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Nisan comments on Open thread, 24-30 March 2014 - Less Wrong Discussion
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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.
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?
The collapse interpretation predicts that you can't do the super-duper quantum erasure. Once the collapse has occurred the wavefunction can't uncollapse.
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.
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?
Calling them parallel worlds is as always dangerous (you can't go all buckaroo bonzai on them), but basically yes.