The highlight of a low-research day was a visit from Roger Blandford (KIPAC), who gave the Physics Colloquium on particle acceleration, especially as regards ultra high-energy particles. He pointed out that the (cosmic) accelerators are almost the opposite of thermal systems: They put all the energy very efficiently into the most energetic particles, with a steep power-law distribution. He made the argument that the highest energy particles are probably accelerated by shocks in the intergalactic media of the largest galaxy clusters and groups. This model makes predicitions, one of which is that the cosmic rays pretty-much must be iron nuclei. In conversations over coffee and dinner we touched on many other subjects, including gravitational lensing and (separately) stellar spectroscopy.

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