Late collision: a history of a semantic ambiguity

Abstract

This article examines a historical case brought to general attention by English theoretical physicist and historian of science Julian Barbour. The events took place at the beginning of the 20th century when Albert Einstein, on his first steps towards the theory of General relativity, formulated what later was to become the famous Mach’s Principle, in the earnest conviction that he was giving expression to Ernst Mach’s ideas on the ‘relativity of inertia’. The irony is that whatever coincidence of opinion there may be between Einstein and Mach on this issue, it exists on the level of terminological expression alone; in other words, either of them, while speaking of ‘inertia’, had a different physical phenomenon in mind. One of the factors to provoke confusion was the semantic ambiguity of the notion of inertia in the language of science both at the time and even to this date. The implicit terminological contradiction between Einstein and Mach may seem superficial and accidental, though in fact, as this analysis shows, it is a manifestation of circumstances which had come into being much earlier but hitherto remained unnoticed. Such a delayed and unexpected event one may call a ‘late collision’, to borrow a term from network science. To address the problem of the possible origins of the semantic ambiguity of the notion ‘inertia’, the present author reassesses the history of its appearance in the work of Johannes Kepler, then gives a detailed account of the terms chosen by Isaac Newton for the definition of inertia and for the verbal expression of his first law of motion, after which he follows the same procedure for Descartes’s first law of nature. New insights, in particular, are offered on the possible meaning of the introductory words (“quantum in se est”) of Newton’s Principia, traditionally known as representing a serious difficulty for translators. Such investigations lead to the conclusion that the semantic ambiguity of the notion of inertia is very likely explained by the role it plays in Newton’s reduction of Descartes’s symbolic forms, which served to grasp knowledge as an event, to an operational language of knowledge.