Crossroads

Roger Penrose won the 2020 Nobel Prize for Physics (with two other people) for the Penrose singularity theorem. I always find it interesting to ask how the idea came to someone, and Penrose has an interview in which he tells us how this one came to him after he had been thinking about how one might give a geometric characterization of singularities:

At that time I was at Birkbeck College, and a friend of mine, Ivor Robinson, who’s an Englishman but he was working in Dallas, Texas at the time, and he was talking to me … I forget what it was … he was a very … he had a wonderful way with words and so he was talking to me, and we got to this crossroad and as we crossed the road he stopped talking as we were watching out for traffic. We got to the other side and then he started talking again. And then when he left I had this strange feeling of elation and I couldn’t quite work out why I was feeling like that. So I went through all the things that had happened to me during the day – you know, what I had for breakfast and goodness knows what – and finally it came to this point when I was crossing the street, and I realised that I had a certain idea, and this idea what the crucial characterisation of when a collapse had reached a point of no return, without assuming any symmetry or anything like that. So this is what I called a trapped surface. And this was the key thing, so I went back to my office and I sketched out a proof of the collapse theorem. The paper I wrote was not that long afterwards, which went to Physical Review Letters, and it was published in 1965 I think.