A story from David Hitlin

In the early seventies at lunch in the cafeteria I asked Pief, who was, of course, always either heading for or returning from the airport, how many miles he had flown. His answer was "Twenty five percent of a mean free path."

This answer has always, for me, encapsulated Pief perfectly. It was amusing, serious, and to the point. He knew the number of accidents per passenger mile, and therefore could answer the question in the most relevant units.

Editorial note: For non-physicist readers, the "mean free path" of a particle is the average distance it covers between collisions.

- David Hitlin
hitlin@hep.caltech.edu