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Baughn comments on Harry Potter and the Methods of Rationality discussion thread - Less Wrong
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I should really have mentioned this back in the appropriate chapter, but..
Remember how Harry complains that adding (consistent) time-travel makes the universe uncomputable? Leaving aside how I'm not exactly convinced of that myself, I thought I should point out that such consistent time-travel has recently been experimentally demonstrated.
Have a look at http://arxiv4.library.cornell.edu/abs/1005.2219 . It was published way too late for Harry to read it, unfortunately. :P
If I'm reading this right, this isn't an experimental demonstration of time travel, rather, it's a theory of time travel, the predictions of which can be determined experimentally, and an example of such an experiment, to determine what would happen in a grandfather paradox case if the theory is correct.
I interpreted it as stating that they had actually performed the experiment, and gotten a positive result. Am I misinterpreting something?
As I understand it, their theory is that time travel is like postselection. Hence, to determine what would happen in the case of the grandfather paradox, they set up an equivalent postselection experiment. So if their theory is correct, the results of an actual grandfather paradox experiment would match the results of this simulated-via-postselection one.
In what sense is post-selection not time-travel? You've still got the state at time T determined by the state at T+1.
Because, if I'm reading this right, they literally manually postselected the results. They didn't come up with some way to get the physics to postselect it for you.
My interpretation matches yours.
The abstract mentions Deutsch November 1991, just a bit too late for Harry. Deutsch is specificaly about quantum mechanics and that the result is computable. He suggests that you could solve hard problems with a CTC, but I don't think he pursues it. I am amused by the caption to figure 5: "Network for finding a fixed point of f?"