[extropy-chat] Singularity heat waste

[Robert Bradbury]

On 7/14/06, Martin Striz <mstriz at gmail.com> wrote:
>
> Heat might only be an issue if computation continues to be performed
> by semiconductors.


No, no, no.  This is not the problem.  You can "compute" effectively for
*free*.  This area was resolved several decades ago by Bennett with some
contributions from Landauer, Bremermann and Bekenstein.  It has nothing to
do with whether or not one uses semiconductors.  It has to do with whether
one is destructively erasing bits (throwing away information).  That is what
produces the heat.  This is the difference between nonreversible computing
and reversible computing.  In nonreversible computing you throw away lots of
bits and produce lots of heat.  In reversible computing you run the
calculation forward, save the result (producing a small amount of heat),
then run the calculation backwards restoring things to their original
state.  As has been pointed out you can't do such a calculation entirely for
free (at least not quickly) -- but the energy lost to the computation
process itself is many orders of magnitude below the energy lost when you
erase bits.

Merkle and perhaps others (Fredkin?) have shown that you can design
reversible computing circuits using existing semiconductor fabrication
methods.  However, the chips that have reversible capabilities are likely to
require more gates and/or operate more slowly -- so they will not be
implemented until chip manufacturers exhaust all other methods in their bag
of tricks for minimizing or removing heat produced in current nonreversible
designs.

Neurons can only be considered semi-reversible designs.  You don't have to
regenerate the Na+/K+ ions in the brain but you do lose the energy needed to
recharge the neurons after they fire.  That wasted energy shows up as heat
and the rest of your body functions as a radiator for the brain.

Neurons can do a lot of computation within minimal
> heat loss.  Your head isn't hot due to neuron inefficiency.  It's kept
> hot on purpose because enzyme kinetics are optimized for 37C.


Actually, the metabolism in the liver and perhaps intestines probably plays
a more important role in keeping one at 37 deg.  Their fundamental raison
d'etre is to produce the glucose which in turn is used by the brain to keep
you alive long enough and figure out how to make copies of those genes you
are carrying around.  As arctic ground squirrels can be cooled very close to
freezing and still restore themselves to normal functioning in the spring
you would have a hard time convincing me that 37C is necessary for most of
the enzymes in the brain.  More likely 37C just happened to be the
temperature that most of the enzymes performed well at without having to
devote excessive energy to keeping the body cool or reaching the limits on
resources like water for evaporative cooling.

Presumably a Singularity event would produce novel computational
> substrates, so there's not way to predict post-Singularity energy/heat
> budgets.


There are perhaps half-a-dozen novel computational substrates in the works
from DNA (chemical) logic to Spin-logic to photonic logic to quantum logic
to Drexler's rod-logic, etc.  They do *not* require the singularity.  They
require that they demonstrate sufficient advantages over the current path to
justify what will presumably be a very large investment that would allow
them to produce significantly better results than the current path.  My bet
would be that only robust nanotechnology has the potential for such an
investment payoff.  *After* the rapid growth phase of the singularity era,
presumably all of these paths will be explored to see if they provide some
unique benefits but at that point they will just be icing on the cake.

Robert
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