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shminux comments on Who thinks quantum computing will be necessary for AI? - Less Wrong
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No serious neurologists actually consider quantum effects inside microtubules or arrangements of phosporyulation on microtubules or whatever important for neuron function. They're all either physicists who don't understand the biology or computer scientists who don't understand the biology. Nothing happens in neural activity or long-term-potentiation or other processes that cannot be accounted for by chemical processes, even if we don't understand exactly the how of some of them. The open questions are mostly exactly how neurons are able to change their excitability and structure over time and how they manage to communicate in large scale systems.
Actually, protein phosphorylation (like many other biochemical and biophysical processes, such as ion channel gating) is based on quantum tunneling. It may well be irrelevant, as the timing of the process can probably be simulated well enough with pseudo-random numbers, but on an off-chance that "true randomness" is required, a purely classical approach might be inadequate.
Quantum tunneling != quantum computing.
Quantum 'randomness' != quantum computing. No one has ever introduced, even in principle, a cognitive algorithm that requires quantum 'randomness' as opposed to thermal noise.
Holy crap that comment (posted very quickly from a tablet hence the typos) produced a long comment thread.
Yes quantum tunneling goes on in a lot of biological processes because it happens in chemistry. There is nothing special about neurology there. I was mostly referring to writings I've seen where someone proposed that humans must be doing hypercomputation because we dont blow up at the godel incompleteness theorem (which made a cognitive scientist in my circle laugh due to the fact that we just don't actually deal with the logic) and another that actually was posted here that proposed that digital information was somehow being stored in the pattern of phosphorylation of subunits of microtubules (which made multiple cell biologists laugh because those structures are so often erased and replaced and phosphorylation is ridiculously dynamic and moderated by the randomness of enzymes hitting substrates via diffusion and not retained on any one molecule for long). In the end it mostly serves to just modify the electrical properties of the membranes and their ability to chemically affect and be affected by each other.
As for 'true randomness', we don't run on algorithms, we run on messy noisy networks. If we must frame the way cells work in terms of simulation of gross behavior its a whole lot more like noisy differential equations than discrete logic. I fail to see any circumstance in which you need quantum effects to make those behave as they usually do.
On top of that, every single cell is a soup of trillions of molecules bouncing off each other at dozens of meters per second like lottery balls. If that's not close enough to 'true randomness', such that you somehow need quantum effects like the decay of atoms, what is?
How could "true randomness" be required, given that it's computationally indistinguishable from pseudorandomness?
If there is a feasible psuedorandomness generator that is computationally indistinguishable from randomness, then randomness is indeed not necessary. However, the existence of such a pseudorandomness generator is still an open problem.
What? No it's not. There are no pseudo-random generators truly ultimately indistinguishable in principle from the 'branch both ways' operation in quantum mechanics, the computations all have much lower Kolmogorov complexity after running for a while. There are plenty of cryptographically strong pseudo-random number generators which could serve any possible role a cognitive algorithm could possibly demand for a source of bits guaranteed not to be expectedly correlated with other bits playing some functional role, especially if we add entropy from a classical thermal noise source, the oracular knowledge of which would violate the second law of thermodynamics. This is not an open problem. There is nothing left to be confused about.
A proof that any generator was indistinguishable from random, given the usual definitions, would basically be a proof that P != NP, so it is an open problem. However we're pretty confident in practice that we have strong generators.
As a pedantic note, if you want to derandomize algorithms it is necessary (and sufficient) to assume P/poly != E, i.e. polynomial size circuits cannot compute all functions computed by exponential time computations. This is much weaker than P != NP, and is consistent with e.g. P = PSPACE. You don't have to be able to fool an adversary, to fool yourself.
This is sometimes sloganized as "randomness never helps unless non-uniformity always helps," since it is obvious that P << E and generally believed that P/poly is about as strong as P for "uniform" problems. It would be a big shock if P/Poly was so much bigger than P.
But of course, in the worlds where you can't derandomize algorithms in the complexity-theoretic sense, you can still look up at the sky and use the whole universe to get your randomness. What this means is that you can exploit much of the stuff going on in the universe to do useful computation without lifting a finger, and since the universe is so much astronomically larger than the problems we care about, this is normally good enough. General derandomization is extremely interesting and important as a conceptual framework in complexity theory, but useless for actually computing things.
Are you referring to this result? Doesn't seem to be identical to what you said, but very close.
Yeah, I was using "derandomize" slightly sloppily (to refer to a 2^n^(epsilon) slowdown rather than a poly slowdown). The result you cite is one of the main ones in this direction, but there are others (I think you can find most of them by googling "hardness vs. randomness").
