Miscellaneous
Colloquium Mathematical Philosophy, J. Brian Pitts (Cambridge) gives a talk at the MCMP Colloquium (6 February, 2013) titled "How Almost Everything in Space-time Theory Is Illuminated by Simple Particle Physics: The Neglected Case of Massive Scalar Gravity". Abstract: Both particle physics from the 1920s-30s and the 1890s Seeliger-Neumann modification of Newtonian gravity suggest considering a “mass term,” an additional algebraic term in the gravitational potential. The “graviton mass” gives gravity a finite range. The smooth massless limit implies underdetermination. In 1914 Nordström generalized Newtonian gravity to fit Special Relativity. Why not do to Nordström what Seeliger and Neumann did to Newton? Einstein started in setting up a (faulty!) analogy for his cosmological constant Λ. Scalar gravities, though not empirically viable since the 1919 bending of light observations, provide a useful test bed for tensor theories like General Relativity. Massive scalar gravity, though not completed in a timely way, sheds philosophical light on most issues in contemporary and 20th century space-time theory. A mass term shrinks the symmetry group to that of Special Relativity and violates Einstein's principles (general covariance, general relativity, equivalence and Mach) in empirically small but conceptually large ways. Geometry is a poor guide to massive scalar gravities in comparison to detailed study of the field equation or Lagrangian. Matter sees a conformally flat metric because gravity distorts volumes while leaving the speed of light alone, but gravity sees the whole flat metric due to the mass term. Largely with Poincaré (pace Eddington), one can contemplate a “true” flat geometry differing from what material rods and clocks disclose. But questions about “true” geometry need no answer and tend to block inquiry. Presumptively one should expect analogous results for the tensor (massive spin 2) case modifying Einstein’s equations. A case to the contrary was made only in 1970-72: an apparently fatal dilemma involving either instability or empirical falsification appeared. But dark energy measurements since 1999 cast some doubt on General Relativity (massless spin 2) at long distances. Recent calculations (2000s, some from 2010) show that instability can be avoided and that empirical falsification likely can be as well, making massive spin 2 gravity a serious rival for GR. Particle physics can let philosophers proportion belief to evidence over time, rather than suffering from unconceived alternatives.