=biology =nanotechnology
"Grey goo" is unlikely.
The nanomachinery builds diamondoid bacteria, that replicate with solar power and atmospheric CHON, maybe aggregate into some miniature rockets or jets so they can ride the jetstream to spread across the Earth's atmosphere, get into human bloodstreams and hide, strike on a timer.
To control these atoms you need some sort of molecular chaperone that can also serve as a catalyst. You need a fairly large group of other atoms arranged in a complex, articulated, three-dimensional way to activate the substrate and bring in the reactant, and massage the two until they react in just the desired way. You need something very much like an enzyme.
My understanding is that anyone who can grasp what "orthos wildly attacking the heterodox without reading their stuff and making up positions to attack" looks like, considers that this is what Smalley did with Drexler - made up an unworkable approach and argued against it.
In this post, I use "nanobots" to mean "self-replicating microscopic
machines with some fundamental mechanistic differences from all biological
life that make them superior".
Various specific differences from
biological cells have been proposed. I've organized this post by those
proposed differences.
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1. localized melting
Most 3d printers melt material to extrude it through a nozzle. Large
heat differences can't be maintained on a small scale.
2. rare materials
If a nanobot consists largely of something rare, getting more of that
material to replicate is difficult outside controlled environments.
Growth of algae and bacteria is often limited by availability of iron, which
is more common than most elements. Iron is the
active catalytic site
of many enzymes, and is needed by all known life. The growth of something
made mostly of iron would be far more limited, and other metals have more
limited availability than that.
3. metal surfaces
Melting material isn't feasible per (1), so material must be built up by
adding to the surface. Since that's the case, the inside of structures must
be chemically the same as what was the exterior.
Metal objects have a
protective oxide layer. In an air or water environment, there's no way to
add individual (eg) aluminum atoms to a metal surface and end up with
metallic aluminum inside; the whole thing will typically be aluminum oxide
or hydroxide.
Corrosion is also a proportionately bigger problem for
smaller objects. A micrometer-scale metal structure will rapidly corrode,
perhaps doing some Ostwald
ripening.
4. electric motors
Normal "electric motors" are all electromagnetic motors, typically using
ferromagnetic cores for windings. Bigger is better for those, up to at
least the point where you can
saturate cores.
On a very small scale, it's better to use electrostatic motors, and you
can make MEMS electrostatic motors with lithography. (Not just
theoretically; people actually do that.) But, per (2) & (3), bulk metals are
a problem for a self-replicating system. If you need to have compounds
floating around, electrical insulation is also difficult. You also need some
way to switch current, and while small semiconductor switches are possible,
per (3) building them is difficult.
Instead of electrostatic charge
of metal objects, it's better to use ions. Ions could bind to some molecule,
and electrostatic forces could cause that to rotate relative to another
molecule. Hmm, this is starting to sound
rather familiar.
5. inorganic catalysts
Lab chemistry and drug synthesis often use metal catalysts in solution,
perhaps with a small ligand.
Palladium
acetate is used for
making drugs, but it's very toxic to humans, because it...catalyzes
reactions.
Life requires control of what happens, which
means selective catalysis of reactions, which means molecules need to be
selectively bound, which requires specific arrangements of hydrogen bond
donors and acceptors and so on, and that requires organic compounds.
Controlled catalysis requires organic compounds.
6. no liquid
Any
self-replicating nanobot must have many internal components. If the interior
is not filled with water, those components will clump together and be unable
to move around, because electrostatic & dispersion interactions are
proportionately much stronger on a small scale. The same is true to a lesser
extent for the nanobots themselves.
Vacuum is even worse. Any
self-replicating cell must move material between outside and multiple
compartments. Gas leakage by the transporters would be inevitable. Cellular
vacuum pumps would require too much energy and may be impossible. Also,
strongly binding the compounds used (eg CO2) to carriers at every step would
require too much energy. ("Too much energy" means too much to be competitive
with normal biological processes.)
7. no water
Most
enzymes maintain their shape because the interior is hydrophobic and the
exterior is hydrophilic. If some polar solvent is used instead of water,
then this stability is weakened; most organic solvents will
denature most
proteins. If you use a hydrophobic solvent, it can't dissolve ions or
facilitate many reactions.
Ester and amide bonds are the best ways to
reversibly connect organic molecules. Both involve making or taking water or
alcohol. Alcohols have no advantages over water in terms of conditions where
they're stable.
Water is by far the best choice of liquid. The
effectiveness of water for dissolving ions is unique. Water can help
catalyze reactions by donating and accepting hydrogen. Water is common on
Earth, easy to get and easy to maintain levels of.
8. high temperatures
Per (5) you need organic molecules to selectively catalyze reactions.
