The One Mystery of Quantum MechanicsIn Richard Feynman's famous Lectures on Physics, he wrote the following on the mystery of quantum behavior.
We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery. We cannot make the mystery go away by “explaining” how it works. We will just tell you how it works. In telling you how it works we will have told you about the basic peculiarities of all quantum mechanics.In his 1967 Messenger Lectures at Cornell which were published as The Character of Physical Law), Feynman famously said that "nobody understands quantum mechanics" and that the two-slit experiment contains "all of the mystery of quantum mechanics."
I will take just this one experiment, which has been designed to contain all of the mystery of quantum mechanics, to put you up against the paradoxes and mysteries and peculiarities of nature one hundred per cent. Any other situation in quantum mechanics, it turns out, can always be explained by saying, 'You remember the case of the experiment with the two holes? It's the same thing'. I am going to tell you about the experiment with the two holes. It does contain the general mystery; I am avoiding nothing; I am baring nature in her most elegant and difficult form.Feynman summarized the one mystery in the two-slit experiment in two propositions