Harry appears to McGonagall and tells her, "If you give me 1 Galleon now, I'll go back in time and hand you 100 Galleons an hour ago." Suppose McGonagall does not recall being handed 100 Galleons an hour ago. What should she do?

Here's my analysis. Suppose McGonagall decides not to give Harry 1 Galleon, then there are two possible consistent timelines for this universe. One where McGonagall gets 100 Galleons, and one where she doesn't. How does the universe "choose" which one becomes reality? I don't know but let's say that the two possibilities have equal chance of being true, or get equal amount of "reality juice".

Given the above it seems clear that McGonagall would prefer to have pre-committed to "give 1 Galleon even if not handed 100 Galleons an hour ago" since that would make the "not get 100 Galleons" timeline inconsistent. I think that's also UDT's output (although I haven't written down the math to make sure).

ETA: I didn't follow the previous discussion closely, so this might not apply at all to it. Hopefully, in that case the above is of interest in its own right. :)

Comment author: topynate 01 July 2010 12:20:02AM * 5 points [-]

Seems straightforward to me. McGonagall knows that she does not recall being handed 100 Galleons an hour ago, so the three states of the world with high probability are: 1) She is not in a universe where she will hand Harry 1 Galleon, 2) She is in a universe where she hands Harry 1 Galleon and Harry breaks the agreement, or 3) She is in a universe where she hands Harry 1 Galleon and Harry keeps the agreement in a way that leaves her unable to recall this happening. By not handing Harry a Galleon, she will ensure that she is in universe 1. By handing Harry a Galleon, she will find herself in universe 2 or 3. She should therefore give Harry a Galleon if she judges it less than 99 times more likely that Harry will break the agreement than fulfil it in a way consistent with her experience.

As Harry has access to a time machine, he doesn't need to decide to give her 100 Galleons before he gets the 1 Galleon, so the situation is quite different to one based on predicting her actions, as Omega does in the Counterfactual Mugging. Rather it has most of the properties of the forward-time version of the gambit: "If you give me 1 Galleon now, I'll hand you 100 Galleons in one hour", except that McGonagall has a big piece of evidence that the promise will be broken, namely that she doesn't remember it being kept.

Comment author: Blueberry 30 June 2010 08:30:13PM 0 points [-]

Suppose McGonagall does not recall being handed 100 Galleons an hour ago. What should she do?

Precommit, give him the Galleon, then reach in her bag to get the 100 Galleons. (She must have been Obliviated; otherwise, she would remember.)

You can actually get around a lot of the problems with time travel by taking advantage of the difference between observation and reality. For instance, if you see one of your friends die, you can go back in time, save him, then plant a fake double so you still have the same observations.

Comment author: Vladimir_Nesov comment score below threshold [+] (2 children)

Comment author: wedrifid 01 July 2010 08:20:20AM * 0 points [-]

ETA: I didn't follow the previous discussion closely, so this might not apply at all to it. Hopefully, in that case the above is of interest in its own right. :)

Vladmir and I agree with the applicability of UDT and have suggested time-travel-with-consistency is a good way to consider Newcomblike problems and the the decision theories that can handle them.

Comment author: red75 comment score below threshold [+] (10 children)

Comment author: wedrifid 01 July 2010 08:17:09AM * 2 points [-]

All of the above. Decision making is just physics.

Comment author: red75 01 July 2010 08:24:11AM * -1 points [-]

As I wrote earlier I am not yet convinced, that this is a kind of physics we can effectively reason of.

Comment author: wedrifid 01 July 2010 08:45:33AM * 0 points [-]

Consider correcting your claim to:

It seems I can't rely on setups, which include decision making inside time loop

If you say 'we' you invite contradiction as your stance implies magical thinking about decision making which not many people would want to admit to. Decisions are not a special case. If gravity works you can build a gravity powered mechanical device that makes a decision.

Either you treat decisions exactly as you usually would or nothing works at all.

Comment author: red75 01 July 2010 08:48:51AM 0 points [-]

No. It's not a decision making that is special. It is closed time loops. They possibly can "magically" create information from nothing.

Comment author: wedrifid 01 July 2010 01:27:45PM 0 points [-]

Can I be in the one where the 'created information' is a a freak quantum conversion of graphite into diamond and not the one where the created information is freak quantum effects making my neurons fire in a way that they normally wouldn't?

Comment author: red75 01 July 2010 02:29:36PM * -1 points [-]

You can be there, no problem. The problem is to setup everyting to get there. I've just considered the question of setting up, and found that I can't say even what causes existence of time loop. Can you say what should I do to be sure that no matter what after 1.01 hour I will timeturn 1 hour back? Being the person that will do that is important, but it will not help if in 20 minutes someone will steal my timeturner.

Returning back to 6(grand)parent, I can say, that imposing any condition on a stable state of time loop (but initial conditions before time loop) seems unphysical. All in all, thought experiments with time loops are interesting, but their validity is highly questionable.

Eliezer can do that, because he is consistency law of Harry's universe.

Comment author: wedrifid 01 July 2010 02:37:43PM * 0 points [-]

You can be there, no problem. The problem is to setup everyting to get there. I've just considered the question of setting up, and found that I can't say even what causes existence of time loop. Can you say what should I do to be sure that no matter what after 1.01 hour I will timeturn 1 hour back? Being the person that will do that is important, but it will not help if in 20 minutes someone will steal my timeturner.

Go think about it some more from the perspective of an integrated physics that happens to include time travel of this kind and not as a special case. That seems to be confusing you.

Eliezer can do that, because he is consistency law of Harry's universe.

Eliezer can have Harry become a Hari Krishna if he really wants. Or he could artificially select for extremely improbable quantum events either inside or outside of time loops. Neither of those would make for an especially good story in this instance.

continue this thread »

Comment author: wedrifid 01 July 2010 08:37:09AM 0 points [-]

In a time loop do things still fall down? Is there gravity? And if I implement a decision making mechanism using a gravity powered system of gears and pulleys where is the magic thing that makes reason no longer apply?

Comment author: red75 01 July 2010 08:43:57AM 0 points [-]

Information from future will affect any kind of decision making. This information is a consequence of consistency law, it even doesn't need to have cause.

Comment author: Vladimir_Nesov 30 June 2010 11:37:57PM * 2 points [-]

Only because you termed that event "real", but the characters can't know that it is.

Comment author: NancyLebovitz 30 June 2010 09:27:45PM 0 points [-]

Is information in other minds what gets stabilized?

Eliezer_Yudkowsky comments on Harry Potter and the Methods of Rationality discussion thread - Less Wrong

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