This man is now my personal hero.
Suddenly I understand why so many bright people were drawn to work with McCarthy. :)
This might have been true in the days before anyone had the concept of macroscopic decoherence (popularly known as many-worlds) but quantum physics is pretty normal these days. See the recent series at Overcoming Bias.
[This is not to say that those researchers are unintelligent, just that a certain bias may exist.]
http://www.overcomingbias.com/2008/04/on-being-decohe.html
Of course, I have a Ph.D. in semiconductor lasers, so I've been trying to think about such stuff off and on for years... which means I'm a terrible test case for articles like this. ;)
It's not as if there aren't many, many great physicists who have failed to understand quantum mechanics. Max Planck invented the field but never understood it. And Einstein, if he did indeed understand it, never stopped wishing that he didn't have to.
Anyway, I haven't read Penrose's arguments -- geez, I barely take time to read credible science these days -- so I'm not dismissing him: I'm outsourcing my dismissal of him to guys like Dennett. Meanwhile, I'll happily accept your assertion that Penrose understands quantum mechanics perfectly well even if he enjoys abusing it for philosophical kicks, kind of like how the screenwriters who wrote Gladiator claim to have actually read real Roman history before they reinvented it for the screen.
From now on I'm outsourcing all my dismissiveness to Daniel Dennett. I just don't have the time to be properly dismissive, and he's so much better at it than I am anyway.
I also think he's wrong, but at least he's not infatuated about it.
edit: I love his books. Well worth the read, and for those concerned he puts his speculations in chapters very clearly marked "speculations". From his books I first learned the details about Turing machines and lambda calculus, and a lot more.
It all hangs, not on quantum mechanics, but on his initial assertion. I've read his explanation in Shadows of the Mind, but the lightbulb didn't go on for me. For McArthy to dismiss the question offhand was a little disappointing. I'd expect him to have a deeper insight.
What the quantum intelligence does is take one unresolved problem (how do we think) and replace it with another (we think with quantum computing, but we don't know exactly how). Its only result is to take away the unknown and replace it with an incomplete explanation.
I also don't think Penrose would have come to this conclusion now. Cognitive psychology has taken enormous strides in the past years, and it's already pretty clear it's on the right track. Take a look for example at "The Emotion Machine" by Marvin Minsky (who by the way is on par with McCarthy in AI but chose cognitive psychology as a main field: http://en.wikipedia.org/wiki/Marvin_Minsky).
[1] Spivey, M., Grosjean, M. & Knoblich, G. (2005). Continuous attraction toward phonological competitors. Proceedings of the National Academy of Sciences, 102(29), 10393-10398.
Q. Is there anything in principle that would prevent a computer from thinking as a human would?
A. No
IOW, there's still no recognition today, on the side of the purveyors of "classical" AI, that anything except digital processing might be needed for a computer to think "as a human would". So the big money is likely to continue being thrown at attempts to emulate animal brains using purely digital means. And I suspect that these funds might largely be better spent elsewhere.
People get confused because it's easy to look at a quantum mechanics problem from the wrong angle and see nondeterminism. We're really well trained in intuitive mechanics ("an electron is like a tiny tennis ball, and tennis balls are always either here or there, right?") so at first glance quantum mechanics seems wacky and random: the electron might be here, or it might be there, with equal probability, and we can't tell which! Whereupon your head explodes. Seriously: The discoverers of quantum exploded in horror, and they started ranting about that crazy cat in the box, or "God playing dice". (Colorful but misleading metaphors. One reason that Bohr and Einstein's early confusion persists today is that they were just so darned eloquent.)
In fact, God does not play dice: God sees all the outcomes of the dice roll at the same time and doesn't understand why we think it's a game, and not a static work of art.
People also think that nondeterminism is somehow an important ingredient in whatever it is we mean by "free will". This doesn't make much sense to me. If you want it to not make sense to you as well, read Daniel Dennett's Freedom Evolves. You might want to budget more than a couple hours for that book, though.
From [http://www.fourmilab.ch/hotbits/how3.html]
"But hidden variables aren't the way our universe works—it really is random, right down to its gnarly, subatomic roots. In 1964, the physicist John Bell proved a theorem which showed hidden variable (little clock in the nucleus) theories inconsistent with the foundations of quantum mechanics. In 1982, Alain Aspect and his colleagues performed an experiment to test Bell's theoretical result and discovered, to nobody's surprise, that the predictions of quantum theory were correct: the randomness is inherent—not due to limitations in our ability to make measurements. So, given a Cæsium-137 nucleus, there is no way whatsoever to predict when it will decay. If we have a large number of them, we can be confident half will decay in 30.17 years; but if we have a single atom, pinned in a laser ion trap, all we can say is that is there's even odds it will decay sometime in the next 30.17 years, but as to precisely when we're fundamentally quantum clueless. The only way to know when a given Cæsium-137 nucleus decays is after the fact—by detecting the ejecta. A Cæsium-137 nucleus which has “beat the reaper” by surviving a century, during which time only one in a thousand of its litter-mates haven't taken the plunge and turned into Barium, has precisely the same chance of surviving another hundred years as a newly-minted Cæsium-137, fresh from the reactor core."
The short answer is that in the many-worlds interpretation, this too is a deterministic process. The total wavefunction of the whole system (atom + observer) evolves deterministically. What isn't deterministic is "your" subjective view of it, but "you" only view a vanishingly small slice of reality.
Sorry, that's the best three-sentence explanation I can come up with right now, and I admit it's only a shade better than "trust me, I'm a physicist". But trust me, I'm a physicist.
Edit. Nevermind, I see what you mean.
I'm game if you are, though. You might start by thinking about a modern digital computer. It's made up of a very large number of very small devices (transistors) that sling populations of electrons back and forth. The brain is made of a much larger number of less-small devices called neurons, that sling bigger populations of bigger objects (organic molecules) back and forth. So according to the basic understanding of QM that tells us relative indeterminacy increases when considering smaller objects, the functioning of the human brain is marginally more predictable than the insides of a modern computer.