X-Message-Number: 21468
Date: Tue, 25 Mar 2003 07:51:31 -0500
From: Thomas Donaldson <>
Subject: CryoNet #21459 - #21467

For Michael C Price:

Someone else's opinion without further argument is unlikely to convince
me. Has this guy actually worked in quantum computing? Published on
the subject?

Moreover (although he speaks loosely enough that he might argue for
a different interpretation) when Tegmark says: "a normal computer ...
is free to output the result of the calculations at the end. A
quantum computer, however, can output only a single result of a 
very special form..." just what does he mean by only outputting 
a single result? Many problems have only a single result. Moreover
how is it that the normal computer outputs only a single result
in such cases, and this is not objectionable, while that feature
of quantum computers IS objectionable? 

And what form must this result take? Without going off into realms
of speculation involving many separate quantum computers, the 
special result is a sentence in logical form ie. 

     ((p1 or (p2 and p3)) or (p7 and (p3 or p4)) ... etc

It hardly takes much thought to see that it might have multiple
answers in the sense that the output is:

      p1 or p2 or p3 or p4 or ...

Basically, a quantum computer will work with any set of propositions
which can be cast into the form of a logical formula. It then 
simultaneously looks at all the different combinations of truth or
falsity of the propositions (which must of course be provided
also beforehand) and finds whether or not that logical formula
is true. In short, it could search through a database to instantly
find a paper dealing with some particular sets of subjects, or
a person with a particular combination of traits, etc etc...with
no restriction other than the human one of writing them all out in
the number and complexity of individual logical forms which 
express the traits of the paper or the person. In fact, if we
first cast a problem into a set of individual states (say, take
a differential equation, and set it up as all possible solutions
satisfying a finite version of the differential equation (a 
finite difference equation)) then quantum computers could solve that
one instantly too.

So just how is Tegmark limiting the abilities of quantum
computers? Sure, you have to cast your problem into a particular
form, but how is that different from programming a normal
computer?

I will add, however, that there ARE some limits. If you have
a problem which for some reason can only be cast in a form like

     for all x, logical.formula(x)

where the variable x cannot have only a finite number of states,
then we need more than a quantum computer to verify it.

            Best wishes and long long life to all,

                 Thomas Donaldson

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