Past exam of the short options paper shared by Parts A and B of the physics course of the University of Oxford / 2019 / S14 / 2 / Solution

Doubt was reasonable if it concerned the physical cause or unlimited extrapolation of Newton's law of universal gravitation; wholesale dismissal of the quantitative evidence was harder to justify. Early eighteenth-century investigators had to distinguish the success of an inverse-square force law in explaining observations from the claim that every material body directly attracts every other one without a mediating mechanism.

The strongest case for Isaac Newton's framework was its unification of phenomena previously treated separately. Kepler's third law and nearly elliptic orbits could be connected to an inverse-square central force. Comparing terrestrial gravity with the centripetal acceleration of the Moon supported continuity between falling bodies and celestial motion. The Philosophiae Naturalis Principia Mathematica by Isaac Newton also developed treatments of interacting bodies, tides and the Earth's figure. These were substantive tests and organized a research programme capable of producing further quantitative comparisons.^[ref]

There were nevertheless rational reservations. A law proportional to $m_1{m}_2/r^2$ inferred from accessible bodies and distances did not logically establish unrestricted universality. Individual celestial predictions involved uncertain observations and approximations to difficult many-body classical mechanics. The Moon's motion and the Earth's shape were demanding tests whose successful treatment was not complete at the beginning of the century. A dissenter could question auxiliary assumptions, numerical accuracy or the inference from special cases without denying every success.

A deeper objection concerned explanation. In traditions requiring contact mechanisms, unexplained attraction across empty space appeared to reintroduce an occult quality. René Descartes's vortices supplied a mechanical alternative, though they faced their own serious dynamical difficulties. Newton himself separated a mathematically established pattern from a claimed cause of gravity; the 1713 General Scholium by Isaac Newton declined to invent an unsupported physical mechanism. Treating every criticism of action at a distance as a failure to understand evidence would therefore misrepresent the available positions.^[ref]

Later confirmations cannot be counted as evidence already possessed by an early-eighteenth-century critic. Subsequent geodetic measurements, improved lunar theory and successful comet predictions helped shift the balance, but their timing matters. Nor did retaining a mechanical alternative remain equally reasonable after every failed prediction: responsible doubt required comparative assessment and willingness to revise it. The balanced conclusion is that selective, explicit scepticism about universality or cause was defensible, while the breadth of Newtonian mechanics' quantitative achievements already imposed a substantial explanatory burden on rival accounts.