Jesse Mazer writes:
> I've sometimes thought that if uploads are ever created, and can be run in a 
> simulation with time-reversible fundamental laws, it would be very 
> interesting to take a snapshot at the end of a simulation and do the trick 
> of reversing everything, but with a tiny perturbation--the simulation might 
> appear to behave like a reversed version of the original run for a little 
> while, but then the butterfly effect would probably kick in and the upload's 
> psychological arrow of time would *reverse* in the middle of the simulation. 
> What would this feel like subjectively, from the upload's point of view? 
> Obviously he wouldn't have a memory of experiencing everything backwards, 
> but it still would be interesting to interview the upload about it 
> afterwards. For example, what would happen if the reversal of the upload's 
> psychological arrow of time happened at the same moment that the entropic 
> arrow of time reversed in the simulated physical world around the upload, 
> and at that moment pieces of a vase were rushing together to reassemble, but 
> instead failed to meet up exactly and just broke apart again? The upload 
> should have a memory of seeing the vase fall, but at the moment it landed it 
> might appear to behave very strangely, assuming the upload didn't just 
> perceive himself blacking out at that moment.

That's a cool problem.  I've given it some thought and here is what
I came up with.  The short answer is that as we are running backwards,
due to the chaotic nature of the physics, the transition from an accurate
backwards one to a locally disrupted one will be nearly instantaneous.
The divergence from the original forwards run will grow exponentially,
meaning that if you have set it so that it "kicks in" after say -5
seconds, then at -4.9999 seconds everything would still look normal.

Then, once you had divergence in a specific location, I think the effects
would spread out at the speed of sound.  We are relying on every atom
moving the opposite of what it did before, and atoms generally move at
the speed of sound, so as soon as one starts misbehaving it will kick
its neighbors, which will kick their neighbors, and the disruption
will spread at that speed.

Once the disruption has occured then I think you are right that time will
effectively start forward again, and probably take a different path than
it did the first time through.

This would imply then that subjectively there are two paths, the one we
ran the first time, and the one which resulted from the alteration. They
would subjectively diverge at the point where the butterfly effect
kicked in during the reverse run.  The transition would be subjectively
instantaneous, with the whole brain flipping in a millisecond or less
from backwards to forwards motion.

>From the measure perspective, I'd say that the first half that was
shared has measure 1, the second half that got run twice (once forward,
once backward) had measure 2, and the alternate second half would have
measure 1.

> And perhaps something like this could help explain the low-entropy big bang, 
> which is apparently the source of the arrow of time in our universe and yet 
> doesn't have any agreed-upon explanation by physicists. It would certainly 
> be interesting if even a complete theory of quantum gravity didn't explain 
> it, so that the only remaining option would be either "intelligent design" 
> or some sort of "meta-physical" explanation in terms of a multiverse with 
> different types of universes having different measure.

Right, that is one of the big selling points of the Tegmark and
Schmidhuber concept, that the Big Bang apparently can be described in
very low-information terms.  Tegmark even has a paper arguing that it
took "zero information" to describe it (but frankly I am getting pretty
turned off on the "zero information" concept since several people here
use it to describe completely different things, and if it really took
zero information then there couldn't be more than one thing described,
could it?).

> >My feeling is that causality, like time, is in the eye of the beholder.
> >It's not an inherent or fundamental property.  Rather, it is a way that
> >we can interpret events in some kinds of universes.  Completely chaotic
> >universes (where every moment is random and uncorrelated with the next)
> >would not have causality in any meaningful sense.  Likewise for static
> >universes.
> But if such a chaotic universe is computable, then for those of us watching 
> the computation from the outside, the read/write head of the Turing machine 
> is still obeying regular laws, in terms of when it decides to flip a 
> particular bit or change its internal pointer-state or move from one 
> location on the bitstring to another...if it's possible to define a 
> mathematical notion of "causal structure" for any particular algorithm, I 
> would think it would be possible to apply it to *all* algorithms. But 
> perhaps no such mathematical notion of causal structure will be 
> forthcoming...the reason I'd guess it is is that such a notion would seem 
> essential for defining what it means to "instantiate" a particular observer 
> in such a way that you don't count things like lookup tables, and also so 
> that you don't end up concluding that random thermal vibrations in a rock 
> actually instantiate all possible algorithms (the problem discussed in 
> Chalmers' paper "Does a Rock Implement Every Finite-State Automaton?" at 
> ).

The model I suggested the other day, basically just the Universal
Distribution, IMO fully solves these two riddles.  First, the question
is not "are they conscious" but "how much measure do they add to the
particular observer-moments which they are putatively experiencing".
And the UDist shows that this can be answered in a straightforward,
quantitative way, by asking what is the shortest program that takes these
data structures as input (the lookup table or the rock) and outputs
something that matches our canonicalized representation of the OMs in
question (perhaps a schematic representation of a neural network with
specified firing patterns).  I can tell you that the rock isn't going
to add any measure.  I don't know about the lookup table, maybe there
is an algorithm to use it to deduce the neural network that would have
created it.  Hans Moravec argued that there was such an algorithm, at
least for a big enough lookup table.  But in principle it is an empirical
question in the framework based on the UDist.  There's nothing fuzzy or
philosophical about it.

> Well, that's what I was talking about in my last post when I said that my 
> intuition of "causal structure" is not a time-asymmetric one, that it would 
> only be about saying two events are causally related without specifying one 
> as the "cause" and the other as the "effect". And as I said, my 
> understanding of loop quantum gravity is that it does involve some notion of 
> building spacetime out of relationships between events without any 
> time-asymmetry being involved.

Maybe so, I don't know much about LCG.

> >Scerir has also posted some interesting paradoxes along these lines
> >relating to QM.  Suppose we have a photon that passes through a
> >polarizer oriented at 20 degrees from vertical, then through one
> >oriented at 40 degrees, and makes it through both.  At the end we would
> >say its polarization was 40 degrees.  But what was it between the two
> >polarizers?  Conventionally we would say that the first polarizer made its
> >polarization become 20 degrees and the second polarizer then changed the
> >polarization to 40 degrees.  But actually you can just as easily argue
> >that the photon polarization was 40 degrees between the two polarizers.
> >That interpretation works just as well, a sort of retroactive causality.
> What would the MWI say about this? Whatever it would say, I'm pretty sure it 
> wouldn't say that there was a single photon in a definite state between the 
> two polarizers.

No, I think it does, but I might be wrong.  I think it says the universe
splits into two when the photon hits the first polarizer; in one the
photon is absorbed and in the other the photon continues in the 20
degree polarization state.  Or you can run time backwards and get the
photon to be in the 40 degree state.  I don't think the MWI helps much
with this.

Hal Finney

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