dChan
Anonymous ID: 11a1ca April 20, 2020, 6:33 a.m. No.8861337   🗄️.is đź”—kun

"Permutation City"

 

1: In the novel, Paul Durham runs a Copy of himself out of temporal order, skipping its mental state forward in time by ten seconds and then computing the intervening states backwards. Surely leaping over ten seconds of time without computing the intervening states would be impossible?

 

A1: Yes, it would almost certainly be impossible to compute the state of a complex computer model of a human brain and body at time t=10 from its state at time t=0, without computing thousands of intermediate states. So why did I include these scenes? Because this seemed like the simplest way to dramatise the notion that the arrangement of the successive states of the Copy in time (or space) should not affect its subjective experience.

 

A version of these computations that would have been possible would be to declare that the ordinary way of representing each state of the Copy was “canonical”, and then to find the t=10 state from the t=0 state by computing all the intermediate states in some “non-canonical” form. To give a trivial example, instead of storing and manipulating all the relevant quantities as binary floating-point numbers, they could be encoded in a variety of different schemes. Moving beyond that, the way in which the representation of the data reflected the three-dimensional layout of the physical objects being modelled could be obfuscated in various ways.

 

Now, it can be argued that it “obviously” makes no difference to the Copy whether the representation of each of its states was the simplest, most transparent data structure possible, or whether the representation was so complex and obscure that an uninformed observer could never have made sense of it in a million years. Then again, if this is so obvious, there’s no need to do the experiments at all (see Q2 below). In the end, the reason I glossed over the difficulty of computing the jump in time was simply that the basic concept was already complicated enough, and I think too many readers would have given up on the whole idea if I’d added further complications at this point.

 

[After I posted this FAQ, Jack Boyce emailed me to point out an algorithm called Hashlife, which sometimes makes it possible to skip over very large numbers of time steps in the evolution of John Conway’s famous Game of Life cellular automaton. This is a very interesting algorithm, but I don’t think there’s much chance that the same thing could be done, efficiently, for something like a simulation of the human body. Life is known to be “Turing complete” (that is, you can mimic any computation at all with some configuration of Life) but that in itself is not enough to make skipping steps in a Copy possible, because Hashlife does not provide a uniform speedup, and those configurations of Life that perform complex computations are unlikely to get any benefit.

 

https://www.gregegan.net/PERMUTATION/FAQ/FAQ.html

 

https://www.goodreads.com/book/show/156784.Permutation_City