Why even physicists still don’t understand quantum theory 100 years on

Elizabeth Gibney, Shamini Bundell

S. P. Boughn

Richard Feynman famously declared, "I think that I can safely say that nobody really understands quantum mechanics." Sean Carroll lamented the persistence of this sentiment in a recent opinion piece entitled, "Even Physicists Don't Understand Quantum Mechanics. Worse, they don't seem to want to understand it." Quantum mechanics is arguably the greatest achievement of modern science and by and large we absolutely understand quantum theory. Rather, the "understanding" to which these two statements evidently refer concerns the ontological status of theoretical constructs. For example, "Do quantum wave functions accurately depict physical reality?" The quantum measurement problems represents a collection of such queries and the conundrums to which they lead. Most physicists are content with foregoing such metaphysical issues, falling back on Bohr's Copenhagen interpretation, and then get on with the business of doing physics. I suspect that Carroll would criticize these physicists as being too dismissive of the wonderful mysteries of quantum mechanics by relegating its role to that of an algorithm for making predictions. Quite to the contrary, I maintain that those who still pursue a resolution to the measurement problem are burdened with a classical view of reality and fail to truly embrace the fundamental quantum aspects of nature.

Thomas Görnitz

David Deutsch

Abstract A collection of classical universes — even if they interact — is still classical. For instance, it doesn't have entanglement; its elements don't have phases to their amplitudes; it doesn't have continuous motion of discrete observables. The multiverse is not classical: it consists of much more than universes. Research into what is there in addition has already been fruitful and promises to be more so. Progress in understanding the quantum world more deeply through such research is also the key to resolving the long-running scandal of the slow take-up of Everett's theory by physicists and philosophers.

Adán Cabello

Motivated by some recent news, a journalist asks a group of physicists: "What's the meaning of the violation of Bell's inequality?" One physicist answers: "It means that non-locality is an established fact". Another says: "There is no non-locality; the message is that measurement outcomes are irreducibly random". A third one says: "It cannot be answered simply on purely physical grounds, the answer requires an act of metaphysical judgement". Puzzled by the answers, the journalist keeps asking questions about quantum theory: "What is teleported in quantum teleportation?" "How does a quantum computer really work?" Shockingly, for each of these questions, the journalist obtains a variety of answers which, in many cases, are mutually exclusive. At the end of the day, the journalist asks: "How do you plan to make progress if, after 90 years of quantum theory, you still don't know what it means? How can you possibly identify the physical principles of quantum theory or expand quantum theory into gravity if you don't agree on what quantum theory is about?" Here we argue that it is becoming urgent to solve this too long lasting problem. For that, we point out that the interpretations of quantum theory are, essentially, of two types and that these two types are so radically different that there must be experiments that, when analyzed outside the framework of quantum theory, lead to different empirically testable predictions. Arguably, even if these experiments do not end the discussion, they will add new elements to the list of strange properties that some interpretations must have, therefore they will indirectly support those interpretations that do not need to have all these strange properties.