(Again) A Philosopher Looks at String Theory
Nicholas Huggett

Abstract: According to Brandenberger and Vafa (1989), because of the 'T-duality' between string theories of with compactified dimensions with radius R and radius 1/R, ‘there is no physical experiment which tells us whether today we live in a Universe of size [10^10 light years], or in a tiny Universe of size [10^-85m]’. This talk sketches the physics behind this remarkable-sounding claim (and others of Greene, 2003), and explains what it means -- it doesn't seem to be the kind of thing we could be mistaken about taken literally, and indeed it involves a kind of pun. The talk will conclude with some remarks about the underdetermination of geometry by experiment in this context (which is rather different from more familiar cases) and about the relation of T-duality to the emergence of spacetime in quantum gravity.