Manfred comments on Open thread, 24-30 March 2014 - Less Wrong

6 Post author: Metus 25 March 2014 07:42AM

You are viewing a comment permalink. View the original post to see all comments and the full post content.

Comments (156)

You are viewing a single comment's thread. Show more comments above.

Comment author: Manfred 25 March 2014 08:12:59PM 1 point [-]

Bob cannot become entangled with the outside world while in the middle of a quantum erasure experiment, or else it doesn't work. So he doesn't really get to do anything :P

If Bob knows that the particle becomes entangled with him, then he still makes the same predictions.

Comment author: Nisan 27 March 2014 09:29:47PM 0 points [-]

If Bob knows that the particle becomes entangled with him, then he still makes the same predictions.

Ok, that's surprising. Here's why I thought otherwise: From Bob's perspective, a particle is prepared in a superposition of states B and C. Then Bob observes or becomes entangled with the particle, thus collapsing its state. Then the super-duper quantum erasure is performed, which preserves the state of the particle. Then the particle strikes the second half-silvered mirror. A collapse interpretation tells Bob to expect two outcomes with equal probability. Is this, then, an experiment where a collapse interpretation and a many-worlds interpretation give different predictions?

Comment author: Oscar_Cunningham 28 March 2014 09:00:04AM 1 point [-]

The collapse interpretation predicts that you can't do the super-duper quantum erasure. Once the collapse has occurred the wavefunction can't uncollapse.

Comment author: Manfred 27 March 2014 11:28:25PM *  1 point [-]

Basically, there are a variety of collapse interpretations depending on where you make the collapse happen. Every time we've tested these hypotheses (e.g. by this sort of experiment), we haven't been able to see an early collapse.

At this point, all actual physicists I know just postpone the collapse whenever necessary to get the right answer.

Comment author: Nisan 28 March 2014 01:05:15AM 1 point [-]

Hm, so that means that quantum physics predicts that our observations depend on the presence of parallel worlds in the universal wavefunction, which in theory might interfere with our experiments at any time, right?

Comment author: Manfred 28 March 2014 03:05:31AM 1 point [-]

Calling them parallel worlds is as always dangerous (you can't go all buckaroo bonzai on them), but basically yes.

Comment author: Oscar_Cunningham 25 March 2014 09:17:30PM 0 points [-]

He can, in theory, make bets. Just so long as the bet he makes doesn't depend on which way he saw the particle go.

Comment author: Manfred 26 March 2014 12:29:28AM 0 points [-]

Hm, good point. We could set aside a few instants for him to send a few photons that wouldn't depend on the state of the particle. From a practical standpoint that's pretty impossible, but forget practicality.

So, sure; Bob should accept the bet. Although if he makes his answer to the bet depend on the state of the particle at all, then he shouldn't accept the bet :P There might be some interesting applications of this, say where the option "don't accept" has some positive payoff if the particle changes directions. Bob can precommit to send out an entangled qubit to get some chance at that reward.