If poly size circuits can't compute E, we can derandomize poly time algorithms with 2^(m^c) complexity for any c > 0, and if 2^(m^c) size circuits can't compute E for sufficiently small c, we can derandomize in poly time. Naturally there are other intermediate tradeoffs, but you can't quite get BPP = P from P/poly < E.
Can you refer me to somewhere to read more about the "usual definitions" that would make this true? If I know the Turing machine, I can compare the output to that Turing machine and be pretty sure it's not random after running the generator for a while. Or if the definition is just lack of expected correlation with bits playing a functional role, then that's easy to get. What's intermediate such that 'indistinguishable' randomness means P!=NP?
You don't sound like you're now much less confident you're right about this, and I'm a bit surprised by that!
I got the ladder down so I could get down my copy of Goldreich's "Foundations of Cryptography", but I don't quite feel like typing chunks out from it. Briefly, a pseudorandom generator is an algorithm that turns a small secret into a larger number of pseudorandom bits. It's secure if every distinguisher's advantage shrinks faster than the reciprocal of any polynomial function. Pseudorandom generators exist iff one-way functions exist, and if one-way functions exist then P != NP.
If you're not familiar with PRGs, distinguishers, advantage, negligible functions etc I'd be happy to Skype you and give you a brief intro to these things.
There are also intros available for free on Oded Goldreich's FoC website.
Here's my simplified intuitive explanation for people not interested in learning about these technical concepts. (Although of course they should!) Suppose you're playing rock-paper-scissors with someone and you're using a pseudorandom number generator, and P=NP, then your opponent could do the equivalent of trying all possible seeds to see which one would reproduce your pattern of play, and then use that to beat you every time.
In non-adversarial situations (which may be what Eliezer had in mind) you'd have to be pretty unlucky if your cognitive algorithm or environment happens to serve as a distinguisher for your pseudorandom generator, even if it's technically distinguishable.
Okay, makes sense if you define "distinguishable from random" as "decodable with an amount of computation polynomial in the randseed size".
EDIT: Confidence is about standard cryptographically strong randomness plus thermal noise being sufficient to prevent expected correlation with bits playing a functional role, which is all that could possibly be relevant to cognition.
Decoding isn't the challenge; the challenge is to make a guess whether you're seeing the output of the PRG or random output. Your "advantage" is
Adv_PRG[Distinguisher] = P(Distinguisher[PRG[seed]] = "PRG") - P(Distinguisher[True randomness] = "PRG")
Note that this is standard notation when one discusses pseudorandom generators. Hence Ciphergoth's comment about "the usual definitions."
For it to be an open problem, there would have to not be a proof either way. Since Eliezer is claiming (or, at least, implying) that there is a proof that there is no PRNG indistinguishable, arguing that there is no proof that there is a PRNG indistinguishable doesn't show that it is an open problem.
Quite. They seem to be agreeing that any PRNG can in principle distinguished, and then Eliezer goes on to say that a mind is a place that will not be able to make that distinction - which ciphergoth didn't begin to address.
You missed the key word "computationally". Of course a pseudorandom generator is a mathematically distinct object, but not in a way that the universe is capable of knowing about (at least assuming that there are cryptographic pseudorandom generators that are secure against quantum adversaries, which I think most people believe).
Even if the Mersenne twister isn't good enough, you could still get a quantum noise generator hooked up. And that's basically a classical device, certainly doesn't need any coherence.
In case anybody needs one: ANU Quantum Random Numbers Server
I suppose we ought to define what "classical" and "quantum" means.
It's a quantum effect, but it's one that's easily taken advantage of, as opposed to the crazy difficult stuff a quantum computer can do. As such, a computer that can do that can be considered classical.
For that matter, transistors work by exploiting quantum effects. We still don't call them quantum computers.
Thanks for the first paragraph. I came here to clarify this, but you beat me to it.
More clearly: a quantum noise generator can have a design such that someone who only understands classical mechanics will understand based on that design that it is a noise generator. They just won't catch the detail that this noise has an additional property.
The above statement may depend on the implementation, but I meant in principle, so there it is.
Someone who only understands classical mechanics will not understand a noise generator. Classical physics is deterministic, so noise generators are impossible.
Only if you're omniscient. A noise generator is a way of controllably injecting your ignorance of some system into a particular channel.
You don't need a quantum computer to exploit quantum effects for random number generation. I've heard it's common to do that by sending electricity backwards through a diode and amplifying it.