To clarify, you are asking whether you need a quantum computer to efficiently simulate a brain, right? (Technically: brain simulation has BQP complexity, but not P.) So, if the brain has some quantum effects, but they aren't computationally relevant, it will resolve to NO, right?

If this understanding is correct, the current probability is unbelievably high. We have a plausible classical models of how brain might work in neural networks, and not even a vaguest idea of how quantum computing could be involved in the brain.

@OlegEterevsky My personal belief is around 5%. But this is one of those questions where it is not clear what will be the resolution source, and I find it quite likely that even the resolution might end up opinion based, so I am not betting it down.

Even among scientists, there are groups that have strong and opposite opinions, so the full consensus seems far. (And it is unlikely we will emulate a brain upload any time soon).

@IsaacKing Can you confirm that efficient classical simulation of functional brain would resolve this NO?

@Irigi Once we understand brain mechanism well enough to simulate it (and I'm hoping we are not too far from that moment), this could be resolved based on whether the brain could be efficiently simulated on a classical computer or not. That's pretty unambiguous.

Here's a somewhat relevant question that I wrote today:

Seems like there should be a distinction between something can be simulated classically and something cannot function without quantum effects. For example, is the vision system considered part of the brain for the purposes of the question?

@bashmaester Quantum mechanics can be simulated classically, only not with so good algorithmic complexity.

Does "then that isn't sufficient to resolve this YES" mean it resolves NO or does it mean it resolves N/A?

@ArmandodiMatteo Depends on what the actual answer is.

@IsaacKing what do you mean exactly?

Out of the four possibilities
1. a bacterium would function in a classical simulation, and so would a brain
2. a bacterium would function in a classical simulation, but a brain wouldn't
3. a bacterium wouldn't function in a classical simulation, so neither would a brain
(and in principle 4. a bacterium wouldn't function in a classical simulation, but a brain would -- though the chances of that are negligible, unless you use different standards for "functioning" and for "classical simulation" in the two cases)

1 (or 4) would resolve NO, and 2 would resolve YES.
You're saying that 3 wouldn't resolve YES, but would it resolve NO, or would it resolve N/A?

I strongly suspect that quantum effects provide the brain with the ability to process information not sequentially or in parallel, but simultaneously. Subjectively, this looks like a so-called gestalt. I think I will live to see the day when this is proven.

@Shalun do you have any ideas of how that could work?

@CodeandSolder I personally think quantum entanglement is relevant. A more detailed answer should be in the book The Relativistic Brain: How it works and why it cannot be simulated by a Turing machine. But I still haven’t read it, because there is no Russian translation, and I procrastinate reading in English.

@Shalun quantum entanglement can be simulated by a Turing machine in exponential time, so the book is wrong or irrelevant if you want to know about quantum effects rather than some kind of spooky uncomputable new physics.

@AMS from an Amazon review of the book:

In the authors’ example, a protein (as a tiny [Oracle] machine) finds its optimal 3-D configuration – an intractable computational problem – in an instant by following the laws of physics in the analog domain.

I count four or five different serious errors in this single sentence:

• Proteins are not oracle machines; they run on Turing-computable physics

• They do not always find the same "optimal" configuration (ever heard of a prion?)

• Finding the lowest energy configuration is Turing computable, and only intractable in the weaker sense of maybe being NP-hard (I'm not certain of the class)

• They don't fold in an instant, the timescale is quite significant on a molecular scale

• (Bonus) The laws of physics may be analog but thermal systems are noisy, so the analog/digital distinction is not important for computational considerations here.

@AMS and the only reason the ones we use are that robust in finding the desired configuration is that using ones that don't is a huge evolutionary disadvantage lol

As others have alluded to, "the classical laws of physics" is extremely ambiguous -- there are many different ways to use non-quantum physics to approximate quantum physics, including most of chemistry. Most "quantum effects" can be treated classically in one way or another. Every argument about whether a phenomenon is a "quantum effect" devolves into muddy semantics, in my experience. Photons are a quantum effect from the perspective of classical electromagnetism (Maxwell's field theory). Electromagnetic fields are a quantum effect from the perspective of classical particle physics (as applied to photons).

