Comment by mjburgess | Hacker News Reader

Comment by mjburgess | Hacker News Reader

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Well I define random to mean whenever P(X|Y) is neither always 1 nor 0, or to put that in causal modal terms:

given an exhaustive description of all possible causes of Y, say X, then if X causes Y, *NECESSARILY* Y given X

causation is a form of necessity, necessarily Y occurs or it doesnt

In this case P(Future|Past) and P(Past|Future) are somewhere between 0 and 1. This isnt consistent with causation.

ie., All possible information about all possible causes of Y (the future), say X (the past) are insufficient to gaurentee any given Y (any particular future).

The "interpretation" element of QM is to say that we are really in this case, ie., that we have X = all possible information.

There are several ways of denying that P(Y|X) in the case of QM should be read this way. You can say X is incomplete, either by saying there is non-local information we don't have, or global information we don't have. Of the "global" kind, which of many worlds we are in is a kind of global ignorance.

So that if we had, P(Y|X, which-world) then we'd observe P(Y|X, whichworld) = either 1 or 0 --- as with classical mechanics

P(CatDead|PoisonBox, worldA) = 1

P(CatDead|PoisonBox, worldB) = 0

etc.

and, eg., P(F|G,M,m,r) = 1 if F=GMm/r^2

what do you mean by the term information? My question is, rather, whether a system needs to be able to know it knows that information for it not to still be causal. Because it doesnt observe its whole wavefunction

I dont understand your use of "knows", knowing isnt part of the problem.

Imagine, for simplicty sake, a world is a series of four switches which we can describe by 0 (off),1 (on).

Eg., world A at time 0 is 0000, at time 1, 0001, ...

Now a change from 0000 to 0001 is causal if in every physically possible world 0001 follows 0000, ie., P(0001|0000) = 1

Now suppose that we enumerate every possible world, ie., all the worlds containing only this kind of switch, and only four of them.

Now if 40% of the worlds had 0000->0001 and 60% had 0000->1000, then P(0001|0000) = 0.4

The question is how can it be that the fourth switch behaves randomly? Ie., without any necessary flip from 0 to 1. Nothing at all can cause that switch to flip, by definition, the world contains nothing else in it.

What property does the switch have such that it turns on 40% of the time? NONE.

Consider two resolutions to this problem: bite the bullet and say the switch is "miraculous in some sense" in that it "just turns on" sometimes; OR, say that this thought experiment is physically impossible and we're missing information.

Eg., that there's a hidden switch: 10000->10001 always, and 00000->11000 always, we just can't see the first switch, so it looks random.

"Imagine, for simplicty sake, a world is a series of four switches which we can describe by 0 (off),1 (on)."

If that's what the world is then there's no "we"

Indeed, by definition, that world consists of four objects with four distinct properties, ie., being on or off.

Then where is the time bit?

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