Fact-checking a silly movie with the help of a physicist



The surviving terranauts manage to outrun the ultimate nuclear blast because the ship’s hull can convert heat into energy. Can you convert heat into energy?

No – that’s the whole “2nd law of thermodynamics” thing. You can turn a heat gradient into energy (hot air inside a piston in an engine pushes against cold air outside to move a car), but grabbing heat out of the surrounding environment to move a ship is straight-up magic.

This is not correct. Heat can be converted into work, that’s how steam engines and nuclear reactors and Stirling engines operate, to list just a few examples. His explanation of how internal combustion works is bizarre too — “inside a piston?” Pushing against cold air? No, the expansion of combustion gases push against the piston, it doesn’t much matter what temperature it is outside the engine (as long as it’s not as hot as the ignited fuel-oxidizer mixture.)

It is true that the ship would have to be hotter or cooler than the surrounding medium for any heat exchange to take place, but he really doesn’t explain this point well at all.


It made sense to me. That difference from the medium is what the term heat gradient refers to. Steam engines, nuclear reactors, and Stirling engines all depend on a heat gradient rather than just the existence of heat; the surroundings need to be cold in a relative sense.


But that's just it—if you grab heat from the surrounding environment then the only way you can make it do any work at all is to play off the differential against the cooler temperature inside the ship, and you'd get very little out of that because the inside temperature would very rapidly heat up to match the outside.


If you have water in a pressurized container it won't rise above boiling until all the water is turned to steam. In other words a heat difference could be managed for some time. (and there must be more sophisticated systems that would last longer.) It won't last forever but the trip presumably has a duration.


You could submerge a pressurized container of water in a heated environment without letting it come to a boil, but the temperature of the water would still heat up all the same. Any object of a specified volume will have a finite capability to absorb heat. The best you could do would probably be to pack the inside of your ship with as much liquid coolant as you could carry and gradually vent it into the outside environment as it evaporated.


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