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The Landau-Pomeranchuk Migdal effect, 1953 to Today: Cosmic Rays to Quark-Gluon Plasmas to String Theory
High-energy cosmic rays lose energy and so eventually stop in the atmosphere/earth by showering: through bremsstrahlung and pair production during scattering from the electric fields of nuclei, one initial particle splits its energy into a increasing number of lower energy particles. In the 1950's, Landau, Pomeranchuk and Migdal (LPM) worked out that the theory of showering has a complicated twist: at very high energy, successive scatterings in even a very dilute system, like the atmosphere, cannot be treated as quantum mechanically independent. I will explain what the LPM effect is and how it arises. Then I will discuss its more recent application to the theory of jet quenching in quark-gluon plasmas created in relativistic heavy ion collisions at the RHIC and the LHC colliders. Finally, I will explain how some unresolved theory issues concerning the LPM effect in strongly-interacting systems are being fruitfully explored by one of string theory's greatest feats of prestidigitation: gauge-gravity duality, which in this case manifests as a mapping of problems in certain types of strongly-coupled finite-temperature quantum field theories in 3+1 space+time dimensions into solvable *classical* field theory problems of general relativity and black holes in 4+1 space-time dimensions.