## Weak Lensing in the Alps!

December 20, 2013 Leave a comment

Post by Tom Kitching, MSSL

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This month I was very fortunate to be a lecturer at the TRR33 winter school in Passo del Tonale in the Italian Alps. The aim of the school was **“Theory for Observers & Observations for Theorists”**.

My task for to educate the PhD students in weak lensing, which is a method that can be used to map dark matter and infer cosmology.

Over the course of the lecture we went through what a lens *actually is*, and I used some inspiration from Richard Feyman in his book QED. As light propagates from point A to point B it can take any possible path, with a particular probability.

In his book Feynman then poses the thought experiment and asks what would happen if we “fooled the light” so that every path took the same amount of time. What we end up with is a lens!

So in a certain sense you can think of a lens as a device for equalising the probability of paths between two points; or down-weighting the most likely paths. Of course, reality is much more complicated than that, but I was struck that this was a very nice way to explain why lenses work the way they do. These explanations, from a mathematical perspective are using the principle of least action, which is a powerful general technique used in physics. The amount of lensing caused by large scale structures in the Universe can be derived in a similar way.

It strikes me an elegant historical path that astronomy itself has taken: it was founded on the technological development of optical lenses, and that now as we are planning on surveying almost the entire sky over a significant fraction of the age of the Universe it is the motivation of observing the lensing caused by the Universe itself that is driving these ambitions (see for example Sami’s Euclid post).

The meetings organisers were amazingly good at arranging fun and engaging activities for the participants (as Peter Coles over at Telescoper eloquently remarked last year), who found themselves skiing, hiking through the night in snow shows, getting guided tours of the night sky (for some students it was the first time the Milky Way had been observed), and having 4 course meals every day for lunch and dinner!

One of the joys of being an astronomer is sharing your knowledge, exploring new places and meeting amazing people.

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