As a clean dividing line, there are some quantum phenomena that can't be efficiently simulated classically, notably quantum computation. If the brain is "a quantum computer" in this sense, then it can't be simulated classically without an exponential-in-brain-size slowdown. This isn't completely rigorous (to a computer scientist) because brains have a constant size, but I expect it to (eventually) be extremely clear that the brain can either be simulated in something like "polynomial time" by a classical computer, or it can't.

So I would propose: the brain is quantum if it cannot be simulated in polynomial(brain_size) time by a classical computer.

@AMS (Penrose seems to be making a stronger claim, but I think it would be totally fair to call the brain quantum even if it were "merely" a BQP machine.)

What if there is some quantum thing that is a key part of how the brain happens to work in practice but a classical brain could replace the quantum thing with some extremely simple modification?

@NoaNabeshima sounds like a quantum brain to me

@NoaNabeshima That would resolve YES. If the brain is computable there's guaranteed to be a way to simulate it non-quantumly, so the question is about whether the actual current implementation of brains is such.

This is already known to be true, electron transport uses quantum tunneling to work. Quantum tunneling is also used by ordinary computers to pass electricity between metallic contacts with an oxide layer (e.g. an ordinary switch). Everything in the universe is quantum in the trivial sense that the universe is quantum. I'm pretty sure this is not what the "quantum consciousness" people mean when they say that quantum effects are needed to explain consciousness.

@adele Do they claim that consciousness is uncomputable?

@IsaacKing AFAICT, essentially yes, at least by a classical computer.

Roger Penrose said:
> A lot of what the brain does you could do on a computer. I'm not saying that all the brain's action is completely different from what you do on a computer. I am claiming that the actions of consciousness are something different. I'm not saying that consciousness is beyond physics, either—although I'm saying that it's beyond the physics we know now.... My claim is that there has to be something in physics that we don't yet understand, which is very important, and which is of a noncomputational character.

All chemistry requires quantum effects but quantum effects are too low a level of abstraction for anything larger than a single serotonin molecule

What are the views of this market's participants in view of this paper? https://arxiv.org/abs/1910.08423

Touches on ideas due to Penrose and Hameroff: https://en.m.wikipedia.org/wiki/Orchestrated_objective_reduction

@TurnipPotato I did not read the article in detail. I did my Ph.D. on quantum effects in photosynthesis mentioned in the study. ("Coherent energy transfer in photosynthesis is reimagined in the tryptophan rings of neural microtubules.") I hold view that many models of coherent energy transfer suffer from confusion in terminology of what is entanglement. Many times, the entanglement is measured on density matrix in basis of molecules before they start interacting, and it corresponds to cross-elements in the density matrix. But the cross elements of the density matrix very often correspond not to the true entanglement, but to the basis redefinition due to interaction of the two molecules. Another point to the coherent energy transfer is that, in fact, loss of coherence is needed to gain true energy transfer to the lower energy states in photosynthetic antennaes. Without it, the mean energy of the excited states remain too high to occupy the lower energy state and the energy transfer is hindered or prohibited.

I am very skeptical to the quantum brain hypothesis in general. Two main points:
1] To receive any quantum effects in our thinking, the superpositions would have to span much bigger spatial scales than microtubules, and last on the timescale of ~0.1 s. This is because that is the scale on which the information is transferred between neurons. Both scales are very much off.
2] I should study the reference 48 in greater detail (if I ever find time), but other references (not only Tegmark, but for example Dieter Zeh) give much shorter decoherence time than 10^-4 s, more like 10^-18 - 10^-20 s. One must be very careful on what exactly is the Hilbert subspace subject to the decoherence. Sure, there might be sub-spaces decohering more slowly (spins of nuclei, for example), but they are not necessarily the ones relevant for the "quantum thinking".