Speaker
Description
Three decades ago, Ted Jacobson surprised us with a very appealing approach to classical gravity. According to him, the gravitational field equations are the consequence of the first law of thermodynamics applied to a Rindler observer. Jacobson’s approach, being formulated for Riemannian geometries, we have wondered what its consequences would be for non-Riemannian geometries. The results of our quest have been particularly appealing: we have found that the theory that derives from the Einstein- Hilbert action, arguably “the simplest one,” does not belong to the pool of gravitational theories available for Nature’s selection (except in the Riemannian case). In the search for a unique alternative, we have considered the hypotheses employed in the formulation of the Lanczos-Lovelock theories of gravity. Together, the two approaches point toward the theory that derives from the Einstein-Hilbert action plus a term quadratic in the torsion vector as the one that would be selected by Nature in the non- Riemannian case without non-metricity (when the energy-momentum tensor is identified as its metric version). The same strategy cannot be followed in the full non-Riemannian case (and in the previous case when the energy-momentum tensor is identified as its canonical version) as the two approaches are mutually inconsistent.