Niels Bohr, one of the fathers of the quantum theory, said in 1922: “If quantum mechanics hasn’t profoundly shocked you, then you haven’t understood it”. And I would add here: Even if you found it disconcerting, you probably didn’t understand it either. This statement can be extended to scientists working in the field as well. Richard Feynman quoted: “I can safely say that nobody understands quantum mechanics”.
The situation hasn’t essentially changed since Feynman’s time, but the question has become more dramatic. The quantum theory was developed at the start of the twentieth century; it has changed enormously the scientific view on nature, the life of many generations and will likely continue to do so in the future. Around the 30% of the gross world product depends on our limited comprehension of quantum mechanics; wherever there is a transistor, a magnetic resonance, a computer or a laser, there will be the quantum theory behind. And yet the strange thing is that we have no clue how any of its fundamental principles (tunnel effect, wave-particle duality, the Copenhagen interpretation…) actually work. Ask a bunch of physicists about the meaning of the quantum theory; their answers will probably display little consensus.
For more than 100 years most physicists and chemists have used quantum mechanics to calculate with incredible precision many different phenomena, but nobody has successfully explained where it comes from. The quantum world continues to be an imaginary world that defies logical sense, a place where anything can happen, and it raises all kinds of weird questions that push the limits of our imagination. It tries to convince us, for example, that objects like electrons exist simultaneously in two places at the same time. That’s why, for some researchers, quantum theory issues are not truly scientific questions that can be analysed by experiment, but philosophical ones that might depend on personal preference.
Quantum mechanics perturbed Einstein so much that he refused to accept it during many years of his life. When the fathers of the quantum theory (Bohr, Heisenberg, Schrödinger…) came up with a description of the quantum world in which certainties were replaced by mere probabilities, Einstein protested with his very famous quote: “God does not play dice”. Einstein may have gone wrong with his particular approach, but he was right in feeling a resolved sense of anxiety regarding the reality which the quantum world encloses. Perhaps he couldn’t bear the feeling that one of the most successful theories of history is, at its deepest level, a complete mystery.
Maybe our mistake is in trying to use everyday common sense to explain an unworldly universe that simply cannot be encapsulated in our finite minds. It’s been said that, as a quantum physicist or chemist, you don’t ever come to understand the quantum universe in any intuitive sense; you just get used to accepting it. In the words of physicist David Mermin, scientists should continue to “shut up and calculate” and give appropriate explanations of the unbelievable agreement with experiment data that the theory provides us. Because, once you accept its weirdness, quantum theory becomes a fantastically useful tool.