"I think I can safely say that nobody understands quantum mechanics." -Feynman

"…what I did can be described simply as an act of desperation… A theoretical interpretation… had to be found at any cost, no matter how high." -Planck, regarding his use of Boltzmann statistics to derive his blackbody radiation formula that solved the ultraviolet catstrophe

"From the point of view of physics in 1900, the logic of Planck’s electromagnetic and thermodynamic steps was impeccable, but his statistical step was wild." A. Pais, biographer of Einstein, regarding the above

J.J. Thomson won the Nobel Prize for showing that an electron is a particle.
His son, G.P. Thomson won the Nobel Prize for showing that an electron is a wave.

"You surely must understand, Bohr, that the whole idea of quantum jumps necessarily leads to nonsense… If we are still going to have to put up with these damn quantum jumps, I am sorry that I ever had anything to do with quantum theory." -Schrödinger

"Quantum mechanics is very impressive. But an inner voice tells me that it is not yet the real thing. The theory produces a good deal but hardly brings us closer to the secret of the Old One. I am at all events convinced that He does not play dice." -Einstein

"Can nature possible be as absurd as it seems…?" -Heisenburg regarding his debates with Bohr over the meaning of his own uncertainty principle

"Experiments are the only means of knowledge at our disposal. The rest is poetry, imagination." -Planck