This is not a joke. It is something I have wondered about for a long time. Nearly every picture of Werner Heisenberg shows him smiling.
This is unusual. The difference between Heisenberg and other physicists is striking. Try an image search for Dirac or Schrödinger and you will see a lot of scowling, sour faces. It is possible that Niels Bohr smiled once but if so there does not seem to be any photographic evidence.
Was Heisenberg noted for being a particularly cheerful guy?