Enzymes need to be able to change shape somewhat. Without
conformational changes, enzymes can't grab their substrate well enough.
Without conformational changes, there's no way to drive an unfavorable
reaction with a favorable reaction, and that's necessary.
Because
enzymes must be able to do conformational changes, they need to have some
strong interactions and some weaker interactions that can be broken or
shifted. Those weaker interactions can't hold molecules together at high
temperatures. Some life
can grow at 100
C but 200 C isn't
possible.
This means that the reactions you can do are limited to
what organic compounds can do at relatively low temperatures - and existing
life can pretty much do anything useful in that category already.
9. diamond
It's
possible to make molecules containing diamond
structures at ambient
temperature. The synthesis involves carbocations or carboanions or carbon
radicals, which are all very unstable. The yields are mediocre and the
compounds involved are reactive enough to destroy any conceivable enzymes.
Some people have simulated structures that could theoretically place
carbon atoms on diamond in specific positions at ambient temperature.
Here's a paper on
that. Because diamond is so kinetically stable, the synthesis must be
exothermic, with high-energy intermediates. So, high vacuum is required,
which per (6) doesn't work.
Also, the chemicals consumed to make
those high-energy intermediates are too reactive to plausibly be made by any
enzyme-like system. And per (1) & (8) you can't use high temperatures to
make them on a small scale.
Also, there is no way to later remove
carbon atoms from the diamond at low temperature. How, then, would a nanobot
with a diamond shell replicate?
10. other rigid materials
CaCO3, silica, and apatite are much easier to manipulate than diamond.
They're used in (respectively) mollusk shells, diatom frustules, and bone.
If it was advantageous to use structures of those inside cells for
reactions somehow, then some organisms would already do that. Enzymes
generally must do conformational changes to catalyze reactions. A completely
rigid diamond shape with functional groups would not make a particularly
good enzyme.
And of course, just a small solid shape, with
nothing attached to it - even if you can make arbitary shapes - isn't useful
for much besides cell scaffolding, and even then, building diatom frustules
out of linked diamond pieces seems worse than what they do now with silica.
Sure, diamond is even stronger than silica, but that doesn't matter.
And that's assuming you can make interlocking diamond pieces, which you can't.
11. 3d structures
"Unlike cells, nanobots could make 3d structures, instead of being limited to a soup of folded linear structures."
Yes, believe it or not, I've seen people say that. But cells have eg
microfilaments.
Again, enzymes must be able to do conformational changes to work. At ambient
temperature, that means they're shaking violently, and if proteins are
flopping around constantly, you can't have a rigid positioner move to a
fixed position and assume you're placing something correctly.
What
you can do is hold onto the end of a linear chain as you extrude
it, then fold up that chain into a 3d structure. What you can do is use an enzyme
that binds to 2 folded proteins and
connects them
together.
And those are methods that are used by all known life.
12. active transport
"Life relies on diffusion and random collisions; nanobots could intentionally move things around."
Yes, I've actually seen people say that, but cells do use myosin to transport proteins sometimes. That uses a lot of energy, so it's only used for large things.
13. combining reaction steps
"Nanobots could put all the sequential reaction steps next to each other, making them much more efficient than cells."
Cells have compartments with proteins that do related reactions. Some proteins form complexes that do multiple reaction steps. Existing life already does this to the extent that it makes sense to.
14. positional nanoassembly
The above sections should be enough background to finally cover what's
perhaps the most central concept of the genre of proposals called
"nanobots".
Some people see 3d printers and CNC routers, and don't
understand enzymes or what changes on a molecular scale very well, and think
that cells that work more like 3d printers or gantry cranes would be better.
Now, a FDM 3d printer has several components:
- sensors that detect the
current position
- drivers that control motors based on sensors
- 3
motors that do 3-axis movement
- a rigid bed and rigid drive system
-
a good connection between the bed and material being printed
- a nozzle
that melts material
Protein-sized position sensors don't exist.
Molecular linear motors do exist, but 1 ATP (or other energy carrier) is
needed for every step taken.
If you want to catalyze reactions,
you need floppy enzymes. Even if you attach them to a rigid bed, they'll
flop all over the place. (On a microscopic scale, normal temperatures are
like a macroscopic 3d printer being shaken violently.)
Suppose you're
printing diamond somehow. You need a seed that's rigidly connected to the
printing mechanism. The connection would need to be removable in order to
detach the product from the printer. In a large 3d printer, you can peel
plastic off a metal surface, but that won't work for covalently bonded
diamond. You would need a diamond seed with functional groups that allow it
to be grabbed, and since you're not starting with a sheet, you'd need a
5-axis printer arm.
