A few weeks ago I reported on my safari to Philosophy. Today, in the Astro seminar at NYU, Tim Maudlin (NYU Philosophy) went on safari to Physics. He works on the question of why mathematics is useful in describing the physical world, and this has led him to the basis of mathematics, or really the basis of the mathematics that is used in physics. He finds (remarkably to me) that if he builds topology (which is really the continuity structure of space or spacetime) on the properties of one-dimensional fundamental objects rather than open sets or neighborhoods, he gets some aspects of the causal structure of spacetime for free. We think (often, informally) of the causal structure as coming from the metric, but Maudlin finds that it can come in far earlier than that if we replace or transpose (in some sense) the foundations of topology. Crazy stuff, and a very lively seminar. My kind of Friday afternoon. Lunch was pretty hilarious too; Maudlin has at his fingertips many paradoxes that get at controversies about probability and information, related to the anthropic principle and the like.
In the morning, Mykytyn showed me that he can fit the intensity images of big galaxies, even in the presence of bright stars, even when those galaxies span different fields taken on different nights, and subsets of the images come from different photometric bandpasses. We are very close to having a system that can re-measure all the (very, very) bright galaxies in SDSS. That could have big impact.