“The essence of Dirac‚Äôs problem was simply that:
√(p2c2 + m02c4) ≠ pc + m0c2
and, in general: √(a2 + b2) ≠ a + b (for non-zero a and b), as everyone learns (or should learn) at school. At which point mere mortals give up. Geniuses try harder.”
(Norman McCubbin, “Beauty in physics: the legacy of Paul Dirac” , in Contemporary Physics, Volume 45, Number 4, July-August 2004, pp. 319-333)