth's motion and found no variation at all. Conviction died hard. The great Victorian physicist Lord Kelvin declared, ``One thing we are sure of, and that is the reality and substantiality of the luminiferous ether.''

But repetitions of the experiment found only an invariant light speed. Albert Einstein would later show why physicists should have expected this.

The fact that light speed is the same for all observers regardless of their relative motion became a key factor in Einstein's relativity theory. That theory rests on the postulate that basic physical laws must be the same for all observers, regardless of their state of relative motion. Thus only absolute quantities, which are the same for all observers, such as light speed, can be ingredients of valid physical law.

As Nick Herbert, a consulting physicist and science philosopher, notes:

``The luminiferous ether - a body that's `standing still in space' - is a manifestly [non-absolute] concept because it is standing still for only one observer. ... [it] can never enter into any correct physical law.'' He adds, ``The ether is a reality that failed.''

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