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Nanites would likely generate a lot of heat. If a large number of nanites infested a human body, multiplied, and began moving / operating what would be the result? Could the composition of the nanites result in Spontaneous Human Combustion?

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    $\begingroup$ Not really spontaneous if it is caused by reproducing nanites jiggling hotly within the body. Sort of like spontaneous human combustion caused by many laser beams aimed at the hapless human. Maybe just "human combustion"? $\endgroup$
    – Willk
    Mar 11, 2021 at 23:34
  • $\begingroup$ @Willke Fair enough. $\endgroup$ Mar 11, 2021 at 23:35
  • $\begingroup$ What are these nanites doing? Are they designed by humans to perform a task within the human body- if so what? How do you define "infest"? Do they hijack DNA to produce copies of themselves or do they have a separate mechanism? Are they virus-sized, cell-sized? What are their capabilities for movement? What "intelligence" do they have? $\endgroup$ Mar 12, 2021 at 2:21
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    $\begingroup$ The drummer nanites in Stephenson's The Diamond Age did this, from recollection $\endgroup$ Mar 12, 2021 at 8:38
  • $\begingroup$ What if each nanite has a sodium molecule inside it's case, could they all just open up and expose all that sodium to water at the same time? $\endgroup$
    – Culyx
    Mar 12, 2021 at 18:40

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It is worth remembering that the average human body has something of the order of 1016 energetic, heat-generating intracellular "nanites" in the form of mitochondria.

These can, under the right circumstances, kill the human they're in... one way of doing this is to interfere with various cooling and regulatory mechanisms using suitable pharmaceuticals, causing death by hyperthermia.

This does not actually involve anyone bursting into flames though, as human bodies are pretty soggy and hard to ignite without some pretty weird chemistry being involved. To do it just via heat, without the presence of any other fuels or accelerants or oxidisers or other trickery you must first boil a human to drive off enough water that the stuff left behind is flammable.

If a human weighs 60kg, and they start out at a uniform 36 degrees C (which is obviously inaccurate, but it'll do), and the specific heat capacity of water is 4200 J/kgK, you'd need to impart ~16MJ to raise their body temperature to 100 degrees. You then need to impart another 2260kJ/kg (the latent heat of vapourisation of water) to actually boil the water... that's another 81.4MJ, if you could deliver it only to the water somehow.

So that's about 100MJ, or the energy in 2-3kg of petrol (or gasoline, if you prefer) or more like 10kg of dynamite. A that point you've got to wonder about the energy density of your nanites. How are they getting this much juice? Where do they store it? How do they release enough of it to boil their host without destroying their own pretty delicate structures and chemical processes? Are your nanites actually just antimatter bombs? (they'd need to share about a microgram of the stuff between the entire swarm, FYI)

So, if you want to incinerate a human, either cheating (by releasing chemicals that can cause them to burn whilst wet) or by using external fuels or explosives to do the job. Trying to do it with heat generated inside their body alone seems like a bit too much like implausibly hard work.

On the other hand, generating enough heat to cause serious or fatal hyperthermia seems pretty straightforward, and an entirely reasonable side effect of various kinds of nanomechanism or cybernetic implant.

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  • $\begingroup$ Another common chemical that induces a sixth type of hyperthermia - uncoupled phosphorylation - is 2,4-dinitrophenol or DNP, sometimes used for weight loss, with the side effect of death by malignant hyperthermia. Rather than interfering with thermoregulation, uncoupling proteins work by short-circuiting ATP energy into heat rather than any useful cellular work. $\endgroup$
    – LSerni
    Mar 12, 2021 at 18:28
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In classical.SciFi and fantasy, nanites are very very small indeed. If there are millions in a body, they need to be. If there are billions, they need to be even smaller (like a lot smaller). So you're discussing an item of tech probably similar size to a human cell, or a very few of them,or maybe even smaller (they need to be able to interact with individual nerves, cross the blood-brain barrier or whatever).

If they are that small, I don't see a reason why we can't handwave them a power consumption similar to a human cell, in which case they won't make much impact on the body's heat output, beyond what ordinary cells could do. The body will regulate its core temperature exactly as it usually does, and compensate.

It would be a very poorly designed nanite which caused its host to quickly die.

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  • $\begingroup$ A nanite would be of the order of a large molecule $10^{-9}$ m, hence the name nanobot $\endgroup$
    – nzaman
    Mar 12, 2021 at 5:08
  • $\begingroup$ @nzaman in your dream, less than 1000 atoms is not enough $\endgroup$
    – MolbOrg
    Mar 12, 2021 at 6:25
  • $\begingroup$ The smaller it is, the less energy used. A "large molecule" would only need energy consumption of the order of a large molecule. And yes, that small is tricky for a functional machine. $\endgroup$
    – Stilez
    Mar 12, 2021 at 6:53
  • $\begingroup$ @MolbOrg:Huh? What are you saying? $\endgroup$
    – nzaman
    Mar 12, 2021 at 12:09
  • $\begingroup$ @nzaman - from wiki - "More specifically, nanorobotics (as opposed to microrobotics) refers to the nanotechnology engineering discipline of designing and building nanorobots, with devices ranging in size from 0.1 to 10 micrometres and constructed of nanoscale or molecular components." - components are nanoscale, not the thing is nanoscale $\endgroup$
    – MolbOrg
    Mar 12, 2021 at 12:51

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