Talk by Brian Pitts at MCMP (Wed. 6th Feb.)

[Karim Thebault]

Speaker: J. Brian Pitts (Cambridge)
Wednesday 6th Feb.
Ludwigstr.31 /II room 225
Time: 6 to 8 pm

Title: 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.
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