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I've been studying this topic for use in a story I'm working on and I've come across various videos and interviews on the topic, but they all seem mostly concerned with assembly of larger objects.

I was just curious if the same actions that would assemble an object could be reversed to disassemble it, or if there were other necessary actions that needed to be taken. I understand that energy needs to be put in to break a molecular bond, so is that something that would have to be taken into account as well?

Also, as a side note, the current idea is to have the nanobots be mostly carbon constructs, if that affects the way things work.

I'm looking for the most realistically plausible answer, as well.

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    $\begingroup$ A nanobot which can dismantle a molecule is itself a molecule. Specifically, an enzyme if we’re talking organics. There is no single molecule that can dismantle all molecules, they obey the laws of thermodynamics. They grab an atom with a bond that is stronger than the bond holding it to the molecule, then fold to remove the atom. But they don’t navigate or see or calculate anything. They are molecules with all the intelligence of a molecule. “Robots” just can’t be believable in a science-based answer. It’s a fun thought excercise, that’s all. $\endgroup$
    – Vogon Poet
    Aug 22, 2023 at 0:12
  • $\begingroup$ Vogon Poet nailed it. If you want "realistically plausible", then the answer is, "they don't". Nanobots, as they appear in science fiction, are fantasy. $\endgroup$
    – jdunlop
    Aug 22, 2023 at 9:21
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    $\begingroup$ Both comments above are wrong. Enzymes are not considered nanobots, though they are certainly nanoscopic. utility fog is an example of nanobots with a diameter of about 100 nanometers. Yes they can disassemble stuff on the molecular level one nanoscopic layer at a time. Only if it's organic or if it's production takes place at standard temperature and pressure. claiming that the nanobot I listed cannot disassemble stuff on the molecular level is the same as claiming people can't control things smaller then our own bodies. Our hands are a small part of our body that we use rather well. $\endgroup$ Aug 22, 2023 at 11:07

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You would see something akin to the human stomach being formed on the outside of the nano-bot blob, producing highly active chemicals that debond and denature the opposition. But its not just limited to chemistry. Nanorobots can produce all sorts of machinery, and this include chemical warfare, physical warfare (explosives and heat) and other denaturing weapons, such as radiation or similar denaturing. So things would start to wither and die, depending on the attack mode- by chemical denaturing(fire, chemistry electricity),physical denaturing (magnetism, radiation, light) or brute force (explosives extruded from tendrils driven into the "enemy". It wouldn't be very quiet either. One can expect to encounter a sensory/laboratory layer on the swarms epidermis. Once they have a sample, one can expect to be passed through various layers. Over time, the swarm might even develop something akin to organs, ripping parts of, to transport them to processing hubs like basins of acid.

If small amounts of nanobots are encountered, they would not have this ability. They would still sample, but then would need time to climb up the exponential arc. They would need to construct factories and energy converters. So you wake up with a industrial park of implants growing around the point of wounding. Similar to cancer, starvation and counter measures might even be able to cure the nanotic plague.

PS: If you want to add some horror element to the nano-robots, they could in theory dissasemble something very slow, carefully and store the information. Which then would make destroying the nanobots, the actual act of killing somebody forever. For bonus horror, the nano-bots could reconstitute a person now and then, to refresh the memory, creating the ultimate horror of having a blink of conciousness every thousand years. Imagine you could watch the earths future history in fast-forward.

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I was just curious if the same actions that would assemble an object could be reversed to disassemble it, or if there were other necessary actions that needed to be taken.

Approximately, yes. In detail, no, because you need slightly different manipulator heads for assembly vs. disassembly of the same molecule, because...

I understand that energy needs to be put in to break a molecular bond, so is that something that would have to be taken into account as well?

...of exactly that.

The definitive works on this subject are Nanosystems (which is more technical) and Engines of Creation (which is more pop-science / aspirational), both by Eric Drexler.

Nanobots would work essentially by catalyzing chemistry by placing reactants in very specific positions--pretty much like natural enzymes do, but with fixed engineered pathways for providing materials to reaction sites, rather than relying on random collisions in a solute. To build something, you use a manipulator site, like a atomic force microscope probe, that has an atom or molecule you want to place bonded to it, and position it such that it will form a stronger bond with the stubstrate and thus be pulled away. To disassemble, you do the same thing in reverse--use an appendage which is designed to bond more strongly to whatever you want to remove than that target bonds to its existing substrate, position the appendage to encourage the reaction, then pull it away. Note that we can actually do all of these steps already with atomic force microscopes to position individual atoms--we just can't do it very quickly or energy-efficiently, because all the machinery around the manipulator tip of an atomic force microscope is rather large!

Note that naively this set of processes would seem to go strictly one way--one kind of nanobot can construct things, one kind can deconstrust them, and whichever one happens to be the energy-producing order for those steps to occur in is the only one that will happen. It's like trying to get sticky stuff off your hands--you try to rub it off, and it just gets stuck to your other hand, so you use it towel, but now the stuff is stuck to the towel, so you wash the towel and now the sticky stuff is disposed of, but you can't get it back and re-use it; you can't push it through the pathway in the opposite direction. Fortunately, however, there are ways around that--the same ways that are employed by living organisms, in fact. The nanobots just have to supply energy, which manifests as altering the physical conformation or electronic state of their manipulator sites, which raise or lower the bond energies and allow passing atomic and molecular materials around in a reversible manner. In most cases, this still requires multiple manipulator types to construct anything particularly complex, and whether construction or destruction consumes vs. produces energy will depend on exactly which kinds of molecules you are trying to construct or destruct, but you can design molecular systems which will do both.

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