How Luminiferous Aether Led to Relativity

**SPEAKER_1** (0:00)
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**Matt O'Dowd** (1:00)
As the 19th century came to a close, physicists were feeling pretty satisfied with the state of their science. The great edifice of physical theory seemed near complete. A few minor experiments remained to verify everything, but little did those physicists know that one of those experiments would bring the entire structure crashing down, paving the way for the physics revolutions of the 20th century.
In 1894, the eminent physicist Albert Abraham Michelson described the state of his science like this.
It seems probable that most of the grand underlying principles have been firmly established, and that further advances are to be sought chiefly in the rigorous application of these principles to all the phenomena which come under our notice. Michelson had lived this principle. In 1887, he and fellow American Edward Morley attempted to verify the existence of one of the oldest concepts in physics. They attempted to measure the luminiferous ether, the invisible or pervasive medium through which light was thought to propagate. Their experimental design was so ingenious it couldn't possibly fail.
And yet, it did fail. Fail to detect the ether, at least. But in so doing it succeeded in cracking the supposedly unassailable foundations of 19th century physics. The death of the ether helped open the way for the acceptance of Einstein's theory of relativity. So today we're going to celebrate the life and death of an idea.
As with so many ideas, the ether was born in the minds of the ancient Greeks. For them, it was the air breathed by the gods and was embodied as a primordial deity of light. In Aristotle's cosmology, earth, air, fire and water are the physical elements of the world, while ether is the immutable and indestructible element that fills space and in crystalline state forms celestial bodies. That's right, ether is the fifth element. Big butter boom. Medieval alchemists were all about the ether, also called quintessence. They believed this spiritually pure element had miraculous capabilities of transmutation, and in some cases that it could be extracted by distilling huge quantities of human urine. It turned out that was phosphorus. Still useful I guess. Fast forward to the mid-17th century, French philosopher René Descartes asserted that there could be no such thing as empty space. And Aristotle's ether therefore both filled and gave reality to the space between the celestial bodies. He imagined the ether as flowing, moving in vortices. These flows carried the celestial bodies, for example, in circles around the sun. The Dutch physicist Christiaan Huygens developed the idea of the universe, and the idea of the universe as a whole.

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**Matt O'Dowd** (4:48)
Huygens developed a detailed mathematical theory around this idea that gravity resulted from the fluid dynamics of the ether. But Huygens is most famous for his wave theory of light. By thinking of light as a wave, he was able to build a theory of optics, explaining how light refracted, bent in its path between different substances. And in Huygens' mind, that theory also required the ether in order to make sense. Consider a wave on a string. Each string segment moves up and down only, tugging on neighboring segments so that a sinusoidal pattern flows forwards, while the segments themselves just oscillate in place. In sound waves, air molecules oscillate back and forth to propagate a pattern in the density of the air.