On the issue private_messaging raises, I think it's a serious philosophical problem, but not necessarily a practical one (as he claims), assuming Solomonoff Induction could be made practical in the first place, because the hypothetical AI could quickly update away even a factor of 2^1000 when it turns on its senses, before it has a chance to make any important wrong decisions. private_messaging seems to have strong intuitions that it will be a practical problem, but he tends to be overconfident in many areas so I don't trust that too much.
Are you picturing AI that has simulated multiverse from big bang up inside a single universe, and then it just uses camera sense data to very rapidly pick the right universe? Well yes that will dispose of 2^1000 prior very easily. Something that is instead e.g. modelling humans using minimum amount of guessing without knowing what's inside their heads, and which can't really run any reductionist simulations at the level of quarks to predict it's camera data, can have real trouble getting right the fine details of it's grand unified theory of everything, and most closely approximate a crackpot scientist. Furthermore, having to include a non-reductionist model of humans, it may even end up religious (feeding stuff into human mind model to build it's theory of everything by intelligent design).
How it would work under any form of a practical bound (e.g. forbidding zillions upon zillions of the quark level simulations of everything from big bang to now to occur within an AI, which seem to me like a very conservative bound), is a highly complicated open problem. edit: and the very strong intuition I have is that you can't just dismiss this sort of stuff out of hand. So many ways it can fail. So few ways it can work great. And no rigour what so ever in the speculations here.
How it would work under any form of a practical bound (e.g. forbidding zillions upon zillions of the quark level simulations of everything from big bang to now to occur within an AI, which seem to me like a very conservative bound), is a highly complicated open problem.
I certainly don't disagree when you put it like that, but I think the convention around here is when we say "SI/AIXI will do X" we are usually referring to the theoretical (uncomputable) construct, not predicting that an actual future AI inspired by SI/AIXI will do X (in part be...
My friend, hearing me recount tales of LessWrong, recently asked me if I thought it was simply a coincidence that so many LessWrong rationality nerds cared so much about creating Friendly AI. "If Eliezer had simply been obsessed by saving the world from asteroids, would they all be focused on that?"
Obviously one possibility (the inside view) is simply that rationality compels you to focus on FAI. But if we take the outside view for a second, it does seem like FAI has a special attraction for armchair rationalists: it's the rare heroic act that can be accomplished without ever confronting reality.
After all, if you want to save the planet from an asteroid, you have to do a lot of work! You have to build stuff and test it and just generally solve a lot of gritty engineering problems. But if you want to save the planet from AI, you can conveniently do the whole thing without getting out of bed.
Indeed, as the Tool AI debate as shown, SIAI types have withdrawn from reality even further. There are a lot of AI researchers who spend a lot of time building models, analyzing data, and generally solving a lot of gritty engineering problems all day. But the SIAI view conveniently says this is all very dangerous and that one shouldn't even begin to try implementing anything like an AI until one has perfectly solved all of the theoretical problems first.
Obviously this isn't any sort of proof that working on FAI is irrational, but it does seem awfully suspicious that people who really like to spend their time thinking about ideas have managed to persuade themselves that they can save the entire species from certain doom just by thinking about ideas.