From: Eliezer S. Yudkowsky (sentience@pobox.com)
Date: Thu Aug 28 1997 - 19:12:03 MDT
[Discussion being split due to 20K letters.]
[Terminology: "Black goo" is goo designed to destroy.]
Anders Sandberg wrote:
> "Eliezer S. Yudkowsky" <sentience@pobox.com> writes:
> > Anders Sandberg wrote:
> > > Building a nuke: you need around 10 kilograms of uranium 235 (or whatever
> > > isotope it was). There is around 2 grams uranium / tonne in the crust of the
> > > earth, of which 0.72% is U235, so to get 10 kg you need to process around
> > > 7000 tonnes of crust. I'm not sure how much energy is required to reduce
> > > the UO2 to pure U, but it is a noticeable amount (are there a chemist
> > > in the house?). Assuming the nanites cover a large patch with solar
> > > collectors, they can get around 500 W/m^2, which has to cover their
> > > replication, search through the crust, reduction, isotope separation and return
> > > to the "base"; how much energy this is is a bit hard to tell right now
> > > (it is 1.30 in the morning here :-), but it looks like it will take
> > > a while for the bomb-mold to blow up. A wild guess would be around a
> > > week.
> >
> > Well, there's a couple of questions I'd pose about nuclear nanotech:
> >
> > 1) Can mini-assemblers build mini-thermonuclear devices? If you have 10
> > grams instead of kilograms, but you still put the deuterium at the center and
> > jacket with U-238, could you get the same effect? What about custom-assembled
> > crystals that will collapse easily and tightly... maybe laced with water to
> > slow neutrons down?
>
> No. A fission bomb is based on the creation of a critical mass, where the
> growth of neutron flux due to fission is larger than the loss into the
> surroundings. Unless nanotech could build a neutron reflector (which I
> doubt is physically possible?) it would still need several kilograms of
> fissible material.
Okay. Well, it'd take more tech than I thought, but a few points:
Modern atomic technology goes like this: You have a mass of U235, with all
the atoms inside being arranged randomly. Atoms fission all the time, but the
material isn't very dense, so most neutrons escape. Then an implosive
explosion occurs which condenses the material down to a tiny, tight ball.
Neutrons collide with other U235 atoms, causing a chain reaction.
We all know this implicitly, but I'm verbalizing it so we can alter the scenario.
Now, my original suggestion was based on a passage from "Surely You're Joking,
Mr. Feynman":
"It turned out that the army had realized how much stuff we needed to make a
bomb - twenty kilograms or whatever it was - and they realized that this much
material, purified, would never be in the plant, so there was no danger. But
they did not know that the neutrons were enormously more effective when they
are slowed down in water. In water it takes less than a tenth - no, a
hundredth - as much material to make a reaction that makes radioactivity. It
kills people around and so on. It was very dangerous, and they had not paid
any attention to the safety at all."
So you don't need neutron reflectors; just ordinary water. I'm not sure
whether "10% to make radioactivity" and "10% to get critical mass" are the same
thing. Could be a power relation I didn't factor in. I'd be surprised to
find out that they were unrelated, though. Still, it could be that no amount
of water decreases the size of a critical mass. Let me know if so.
My guess is that current bombs don't use water amplifiers, because the bombs
would go around radiating lethal radiation all the time, and the material
inside would melt.
So. The bomb is made of multiple concentric shells, with a small space left
between each shell. A millisecond before the bomb goes off, water is pumped
into the space between shells. Then the bomb crushes, and the water decreases
the amount necessary for critical mass. Or, if that won't work, you can have
oxygen and hydrogen in close proximity. The bomb crushes, and then nanotech
very quickly turns the oxygen and hydrogen into water. Or with nano-precision
shaped charges and precisely positioned oxyhydro, you might be able to set up
the reaction to occur naturally as a result of the explosion.
And, getting a bit speculative, what if you take U235, store it in ordinary
water, and then use nano to "catch" the slowed neutrons that emerge? Could
you store the neutrons and use them as a catalyst for the bomb?