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Drexler wrote
a
book that proposed mechanical computers which control positioner arms by
lever assemblies. An obvious problem there is mechanical wear - yes, some
MEMS devices have adequate lifetimes, but those just vibrate; their sliding
friction is negligible. But suppose you can solve this by making everything
out of diamond or using something like
lubricin.
So, suppose
you have a mechanical computer that moves arms that control placement of
something. Diamond is impractical, so let's say silica is being placed.
Whatever you're placing, you need chemical intermediates that go on the
arms, and you need energy to power everything. Making energy from fuel or
photosynthesis requires more specific chemicals, not just specific
arrangements of some solid. To do the reactions needed for energy and
intermediate production, you need things that can do conformational changes
- enzymes.
Without conformational changes, enzymes can't grab their
substrate well enough. Without conformational changes, there's no way to
drive an unfavorable reaction with a favorable reaction, and that's
necessary. You can't just use rigid positioners to drive reactions that way,
because they have no way to sense that the reaction has happened or
not...except through conformational changes of a flexible enzyme-like
tooltip on the positioner, which would have the same issues here.
At
ambient temperature, enzymes that can do conformational changes are shaking
violently, and if proteins are flopping around constantly, you can't have a
rigid positioner move to a fixed position and assume you're placing
something correctly. Since you need enzymes, you need a ribosome, and
production of monomers - and amino acids are the best choice, chemical
elements are limited and there is no superior alternative.
Since all
that is still needed, what are the positioners actually accomplishing?
They'd only be needed to build positioners. The whole thing would be a
redundant side system to enzymatic life.
OK, maybe you want to build
some kind of mechanical computers too. Clearly, life doesn't require that
for operation, but does that even work? Consider a mechanical computer
indicating a position. It has some number, and the high bit corresponds to a
large positional difference, which means you need a long lever, and then the
force is too weak, so you'd need some mechanical amplifier. So that's a
problem.
Consider also that as vacuum is impractical per (6), and
enzymes and chemical intermediates are needed, you'd have stuff floating
around. So you have all these moving parts, they need to interface with the
enzymes so they can't just be separated by a solid barrier, and stuff could
get in there and jam the system.
The problems are myriad, and I'd be
well-positioned to see solutions if any existed. But suppose you solve them
and make tiny mechanical computers in cells - what's the hypothetical
advantage of that? The ability to "do computation"? Brains are more
energy-efficient than semiconductor computers for many tasks, and the
total
embodied
computation in
cells is far greater than that of neurons' occasional spikes.
15. everything else
When someone has an idea about something cells could do, it's often
reasonable to presume that it's either impossible, useless, or already used
by some organism - but there are obviously cases where improvement is
possible. It's certainly physically possible to correct harmful mutations
with genetic engineering. There are also ongoing arms races between
pathogens and hosts where each step is an informational problem.
But what about more basic mechanisms? Have basic mechanisms for typical
Earth conditions been optimized to the point that no improvement is
possible? That depends on their complexity. For example, glycolysis and the
citric acid cycle are optimal, but
here's a
more-efficient CO2 fixation pathway I designed. (Yes, you'd want to
assimilate the glycolaldehyde synthons by (erythrose 4-phosphate -> glucose
6-phosphate -> 2x erythrose 4-phosphate). I left that as a way for people
to show they understood my blog.) See also my
post on the
origin of life for some reasons life works the way it does. (You can see
I'm a big blogger - that's a good
career plan,
right?)
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I wrote this post now as a sort of side note to my
post on AI
risks.
But...what if a superintelligence finds something I didn't think of?
I know, right? What if it finds a way to travel faster than light and sets
up in Alpha Centauri, then comes back? What if it finds a way to make
unlimited free energy? What if it finds a friendly unicorn that grants it 3
wishes?
There's a gap between seeing that something is conceivably
possible and seeing how to do it, and that's the only reason that things
like research and planning and prediction about the future are possible. I
understand Eliezer Yudkowsky thinks that someone a little smarter than von
Neumann (who didn't invent the "von Neumann architecture" or half the other
stuff he took credit for, but that's off topic) would be able to invent
"grey goo" type nanobots. If that was the case, even I would at least be
able to see how it would be done.
To be clear, I'm not trying to
imply that a superintelligent AI wouldn't have any plausible route to taking
over societies or killing most of humanity or various other undesirable
outcomes. I'm only saying that worrying about "grey goo" is a waste of time.
On the other hand, Smalley was mad at Drexler for scaring people away from
research into carbon nanotubes, but carbon nanotubes would
be a health
hazard if they were used widely, and the applications Smalley hoped for
weren't
practical.
Perhaps I would thank Drexler if he actually pushed people away from working
on carbon nanotubes, but he didn't.
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This post has some comments at LessWrong.