Thomas Spencer (mathematical physicist)

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• Together with James Glimm and Arthur Jaffe he invented the cluster expansion approach to quantum field theory that is widely used in constructive field theory.^[2]
• Together with Jürg Fröhlich and Barry Simon, he invented the approach of the infrared bound, which has now become a classical tool to derive phase transitions in various models of statistical mechanics.^[3]
• Together with Jürg Fröhlich, he devised a 'multi-scale analysis' to provide, for the first time, mathematical proofs of: the Kosterlitz–Thouless transition,^[4] the phase transition in the one-dimensional ferromagnetic Ising model with interactions ${\displaystyle J_{x,y}\sim |x-y|^{-2}}$^[5] and Anderson localization in arbitrary dimension.^[6]
• Together with David Brydges, he proved that the scaling limit of the self-avoiding walk in dimension greater or equal than 5 is Gaussian, with variance growing linearly in time.^[7] To achieve this result, they invented the technique of the lace expansion that since then has had wide application in probability on graphs.^[8]

Spencer is a member of the United States National Academy of Sciences,^[1] and the recipient of the Dannie Heineman Prize for Mathematical Physics (joint with Jürg Fröhlich, "For their joint work in providing rigorous mathematical solutions to some outstanding problems in statistical mechanics and field theory.").^[9][10]