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Comment by blackhole | Hacker News Reader

The part of the article that bothers me here the most is a leading nueroscientist asserting that the brain is not computable. That is demonstrably false: The brain is a quantum system, just like everything else in the universe. All quantum systems containing n qubits can be simulated by 2^n classical bits. It may very well be impractical to compute a decent sized brain, but that's still technically computable, which is a serious problem in his whole "you can't reduce human nature to a computer algorithm" tirade. Yes, you can. It will be impossibly huge and slow, but you can.

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The whole question of whether everything in the universe is computable is usually regarded as an open question, sometimes called the Church-Turing-Deutsch thesis. This wikipedia article has a little more: http://en.wikipedia.org/wiki/Church%E2%80%93Turing%E2%80%93D...

Lots of possibilities allowed by general relativity (e.g. computers orbiting black holes and information traveling backwards through time) that may or may not correspond to our universe allow for computation beyond that of a Turing machine. Until we understand quantum gravity it seems premature to write off those possibilities.

Even simpler, there is the question of whether the universe allows for arbitrary precision measurement of a physical quantity. If it does, then it may turn out that an observable physical quantity (e.g. the mass or charge of a fundamental particle) is a noncomputable real number. If it doesn't, then it may be possible to represent a neural network with noncomputable real weights, but impossible to actually measure them to enough accuracy to simulate the network on a given input.

But that's completely irrelevant when it comes to the functioning of the brain, which operates in the oh so mundane medium size, medium velocity, medium energy scale which modern physics is able to model extremely accurately.

I know that a few folks (Penrose, most notably) think that quantum gravity may be relevant to the brain, but that's an extreme fringe opinion, and in Penrose's case it's practically earned him full fledged crackpot status despite his numerous indisputable successes in physics.

While it seems unlikely that relativistic effects would play a role in the brain, the original argument also said that the same logic should apply to anything in the universe, e.g. a solar system or a galaxy, which is why I mentioned it.

>Yes, you can. It will be impossibly huge and slow, but you can.

A major problem is many of the brain's processes use "chaotic models". These are what we associate with the "butterfly effect" where extremely tiny deviations from the true value make the model diverge from actual behavior. Most models tend to have some tiny deviation from the true value but it usually doesn't matter. In this case it does, and it may be an insurmountable problem.

The brain might be computable but that doesn't mean a classic turing machine would work. Maybe one day we can build a biological protein-based computer that shares the "irreducible properties" of brains, but then all we've really built is a brain.

I'm just wondering, how does this statement make sense if we replace the "brain" with "a block of wood" since they are both a "quantum system, just like everything else in the universe":

"The part of the article that bothers me here the most is a leading lumberjack asserting that a block of wood is not computable. That is demonstrably false: a block of wood is a quantum system, just like everything else in the universe. All quantum systems containing n qubits can be simulated by 2^n classical bits. It may very well be impractical to compute a decent sized block of wood, but that's still technically computable."

What would it mean to say that a block of wood is computable?

What you're saying is merely postulation, it's not "demonstrably" true, not to mention your use of the term "impractical" is probably the most misleading use of that word I've read today.

You're writing off thousands of years of philosophy and dozens of years of experimental results in maybe a paragraph, if I'm being generous.

It actually IS demonstrably true that the brain is made of atoms. That makes it a quantum system, which our modern physics tells us how to simulate. The use of the term "impractical" here refers to the fact that we don't have quantum computers and that we don't have the computing power required to simulate such a large quantum system.

I'd say dismissing thousands of years of philosophy here is relevant, since it was likely produced before we had the tools to understand what we're dealing with. All experimental physics points towards an understanding of how quantum systems work, and that's all we need to model any quantum system. The brain is not any different because it's a brain.

quantum theory is a hypothesis. perhaps it's the best one so far for what the brain is made from. but it's truth is only as demonstrable as this: it's not yet been falsified.

You can say the exact same thing about any scientific model. My view on this has always been that as long as the model accurately predicts experimental results, assume it is correct for your calculations until it is proven to be wrong.

Even then, we never stopped using classical mechanics even though they were proven to be wrong at a variety of scales. They just happen to very closely approximate reality in some contexts and are useful.

