gregconen comments on A problem with Timeless Decision Theory (TDT) - Less Wrong

36 Post author: Gary_Drescher 04 February 2010 06:47PM

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Comment author: gregconen 05 February 2010 04:11:36AM 1 point [-]

Actually, there is an optimal solution to this dilemma. Rather than use any internal process to decide, using a truly random process gives a 50% chance of survival. If you base your decision on a quantum randomness source, in principle no simulation can predict your choice (or rather, a complete simulation would correctly predict you fail in 50% of possible worlds).

Knowing how to use randomness against an intelligent adversary is important.

Comment author: loqi 05 February 2010 04:37:20AM *  0 points [-]

Gary postulated an infallible simulator, which presumably includes your entire initial state and all pseudorandom algorithms you might run. Known quantum randomness methods can only amplify existing entropy, not manufacture it ab initio. So you have no recourse to coinflips.

EDIT: Oops! pengvado is right. I was thinking of the case discussed here, where the random bits are provided by some quantum black box.

Comment author: pengvado 05 February 2010 05:58:02AM 3 points [-]

Quantum coinflips work even if Omega can predict them. It's like a branch-both-ways instruction. Just measure some quantum variable, then measure a noncommuting variable, and voila, you've been split into two or more branches that observe different results and thus can perform different strategies. Omega's perfect predictor tells it that you will do both strategies, each with half of your original measure. There is no arrangement of atoms (encoding the right answer) that Omega can choose in advance that would make both of you wrong.

Comment author: wedrifid 05 February 2010 06:46:51AM 2 points [-]

I agree, and for this reason whenever I make descriptions I make Omega's response to quantum smart-asses and other randomisers explicit and negative.

Comment author: gregconen 05 February 2010 02:12:53PM *  3 points [-]

If Omega wants to smack down the use of randomness, I can't stop it. But there are a number of game theoretic situations where the optimal response is random play, and any decision theory that can't respond correctly is broken.

Comment author: wedrifid 05 February 2010 05:38:01PM 0 points [-]

EDIT: Oops! pengvado is right. I was thinking of the case discussed here, where the random bits are provided by some quantum black box.

Does putting the 'quantum' in a black box change anything?

Comment author: loqi 05 February 2010 07:09:25PM *  0 points [-]

Not sure I know which question you're asking:

  1. A black box RNG is still useless despite being based on a quantum mechanism, or
  2. That a quantum device will necessarily manufacture random bits.

Counterexamples to 2 are pretty straightforward (quantum computers), so I'm assuming you mean 1. I'm operating at the edge of my knowledge here (as my original mistake shows), but I think the entire point of Pironio et al's paper was that you can verify random bits obtained from an adversary, subject to the conditions:

  • Bell inequality violations are observable (i.e., it's a quantum generator).
  • The adversary can't predict your measurement strategy.

Am I misunderstanding something?

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