Re: Simulation: digital vs analogue

In article <DsMmj.283013$S37.84001@xxxxxxxxxxxxxxxxxxxxxxxxx>, Tim Tyler
<seemysig@xxxxxxxxxxxxxx> writes
``We should keep in mind as well that you digital computing
can be functionally equivalent to analog computing - that
is we can perform any of the functions of a hybrid
digital-analog network with an all digital computer.
The reverse is not true: we can't simulate all of the
functions of the digital computer with an analogue one.''

- The Singularity is Near, Ray Kurzweil, page 130

What?!?

Both can do universal computation - and so can simulate
any other type of system with an arbitrary degree of
precision.
Quantum computing is being investigated precisely because it cannot be
simulated efficiently with traditional digital computers, so, quoted out
of context, Kurzweil appears to be 180 degrees wrong.

But if his context identifies analog computing with consensus models of
neurones he may be practically correct; very few people believe that
neurones are exploiting quantum computing, and they appear to be suffer
too much from noise and general irreproducibility to make it practical
to use them as building blocks for general purpose digital computing.

While there are theoretical schemes for building reliable digital
computers from unreliable components, in practice the cost is high and
the payoff low. Systems faced with this problem (e.g. satellite
electronics, which face radiation-induced errors) tend to put a lot of
effort into physical hardening to stop the problem at source, leaving
them with fairly simple dual redundancy schemes for the main computer,
backed up with a core of hardwired electronics sufficient to reboot and
reload the computer under orders from the ground if necessary. Even
where dual redundancy is used, some very small part of the control logic
usually remains a single point of failure.
--
A.G.McDowell
.

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