The fact of the matter is, we have tools that are correct as far as we know and they point towards thinking that every quantum system is computable. Until this has been proven wrong, the fallacy is believing the brain is different, not the other way around.

Theories not only have to predict outcomes of events, they must also be falsifiable (and must expand on something thus far unexplained by other theories, you can't just recreate gravitational theory, for example).

You are, by your own admission, working with an incomplete understanding of how a scientific model functions. So I ask you, why should you be even commenting on this topic? Why should anyone take what you have to say seriously on this specific topic?

So no one should be commenting on this topic unless they have a perfect understanding of scientific theory? That seems terribly counter-productive.

I'm commenting on this topic to share my opinion and, to the extent of my knowledge, try to explain why I believe someone else's reasoning is flawed.

Now if you believe my reasoning is false, you're free to call that out. You're not free, however, to dismiss my contribution to the discussion simply because I'm not operating under perfect understanding of a field that isn't mine.

Call it out, explain why, participate in the discussion, and drop the personal attacks. I think at least part of my point is valid, even after what you pointed out.

I'm pretty sure I am free to dismiss your contribution "simply" because you don't know what you're talking about.

But let's not get caught in the weeds here; I don't think you're correctly conveying the level of certainty with which we understand quantum mechanics. There's a ton we don't have the slightest idea about in this area of science, so let's not forget that.

Quantum theory is a "hypothesis"? The entire foundation of modern electronics is based on quantum theory. It's something that has been tested over and over in labs and the regular world for over a century.

It's been tested and proven FALSE, just look at relativistic effects. We just don't have anything better.

You're confusing hypothesis with theory. Hypothesis is a proposed explanation that can be tested (i.e. is falsifiable). Theory, on the other hand, is a hypothesis that has undergone extensive testing and has been proven to be a plausible explanation for observed phenomena. Quantum mechanics has undergone rigorous testing, and has proven time and time again that it can accurately explain many of the properties of our universe.

Yes, the brain is made up of atoms, but our modern physics does not, at least not with the degree of certainty you're waving about, tell us that it is therefore, necessarily, simulatable.

I believe this to be the case, and there is some evidence that this is the case, but it is not anywhere near as certain as you're claiming.

As for your word choice re: impractical, the word shouldn't be used in place of 'impossible', which is the correct word you're looking for.

> What you're saying is merely postulation, it's not "demonstrably" true

I don't think the parent disagrees with you. The question of whether or not something is _computable_ in the technical sense of the word does not need any sort of demonstration or experimental results.

Actually, when we assume that any particular physical system is computable, that's exactly what it is - an assumption, there is currently no strong evidence that all physical systems are computable, and despite the fact that it seems most people (including experts) believe that they are, it's still usually considered an open question.

> Viewed from the right angle, the CTD Principle still is shocking. All we have to do is look at it anew. How odd that there is a single physical system – albeit, an idealized system, with unbounded memory – which can be used to simulate any other system in the Universe!

http://michaelnielsen.org/blog/interesting-problems-the-chur...

> You're writing off thousands of years of philosophy and dozens of years of experimental results in maybe a paragraph, if I'm being generous.

I'm not the parent poster, but I'd like a citation re: experimental results. What are you saying exactly?

You're being overly tight with your definition. We still don't completely know how the brain works, and while we have a top-down discipline and a bottom-up discipline, we still haven't figured out how they meet in the middle - and the fundamental point he's making is that there's nothing to say that we will get those algorithms. Until we get to that stage, regardless of whether it's theoretically feasible if you had a magic emulation tool, it's not feasible because we don't have the algorithms to emulate. And if we get to that stage, 'we made a brain emulator!' is one of the smaller stories, since things like 'we have answered the questions of whether free will and the soul exist' will be far more profound.

Technically, you'd have to say that brain's output is combination of computable system and a perfect random noise source.

That is because a simple source of quantum noise should not be computable. Any computable pseudo-random noise generator produces strings of digit with asymptotically constant Kolmagorov complexity while the Kolmagorov complexity of a true random number generator goes to infinity.

All that said, the person is article is certainly overreaching to imply we can know the brain definitely isn't computable. Clearly he does have any kind of knowledge that would tell him that. This might enough to say since the rest of his argument seems to hinge on this.

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