[extropy-chat] Singularity heat waste

[Eugen Leitl]

On Sat, Jul 15, 2006 at 07:27:30AM -0400, Martin Striz wrote:

> 1) Why do we use silicon for computation in most microprocessors, as
> opposed to some other substrate?

Silicon is a particular sweet spot (single-element 
semiconductor with a particular bandgap, easy to purify, 
forms solid oxides, plays well with other elements, etc) 
for a particular technology (photolithography, the only
massively parallel processing technology apart from classical
chemistry and biotechnology we currently command).
In fact, we're already using some 20-odd other elements in a modern
(CPU or memory) process. The next major advance will most likely
use carbon (both as diamond and graphene), with minor additives
(just as life does CHNOPS, and almost the entire PSE, in traces).
 
> 2) Is there a more efficient substrate that we could use, what is it,
> and why don't we use it?

Because we can't yet do self-assembly of molecular electronics, we will
continue using silicon massively for at least the next 20 years.
Once we have self-assembly, or even machine-phase self-assembly (nanoscale
factory, with nanorobots manning the processing pipeline), silicon
will rapidly disappear, similiarly as germanium before, vacuum tubes and
ferrite rings on copper mesh, before, and zink sulphide before, and 
electromechanical relais even before. I hence really object to people
using 'in silico' instead of 'in machina', because that expression won't
age gracefully.
 
> 3) If neurons are nonreversible (and they are), why are they so much
> more efficient (in your view)?

Both CMOS and neurons are so far removed from reversible computing
that other effects dominate. In principle, classical electronics is
a joke at the nanoscale, but we've been doing it for so long we've
grown pretty good at it.
 
> > 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.
> 
> 4) Why do they require more gates and/or operate more slowly?

Basically, they're catching the energy and reuse it, instead of just
dissipating it away. As to slowness, it is probably related to reversibility
requring equilibrium at each step, which needs time. The faster you operate,
the less the system has time to equilibrate.

There are many things that waste energy in current CMOS. There's
leak current, clock distribution (instead of asynchronous logic),
drivers for scaling up from nano to macro, inability to hold
state statically (which is a major advantage of spintronics). After
you kill this all off, and work with SETs, ballistic transport
and spin-polarized current you can consider going reversible, not
before. We could build reversible electronics today, but the power
saving would not be measurable. It would completely disappear in
the noise floor.
 
> No.  It takes a goot bit of energy to reinstate the action potential.
> It doesn't show up as heat though because chemical transformations are
> efficient.  Silicon literally RELIES on internal resistance to do
> computation, which produces waste energy by its nature.  What I'm
> asking you is why we use that to do computation in the first place.

It's an evolutionary fluke. Biology has been using gradients for a very
long time, and with switchable ion channels you can make gradients
collapse temporarily.
 
> Yes, neurons don't warm your head up.  They don't produce a lot of

There are some more metabolically active tissues in the body, but
the CNS and the brain especially are pretty close to the top (sorry,
forgot the numbers, and don't have time to look them up right now)

> waste heat, as I said. Your head is kept warm through fluid conduction
> thanks to the vasculature.  The point is that there's a reason for
> that: enzyme kinetics.  If you drop the system below ~34C, enzyme
> rates slow down enough that the system goes kaput.  If you take it
> above ~40C, enough enzymes start denaturing that that system also goes
> kaput.  They are remarkably temperature sensitive (trust me, I have
> loads of empirical evidence to back this up :), and that's why
> thermoregulation matters so much to warm blooded species.

Extremophiles manage to operate at 120 C or so, but they don't
form complex tissues. There's no particular reason we operate
at 37 C but that our normothermic homeostasis is adapted to
a specific heat loss rate, which is a function of the environment
temperature. If you cool too far down, the kinetics of chemical
reactions plummets too far down to homeostate the system.
 
> Cart before the horse.  In the vast design space of protein chemistry,
> you can design enzymes to function optimally at virtually any
> temperature between 0 and 100C.  Arctic species function at close to
> 0C.  Thermophilus aquaticus functions at 60C in hot water geysers.
> The question of why mammals have converged on 35-40C is an interesting
> one.  I personally hypothesize that it has to do with fighting
> pathogens, but one way or another, it's dictated by environment, not

Possible, but I think it's a co-evolution drive for fast fight/flight
reflexes. If you run too hot, however, you need more food for metabolism
to maintain, and might starve.

-- 
Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org
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