Re: Read any good books lately?

hansen (
Mon, 16 Aug 1999 22:48:13 -0700

On Sun, Aug 15, 1999 at 06:00:35PM -0700, wrote:
> hansen, <>, quotes David Deutsch:
> > "To those who still cling to a single-universe world-view, I
> > issue this challenge: explain how Shor's Algorithm works. ....
> > When Shor's Algorithm has factorized a number, using 10^500 or
> > so times the computational resources that can be seen to be
> > present, where was the number factorized? There are only about
> > 10^80 atoms in the entire visible universe, an utterly
> > miniscule number compared with 10^500. So if the visible
> > universe were the extent of physical reality, physical reality
> > would not even remotely contain the resources required to
> > factorize such a large number. ..." p 217
> I don't agree with Deutsch's suggestion that quantum computers would imply
> that the Many-Worlds interpretation must be true. He is not correct
> to say that running Shor's algorithm has used many times the computing
> resources that can be seen to be present. The quantum computer is
> clearly seen to be present; it is put into a superposition of states and
> run through a series of transforms, and at the end the answer comes out.

All of the states making up the superposition are most definitely not present (in this "universe"). They are not there until measured, in which case only one of them is there.

> The quantum computer is more powerful than an ordinary computer, and
> requires correspondingly more care in its preparation and use, but it
> does not use invisible resources.
> The reason for the computational power is that the machine is put into
> a superposition of states. But such superpositions are observed every
> day in quantum experiments, like double slit diffraction.

Consider the argument of the next sentence:

>Since most
> scientists do not find that this common diffraction effect forces them to
> adopt a many-universe model, there is no more reason to expect it from
> a successful quantum computer. The idea is the same, only the degree
> of superposition is different.
> Hal

We already know the majority of physicists do not accept the many worlds interpretation. You can't (logically) use that fact to be an argument against the very issue under discussion.

Let me run the following argument by you, for discussion purposes:

If a QC carries out a computation requiring 10^500 superimposed possible states, and actually carries out the computation, then those states have to be real, since they had a physical effect, as real as the state (after measurement) we observe in this universe. Since those states are real, and don't (and can't) exist in this (visible) "universe", then those alternate "universes" have to be real. (This is the essence of Deutsch's challenge.) Then, running your last paragraph backwards, "The idea is the same, only the degree of superposition is different." "Common diffraction effects" have same status as QC, therefore common diffraction effects are also manifestations of the multiverse, (one would therefore seem to be lead to believe).

(Terminological clarification: Deutsch uses "multiverse" to refer to the entire collection of alternate "universes", even though "universe" is a more logical word for the collection...)

John K Clark said:

   >Hal said that a working quantum computer would not prove that the
   >many world interpretation is correct and he's right, but it would
   >make it much more popular.  You'd have to understand such a beast
   >pretty well to program it and the many world way would probably be
   >the clearest way to think about it.


Anyway, if I'm wrong, tell me. I don't mind.

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