The book Elegance and Enigma: The Quantum Interviews gives a shocking account of the present state of quantum mechanics as foundation of modern physics:
- Quantum theory makes the most accurate empirical predictions. Yet it lacks simple, comprehensible physical principles from which it could be uniquely derived. Without such principles, we can have no serious understanding of quantum theory and cannot hope to offer an honest answer—one that’s different from a mere “The world just happens to be that way”—to students’ penetrating questions of why there is indeterminism in quantum physics, or of where Schrödinger’s equation comes from.
- The standard textbook axioms for the quantum formalism are of a highly abstract nature, involving terms such as “rays in Hilbert space” and “selfadjoint operators.” And a vast majority of alternative approaches that attempt to find a set of physical principles behind quantum theory either fall short of uniquely deriving quantum theory from these principles, or are based on abstract mathematical assumptions that themselves call for a more conclusive physical motivation.
- We don’t really understand the notion of a quantum state, in particular an entangled quantum state, and the peculiar role of measurement in taking the description of events from the quantum level, where you have interference and entanglement, to an effectively classical level where you don’t.
- John Wheeler would ask, “Why the quantum?” To him, that was the single most pressing question in all of physics. You can guess that with the high regard I have for him, it would be the most pressing question for me as well. And it is. But it’s not a case of hero worship; it’s a case of it just being the right question. The quantum stands up and says, “I am different!” If you really want to get to the depths of physics, then that’s the place to look.
- I believe that the most pressing problems are still those that have been debated for more than eighty years by some of the brightest scientists and deepest thinkers of the past century: Niels Bohr, Werner Heisenberg, John von Neumann, Albert Einstein, Erwin Schrödinger, John Bell.
- To characterize these problems in a nutshell, I cannot do better than stressing the totally unsatisfactory conceptual status of our best theory by reporting the famous sentence by Bell: “Nobody knows what quantum mechanics says exactly about any situation, for nobody knows where the boundary really is between wavy quantum systems and the world of particular events.”
- I don’t think the measurement problem will be solvable soon, or possibly ever.
- The most well-known problem in quantum foundations is the measurement problem—our basic conception of reality depends on how we resolve this. The measurement problem is tremendously important.
- But there is another problem that is even more important—and that may well lead to the solution of the measurement problem. This is to find a theory of quantum gravity. The problem of quantum gravity is easy to state: Find a theory that reduces to quantum theory and to general relativity in appropriate limits. It is not so easy to solve. The two main approaches are string theory and loop quantum gravity. Both are deeply conservative, in the sense that they assume it will be possible to formulate a theory of quantum gravity within the quantum formalism as it stands. I do not believe this is the right approach.
- The most pressing problem today is the same as ever it was: to clearly articulate the exact physical content of all proposed “interpretations” of the quantum formalism. This is commonly called the measurement problem, although, as Philip Pearle has rightly noted, it is rather a “reality problem.”
- Physics should aspire to tell us what exists (John Bell’s “beables”), and the laws that govern the behavior of what exists. “Observations,” “measurements,” “macroscopic objects,” and “Alice” and “Bob” are all somehow constituted of beables, and the physical characteristics of all things should be determined by that constitution and the fundamental laws.
- What are commonly called different “interpretations” of quantum theory are really different theories—or sometimes, no clear theory at all. Accounts that differ in the beables they postulate are different physical theories of the universe, and accounts that are vague or noncommittal about their beables are not precise physical theories at all. Until one understands exactly what is being proposed as the physical structure of the universe, no other foundational problem, however intriguing, can even be raised in a sharp way.
- In the words of Chris Fuchs, “quantum states: what the hell are they?” Quantum states are not objective properties of the systems they describe, as mass is an objective property of a stone. Given a single stone, about which you know nothing, you can determine its mass to a high precision. Given a single photon, in a pure polarization state about which you know nothing, you can learn very little about what that polarization was. (I say “was,” and not “is,” because the effort to learn the polarization generally results in a new state, but that is not the point here.)
- But I also find it implausible that (pure) quantum states are nothing more than provisional guesses for what is likely to happen when the system is appropriately probed. Surely they are constrained by known features of the past history of the system to which the state has been assigned, though I grant there is room for maneuver in deciding what it means to “know” a “feature.”
- The only interpretations of quantum mechanics that make sense to me are those that treat quantum mechanics as a theory of the information that observers in one subsystem of the universe can have about another subsystem. This makes it seem likely that quantum mechanics is an approximation of another theory, which might apply to the whole universe and not just to subsystems of it. The most pressing problem is then to discover this deeper theory and level of description.
- The interpretation of quantum mechanics is a wide open question, so we can’t say in advance what the most pressing problems are...What’s important is that we leave the smoke screen of the Copenhagen interpretation well behind us, and that talented and knowledgeable people think hard about this subject from a realist perspective.
- Just how are we to understand the apparently greater efficiency of quantum computers over classical ones?
- We have learned from quantum mechanics that naive realism is not tenable anymore. That is, it is not always possible to assume that the results of observation are always given prior to and independent of observation. To me, the most important question is to find out what exactly the limitations are.
- This can only be found out by carefully exploring quantum phenomena in more complex situations than we do today.
PS1 With measurement being replaced by computation in the model (some form of Schrödinger's equation), physics can be studied and inspected without interference from the observer. This changes the game completely by reducing the importance of the unsolvable measurement problem.