he described his own ideals, the desire for fulfillment not just as a theorist but also as an experimental physicist. (In the second respect, he, of … - Abraham Pais

Special relativity killed the classical dream of using the energy-momentum-velocity relations of a particle as a means of probing the dynamic origins of its mass. The relations are purely kinematic. The classical picture of a particle as a finite little sphere is also gone for good. Quantum field theory has taught us that particles nevertheless have structure, arising from quantum fluctuations. Recently, unified field theories have taught us that the mass of the electron is certainly not purely electromagnetic in nature. But we still do not know what causes the electron to weigh.

I lived altogether in nine different places while in hiding, because whenever something happened, either someone betrayed the place or something happened to someone who knew where I was, I had to move. The rule of the game was never assume that anybody, however honorable, would be able to stand up under torture. If Mr. X, who knew where I was, was caught for some reason, I should move.

To make a discovery is not necessarily the same as to understand a discovery. Not only Planck but also other physicists were initially at a loss as to what the proper context of the new postulate really was.