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A cell is a physical system. A physical system can either: 1) Be sufficiently large that quantum effects are irrelevant, like a billiard ball, and thus it can

by bascule 14y ago

A cell is a physical system. A physical system can either: 1) Be sufficiently large that quantum effects are irrelevant, like a billiard ball, and thus it can be modeled deterministically inside of a computer. The Tegmark paper proposes this is how the brain works. 2) Rely on quantum mechanical effects (which are non-deterministic, but can still be simulated inside a computer if you trust a computer's opinion of "random"). Penrose, Searle, etc. would argue otherwise, that there's a method to the madness, and that there's something "special" about the quantum mechanical effect on microtubules. We still don't understand quantum mechanics very well, so if human consciousness really relies on quantum mechanics, there is arguably a bit of wiggle room here, especially if you're a physicist of the same caliber as Penrose. Microtubules are specifically of interest because they're one of the few brain structures small enough to arguably be subject to quantum mechanical events. Tegmark argues that they're not small enough and that it's irrelevant. The real question is: Is it possible to simulate a brain within a computer? - not - Is it really hard? Is it too hard for modern technology? Is this problem simply too complex for us to understand? Do we lack the technology to build sufficient understanding today? Etc. Etc. This article is claiming it's IMPOSSIBLE to simulate a brain within a computer. Proving that is a tall order.

lutusp 14y ago
> Be sufficiently large that quantum effects are irrelevant ... This misunderstands the role of quantum theory in macroscopic systems. There is never a scale so large that quantum effects can be safely ignored. All that happens is that the specific effects, and probabilities, change. The Heisenberg Uncertainly principle doesn't build a wall between the microscopic and macroscopic realms, it makes a probabilistic prediction about quantum events at all scales -- larger scale, lower probability. But a macroscopic system that has a very low probability of exhibiting classic quantum behaviors as a whole, will nevertheless show quantum behaviors at some level. An easily understood example is a radioactive sample -- let's say a kilogram of uranium. The sample isn't going to behave like Schrodinger's cat, but its constituent atoms certainly will. In one sense, the uranium is a classical mass with no contribution from quantum theory. In another sense, it's highly influenced by quantum theory -- were this not so, there would be no nuclear disintegrations. An observer can examine the sample and, intent on demonstrating that it's a classical system, use the half-life equation to predict its future -- a' = a 2^-t/f a = activity level at time zero a' = activity level at time t t = time, consistent units f = half-life factor -- And the outcome looks very classical, but only because the sample is large. But the timing of the next disintegration is quantum-deterministic. So the uranium is a chimera -- part classical, part quantum. This is an example that makes the point very clearly, but all classical systems (and scales) possess quantum properties, usually not so obvious as it is with a radioactive sample. > Rely on quantum mechanical effects (which are non-deterministic, but can still be simulated inside a computer if you trust a computer's opinion of "random"). Quantum effects aren't merely random. What connects an event to quantum theory isn't its randomness, but the nature of the randomness -- its genesis. No matter how carefully I design a random number generator, I won't be able to imitate quantum entanglement unless the system is actually capable of this specific physical behavior.
api 14y ago
I agree somewhat. I would not be willing to say impossible, but I am also not convinced it's possible. I would like to see people try. But it could be impossible for a discrete classical digital computer. It's a distinct possibility, and without "magic." I think what I was getting at was this: I'd take the Kurzweil crowd more seriously if I saw them really engaging seriously with the depth and breadth of biology... if I saw them talking about the sorts of things I was talking about seriously instead of hand-waving them away. I get the impression (I'm knowledgeable in biology BTW, and a programmer) that they are making the frequent programmer mistake of failing to have respect for the problem domain they are entering. A similar error often occurs in creative product development. In this case you have a software effort to subsume/encapsulate an informal industrial process that is approximately five billion years old. There are lots of things going on that we don't understand yet, and that will not yield to a cavalier, dismissive approach.

by bascule 14y ago

lutusp 14y ago

> Be sufficiently large that quantum effects are irrelevant ... This misunderstands the role of quantum theory in macroscopic systems. There is never a scale so large that quantum effects can be safely ignored. All that happens is that the specific effects, and probabilities, change. The Heisenberg Uncertainly principle doesn't build a wall between the microscopic and macroscopic realms, it makes a probabilistic prediction about quantum events at all scales -- larger scale, lower probability. But a macroscopic system that has a very low probability of exhibiting classic quantum behaviors as a whole, will nevertheless show quantum behaviors at some level. An easily understood example is a radioactive sample -- let's say a kilogram of uranium. The sample isn't going to behave like Schrodinger's cat, but its constituent atoms certainly will. In one sense, the uranium is a classical mass with no contribution from quantum theory. In another sense, it's highly influenced by quantum theory -- were this not so, there would be no nuclear disintegrations. An observer can examine the sample and, intent on demonstrating that it's a classical system, use the half-life equation to predict its future -- a' = a 2^-t/f a = activity level at time zero a' = activity level at time t t = time, consistent units f = half-life factor -- And the outcome looks very classical, but only because the sample is large. But the timing of the next disintegration is quantum-deterministic. So the uranium is a chimera -- part classical, part quantum. This is an example that makes the point very clearly, but all classical systems (and scales) possess quantum properties, usually not so obvious as it is with a radioactive sample. > Rely on quantum mechanical effects (which are non-deterministic, but can still be simulated inside a computer if you trust a computer's opinion of "random"). Quantum effects aren't merely random. What connects an event to quantum theory isn't its randomness, but the nature of the randomness -- its genesis. No matter how carefully I design a random number generator, I won't be able to imitate quantum entanglement unless the system is actually capable of this specific physical behavior.

api 14y ago

I agree somewhat. I would not be willing to say impossible, but I am also not convinced it's possible. I would like to see people try. But it could be impossible for a discrete classical digital computer. It's a distinct possibility, and without "magic." I think what I was getting at was this: I'd take the Kurzweil crowd more seriously if I saw them really engaging seriously with the depth and breadth of biology... if I saw them talking about the sorts of things I was talking about seriously instead of hand-waving them away. I get the impression (I'm knowledgeable in biology BTW, and a programmer) that they are making the frequent programmer mistake of failing to have respect for the problem domain they are entering. A similar error often occurs in creative product development. In this case you have a software effort to subsume/encapsulate an informal industrial process that is approximately five billion years old. There are lots of things going on that we don't understand yet, and that will not yield to a cavalier, dismissive approach.