> > 2) What about direct fusion devices? Inertial confinement would be my guess.
> > Take the deuterium and imprison it inside a fullerene-bracketed block of
> > diamond, then apply a large energy pulse, perhaps from an enormously efficient
> > quantum-well laser.
>
> Doesn't sound very likely, although I'm more iffy on the physics here. First,
> the entire structure would have to be macroscopic in order to do any damage,
> so there is no real need to use fancy fullerenes to keep the hydrogen in.
> And you would need to store enough energy for the laser, and somehow compress
> almost all of the hydrogen, not just a small pellet (ah, thats the problem!)
> which seems to be nearly impossible to do using laser light.
The fullerenes would be macroscopic too. (Arbitrary size, at least in
theory.) The fullerenes are there to help hold everything in place - for
confinement. Now that I think about, they'd hurt more than help, so forget 'em.
Inertial confinement only operates on a single pellet, the fuse. (Ha ha!)
The problem with inertial confinement is delivering enough energy.
Quantum-well lasers are something like 10 to 100 times as efficient, and I
believe they aren't difficult to construct given nano. Englobe the whole
pellet with lasers instead of using one. Then fire. I think that you could
probably deliver at least 1,000 times as much energy as in modern inertial
confinement. It'd take a gigajoule to set the thing off, sure, but then you
have a fusion explosion and you can use it to set off arbitrarily larger ones.
Or maybe you could set each and every photon to hit a designated atom at the
designated time. I'm not sure that would be too hard in theory, given atomic
positioning and some quantum electrodynamics. But (IMHO) it would take a HELL
of a lot of computing power. FAR more than the mere exaflops the human brain
is estimated to use. You'd probably need an entire quantum computer for each
atom, a quantum continent for the whole pellet. So chalk this up as a
possible strategic advantage for black goo; if you can get a whole continent
as base, you might be able to get arbitrary amounts of energy from fusion.
> > 3) Given either of the two above, could you build a REALLY HUGE thermonuclear
> > bomb, perhaps by surrounding the item with a huge U-238 crystal? Direct the
> > blast in a specific direction using the properties of crystals, maybe with a
> > bit of quantum mechanics mixed in? Use the blast to pump a graser?
>
> Well, you could always pile up as much U and deuterium as you wanted around
> it. The extra U would not really do much except make it dirtier (it will
> mainly be blown away by the explosion instead of fissioned), but the
> deuterium would add to the blast (that is why you can make fusion bombs
> as large as you want).
Huh. As I recall, the U238 fizzed when the deuterium fused, which is how
themonuclears got up to 50 megatons. Well, either way, we're pretty much
agreed that given a small sun to start with, you can make arbitrarily larger ones.
> Directed nuclear explosions doesn't sound likely, since at present we
> are relying on random nuclear fission with no preferred direction. If
> we could make neutron mirrors things might get different.
It seems to me that this all depends on what kind of quantum properties we can
play with. Light is very easy to play with. Quantum electrodynamics lets us
make it sit up, roll over, and play dead. We can send it pretty much anywhere
we want if we can make all the little arrows line up properly. Now, are there
quantum reasons why we can't do this with neutrons or protons? Or is it just
that we'd need to arrange atoms precisely? If the former, nanonukes won't be
much improved. If the latter, nanonukes will be continent-wreckers.
> > In other words, nanotechnology operates as close to the level of nuclear
> > reactions as we operate to the level of chemical explosives. I can't help but
> > think that there will be similar improvements in technology.
>
> That is likely, if there is an interest. But remember that nanotech isn't
> much closer to the nuclear level than we are - it still cannot interface
> directly with it, just move the atoms around. There are fundamental
> physical problems here we need to look into to get the nano-nukes.
>
> > > Doing the same work as a nuke with nanites (i.e. disassembling everything
> > > within a few hundred meters and blasting everything within a few kilometers)
> > > is rather tricky, since it is extremely energy intensive. You need plenty
> > > of energy to do the disassembly (essentially you have to break most molecular
> > > bonds), and nanites are bad at making blast waves.
