X-Message-Number: 11561
From: 
Date: Fri, 16 Apr 1999 00:38:25 EDT
Subject: BB bull

John de Rivaz writes,

>The Fabric of Reality by David Deutsch Is worth reading in this context 
[simulated >people and worlds], and it is also highly entertaining and 
readable. Do you know >that it is physically impossible simulate certain 
things in virtual reality? So those of >you who think this world is a 
computer simulation, should read on. 

A few people may be interested in a few more comments:

First, Deutsch talks about "rendering virtual realities" rather than 
simulating people in their worlds--the brain is real, the inputs 
manipulated--but the difference is not important for most of this discussion.

Now, Deutsch notes that a finite state computer--a Turing universal computer 
or any ordinary digital computer, regardless of speed and capacity--can only 
have a denumerable number (even if an unlimited number) of commands in its 
program, and therefore all possible programs could in principle be numbered. 
But then one can show (a la Cantor) that the number of possible environments 
(including histories) is not denumerable. Therefore almost all possible 
environments are beyond the reach of any computer. (Renderable environments 
are of "measure zero.")

In seeming contrast, recall that, according to Tipler and others, the 
Bekenstein Bound implies that, in a brain of fixed mass and volume, the total 
number of possible quantum states, hence also the total number of possible 
human experiences or brain states, is finite. 

Is there a contradiction? Not necessarily--if you have a strong stomach. 
Perhaps one might consider it possible that a nondenumerable number of 
histories could map onto a finite number of brain states. That would be a set 
of histories of cardinal number c mapping onto a finite number of brain 
states. This would be an instance of "eternal return," with a vengeance, in a 
sense.

If your stomach is as weak as mine, you will look for other answers--problems 
with the premises of Tipler or Deutsch or both. I find problems with both. 
(Both of them are smarter and better informed than I am, but I still have to 
follow my train of thought.)

Today I will just focus on the problems of Tipler/Bekenstein.   

First, there is the apparent premise that a "quantum state" of the brain is 
equivalent to a particular person at a particular stage of life, and also to 
an experience (quale). I doubt this, because I think an experience must be 
time-binding, spanning more than an objective moment.

Second, there seem to be problems with the implied "phase space" within which 
quantum states are defined according to Bekenstein/Tipler. In view of the 
undecided character of the underlying physics--superstrings or whatever, 
churning in "empty" space, the character of time, on and on--any purported 
calculations of points in phase space surely must be of the most tentative 
nature. (For example, why should the space coordinates of the probable center 
of mass of an atom constitute part of the designation of a point in the phase 
space of a system, if the atom also has parts or subdivisions?)

Indeed, the whole question of the interpretation of quantum mechanics is 
still very much up in the air; after almost a century the disagreements show 
no signs of being resolved or even abating. So anyone who takes too seriously 
any conclusion based on a particular interpretation of quantum mechanics--as 
opposed to the physical predictions in specified experiments--is going too 
far.

Robert Ettinger
Cryonics Institute
Immortalist Society
http://www.cryonics.org

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