Source: Quora question – “Scientifically speaking, how serious is the measurement problem concerning the validity of the various interpretations in quantum mechanics?”

David Pearce responds [emphasis mine]:

It’s serious. Science should be empirically adequate. Quantum mechanics is the bedrock of science. The superposition principle is the bedrock of quantum mechanics. So why don’t we ever experience superpositions? Why do experiments have definite outcomes? “Schrödinger’s cat” isn’t just a thought-experiment. The experiment can be done today. If quantum mechanics is complete, then microscopic superpositions should rapidly be amplified via quantum entanglement into the macroscopic realm of everyday life.

Copenhagenists are explicit. The lesson of quantum mechanics is that we must abandon realism about the micro-world. But Schrödinger’s cat can’t be quarantined. The regress spirals without end. If quantum mechanics is complete, the lesson of Schrödinger’s cat is that if one abandons realism about a micro-world, then one must abandon realism about a macro-world too. The existence of an objective physical realm independent of one’s mind is certainly a useful calculational tool. Yet if all that matters is empirical adequacy, then why invoke such superfluous metaphysical baggage? The upshot of Copenhagen isn’t science, but solipsism.

There are realist alternatives to quantum solipsism. Some physicists propose that we modify the unitary dynamics to prevent macroscopic superpositions. Roger Penrose, for instance, believes that a non-linear correction to the unitary evolution should be introduced to prevent superpositions of macroscopically distinguishable gravitational fields. Experiments to (dis)confirm the Penrose-Hameroff Orch-OR conjecture should be feasible later this century. But if dynamical collapse theories are wrong, and if quantum mechanics is complete (as most physicists believe), then “cat states” should be ubiquitous. This doesn’t seem to be what we experience.

Everettians are realists, in a sense. Unitary-only QM says that there are quasi-classical branches of the universal wavefunction where you open an infernal chamber and see a live cat, other decohered branches where you see a dead cat; branches where you perceive the detection of a spin-up electron that has passed through a Stern–Gerlach device, other branches where you perceive the detector recording a spin-down electron; and so forth. I’ve long been haunted by a horrible suspicion that unitary-only QM is right, though Everettian QM boggles the mind (cf. UniverseSplitter). Yet the heart of the measurement problem from the perspective of empirical science is that one doesn’t ever see superpositions of live-and-dead cats, or detect superpositions of spin-up-and-spin-down electrons, but only definite outcomes. So the conjecture that there are other, madly proliferating decohered branches of the universal wavefunction where different versions of you record different definite outcomes doesn’t solve the mystery of why anything anywhere ever seems definite to anyone at all. Therefore, the problem of definite outcomes in QM isn’t “just” a philosophical or interpretational issue, but an empirical challenge for even the most hard-nosed scientific positivist. “Science” that isn’t empirically adequate isn’t science: it’s metaphysics. Some deeply-buried background assumption(s) or presupposition(s) that working physicists are making must be mistaken. But which? To quote the 2016 International Workshop on Quantum Observers organized by the IJQF,

“…the measurement problem in quantum mechanics is essentially the determinate-experience problem. The problem is to explain how the linear quantum dynamics can be compatible with the existence of our definite experience. This means that in order to finally solve the measurement problem it is necessary to analyze the observer who is physically in a superposition of brain states with definite measurement records. Indeed, such quantum observers exist in all main realistic solutions to the measurement problem, including Bohm’s theory, Everett’s theory, and even the dynamical collapse theories. Then, what does it feel like to be a quantum observer?“

Indeed. Here I’ll just state rather than argue my tentative analysis.
Monistic physicalism is true. Quantum mechanics is formally complete. There is no consciousness-induced collapse the wave function, no “hidden variables”, nor any other modification or supplementation of the unitary Schrödinger dynamics. The wavefunction evolves deterministically according to the Schrödinger equation as a linear superposition of different states. Yet what seems empirically self-evident, namely that measurements always find a physical system in a definite state, is false(!) The received wisdom, repeated in countless textbooks, that measurements always find a physical system in a definite state reflects an erroneous theory of perception, namely perceptual direct realism. As philosophers (e.g. the “two worlds” reading of Kant) and even poets (“The brain is wider than the sky…”) have long realised, the conceptual framework of perceptual direct realism is untenable. Only inferential realism about mind-independent reality is scientifically viable. Rather than assuming that superpositions are never experienced, suspend disbelief and consider the opposite possibility. Only superpositions are ever experienced. “Observations” are superpositions, exactly as unmodified and unsupplemented quantum mechanics says they should be: the wavefunction is a complete representation of the physical state of a system, including biological minds and the pseudo-classical world-simulations they run. Not merely “It is the theory that decides what can be observed” (Einstein); quantum theory decides the very nature of “observation” itself. If so, then the superposition principle underpins one’s subjective experience of definite, well-defined classical outcomes (“observations”), whether, say, a phenomenally-bound live cat, or the detection of a spin-up electron that has passed through a Stern–Gerlach device, or any other subjectively determinate outcome. If one isn’t dreaming, tripping or psychotic, then within one’s phenomenal world-simulation, the apparent collapse of a quantum state (into one of the eigenstates of the Hermitian operator associated with the relevant observable in accordance with a probability calculated as the squared absolute value of a complex probability amplitude) consists of fleeting uncollapsed neuronal superpositions within one’s CNS. To solve the measurement problem, the neuronal vehicle of observation and its subjective content must be distinguished. The universality of the superposition principle – not its unexplained breakdown upon “observation” – underpins one’s classical-seeming world-simulation. What naïvely seems to be the external world, i.e. one’s egocentric world-simulation, is what linear superpositions of different states feel like “from the inside”: the intrinsic nature of the physical. The otherwise insoluble binding problem in neuroscience and the problem of definite outcomes in QM share a solution.

Absurd?
Yes, for sure: this minimum requirement for a successful resolution of the mystery is satisfied (“If at first the idea is not absurd, then there is no hope for it”– Einstein, again). The raw power of environmentally-induced decoherence in a warm environment like the CNS makes the conjecture intuitively flaky. Assuming unitary-only QM, the effective theoretical lifetime of neuronal “cat states” in the CNS is less than femtoseconds. Neuronal superpositions of distributed feature-processors are intuitively just “noise”, not phenomenally-bound perceptual objects. At best, the idea that sub-femtosecond neuronal superpositions could underpin our experience of law-like classicality is implausible. Yet we’re not looking for plausible theories but testable theories. Every second of selection pressure in Zurek’s sense (cf. “Quantum Darwinism”) sculpting one’s neocortical world-simulation is more intense and unremitting than four billion years of evolution as conceived by Darwin. My best guess is that interferometry will disclose a perfect structural match. If the non-classical interference signature doesn’t yield a perfect structural match, then dualism is true.

Is the quantum-theoretic version of the intrinsic nature argument for non-materialist physicalism – more snappily, “Schrödinger’s neurons” – a potential solution to the measurement problem? Or a variant of the “word salad” interpretation of quantum mechanics?
Sadly, I can guess.
But if there were one experiment that I could do, one loophole I’d like to see closed via interferometry, then this would be it.