Speaker
J. Brian Pitts
Description
Scientific principles in physics can be understood as useful in the context of discovery, but rarely crucial in the context of justification (to recall a distinction due to Hans Reichenbach)---at least not in the physics of space-time and gravity, where principles have seemed particularly important. Consequently neither the need to evaluate principles philosophically nor the apparent failure of some attractive principle(s) generates a crisis for physics or the philosophy of science.
The dispensability of principles in the justification of space-time physics can be understood using two converging lines of research: (1) the particle physics tradition of gravity, which during the late 1930s-early 1970s generated a more compelling nuts-and-bolts argument for Einstein’s equations, something like an eliminative induction, than is yielded by such principles as relativity, equivalence, general covariance, etc., and (2) historiographic work on the history of General Relativity, which uncovered Einstein’s dual strategy, including his now-famous principles and a less remembered “physical strategy” involving criteria such as an analogy to electromagnetism, a link between energy conservation and gravitational field equations, and an analogy to Newtonian gravity.
The particle physics tradition in gravity, involving key contributions by Pauli, Fierz, Kraichnan, Gupta, Feynman, Weinberg, van Nieuwenhuizen, Deser, Duff, van Dam, Veltman, etc., explored gravity assuming (at least) Poincare invariance, stability (a key criterion ruling out vector potentials and vector parts of tensor potentials), gravity as described by a symmetric rank 2 tensor potential (the simplest possibility once the bending of light refuted scalar theories such as Nordström’s), and long or infinite range. Excluding negative-energy vector components largely fixes the free gravitational equation to have gauge freedom, which in turn requires that any source by conserved, which leaves the total (material + gravitational) stress-energy as the only plausible candidate. A change of variables then shows that gravity and space-time merge in the field equations. “Graviton mass terms,” though prima facie possible, encounter various devils in the details, a long story with no little historical contingency and much modern (2010s) attention in physics. “Principles” such as equivalence and general covariance emerge rather as theorems starting from premises involving modest empirical facts and mathematical requirements for viable field theories. Much of this reasoning, one notices, parallels Noether’s discussion of the converse Hilbertian assertion, that the stress-energy complex involves a term vanishing with the field equations and an identically conserved term.
Recent historiography on Einstein’s process of discovery, due to Stachel, Renn, Janssen, Norton, Sauer, van Dongen, etc., has recovered Einstein’s above-mentioned “physical strategy.” Van Dongen has argued that Einstein suppressed his physical strategy, which he deemed a failure, in order to bolster his unified field theory program. One notices that Einstein’s physical arguments resemble the ideas later developed in the particle physics tradition, so the physical strategy was indeed viable.
Hence the prominence of principles in space-time physics is historically contingent; physics still primarily rests on mathematics, logic and experiment.
Author
J. Brian Pitts
