Most truly radical intellectual progress depends on “crazy” conjectures. Unfortunately, few folk who make crazy conjectures give serious thought to extracting novel, precise, experimentally falsifiable predictions to confound their critics. Even fewer then publish the almost inevitable negative experimental result when their crazy conjecture isn’t confirmed. So kudos to Andrés for doing both!!
What would the world look like if the superposition principle never breaks down, i.e. the unitary Schrödinger dynamics holds on all scales, and not just the microworld? The naïve – and IMO mistaken – answer is that without the “collapse of the wavefunction”, we’d see macroscopic superpositions of live-and-dead cats, experiments would never appear to have determinate outcomes, and the extremely well tested Born rule (i.e. the probability of a result is the squared absolute value of the inner product) would be violated. Or alternatively, assuming DeWitt’s misreading of Everett, if the superposition principle never breaks down, then when you observe a classical live cat, or a classical dead cat, your decohered (“split”) counterpart in a separate classical branch of the multiverse sees a dead cat or a live cat, respectively.
In my view, all these stories rest on a false background assumption. Talk of “observers” and “observations” relies on a naïve realist conception of perception whereby you (the “observer”) somehow hop outside of your transcendental skull to inspect the local mind-independent environment (“make an observation”). Such implicit perceptual direct realism simply assumes – rather than derives from quantum field theory – the existence of unified observers (“global” phenomenal binding) and phenomenally-bound classical cats and individually detected electrons striking a mind-independent classical screen cumulatively forming a non-classical interference pattern (“local” phenomenal binding). Perception as so conceived – as your capacity for some sort of out-of-body feat of levitation – isn’t physically possible. The role of the mind-independent environment beyond one’s transcendental skull is to select states of mind internal to your world-simulation; the environment can’t create, or imprint its signature on, your states of mind (“observations”) – any more than the environment can create or imprint its signature on your states of mind while you’re dreaming.
Here’s an alternative conjecture – a conjecture that holds regardless of whether you’re drug-naïve, stone-cold sober, having an out-of-body experience on ketamine, awake or dreaming, or tripping your head off on LSD. You’re experiencing “Schrodinger’s cat” states right nowin virtue of instantiating a classical world-simulation. Don’t ask what’s it like to perceive a live-and-dead Schrödinger’s cat; ask instead what it’s like to instantiate a coherent superposition of distributed feature-processing neurons. Only the superposition principle allows you to experience phenomenally-bound classical objects that one naively interprets as lying in the mind-independent world. In my view, the universal validity of the superposition principle allows you to experience a phenomenally bound classical cat within a seemingly classical world-simulation – or perform experiments with classical-looking apparatus that have definite outcomes, and confirm the Born rule. Only the vehicle of individual coherent superpositions of distributed neuronal feature-processors allows organic mind-brains to run world simulations described by an approximation of classical Newtonian physics. In the mind-independent world – i.e. not the world of your everyday experience – the post-Everett decoherence program in QM pioneered by Zeh, Zurek et al. explains the emergence of an approximation of classical “branches” for one’s everyday world-simulations to track. Yet within the CNS, only the superposition principle allows you to run a classical world-simulation tracking such gross fitness-relevant features of your local extracranial environment. A coherent quantum mind can run phenomenally-bound simulations of a classical world, but a notional classical mind couldn’t phenomenally simulate a classical world – or phenomenally simulate any other kind of world. For a supposedly “classical” mind would just be patterns of membrane-bound neuronal mind-dust: mere pixels of experience, a micro-experiential zombie.
Critically, molecular matter-wave interferometry can in principle independently be used to test the truth – or falsity – of this conjecture (see: https://www.physicalism.com/#6).
OK, that’s the claim. Why would (almost) no scientifically informed person take the conjecture seriously?
In a word, decoherence.
On a commonsense chronology of consciousness, our experience of phenomenally bound perceptual objects “arises” via patterns of distributed neuronal firings over a timescale of milliseconds – the mystery lying in how mere synchronised firing of discrete, decohered, membrane-bound neurons / micro-experiences could generate phenomenal unity, whether local or global. So if the lifetime of coherent superpositions of distributed neuronal feature-processors in the CNS were milliseconds, too, then there would be an obvious candidate for a perfect structural match between the phenomenology of our conscious minds and neurobiology / fundamental physics – just as I’m proposing above. Yet of course this isn’t the case. The approximate theoretical lifetimes of coherent neuronal superpositions in the CNS can be calculated: femtoseconds or less. Thermally-induced decoherence is insanely powerful and hard to control. It’s ridiculous – intuitively at any rate – to suppose that such fleeting coherent superpositions could be recruited to play any functional role in the living world. An epic fail!
Let’s step back.
Many intelligent people initially found it incredible that natural selection could be powerful enough to throw up complex organisms as thermodynamically improbable as Homo sapiens. We now recognise that the sceptics were mistaken: the human mind simply isn’t designed to wrap itself around evolutionary timescales of natural selection playing out over hundreds of millions of years. In the CNS, another form of selection pressure plays out – a selection pressure over one hundred of orders of magnitude (sic) more powerful than selection pressure on information-bearing self-replicators as conceived by Darwin. “Quantum Darwinism” as articulated by Zurek and his colleagues isn’t the shallow, tricksy metaphor one might naively assume; and the profound implications of such a selection mechanism must be explored for the world-simulation running inside your transcendental skull, not just for the extracranial environment. At work here is unimaginably intense selection pressure favouring comparative resistance to thermally (etc)-induced decoherence [i.e. the rapid loss of coherence of complex phase amplitudes of the components of a superposition] of functionally bound phenomenal states of mind in the CNS. In my view, we face a failure of imagination of the potential power of selection pressure analogous to the failure of imagination of critics of Darwin’s account of human evolution via natural selection. It’s not enough lazily to dismiss sub-femtosecond decoherence times of neuronal superpositions in the CNS as the reductio ad absurdum of quantum mind. Instead, we need to do the interferometry experiments definitively to settle the issue, not (just) philosophize.
Unfortunately, unlike Andrés, I haven’t been able to think of a DIY desktop experiment that could falsify or vindicate the conjecture. The molecular matter-wave experiment I discuss in “Schrodinger’s Neurons” is conceptually simple but (horrendously) difficult in practice. And the conjecture it tests is intuitively so insane that I’m sometimes skeptical the experiment will ever get done. If I sound like an advocate rather than a bemused truth-seeker, I don’t mean to be so; but if phenomenal binding _isn’t _quantum-theoretically or classically explicable, then dualism seems unavoidable. In that sense, David Chalmers is right.
How come I’m so confident that superposition principle doesn’t break down in the CNS? After all, the superposition principle has been tested only up to the level of fullerenes, and no one yet has a proper theory of quantum gravity. Well, besides the classical impossibility of the manifest phenomenal unity of consciousness, and the cogent reasons that a physicist would give you for not modifying the unitary Schrödinger dynamics, the reason is really just a philosophical prejudice on my part. Namely, the universal validity of the superstition principle of QM offers the only explanation-space that I can think of for why anything exists at all: an informationless zero ontology dictated by the quantum analogue of the library of Babel.
We shall see.
– David Pearce, commenting on the latest significant article published on this blog.