From: hal@finney.org
Date: Sat Aug 28 1999 - 13:31:06 MDT
Robert J. Bradbury, <bradbury@www.aeiveos.com>, writes:
> Well, my questions to Robert Freitas regarding the "hypsithermal limit"
> (heat tolerance of the planet), have drawn a response.
>
> [Yes, I am an angel from god... :-)]
Or perhaps the Pope?
> And the conclusions are not much different. He basically says
> that from the conservative to the optimistic sides we are limited
> to 100-1000 KW/person. That is pretty much what I had said
> with my 400 KW. If you want more, you have to borrow or buy
> from someone else (if you are going to be ethical at any rate).
Well, without access to his reasoning I can't judge this. I suggested
that 10,000 kW/person would be available based purely on energy
considerations, and it might well be that you could only get 1/10 of
this in practice, which would be at the high end of Robert's range.
Particularly if you wanted to leave the oceans alone (water evaporation
is a significant part of the energy budget, not to mention affecting
the weather), you lose 75% right there. So there is not that much
disagreement here.
I did look at http://asd-www.larc.nasa.gov/erbe/components2.gif
which shows the Earth's energy budget, and I notice that there is no
allocation for binding solar energy chemically. The Earth is, according
to this diagram, at energetic equilibrium, radiating as much energy
as it receives, partially as reflected visible light and partially as
radiated infrared.
Presumably the lack of a budget item for life or for chemical energy
binding reflects the fact that the total biomass is approximately
constant, and that the chemical reactions are also at a steady state.
Now it seems to me that with nanotech we threaten to affect this energy
balance in three ways. One is to change the albedo, covering the earth
with dark, energy-absorbing solar cells. Then we would be radiating
less at visible wavelengths and more as heat, which could increase the
temperature and would in any case alter the energy balance. Of course,
to some extent we have been changing the albedo for thousands of years,
and with nanotech we could fine-tune this effect. If we simply replaced
plant life with equal-albedo solar collectors which captured the energy
in a more useful form, we could probably make a lot of energy available.
I note on the NASA diagram that 51% of incoming solar energy is absorbed
by land and ocean and later radiated as heat, while only 4% is reflected
from the surface as visible light. So it does not appear that albedo
effects are very important in the overall scheme. We could make the
entire earth flat black and it would only change the energy budget by 4%.
Another effect we could have would be a one-time release or absorption
of energy as we convert chemicals from their natural form (rock and
atmospheric gasses) into manufactured forms (which are anticipated
to be crystalline structures that are essentially rocks as well).
I don't actually know whether this will be a net energy release or not.
I suspect that the natural rocks are chemically bound with very similar
energy levels to the kinds of structures we would build with nanotech,
for the most part. Rocks are mostly made of small crystals of oxides of
silicon, aluminum, iron, and a few other metals. This is very similar
to what Drexler envisions making nanotech devices out of. So the net
effects of this transition from natural to manufactured forms are probably
not that large overall.
I see this as a one-time change because after a while we will have found
most of the easy-to-get atoms that we want to use, and from then on we
will take things apart to build new ones, just as life does today.
Then the third effect would be heat dissipation as a side effect of
our use of nanotech devices, both for manufacturing (and disassembling),
as well for simply operating our computers, utility fog, and other nifty
nano devices. This energy-waste cost would also be present during the
transition period as we convert from natural elements to manufactured
devices for the first time.
However as I noted originally, the energy to waste in this way all
comes from solar, and should already have been accounted for in my
first category.
Until I can see Robert Freitas' figures in more detail it still looks
to me like we might hope for a factor of 2, 5, or perhaps 10 above his
high figure of 1000 kW/person. We might have to do some work so that
we continue to evaporate the oceans, etc., with the waste heat in the
same way it happens naturally. But in principle it could be done.
Hal
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