In the year 1851 cultivated persons in cities throughout Europe went to the largest cathedrals to attend an unusual sort of worship. They were coming to witness Jean Foucault’s pendulum experiment, which he had first performed for the public in that year under the dome of the Pantheon in Paris.
From the highest point in the cathedral a heavy weight hung suspended on a thin rope, so that it was free to swing in all directions. it was given a push in a northerly direction, and began to swing in a north-south line. It continued to swing for days, but ever so slowly the direction of its swing shifted. And it continued to do so visibly. Those who waited long enough were able to see the plane of the pendulum’s swing turn in a full circle in the course of a day.
Actually, however, the plane of oscillation had not change d at all. A pendulum retains the direction of its original motion, as stated by Galileo’s law of inertia. Thus the pendulum provided visible proof of Copernicus’ doctrine: the Earth was turning underneath the swinging pendulum.
How unfortunate it was that Galileo did not notice this when he observed the chandelier swinging in the Duomo at Pisa. He would have been spared his troubles with the Inquisition; such tangible proof of the Earth’s rotation would have silenced all doubts.
Nevertheless, the Frenchman’s ingenious experiment stimulated other ideas, for which the times and the place were ready: ideas on one of the fundamental problems of both philosophy and religion.
Originally Newton had asked himself whether absolute movement existed in the universe, that is to say, movement in itself which we could determine without reference to other movements. His answer was that there was only one such motion: the rotation of the Earth. Ultimately, he maintained, we would have discovered this motion even if there had not been a sky full of stars circling about the polestar above our heads. Even without the polestar we would have found the flattened poles of our globe; we would have understood that they had been caused by the distorting effect of centrifugal force.
From this reasoning Newton drew a profound conclusion. If we imagine our universe with no other bodies beside the Earth, there must still be something to which we can refer the motion of the Earth, something that is at rest in relation to the Earth. Absolute motion presupposes something absolutely at rest. Only space can be this something. Hence, space ceases to be solely a philosophical concept, a mere word; it must have physical existence, for all that its only characteristic is being at rest. This idea of something at rest, ubiquitous, absolutely fixed, suggested the attributes of the Supreme Being; physical space of its own accord intruded itself into the sphere of religion.
Similar reflections may have occurred to the spectators who witnessed the Foucault experiment in 1851. The rotation of the Earth, absolute motion, the existence of space and hence of divine ubiquitousness-they were being granted ,an actual demonstration of these concepts, not only an intellectual apprehension such as Newton had had.
Ernst Mach, who was a boy of thirteen in 1851, was destined to expose the fallacy of the experiment and demolish the Newtonian argument. Mach was a scientist possessed of an unusual need to unify scientific thought. He challenged Newton’s ideas with the Newtonian principle that no superfluous assumptions ought to be made. Among such superfluous assumptions he considered the “persistence in a straight line,” which held Foucault’s pendulum to its original plane of vibration, and even “centrifugal force,” which everyone could personally experience by twirling a stone on a string. These concepts were needless, he maintained; they could be eliminated, and everything explained by the law of universal gravitation. The force that flattened out the Barth and held Foucault’s pendulum to its plane of vibration, so that the Earth turned underneath it, thus betraying its rotation-this force was nothing less than the attraction of all the bodies in the universe, stars and remote galaxies: in short, the total gravitation of the universe. Undoubtedly the Earth turns, Mach argued, but we are not logically forced to regard its rotation as absolute; we may also take it to be relative to the totality of matter in the universe. Inexplicable centrifugal force, the equally mysterious persistence in a straight line, and even Galileo’s inertia, could be dropped. All these fundamental concepts of physics could be explained as effects of a single universal force, gravitation.
Newton himself might have assented to this overwhelming simplification of the mechanism of the universe; he probably would have done so under protest, but bowing to his own principles of reasoning. The simpler explanation must always be accepted in preference to one more complex. Ernst Mach was ennobling the universe by reducing all motion to a single principle. But if the Barth’s rotation were considered in reference to the totality of matter in the universe, if absolute motion were thus eliminated, then absolute space could no longer be demonstrated; the factor of rest and ubiquity would have to be put aside, and therewith one religious value that Newton had wrested from physics. There was, however, a second logical avenue one might take in order to establish absolute motion in an absolute: the motion of light in the ether. .
Once again it was Foucault who supplied the means for testing this possibility. He succeeded in measuring the velocity of light in the laboratory. In order to appreciate this achievement, we must remember that light can rush around the globe seven times in a second. How can it possibly be timed over a reasonable distance? Foucault set up a tiny mirror to revolve around an axis 800 times a second, so that it tore an impinging ray of sunlight and scattered fragments of light. One such fragment was caught by a concave mirror; the time it took to go and come was measurable. The velocity of light proved to be very nearly 186,000 miles per second, with a possible error of only one per cent.
With this figure to work with, it was hoped to prove the existence of the motionless ether. The Earth not only revolves around its axis, but moves around the Sun at a velocity of 18.5 miles per second. Hence, a ray of sunlight ought to move 18.5 miles per second faster when measured along the Barth’s orbit than when measured perpendicularly to it, because the Earth’s speed would be added to that of the light.
Such an experiment would not only prove conclusively the Barth’s revaluation around the Sun (if this were not proved by other methods), but would also demonstrate the existence of something at rest in the universe, the “luminiferous ether.” The test of this concept, the most important experiment of modern times, was undertaken by A. A. Michelson. His first attempts, with apparatus so sensitive that the footstep of a passer-by upset it, yielded a negative result. In 1887, working with Edward W. Morley, he perfrmed the experiment with such accuracy that the final negative result was without question: the sought-after change in the velocity of light did not occur. The Michelson- Modey experiment showed that the speed of light is independent of the motion of the observer and the motion of the source of light.
Consequently, no ether at rest, no ether at all, existed. The physicists were not sorry; they had never felt at ease with this clumsy concept. But when the ether vanished, there vanished with it the last prospect for proving that Newton’s absolute space existed. And physics was promptly confronted with a new set of problems. if ether did not exist, how could light be transmitted in the form of waves? And was it conceivable that the velocity of light was entirely independent of the Earth’s velocity, that the two sums could not be added?
No solution to the second problem was needed.
A young mathematician in Zurich proposed giving up .all attempt at explanation, accepting the fact as inherent in nature, and raising it to the status of an axiom of physics.
The velocity of light, and it alone, would then be the sought for absolute, he posited. Everything else, motion, space, even time, must be regarded as relative, no matter what conceptual difficulties such a position gave rise to.
Accordingly, the only absolute we can cling to is not something at rest, but something in motion-the fastest-moving thing we know. Here was a hypothesis fully in consonance with our modern urge to see movement in everything -the very ultimate conclusion of that impulse, as it were.
The young mathematician who enunciated it was named Albert Einstein.