Harry Potter and the Methods of Rationality discussion thread, part 16, chapter 85

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The next discussion thread is here.

This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 85. The previous thread has long passed 500 comments. Comment in the 15th thread until you read chapter 85.

There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)

The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15.

As a reminder, it’s often useful to start your comment by indicating which chapter you are commenting on.

Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically:

You do not need to rot13 anything about HP:MoR or the original Harry Potter series unless you are posting insider information from Eliezer Yudkowsky which is not supposed to be publicly available (which includes public statements by Eliezer that have been retracted).

If there is evidence for X in MOR and/or canon then it’s fine to post about X without rot13, even if you also have heard privately from Eliezer that X is true. But you should not post that “Eliezer said X is true” unless you use rot13.

The next discussion thread is here.

As a reminder, it’s often useful to start your comment by indicating which chapter you are commenting on.

Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically:

You do not need to rot13 anything about HP:MoR or the original Harry Potter series unless you are posting insider information from Eliezer Yudkowsky which is not supposed to be publicly available (which includes public statements by Eliezer that have been retracted).

If there is evidence for X in MOR and/or canon then it’s fine to post about X without rot13, even if you also have heard privately from Eliezer that X is true. But you should not post that “Eliezer said X is true” unless you use rot13.

So the issue here is that we might have three points x, y and z, where x and y have a space-like separation, y and z have a time-like or null separation (which is future pointing from y to z) and x and z have a space-like separation. Further, d(x,y) < 6 (measured in light hours), but d(x,z) > 6.

If so, then the principle I described would prevent information passing from x to z. So it either prevents information transmission from x to y; or if a Time-Turner has already been used to get info from x to y, prevents the further transmission from y to z. The last would be a very interesting effect, because either there is no communication attempt from y to z at all, or the usual communication methods - like light-beams - are attempted, but fail for some reason.

Alternatively, suppose the principle I described is wrong, and info can move from x back in time to x', then forward to y, then to z by the usual physical means. Then I believe we can make d(x,z) arbitrarily large and this opens up some even more exciting possibilities.

Consider a two-dimensional example, with one space and one time co-ordinate - space first then time. Fix a particular inertial reference frame. Point x is (0,0), point x' is (0, -delta) i.e. a tiny bit in the past of x. Point y is (delta, 0) i.e. a tiny bit spatially separated from x, and then point z is (delta+t, t) i.e. t hours in the future from y along a light beam through x' and y.

Then d(x,z) = Sqrt( (delta+t)^2 - t^2 ) = Sqrt(2 x delta x t + delta^2) which of course grows without limit as t gets arbitrarily large. And there is some reference frame in which x and z are that distance apart spatially. So now here's a neat trick. Imagine that Harry wants to send a message to the Andromeda galaxy, but without it taking two millions years to arrive. Let's say that x is the Earth now, and z is somewhere in Andromeda which is approximately "now" in the Earth's reference frame, and in cosmological terms (e.g. at z, measurements of the temperature of the background radiation and the Hubble constant are the same as at x). Then Harry finds a suitable alternative reference frame in which to pick the points x' and y, uses a Time Turner to send a message from x to x', then uses ordinary light to send it through y to z. Instant magical faster-than-light signalling out to any distance! And indeed faster-than-light space travel as well, provided he can transfigure rockets that travel at near-light speed...