In The Hitchhiker's Guide to the Galaxy, by Douglas Adams, the number 42 was revealed to be the "Answer to the Ultimate Question of Life, the Universe, and Everything". But he didn't say what the question was! I will reveal that here. In fact it is a simple geometry question, which then turns out to be related to the mathematics underlying string theory.
You can see the slides and watch a video of my public talk at the Canadian Mathematical Society, or watch this video of my Leverhulme Lecture on the same topic:
For related material, see: