Not an expert in chemistry or biochemistry, but this post seems to basically not engage with the feasibility studies Drexler has made in Nanosystems, and makes a bunch of assertions without justification, including where Nanosystems has counterarguments. I wish more commenters would engage on the object level because I really don’t have the background to, and even I see a bunch of objections. Nevertheless I’ll make an attempt. I encourage OP and others to correct me where I am ignorant of some established science.
Points 1, 2, 3, 4 are not relevant to Drexlerian nanotech and seem like reasonable points for other paradigms.
Regarding 5, my understanding is that mechanosynthesis involves precise placement of individual atoms according to blueprints, thus making catalysts that selectively bind to particular molecules unnecessary.
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.)
If your components are all fixed in place by covalent bonds, they can’t clump together.
Nanosystems section 11.4 gives arguments against gas leakage being inevitable, and a proposed “turbomolecular pump” design.
It’s not clear why a cellular vacuum pump must have a low efficiency, and the actual work (P*V) that must be done by a vacuum pump should be well below the kcal/gram range that biological cells need to replicate.
7 is not relevant given that I’m imagining hard vacuum systems.
8. high temperatures
[...] 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.
If the “enzymes” are not made of proteins, their weaker interactions can be stronger than the hydrogen bonds that hold proteins together. If they’re roughly twice as strong (say by using covalent bonds or twice as many hydrogen bonds), they wouldn’t denature even at 200C.
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?
The shell could be made of diamond panels with airtight joints. The daughter cell’s internal components and membrane are manufactured inside the parent cell, then the membrane is added to the parent cell’s membrane, it unfolds in an origami fashion into two membranes of original size, then the daughter cell separates.
This is a pretty obvious idea to me, so unless there’s some obvious reason why something like this doesn’t work, I get the feeling that the post isn’t engaging with the strongest arguments.
10-13 seem pretty reasonable to me.
14. positional nanoassembly
[...] 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.)
It seems to me that the “step” for molecular linear motors could be an arbitrarily long distance. The moving part randomly moves about the stator, and there are ratchets every, say, 100 nm that let the moving part pass in one direction when an ATP is consumed. Then when it needs to be fixed in position, a different mechanism does that.
The “floppy enzymes” has the same solution as section 8. In chapter 13 of Nanosystems Drexler also gives three different ways this problem is solved, two of which involve molecular manipulators:
Aside from differences of scale and component properties, molecular manipulators differ from macroscale devices in that they must maintain positional accuracy despite thermal excitation. This problem can be minimized either (1) by operation at reduced temperatures, which receives no further attention here; or (2) by the use of a stiff mechanism, as described in Section 13.4.1:or (3) by use of local nonbonded contacts to align reagent devices to workpieces immediately before reaction, as discussed in Section 13.4.2.
My current impression is that this post is perfectly consistent with the problem of making a self-replicating diamondoid bacterium being easy enough that unamplified humans could do it in <500 years given good research practices, software, and perhaps some narrow AI tools like some future version of AlphaFold. It’s true that Drexlerian nanotech is not really optimized for being self-replicating nanomachines, as Drexler envisions it’s most useful for mass manufacturing and computing. They might be less evolvable, consume more energy, or be specialized to certain environments like the atmosphere or ocean, especially when designed by human-level intelligences. But there are also potentially huge advantages like being undigestible to biological life and viruses, being impervious to more forms of damage, or a wider range of metabolic pathways. But it requires a different argument to say that either (a) nanotech is impossible, or (b) the disadvantages of nanotech-based life forms outweigh the advantages.
If I were in the mindset I get from this post, I would have a hard time not asserting that powered flight would be impractical for airplanes larger than birds, or that modern semiconductor manufacturing were impossible due to the precision required. I would probably deny other possibilities currently thought extremely plausible, like fusion energy and immortality. Maybe the author isn’t making the same mistake, but I’m doubtful nonetheless.
Regarding 5, my understanding is that mechanosynthesis involves precise placement of individual atoms according to blueprints, thus making catalysts that selectively bind to particular molecules unnecessary.
No, that does not follow.
The shell could be made of diamond panels with airtight joints. The daughter cell’s internal components and membrane are manufactured inside the parent cell, then the membrane is added to the parent cell’s membrane, it unfolds in an origami fashion into two membranes of original size, then the daughter cell separates.
...for one thing, that’s not airtight.
It seems to me that the “step” for molecular linear motors could be an arbitrarily long distance.
No, the steps happen by diffusion so they become slower. That’s why slower muscles are more efficient.
The “floppy enzymes” has the same solution as section 8. In chapter 13 of Nanosystems Drexler also gives three different ways this problem is solved, two of which involve molecular manipulators:
I don’t know how to engage with the first two comments. As for diffusion being slow, you need to argue that it’s so slow as to be uncompetitive with replication times of biological life, and that no other mechanism for placing individual atoms / small molecules could achieve better speed and energy efficiency, e.g. this one.
I don’t have the expertise to evaluate the comment by Muireall, so I made a Manifold market.
