Newton’s answer was to define a fixed, universal reference frame, called “absolute space,” placed in an exalted position above any other framework. “Absolute space,” he wrote, “in its own nature, without relation to anything else, remains always similar and immovable.”

To the concept of absolute space, Newton added another expression, called “absolute time.” Absolute time represents the uniform ticking of an ideal universal clock. Together, absolute space and time serve to define absolute motion—an inviolate description of movement through the cosmos.

Common parlance, Newton pointed out, fails to distinguish between relative motion (measured with respect to any fleeting frame) and absolute motion (defined with regard to the steel scaffolds of absolute space and time). The bucket example, however, demonstrates why such confusion of terms won’t do. To understand dynamics properly, he emphasized, one must reject the ephemeral and take a firm universal perspective. “Relative quantities,” he wrote, “are not the quantities themselves whose names they bear….” Those who mistake transient measures for true quantities, he continued, “violate the accuracy of language, which ought to be kept precise….”

Despite Newton’s admonition, in the centuries after his death a growing community of scholars came to find his distinction rather artificial. With everything in the cosmos in ceaseless motion, why should any one reference frame stand still? By the 19th century, a number of scientists replaced Newton’s artifice with an all-pervading invisible substance, known as the aether. Absolute motion could thereby be defined with respect to the aether stream. Nobody, however, could detect the aether; it seemed as elusive as a ghost.

Viennese physicist Ernst Mach took a different approach. In his popular book on mechanics, he dismissed the notion of an absolute frame. Rather, he argued that it is the combined pull of distant stars that keeps inertia’s hammock aloft. “Instead of referring a moving body to [absolute] space,” he wrote, “let us view directly its relation to the bodies of the universe, by which alone such a system of coordinates can be determined.” Hence, objects resist acceleration

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