This is a reference to Maxwell's Demon, a thought experiment related to the philosophy of thermal and statistical physics. Created by James Clerk Maxwell, the experiment aims to demonstrate that the Second Law of Thermodynamics has only a statistical certainty.
In the imagined experiment, a container is split by an inner partition with a door in it. Within the container resides a demon, who can open the door selectively whenever a fast-moving (hot) molecule approaches, so causing one side to get warmer than the other.
This activity would appear to contradict the Second Law of Thermodynamics, which states that under such circumstances, the two sides would eventually reach an equilibrium temperature.
Maxwell's demon also makes an appearance in Pynchon's previous novel, The Crying of Lot 49.