Scott_Aaronson2
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Silas: Look, as soon you find a 1 bit, you've solved the problem with certainty. You have no remaining doubt about the answer. And the expected number of queries until you find a 1 bit is O(1) (or 2 to be exact). Why? Because with each query, your probability of finding a 1 bit is 1/2. Therefore, the probability that you need t or more queries is (1/2)^(t-1). So you get a geometric series that sums to 2.
(Note: I was careful to say you succeed with certainty after an expected number of queries independent of n -- not that there's a constant c such that you... (read more)
Silas: "Solve" = for a worst-case string (in both the deterministic and randomized cases). In the randomized case, just keep picking random bits and querying them. After O(1) queries, with high probability you'll have queried either a 1 in the left half or a 1 in the right half, at which point you're done.
As far as I know this problem doesn't have a name. But it's how (for example) you construct an oracle separating P from ZPP.
Don: When you fix the goalposts, make sure someone can't kick the ball straight in! :-) Suppose you're given an n-bit string, and you're promised that exactly n/4 of the bits are 1, and they're either all in the left half of the string or all in the right half. The problem is to decide which. It's clear that any deterministic algorithm needs to examine at least n/4 + 1 of the bits to solve this problem. On the other hand, a randomized sampling algorithm can solve the problem with certainty after looking at only O(1) bits on average.
Eliezer: I often tell people that theoretical computer science is... (read more)
I am interested in what Scott Aaronson says to this.
I fear Eliezer will get annoyed with me again :), but R and Stephen basically nailed it. Randomness provably never helps in average-case complexity (i.e., where you fix the probability distribution over inputs) -- since given any ensemble of strategies, by convexity there must be at least one deterministic strategy in the ensemble that does at least as well as the average.
On the other hand, if you care about the worst-case running time, then there are settings (such as query complexity) where randomness provably does help. For example, suppose you're given n bits, you're promised that either n/3 or 2n/3 of... (read more)
(the superscripts didn't show up: that was N^googol and 2^N)
Um, except that we also don't know whether there are computations that can be checked in N time but only performed in Ngoogol time. The situation is qualitatively the same as for N versus 2N.
Otherwise, of course a larger environment can outsmart you mathematically.
No, not of course. For example, suppose P were equal to PSPACE. Then even though a larger environment could fundamentally outsmart you mathematically (say by solving the halting problem), it couldn't prove to you that it was doing so. In other words, the situation with polynomial-time computation would be more-or-less the same as it is with unlimited computation: superintelligent machines could only prove their superintelligence to other superintelligent machines.
That the situation with efficient computation appears to be different---i.e., that it appears superintelligent machines can indeed prove their superintelligence to fundamentally dumber machines---is (if true) a profound fact about the world that seems worth calling attention to. Sure, of course you can nullify it by assuming away all complexity considerations, but why? :-)
In fact, it's just bloody hard to fundamentally increase your ability to solve math problems in a way that "no closed system can do" just by opening the system. So far as I can tell, it basically requires that the environment be magic and that you be born with faith in this fact.
As Wei mentioned, P≠NP is basically the conjecture that this isn't true: i.e., that you can exponentially increase your ability to solve math problems by your environment being magic and your not being born with faith in that fact. So for example, if your environment immediately inverted any one-way function, that would be evidence (no faith required) that your environment is not merely 'slightly' smarter than you are, but astoundingly smarter. In qualitative terms, I think it would be almost as astounding as if the environment solved the halting problem.
Carl: I'm not sure, but I'd certainly try such a pill were the effects reversible.
Will: Yeah, it's 1/4, thanks. I somehow have a blind spot when it comes to constants. ;-)