Interpretations of quantum mechanics

set of statements which attempt to explain how quantum mechanics informs our understanding of nature

An interpretation of quantum mechanics is a set of statements which attempt to explain how quantum mechanics informs our understanding of nature.

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  • Christian Imbert, to support my project and to act as my thesis advisor. He had advised me to go first to Geneva, to discuss my proposal with John Bell. I got an appointment without delay, and I showed up in John's office at CERN, quite nervous. While I explained my planned experiment, he listened silently. Eventually, I stopped talking, and the first question came: "Have you a permanent position?" After my positive answer, he started talking of physics, and he definitely encouraged me, making it clear that he would consider the implementation of variable analysers a fundamental improvement. Remembering this first question reminds me both of his celebrated sense of humour and of the general atmosphere at that time about raising questions on the foundations of quantum mechanics. Quite frequently there was open hostility, and in the best case, irony: "quantum mechanics has been vindicated by such a large amount of work by the smartest theorists and experimentalists; how can you hope to find anything with such a simple scheme, in optics, a science of the 19th century?" In addition to starting the experiment, I had then to develop a line of argument to try to convince the physicists I met (and among them some had to give their opinion about funding my project).
    • Alain Aspect, "Bell's Theorem: The Naive View of an Experimentalist", in Quantum [Un]speakables (2002) edited by Reinhold A. Bertlmann and Anton Zeilinger


  • This particular question of locality is still open, in my opinion. I think we have not found a way of digesting this situation. I think we have not found a way of digesting this situation. We have the formulas of quantum mechanics, and they work extremely well; but I have not digested them. There certainly remains something to be said, some illumination to be found.


  • Unlike Newton's mechanics, or Maxwell's electrodynamics, or Einstein's relativity, quantum theory was not created—or even definitively packaged—by one individual, and it retains to this day some of the scars of its exhilarating but traumatic youth. There is no general consensus as to what its fundamental principles are, how it should be taught, or what it really "means." Every competent physicist can "do" quantum mechanics, but the stories we tell ourselves about what we are doing are as various as the tales of Scheherazade, and almost as implausible.
    • David J. Griffith, Introduction to quantum mechanics (2nd ed., 2005), Preface


  • Of course, the apparent disarray could have stemmed entirely from my own ignorance. But when I revealed my impression of confusion and dissonance to one of the attendees, he reassured me that my perception was accurate. “It’s a mess,” he said of the conference (and, by implication, the whole business of interpreting quantum mechanics). The problem, he noted, arose because, for the most part, the different interpretations of quantum mechanics cannot be empirically distinguished from one another; philosophers and physicists favor one interpretation over another for aesthetic and philosophical—that is, subjective—reasons.
    • John Horgan, The End of Science (1996), Ch. 3: The End of Science


  • I argue that what breathes fire into the QM equations is field-theoretic what-it's-likeness: "microqualia" to use a philosopher's term of art. The different values of the solutions to the ultimate physical equations exhaustively yield the abundance of different values of subjectivity. There is no room for dualism; "nomological danglers"; causally inert epiphenomena; classical, porridge-like lumps of otherwise insentient but magically mind-secreting matter, etc. There is no "explanatory gap" because there aren't any material objects - not even brains or nerve cells as commonly (mis)perceived. Instead, over millions of years, non-equilibrium thermodynamics and universal, (neo-)Darwinian principles of natural selection have contrived to organise a minimal and self-intimating subjective sludge of microqualia into complex functional living units. Initially, these units have taken the form of self-replicating, information-bearing biomolecular patterns. Eventually, selection-pressure has given rise to complex minds as well, albeit as just one part of the throwaway host vehicles by which our genes leave copies of themselves. Conscious mind, on this proposal, is a triumph of organisation: our egocentric virtual worlds are warm and gappy QM-coherent states of consciousness. Contra materialist metaphysics, sentience of any kind is not the daily re-enactment of an ontological miracle. Moreover the idea that what-it's-like-ness is the fire in the equations is (at least) consistent with orthodox relativistic quantum field theory - because the theorists' key notions (e.g. that of a field, string, brane, etc) are defined purely mathematically. In other cases, they readily lend themselves to such a reconstruction. Using the word "physical" doesn't add anything of substance.



  • It is truly surprising how little difference all this makes. Most physicists use quantum mechanics every day in their working lives without needing to worry about the fundamental problem of its interpretation. Being sensible people with very little time to follow up all the ideas and data in their own specialties and not having to worry about this fundamental problem, they do not worry about it. A year or so ago, while Philip Candelas (of the physics department at Texas) and I were waiting for an elevator, our conversation turned to a young theorist who had been quite promising as a graduate student and who had then dropped out of sight. I asked Phil what had interfered with the ex-student’s research. Phil shook his head sadly and said, “He tried to understand quantum mechanics.”
    So irrelevant is the philosophy of quantum mechanics to its use, that one begins to suspect that all the deep questions about the meaning of measurement are really empty, forced on us by our language, a language that evolved in a world governed very nearly by classical physics. But I admit to some discomfort in working all my life in a theoretical framework that no one fully understands. And we really do need to understand quantum mechanics better in quantum cosmology, the application of quantum mechanics to the whole universe, where no outside observer is even imaginable. The universe is much too large now for quantum mechanics to make much difference, but according to the big-bang theory there was a time in the past when the particles were so close together that quantum effects must have been important. No one today knows even the rules for applying quantum mechanics in this context.
    • Steven Weinberg, Dreams of the Final Theory (1993), Chap. 4. Quantum Mechanics and Its Discontents
  • My own conclusion is that today there is no interpretation of quantum mechanics that does not have serious flaws. This view is not universally shared. Indeed, many physicists are satisfied with their own interpretation of quantum mechanics. But different physicists are satisfied with different interpretations. In my view, we ought to take seriously the possibility of finding some more satisfactory other theory, to which quantum mechanics is only a good approximation.
    • Steven Weinberg, Lectures on Quantum Mechanics (2nd ed., 2015), Ch. 3 : General Principles of Quantum Mechanics
  • I had the feeling that the stuff was beautiful. I learned it from Weyl, and Weyl had the art of putting things in a lovely perspective. More so than anybody else I have ever read. That book was just a treat. So the feeling of ‘rotten’ would be the absolutely last feeling I would ever have about it. ‘Beautiful’ is what I would call it. To me it’s the magic way to do it. I think that having started early and having used it in lots of different contexts, all the way from my doctor’s thesis on the dispersion and absorption of light in a helium atom, to nuclear physics, to the decay of elementary particles, I feel absolutely at home with it. But John Bell’s question I certainly sympathize with. An ‘irreversible act of amplification’? As Eugene Wigner always says, ‘What means it "irreversible"?’ [...] I think it is just wonderful to have puzzles like that staring us in the face. You’d be amused. Every day I try to write down something in my notebook, although I don’t always succeed, pushing things ahead just a little bit. I only got in two or three sentences this morning. ‘Nada. The photon doesn’t exist in the atom. It doesn’t exist in the photodetector after the act of emission, and you have no right to talk of what it’s doing in between. Nada—it’s nothing.’ Then there’s the irreversible act of amplification where you’ve got a whole lot of things. It’s nada to nada.

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