# Re: BIBLE: Quantum computers & Many Worlds Hypoth. (fwd)

David Campbell (bivalve@email.unc.edu)
Wed, 28 Aug 1996 10:50:11 -0500

>David Campbell wrote:
>
>>10^500 calculations do not need multiple universes worth of atoms if you
>>can recycle them.
>
>I agree. I had somewhat presumed (probably erroneously) that what they were
>really meaing was the need for lots and lots of memory for such a problem.
>Does anyone know if factoring such large numbers requires large amounts of
>memory say more than 10^80 bits (one atom=1 bit)? However, my calculation
>below would seem to say that memory is not what is needed.
>
>
>Now,the way I calculate things, if each particle could be used only once,
>10^500 calculations requires 10^420 universes. Alternatively, assuming that
>each particle in each universe (of 10^80 particles) can operate at 10^100
>operations per second, then each universe can perform 10^180 operations per
>second. This means you need 10^320 seconds to complete the calculation. The
>18 billion year old universe is only 10^17 seconds old. Thus you need about
>10^300 times as long as the current universe has existed to solve the problem.
>
>If the quantum computer can solve the problem in 5 minutes, where and how did
>it perform this magic? Once again, the major premise in that author's
>argument was that a calculation required the manipulation of a physical
>object. If this is untrue, then his argument fall flat on its face.
>
Certainly, reusing a low number of particles would be slow; however, for
the problem of factoring a large number, it is only necessary to save
factors and not non-factors, so less memory is required. I successfully
ran a computation involving 10^46 possible options on a desktop computer
(over several days), because several options could be eliminated rather
quickly. Similarly, if the number to be factored is odd, all even numbers
can be eliminated as possible factors, and so forth.
To me, this particular arguement does not seem compelling because
it is based on the assumption that this quantum computer works fast enough
to be puzzling, yet it's not known whether it can be built.