In the same way that the timing of a pendulum provided a more rigorous test of Galileo’s kinematical theory than could be achieved by direct testing with balls rolling down planes, so with Newton’s laws the most searching tests are indirect and based on mathematically derived consequences. Kepler’s laws of planetary motion are just such an example, and in the two centuries after Newton’s Principia the laws were applied to elaborate and arduous computations of the motion of all planets, not simply as isolated bodies attracted by the Sun but as a system in which every one perturbs the motion of the others by mutual gravitational interactions. (The work of the French mathematician and astronomer Pierre-Simon, marquis de Laplace, was especially noteworthy.) Calculations of this kind have made it possible to predict the occurrence of eclipses many years ahead. Indeed, the history of past eclipses may be written with extraordinary precision so that, for instance, Thucydides’ account of the lunar eclipse that fatally delayed the Athenian expedition against Syracuse in 413 bce matches the calculations perfectly (see eclipse). Similarly, unexplained small departures from theoretical expectation of the motion of Uranus led John Couch Adams of England and Urbain-Jean-Joseph Le Verrier of France to predict in 1845 that a new planet (Neptune) would be seen at a particular point in the heavens. The discovery of Pluto in 1930 was achieved in much the same way.

There is no obvious reason why the inertial mass m that governs the response of a body to an applied force should also determine the gravitational force between two bodies, as described above. Consequently, the period of a pendulum is independent of its material and governed only by its length and the local value of g; this has been verified with an accuracy of a few parts per million. Still more sensitive tests, as originally devised by the Hungarian physicist Roland, baron von Eötvös (1890), and repeated several times since, have demonstrated clearly that the accelerations of different bodies in a given gravitational environment are identical within a few parts in 10¹². An astronaut in free orbit can remain poised motionless in the centre of the cabin of his spacecraft, surrounded by differently constituted objects, all equally motionless (except for their extremely weak mutual attractions) because all of them are identically affected by the gravitational field in which they are moving. He is unaware of the gravitational force, just as those on the Earth are unaware of the Sun’s attraction, moving as they do with the Earth in free orbit around the Sun. Albert Einstein made this experimental finding a central feature of his general theory of relativity (see relativity).

Ensuing developments and their ramifications

Newton believed that everything moved in relation to a fixed but undetectable spatial frame so that it could be said to have an absolute velocity. Time also flowed at the same steady pace everywhere. Even if there were no matter in the universe, the frame of the universe would still exist, and time would still flow even though there was no one to observe its passage. In Newton’s view, when matter is present it is unaffected by its motion through space. If the length of a moving metre stick were compared with the length of one at rest, they would be found to be the same. Clocks keep universal time whether they are moving or not; therefore, two identical clocks, initially synchronized, would still be synchronized after one had been carried into space and brought back. The laws of motion take such a form that they are not changed by uniform motion. They were devised to describe accurately the response of bodies to forces whether in the heavens or on the Earth, and they lose no validity as a result of the Earth’s motion at 30 km per second in its orbit around the Sun. This motion, in fact, would not be discernible by an observer in a closed box. The supposed invariance of the laws of motion, in addition to standards of measurement, to uniform translation was called “Galilean invariance” by Einstein.

The impossibility of discerning absolute velocity led in Newton’s time to critical doubts concerning the necessity of postulating an absolute frame of space and universal time, and the doubts of the philosophers George Berkeley and Gottfried Wilhelm Leibniz, among others, were still more forcibly presented in the severe analysis of the foundations of classical mechanics by the Austrian physicist Ernst Mach in 1883. James Clerk Maxwell’s theory of electromagnetic phenomena (1865), including his description of light as electromagnetic waves, brought the problem to a state of crisis. It became clear that if light waves were propagated in the hypothetical ether that filled all space and provided an embodiment of Newton’s absolute frame (see below), it would not be logically consistent to accept both Maxwell’s theory and the ideas expressed in Galilean invariance, for the speed of light as it passed an observer would reveal how rapidly he was traveling through the ether.

Ingenious attempts by the physicists George FitzGerald of Ireland and Hendrik A. Lorentz of the Netherlands to devise a compromise to salvage the notion of ether were eventually superseded by Einstein’s special theory of relativity (see relativity). Einstein proposed in 1905 that all laws of physics, not solely those of mechanics, must take the same form for observers moving uniformly relative to one another, however rapidly. In particular, if two observers, using identical metre sticks and clocks, set out to measure the speed of a light signal as it passes them, both would obtain the same value no matter what their relative velocity might be; in a Newtonian world, of course, the measured values would differ by the relative velocity of the two observers. This is but one example of the counterintuitive character of relativistic physics, but the deduced consequences of Einstein’s postulate have been so frequently and so accurately verified by experiment that it has been incorporated as a fundamental axiom in physical theory.

With the abandonment of the ether hypothesis, there has been a reversion to a philosophical standpoint reluctantly espoused by Newton. To him and to his contemporaries the idea that two bodies could exert gravitational forces on each other across immense distances of empty space was abhorrent. However, attempts to develop Descartes’s notion of a space-filling fluid ether as a transmitting medium for forces invariably failed to account for the inverse square law. Newton himself adopted a pragmatic approach, deducing the consequences of his laws and showing how well they agreed with observation; he was by no means satisfied that a mechanical explanation was impossible, but he confessed in the celebrated remark “Hypotheses non fingo” (Latin: “I frame no hypotheses”) that he had no solution to offer.

A similar reversion to the safety of mathematical description is represented by the rejection, during the early 1900s, of the explanatory ether models of the 19th century and their replacement by model-free analysis in terms of relativity theory. This certainly does not imply giving up the use of models as imaginative aids in extending theories, predicting new effects, or devising interesting experiments; if nothing better is available, however, a mathematical formulation that yields verifiably correct results is to be preferred over an intuitively acceptable model that does not.