[ExI] Lethal future was Watson on NOVA
anders at aleph.se
Tue Feb 22 14:20:10 UTC 2011
Kelly Anderson wrote:
> On Fri, Feb 18, 2011 at 5:51 AM, Eugen Leitl <eugen at leitl.org> wrote:
>> Not even for human equivalent, nevermind at 10^6 to 10^9 speedup.
>> I don't think you can go below 1-10 W for a human realtime equivalent.
> According to several sites on the Internet the human brain uses 20-40
> Watts. Some of that undoubtedly goes for biological purposes that are
> not directly supportive of computation.
> It seems very pessimistic to say that we could only improve by 2-40
> times over nature. Granted nanowatts may be overly optimistic, and is
> based on no currently known technology. Nevertheless, I see no reason
> to believe that that the bottom is 1 Watt.
The basal metabolic rate for humans is about 70-80 Watts, so assuming an
average weight of 70-80 kg, we get a basic dissipation of about 1
Watt/kg. The brain dissipates the rest because it runs lots of ion pumps
to restore membrane potentials, as well as some possibly costly synaptic
remodeling. It is a horrendously inefficient Rube-Goldberg scheme, yet
surprisingly tough to beat.
The real issue is how much computation you need to replace brains and
how much this has to dissipate. I have made some estimates that the
likely range for brain emulation is 10^22 to 10^25 flops. Right now the
Roadrunner does 376 Mflops/W, so we are *far* away. But the Darpa
exascale study suggests we can do 10^12 flops per watt using
extrapolated but not blue sky technology - a lot of current computation
is very wasteful, and it is just recently heat dissipation has become a
towering problem. Quantum dot cellular automata could give 10^19 flops
per watt, putting the energy needs at 200-2000 watts per brain.
As I noted in my essay on this,
while this energy demand is higher than the biological brain it can be
supplied more efficiently than growing organisms, harvesting them,
possibly passing them through other animals, and then digesting them.
Even this kind of not-Drexlerian nanotech computing would be very green.
Estimating the ultimate limits is hard, since we do not know how many
dissipative calculations we need. Assuming one irreversible operation
every millisecond at every synapse leads to 10^17 dissipating operations
per second and an energy dissipation of 3*10^-6 watts per degree (colder
computers are more efficient). So even here nanowatts is going to be
tough (cooling below a few Kelvin is expensive), but less than a
milliwatt per brain seems entirely feasible using LN- if we have
reversible computers with little need for error correction.
>> Reversible logic is slow, and it's not perfectly reversible.
Not necessarily, just a lot of the current proof-of-concept designs. I
expect that once we actually start working on it seriously we are going
to optimize it quite a lot, including how to get the error correction
(which dissipates) done in a clean fashion. It wouldn't surprise me if
there was a practical tradeoff between speed and dissipation, though
(all those quantum limits to computation involve energy, and fast
changes do involve high wattages that are hard to keep dissipationless).
> An interesting question to be answered is what is the most limiting
> factor? Is it matter out of which to build intelligence? Is it energy
> to power it? Time to run it? Or space to house it? Or is there some
> other limiting factor? I think it will take a while for the
> exponential growth to stop, but it must eventually stop. I'm just not
> sure which of the above is the most limiting factor. Only time and
> technology will tell. I'm not sure we can even guess at this point
> what the most limiting factor will be.
In the really long run you cannot get more mass than around 10^52 kg,
due to the accelerated expansion of the universe. And there are time
limits due to proton decay and quantum noise. But long before that
lightspeed lags will make it hard to maintain cohesive thinking systems
when the communications delays become much longer than the local
A lot of the limits depend on what you *want* minds to do. Experiencing
pleasure doesn't require long-range communications or even much storage
space, while having the smartest possible mind requires a lot of
communications and resources.
Future of Humanity Institute
James Martin 21st Century School
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