Upcoming talks

2 November 2024

**Craig Callender (UCSD), “**The Toll of the Tolman Effect: On the Status of Classical Temperature in General Relativity” (co-authored with Eugene Chua)

The Tolman effect is well-known in relativistic cosmology but rarely discussed outside it. That is surprising because the effect – that systems extended over a varying gravitational potential exhibit temperature gradients while in thermal equilibrium – conflicts with ordinary classical thermodynamics. In this paper we try to better understand this effect and make five claims. First, we show that it was Einstein – not Tolman – who first discovered the Tolman effect, and furthermore, that Einstein’s derivation helps us appreciate how robust the effect is. Second, we demonstrate that the standard interpretation of the Tolman effect, in terms of ‘local temperature’, leads to the breakdown of much of classical thermodynamics. Third, to rescue thermodynamics we can turn to Einstein’s preferred interpretation in terms of the ‘wahre Temperatur’, but it too has costs, e.g., local inaccessibility. Fourth, we clarify the effect’s interpretation. Like the gravitational redshift, it is often understood in terms energy loss. Inspired again by Einstein, we provide an alternative clock interpretation of both effects. Fifth, this discussion leads us to propose a third, novel, temperature with connections to a proposal of Einstein’s elsewhere, the ‘wahre-local temperature’. On this view, temperature – and thermodynamics – is defined only in relation to local clocks. Put together, we view the fragmentation of temperature in thermodynamics as a natural and expected result of the fragmentation of time in general relativity.

Past Events

12 October 2024

**Josh Eisenthal (Caltech)**, **“The Absolute Motion Detector”**

The central idea of this paper is that the impact of spatial curvature on bodily motion — a familiar phenomenon from general relativity — is apparent in any space which deviates from “flat” Euclidean geometry, including, in particular, in spaces of constant non-zero curvature. My argument will be that, despite the relative simplicity of this idea, it was completely out of sight to the figures who grappled with physical geometry in the nineteenth century (not to mention most of the rest of us more recently).

My more specific claim will be that all the major figures who engaged with the philosophy of geometry in the nineteenth century failed to recognize that absolute motion is detectable in the classical non-Euclidean geometries. I will examine the implications of this for the main positions in the philosophy of geometry that arose in this period, and suggest three responses that Helmholtz, Poincaré, and Russell might have had. The first (empiricist) response would have been to accept that absolute motion might indeed be detectable, and regard it as a way to measure the curvature of space. The second (conventionalist) response would have been to insist that the choice of a physical geometry was a convention, but similar to the “convention” of adopting the Copernican system over the Ptolemaic. The third (metaphysical) response would have been to deny on a priori grounds that absolute motion is possible, and conclude that space must be necessarily Euclidean after all.