History and Meaning of the Principle of Energy Conservation: going back to the Classics

Section 6. Epistemology and the History of Science History and Meaning of the Principle of Energy Conservation: going back to the Classics F.Bevilacqua, Dipartimento di Fisica”A.Volta, Università di Pavia, via Bassi 6, 27100, Pavia In 1959 Thomas Kuhn published a long and famous article on the so-called “simultaneous discovery” in the 1840’s of the Principle of Energy Conservation where he claimed: “Energy is conserved; nature behaves that way”. Paradoxically few years later (1963) the physics Nobel laureate Richard Feynman asserted that we do not know which are the forms of energy, nor its total quantity, “we do not understand energy conservation”. Kuhn’s assertion more than an (erroneous) presentism is perhaps indication of an historiographic debt with E.Meyerson (1908); moreover the whole paper shows an enormous debt with the book of H.A.Haas (1909). These last two contributions are part of an in depth “historic-critical” debate, that took place between the end of the 19th century and the beginning of the 20th, aiming at discussing competing formulations of the principle of conservation and of “energy” models . A main role in this debate was played by Planck’s 1887 contribution, praised by Haas himself, that saw a slightly modified but more widespread edition in 1908. Among the main points discussed by Planck: the old assumptions on the ex nihilo and ad nihilum, the struggle towards the acceptance of the “impossibility of perpetual motion”, the conceptual and not just formal significance of the long lasting vis viva controversy, the definition of the concept of “work”, the role of S.Carnot’s caloric model and cycle and its impact on the definition of absolute temperature, the constant conversion rates of Mayer and Joule, the problems of a correct identification of theoretical and experimental terms in the famous 1847 Erhaltung of Helmholtz, the difference between the universality of the definition of energy for external effects and the difficult but fertile search of a definition for internal energy, the superposition of expressions of energies referring to different phenomena, and finally the impossibility of a primary (complete) definition. Of great relevance is Planck’s approach to the solution of the electromagnetic debate between contiguous action and action at a distance: the comparison is not made on experimental grounds, but on the theoretical alternative expressions of the conservation principle (local and global respectively). Planck’s evaluation, that predates Hertz’s electromagnetic experiments of the late eighties, even if not Poynting’s theorem (1885), is based on philosophical grounds and goes along the same lines of what will be Hertz’s 1894 approach: the contiguous approach has to be preferred on simplicity grounds, but also because it vindicates causality against teleology. Extremely valuable also the remarks on the increasing similarities between the concepts of matter and energy and on the necessity to extend contiguous action to gravitational phenomena (thirty years before the theory of General Relativity). Hertz, like Planck a student of Helmholtz, made a serious attempt (1894) towards decoupling mechanics and potential energy, Mach significantly preferred the expression “conservation of work” for his contribution of 1872 (and later regretted not being cited by Planck), Poincarè underlined the existence of a number of first integrals of the equations of motion and the risk that the principle be reduced to a tautology (“something” is conserved), Helm, Ostwald and Duhem favoured the “energetist” approach (memorable the 1895 debate with Boltzmann at Lubeck) based on thermodynamic potentials and factorization of energy (still adopted in Sommerfeld’s Electrodynamics). It is a time when energy conservation has a role in other disciplines, from economics to psychoanalysis to philosophy (as can be seen in the Meyerson-Cassirer controversy). In the twentieth century the role of the principle has another twist with the mass-energy equivalence, the quantization but also with Noether’s theorem that shifts energy conservation in the symmetry perspective. The invariance for time traslation is the formal result of a long search (the constancy amid change) but certainly not the solution to the debates on the irreversibility of phenomena and on the arrow of time. The debates on energy conservation at the turn of the 19th-20th centuries show the power of the “now mighty theoretical physics” and are a good starting point not only for historical research on previous periods, but also for the interpretation of more recent controversies, beyond the “identitarian” approach of Kuhn and the skeptical one of Feynman.