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Essentially, what would be the most efficient way of causing the Earth's core to cool as much as possible without damaging human civilization in any other way (the outcome can damage it, but not the process), given our current understanding of physics? Assume financial and political obstacles are not an issue.

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  • $\begingroup$ large scale geothermal $\endgroup$ – nzaman Mar 26 '18 at 5:11
  • $\begingroup$ Is there a worldbuilding context to this ? It sounds off topic at present - science wild idea rather than significant element in world building. $\endgroup$ – StephenG Mar 26 '18 at 6:07
  • $\begingroup$ Not too many ways. The most efficient is @nzaman's. Even so, the core is well insulated by a lot of rock, and the heat is produced by radioactive decay on a scale that can't reasonably be interfered with given current or foreseeable technology. $\endgroup$ – LSerni Mar 26 '18 at 6:18
  • $\begingroup$ @StephenG - I think its on topic... i.e. a world where people are dealing with this, the process of it happening in the first place might well be relevant - even if just for describing the background of the story. $\endgroup$ – colmde Mar 26 '18 at 8:36
  • $\begingroup$ How much do you want to accelerate it (at least)? You might want as much acceleration as possible, but can you give a broad rate you are aiming for? I think this is absolutely needed because it makes a difference if you want it cold by December or in 3 billion years. Btw, you always have to assume finite resources, otherwise you can end up with pretty stupid scenarios (just a mandatory comment, it's not as bad here, but your question might be strictly better without that addition) $\endgroup$ – Raditz_35 Mar 26 '18 at 9:18
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I suspect that cooling the Earth's core significantly without destroying civilization may not be possible.

Roughly speaking, the core (inner and outer) is about half the mass of the planet (so 3x10^24kg). Assuming a specific heat capacity of 500J/kg, and a temperature change of 1000K, that gives us:

500x1000x3x10^24J = 1.5x10^30 Joules to get rid of. That's a lot.

The oceans weigh about 1.4*10^21Kg, anf would therefore require roughly

4200x100x1.4x10^21J =6x10^26J to raise to boiling point.

So you are dealing with an amount of energy sufficient to raise the oceans to boiling point about 2000 times over. Doing it once would be catastrophic for life on the planet, obviously.

So, no matter how you get the heat out of the Earth - and I can't think of any even semi-plausible mechanisms to do so* - the chances are that you'd destroy everything on the surface by doing so. Even dumping the heat near to the Earth would probably cause overheating by radiation.

*Physical possibilities:

  1. Conduction: Put many, many rods of come highly conductive material all the way to the core. Except that mantle convection would quickly shear them apart, and any density contrast with the mantle would cause them to collapse.

  2. Convection: Stir the mantle more vigorously to allow more cooling? Would disintegrate the crust and everyone on it.

  3. Radiation : Expose the core directly to space by digging a very big hole (not physically plausible). Place a black hole at the Earth's core, and somehow stop it swallowing matter, just radiation. Would work, but probably destroy the planet, again.

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  • $\begingroup$ You should also mention that successfully cooling the core would make it contract with "amusing" effects to people on top of the sphere around it and that it would shut down the planets magnetic field with equally "amusing" effects to any civilizations relying on electronics and radio communications. $\endgroup$ – Ville Niemi Mar 26 '18 at 9:45
  • $\begingroup$ Yes.. I sort-of assumed that the whole idea behind quietly cooling the core was to shut down the magnetic field.. but yes, if every plate boundary became a site of rapid convergence at the same time it wouldn't be fun. $\endgroup$ – Andrew Dodds Mar 26 '18 at 12:10
  • $\begingroup$ Now that you mention it, I cannot think of any other reason for cooling the core myself. $\endgroup$ – Ville Niemi Mar 29 '18 at 8:04
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Utterly insane method: nested heat pumps.

Use a geothermal heat pump to power another geothermal heat pump, which is used to directly cool the core, or whatever depth of the mantle you can reach. Localised cooling and compression would crush the pipes within weeks, if not days, but with large enough pumps (and enough pumps), you could probably set off some major earthquakes on the surface.

Of course, that leaves the issue of what to do with all that waste heat...

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