Lectures, “It is to be hoped that farther experiments will be made.”
They were. In 1887 Michelson tried again, this time with the help of the chemist Edward W. Morley. Together they constructed an interferometer far more elaborate and sensitive than the ones Michelson had used in Germany, secured it in an essentially tremor-free basement at the Case School of Applied Science in Cleveland, and set it floating on a bed of mercury for, literally, good measure. Michelson had in mind a specific number for the wavelength displacement he expected the ether would produce, and he further decided that a reading 10 percent of that number would conclusively indicate a null result. What he got was a reading of 5 percent of the displacement he thought the ether might produce—a blip attributable to observational error, if anything. Michelson found himself forced to reach the same conclusion he’d previously reported: “that the luminiferous ether is entirely unaffected by the motion of the matter which it permeates.”
“I cannot see any flaw,” said Kelvin of this experiment, in a lecture he delivered in the summer of 1900. “But a possibility of escaping from the conclusion which it seemed to prove may be found in a brilliant suggestion made independently by FitzGerald, and by Lorentz of Leiden.” Kelvin was referring to the physicists George Francis FitzGerald of Dublin, who had submitted a brief conjecture regarding the ether to the American journal Science in 1889, and Hendrik Antoon Lorentz, who in an 1892 paper and then in an 1895 book-length treatise had elaborated an entire argument along nearly identical lines: The ether compresses the molecules of the interferometer—as well as those of the Earth, for that matter—to the exact degree necessary to render a null result. In which case, the two beams of light in Cleveland actually did travel at two separate speeds, as the measurements of their multiple-mirror-deflected journeys would have shown, if only the machinery hadn’t contracted just enough to make up the difference. “Thus,” Lorentz concluded, “one would have to imagine that the motion of a solid body (such as a brass rod or the stone disc employed in the later experiments) through the resting ether exerts upon the dimensions of that body an influence which varies according to the orientation of the body with respect to the direction of motion.”
“An explanation was necessary, and was forthcoming; they always are,” the French mathematician and philosopher Henri Poincaré wrote of Lorentz in 1902 in his Science and Hypothesis; “hypotheses are what we lack the least.” Lorentz himself conceded as much. Two years later he proposed a mathematical basis for his argument while virtually sighing at the futility of the whole enterprise: “Surely this course of inventing special hypotheses for each new experimental result is somewhat artificial.”
Like other physicists at the time, Einstein thought about ways to describe the ether, as in the precocious paper he had sent to his uncle in 1895. Also like other physicists, Einstein thought about ways to detect the ether. During his second year at college, 1897–98, he proposed an experiment: “I predicted that if light from a source is reflected by a mirror,” he later recalled, “it should have different energies depending on whether it is propagated parallel or antiparallel to the direction of motion of the Earth.” In other words: the Michelson-Morley experiment, more or less—though news of that effort, a decade earlier, had reached Einstein only indirectly if at all, and then only as a passing reference in a paper he read. In any case, the particular professor he’d approached with this proposal treated it in “a stepmotherly fashion,” as Einstein reported bitterly in a letter. Then, during a brief but busy job-hunting period in 1901, after he’d left school but hadn’t yet secured a position at the patent office, Einstein proposed to a more receptive professor at the University of Zurich, “a very much simpler method of investigating the relative motion of matter against the luminiferous ether.” On this occasion it was Einstein who didn’t deliver. As he wrote to a friend, “If only relentless fate would give me the necessary time and peace!”
Like a few other physicists at the time, Einstein was even beginning to wonder just what purpose the ether served. What purpose it was supposed to serve was clear enough. Physicists had inferred the ether’s existence in order to make the discovery of light waves conform to the laws of mechanics. If the universe operated only through matter moving immediately adjacent matter in an endless succession of cause-and-effect ricochet shots—like balls on a billiard table, in the popular analogy of the day—then the ether would serve as the necessary matter facilitating the motion of waves of light across the vast and otherwise empty reaches of space. But to say that the ether is the substance along which electromagnetic waves must be moving because electromagnetic waves must be moving along something was as unsatisfactory a definition as it was circular. As Einstein concluded during this period in a letter to the fellow physics student who later became his first wife, Mileva Maric, “The introduction of the term ‘ether’ into the theories of electricity led to the notion of a medium of whose motion one can speak without being able, I believe, to associate a physical meaning with this statement.”
The problem of the ether was starting to seem more than a little familiar. It was, in a way, the same problem that had been haunting physics since the inception of the modern era three centuries earlier: space. To be precise, it was absolute space—a frame of reference against which, in theory, you could measure the motion of any matter in the universe.
For most of human history, such a concept would have been more or less meaningless, or at least superfluous. As long as Earth was standing still at the center of the universe, the center of the Earth was the rightful place toward which terrestrial objects must fall. After all, as Aristotle pointed out in establishing a comprehensive physics, that’s precisely what terrestrial objects did. An Earth in motion, however, presented another set of circumstances altogether, one that—as Galileo appreciated—required a whole other set of explanations.
Nicolaus Copernicus wasn’t the first to suggest that the Earth goes around the sun, not vice versa, but the mathematics in his 1543 treatise De revolutionibus orbium coelestium (On the Revolutions of Celstial Orbs) had the advantage of being comprehensive and even useful—for instance, in instituting the calendar reform of 1582. Still, for many natural philosophers its heliocentric thesis remained difficult, or at least politically unwise, to believe. Galileo, however, not only found it easy to believe but, in time, learned it had to be true because he had seen the evidence for himself, through a new instrument that made distant objects appear near. His evidence was not the mountains on the moon that he first observed in the autumn of 1609, though they did challenge one ancient belief, the physical perfection of heavenly bodies; nor the sight of far more stars than were visible with the naked eye, though they did hint that the two-dimensional celestial vault of old might possess a third dimension; not even his January 1610 discovery around Jupiter of “four wandering stars, known or observed by no one before us,” because all they proved was that Earth wasn’t unique as a host of moons or, therefore, as a center of rotation. Instead, what finally decided the matter for Galileo was the phases of Venus. From October to December 1610, Galileo mounted a nightly vigil to observe Venus as it mutated from “a round shape, and very small,” to “a semicircle” and much larger, to “sickle-shaped” and very large—exactly the set of appearances the planet would manifest if it were circling around, from behind the sun to in front of the sun, while also drawing nearer to Earth.
Galileo’s discovery of the phases of Venus didn’t definitively prove the existence of a sun-centered universe. It didn’t even necessarily disprove an Earth-centered universe. After all, just because Venus happens to revolve around the sun doesn’t mean that the sun itself can’t still revolve around Earth. But such a contortionistic interpretation of the cosmos—a Venus-encircled sun in turn circling Earth—had nothing to recommend it other than an undying allegiance to Earth’s central position in it. And so “Venus revolves around the Sun,” Galileo finally declared with virtual certainty, in a letter he wrote in January 1612 and published the following year, “just as do all the other planets”—a category of celestial object that, he could now state with a confidence verging on nonchalance, included the heretofore terrestrial-by-definition Earth.
An Earth spinning and speeding through space, however, required
