On November 9, 1919, The New York Times reported solar eclipse observations confirming a prediction of Einstein's general theory of relativity: rays of starlight bend near the sun. It also reported that when Einstein sent his theory to the publishers, "he warned them that there were not more than twelve persons in the world who would understand it . . . ." Was there a time when only "twelve wise men" understood the general theory of relativity? "I do not believe there ever was such a time," commented Feynman. "There might have been a time when only one man did, because he was the only guy who caught on, before he wrote his paper. But after people read the paper a lot of people understood the theory of relativity in some way or other, certainly more than twelve. On the other hand, I think I can safely say that nobody understands quantum mechanics." [1] What is the problem with quantum mechanics? It is a spectacularly successful theory. It governs the structure of all matter. Measurements of Planck's constant are accurate to better than a part in a million, and still more accurate measurements confirm predictions of quantum electrodynamics. But along with the spectacular successes of quantum mechanics come spectacular difficulties of interpretation. "Do not keep saying to yourself, if you can possibly avoid it, 'But how can it be like that?"' Feynman continued, "because you will get 'down the drain', into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that."
We can stop asking ourselves, "But how can it be like that?" We may indeed despair of asking a question that Einstein, SchrΓΆdinger and Feynman could not answer. But we cannot stop using quantum mechanics. So the problem is that everybody uses quantum mechanics and nobody knows how it can be like that. Our relationship with quantum mechanics recalls a Woody Allen joke:
This guy goes to a psychiatrist and says, "Doc, my brother's crazy -he thinks he's a chicken! And, uh, the doctor says, "Well, why don't you turn him in?" And the guy says, "I would, but I need the eggs!"
We say, "Quantum mechanics is crazy -but we need the eggs!" Such a relationship with quantum mechanics is paradoxical. In this book, we will not be satisfied to have a paradoxical relationship with quantum mechanics. We will not stop asking, "How can it be like that?" But we will use paradox repeatedly in order to understand quantum mechanics better.