One of the lessons of General Relativity is supposed to be that gravitation is the science of geometry. John Wheeler characterised it as, “a geometrodynamical universe”: a world whose properties…
Is Relativistic Geometry Conventional?
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Ice water on closed timelike curves
Today, let’s kick off our shoes and relax our standards for what counts as a reasonable spacetime. Let’s talk about spacetimes allowing for a certain kind of time travel, called…
Hyper-intelligent fish and black hole thermodynamics
Bill Unruh’s recent collection on black hole analogues begins, Deep beneath the great encircling seas of the Discworld lived a species of hyper-intelligent fish. (Unruh 2007, p.1) Unusual, but inspiring:…
Why does matter follow geodesics?
In the very first lessons on General Relativity, we learn that free particles follow geodesics — the equivalent of straight lines in curved spacetimes. Why? Well, it’s easy to show…
Two Tough Cases of Underdetermination
John Earman once pointed out two tough cases of underdetermination, neither of which arises in a silly algorithmic way. Today, I’d like to argue for an important difference between these…