The morning began breakfastless, and a little breathless, rushing from the shower to the conference. By day four conference fatigue was beginning to set in. It had absoulutely nothing to do with the Belgian-beer filled discussions. None whatsoever. However day four proved to give second wind to the meeting, filled with very interesting talks that I hope to give a flavour of here.
The morning review lecture was given by Samir D. Mathur, he does not like horizons, well at least those around black holes. One has to sympathise - is it really acceptable to cut a singularity out of a theory? Mathur prefers a fuzzball picture, where the black hole horizon becomes a statistical entity, emerging macrosopically - the canonical comparison is with temperature in the thermodynamical picture. In thermodynamics the temperature is a statistical quantity that can be measured over a large number of microscopic states, but if you sat on a hydrogen molecule (well maybe you already are, but what I mean is, if you had the molecule's view of a gas) you would be able to say a few things about your nearest neighbours relative velocities, and only with a large amount of time would you collect enough information to speak with confidence of the average molecular speed, or temperature. To a microscopic state the temperature is an odd concept, supposedly the black hole horizon is also an odd concept to a gravitational microstate. The fuzzball proposal functionally aims to reproduce the macrosopic black hole phenomena from collections of microstates. The brane microstates themselves do not have horizons in this setup, the horizon appears in the averaging over a large number of brane states. Old and familiar properties of black holes are reproduced in this picture, light can be trapped behind the horizon by an elaborate setup up of light deflecting states, Hawking temperatures can be reproduced and lately Hawking radiation can be produced by pair-production. For an introduction to the proposal you can read his papers here and here. The proposal offers a way to side-step Hawking's information paradox. Mathur's discussion of the information paradox can be read in this preprint, where he aims to make a review using pictures.
Kurt Vonnegut used to use a technique of repeating a small, catchy phrase when something of particular note happened in a sentence of his (e.g. in Cat's Cradle each reference to slipping off the mortal coil earns a: so it goes, or in Timequake ting-a-ling is the catchphrase). I think everytime someone tries to explain something with pictures I would like to insert a cowbell noise. So here's to Mathur: *cowbell*.
Mathur's title this morning was "Lessons from resolving the information paradox". He threw out the notion two charge non-extremal black holes have a singular throat in the spacetime, the geometry may become complicated but not singular. We heard about tunnelling in fuzzball geometries, radiation and pair-creation, which you can read about in the links.
After coffee, we had talks from Eric D'Hoker ("Exact 1/2 BPS solutions in type IIB and M-theory"), Dario Francia ("Unconstrained higher spins and current exchanges") and Diego Chialva ("Chain inflation revisited").
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