My PhD advisor was John Preskill (Caltech, 1988).

   He finished his PhD at Harvard in 1980 with Steven Weinberg as his advisor.

   Weinberg did his thesis at Princeton in 1957 with Sam Treiman as his advisor.

   Treiman did his thesis at Chicago in 1952 having two advisors: John Simpson (official) and Enrico Fermi (unofficial).

My Erdos number is 3. You can compute this or any other collaboration distance using the calculator of AMS.