The Speed of Light: Constancy and Cosmos

2. Special Relativity

Space, Time, and Light Speed Constancy · 15 As Newton developed his ideas, he dismantled Aristotle’s theory of motion and erected new categories of thought. It no longer made sense to talk of natural and unnatural (forced) motion because these implied sentience and innate preference on the part of material bodies. One could, however, distinguish between inertial (uniform) and non-inertial (non-uniform) motion. Bodies moved inertially in the absence of outside forces; when forces impinged on them, noninertial or accelerated motion occurred—as evidenced by changes in their states of motion. One surprising aspect of this new distinction was the equivalence of inertial motion and rest. Newton realized that since inertial motion is force-free, there is nothing to set it apart from rest. A person moving inertially could not detect that motion. She would experience nothing (no force) to suggest motion—no wind or jostling. She might, of course, see things passing by, but since every experiment performed within her reference frame yields a result exactly like that obtained from a stationary platform, she has no way of demonstrating that her motion is real rather than apparent. But what counts as a stationary platform? Having lost the stationary earth, Newton identified a new benchmark: he equated space with rest and thereby turned space into a universal rest frame: “Absolute space, in its own nature, without relation to anything external, remains always similar and immovable.”7 So defined, space became the stationary backdrop against which the motion of objects could be precisely charted, at least in principle. The scientific value of this backdrop can hardly be overstated. Something in the world must be absolutely fixed—otherwise distinctions between real and apparent inertial motion become impossible. But Newton’s space was not quite like Aristotle’s fixed earth. The earth engages the senses, and no one can deny its brute existence. Space, by contrast, is not part of our sense experience; it is, at best, the conspicuous absence of sense data. What this means is that although space may be idealized as a stationary backdrop against which to measure motion, no such measurement is possible. One cannot take space and mark it out as a coordinate system. Newton recognized this drawback while yet holding to the prop­ osition that “absolute space,” in principle if not in practice, allows scientists to coordinate their experience with God’s. Newton aspired to a God’s-eye perspective, and he imagined that God’s 16 · The Speed of Light vantage point on space afforded Him faultless understanding of motion with­ in the universe. And while it was true that space offered no sense data to the human mind, it was possible that science might detect something fully coincidental with space. If so, that substance could be exploited as a universal rest frame, thereby allowing scientists to comprehend events in a manner approximating God’s comprehension. The Universal Ether The substance of interest was ether. Newton had proposed its existence while responding to critics disturbed by the thought of New­ ton’s gravity propagating across empty space—that is, without a supporting medium. How could the earth hold the moon in its orbit when 240,000 miles of apparent nothingness separated the two bodies? By what physical or mechanical means would each body’s gravitational force be transmitted outward? While Newton was largely content to let these questions go unanswered, believing that nature was not fully transparent to human reason, he nevertheless felt obliged to respond to those who demanded mechanical explanations for all of nature’s operations. Descartes had earlier tried to rid the cosmos of all non-contact or action-at-a-distance forces, feeling that such savored of magic and astrology, by insisting that space is a plenum: material particles fill every part of space, and these, then, mediate force or impact from one body to another. While young, Newton appreciated this approach, but he eventually concluded that Descartes populated his universe with too many imaginary mechanisms. Magnetic attraction , for example, was said to occur as threaded particles rotated into similarly threaded pores of iron or lodestone, thereby drawing the object toward the magnet. This mechanical explanation rid magnetism of long-standing occult associations, but it had no empirical support. Not wishing to “feign hypotheses” or speculate as Descartes had done, Newton stated that the deeper workings of gravity were yet “to be found out.”8 In the meantime, he would stick to incontrovertible explanations arising from the logical elucidation of empirical...