> >
> > I don't see why. First of all, they'll be able to construct explosives
> > completely to order.
>
> But where does the energy in the explosives come from? Remember conservation
> of energy - when you make explosives you have to add energy beside the
> chemical energy of the raw material to make the energetic but unstable
> TNT molecules (electrical energy -> chemical energy). The same is true for
> the nanites - if they want to turn a lawn into a bomb, they need more
> energy than will be released in the eventual blast.
So they launch a few million square miles of solar power satellites.
Admittedly, those are then vulnerable. Do you really think energy is going to
be a problem? Maybe some of the black goo will be suicide exploding nanites,
or battery nanites, imported from the Solar Collection Continent. Current
technology can always be taken as a lower limit. Does nano imply fusion? Ask
some fusion researcher: "If you had complete control over the molecular
structure of matter, and could get anything built of available atoms just by
wishing for it, could you build a powerful fusion reactor?"
> > I'm far from an expert on explosives... but couldn't you get a bit of an
> > improvement by detonating the ENTIRE bomb with electronic synchronization, so
> > that the entire blast arrives time-on-target? A shock wave is just that, a
> > wave. So modern TNT is a huge splash made up a lot of little uncoordinated
> > splashes, while nanotech TNT is the precisely synchronized sum of all those
> > splashes. Almost exactly like the distinction between a light bulb and a laser.
>
> I think something like this is done already, although not with the nano
> precision you suggest. You might improve the current state a bit, but it
> is likely not a quantum leap.
I note we've gone from "Nanites can't handle blast waves" to "There won't be
any *major* improvements." :)
> > Again, I'm trying to point out that a bit of
> > imagination is necessary to see the enormous destructive potentials here. I
> > think we can safely extrapolate a *little* bit beyond current capabilities.
>
> What I am trying to do is theoretical applied science - using known laws
> of physics to discuss what can and cannot be done. You can always say that
> unknown quantum effects will circumvent all that, but it is an empty
> speculation until you show that these unknown effects really do exist.
> Note that we should speculate beyond current *capabilities*, not current
> *physics*.
When I invoke quantum nanotech, I'm taking something you can do with photons,
and assuming you can do it with neutrons or protons or atoms. I truly don't
know how plausible that is. Probably some of the stuff I mentioned will work,
and some won't. I'm not a physicist. What I know of Heisenberg and Penrose
makes it seem that neutrons and protons are subject to the same tricks as
photons, but only over very short distances and very short times. So, unlike
photons, you need to be operating at a much lower scale. How much lower, I do
not know. It would probably depend on the application.
Feynman diagrams show a photon hitting an electron, but when we make a mirror,
we don't know which electron the photon will hit. Nanotech might let us
invoke specific Feynman diagrams; fire one specific photon at one specific
electron; position one specific nucleus to be hit by one specific neutron.
Even if you can't quite do THAT, the ability to create precise crystals and
precise positioning has moved you quite a bit Closer To The Bottom. It's
precise spacings of one type or another that let us play so many quantum
tricks with photons.
> > Most of what you're saying has nanotech being less dangerous than modern
> > technology. I can't help but compare this to the old saw about "Atomic
> > technology may someday equal the capabilities of present bombs, but it is
> > unlikely to produce anything more dangerous." [Paraphrase.]
>
> Well, if that turns out to be history's judgement on my part in this thread,
> so be it. But remember, there were people expecting a cure for the common
> cold in the early 70's too...
Underestimating a technology is Clark's Law.
The kind of error distinguished scientists make.
You named a timelike error, like nano arriving next Tuesday or in 2070.
The kind of error bash young scientists make.
Finally, I'm not history, but this argument can't happen without two sides.
Which side is right is not relevant to the value of your contribution.
--
sentience@pobox.com Eliezer S. Yudkowsky
http://tezcat.com/~eliezer/singularity.html
http://tezcat.com/~eliezer/algernon.html
Disclaimer: Unless otherwise specified, I'm not telling you
everything I think I know.
This archive was generated by hypermail 2.1.5 : Fri Nov 01 2002 - 14:44:47 MST