Re: Read any good books lately?
Sun, 15 Aug 1999 18:00:35 -0700

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. 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. 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.