Such actuator design specifics aren’t relevant to my point. If you want to move a large distance, powered by energy from a chemical reaction, you have to diffuse to the target point, then use the chemical energy to ratchet the position. That’s how kinesin works. A chemical reaction doesn’t smoothly provide force along a range of movement. Thus, larger movements per reaction take longer.
Biological life uses an ATP system. This is an energy currency, but it’s discrete. Like having batteries that can only be empty or full. It doesn’t give a good way to apply smaller amounts of energy than 1 atp molecule carries, even if less energy is needed.
Nanobots could have a continuous energy system, or smaller units of energy.
Not an expert in chemistry or biochemistry, but this post seems to basically not engage with the feasibility studies Drexler has made in Nanosystems, and makes a bunch of assertions without justification, including where Nanosystems has counterarguments. I wish more commenters would engage on the object level because I really don’t have the background to, and even I see a bunch of objections. Nevertheless I’ll make an attempt. I encourage OP and others to correct me where I am ignorant of some established science.
Points 1, 2, 3, 4 are not relevant to Drexlerian nanotech and seem like reasonable points for other paradigms.
Regarding 5, my understanding is that mechanosynthesis involves precise placement of individual atoms according to blueprints, thus making catalysts that selectively bind to particular molecules unnecessary.
If your components are all fixed in place by covalent bonds, they can’t clump together.
Nanosystems section 11.4 gives arguments against gas leakage being inevitable, and a proposed “turbomolecular pump” design.
It’s not clear why a cellular vacuum pump must have a low efficiency, and the actual work (P*V) that must be done by a vacuum pump should be well below the kcal/gram range that biological cells need to replicate.
7 is not relevant given that I’m imagining hard vacuum systems.
If the “enzymes” are not made of proteins, their weaker interactions can be stronger than the hydrogen bonds that hold proteins together. If they’re roughly twice as strong (say by using covalent bonds or twice as many hydrogen bonds), they wouldn’t denature even at 200C.
The shell could be made of diamond panels with airtight joints. The daughter cell’s internal components and membrane are manufactured inside the parent cell, then the membrane is added to the parent cell’s membrane, it unfolds in an origami fashion into two membranes of original size, then the daughter cell separates.
This is a pretty obvious idea to me, so unless there’s some obvious reason why something like this doesn’t work, I get the feeling that the post isn’t engaging with the strongest arguments.
10-13 seem pretty reasonable to me.
It seems to me that the “step” for molecular linear motors could be an arbitrarily long distance. The moving part randomly moves about the stator, and there are ratchets every, say, 100 nm that let the moving part pass in one direction when an ATP is consumed. Then when it needs to be fixed in position, a different mechanism does that.
The “floppy enzymes” has the same solution as section 8. In chapter 13 of Nanosystems Drexler also gives three different ways this problem is solved, two of which involve molecular manipulators:
My current impression is that this post is perfectly consistent with the problem of making a self-replicating diamondoid bacterium being easy enough that unamplified humans could do it in <500 years given good research practices, software, and perhaps some narrow AI tools like some future version of AlphaFold. It’s true that Drexlerian nanotech is not really optimized for being self-replicating nanomachines, as Drexler envisions it’s most useful for mass manufacturing and computing. They might be less evolvable, consume more energy, or be specialized to certain environments like the atmosphere or ocean, especially when designed by human-level intelligences. But there are also potentially huge advantages like being undigestible to biological life and viruses, being impervious to more forms of damage, or a wider range of metabolic pathways. But it requires a different argument to say that either (a) nanotech is impossible, or (b) the disadvantages of nanotech-based life forms outweigh the advantages.
If I were in the mindset I get from this post, I would have a hard time not asserting that powered flight would be impractical for airplanes larger than birds, or that modern semiconductor manufacturing were impossible due to the precision required. I would probably deny other possibilities currently thought extremely plausible, like fusion energy and immortality. Maybe the author isn’t making the same mistake, but I’m doubtful nonetheless.
No, that does not follow.
...for one thing, that’s not airtight.
No, the steps happen by diffusion so they become slower. That’s why slower muscles are more efficient.
see this reply
I don’t know how to engage with the first two comments. As for diffusion being slow, you need to argue that it’s so slow as to be uncompetitive with replication times of biological life, and that no other mechanism for placing individual atoms / small molecules could achieve better speed and energy efficiency, e.g. this one.
I don’t have the expertise to evaluate the comment by Muireall, so I made a Manifold market.
Such actuator design specifics aren’t relevant to my point. If you want to move a large distance, powered by energy from a chemical reaction, you have to diffuse to the target point, then use the chemical energy to ratchet the position. That’s how kinesin works. A chemical reaction doesn’t smoothly provide force along a range of movement. Thus, larger movements per reaction take longer.
Biological life uses an ATP system. This is an energy currency, but it’s discrete. Like having batteries that can only be empty or full. It doesn’t give a good way to apply smaller amounts of energy than 1 atp molecule carries, even if less energy is needed.
Nanobots could have a continuous energy system, or smaller units of energy.