# THE DEMON THAT CHALLENGED THERMODYNAMICS:

Thermodynamics is one of the most essential fields in the study of Physics which establishes a relationship between physical quantities like work, energy, heat, and temperature, etc. One of the pioneers in this field of research was Fellowship of UK Royal Society awardee James Clerk Maxwell who had some important contributions to the field, one of them being, the most debated and often questioned: Maxwell’s demon.

## MAXWELL’S DEMON OF THERMODYNAMICS:

Well, this demon doesn’t belong to any cult or anything. It’s just the part of the thought experiment devised by James Clerk Maxwell where entropy decreases. You’ll be now wondering how entropy can decrease! It always increases, right? If the entropy here decreases, then it certainly violates the second law of thermodynamics. Well, nothing like this is gonna happen.

The thought experiment involves two gas chambers placed adjacent to each other with a door in between them controlled by a being or a creature called the demon. Both the gas chambers, say A & B, have particles moving at both high and low speeds. So when the demon sees the fast-moving particles of chamber approaching the door, he opens it so that they’ll flow to the other chamber,i.e., B. Again when he observes the slow-moving particles approaching the door, he opens so that they will move to A.Well what happens here is, now the A chamber has all the particles moving slow and vice versa for chamber B. As the faster-moving molecules are hotter, they increase the temperature of chamber B, and the slower ones decrease the temperature of chamber A, all occurred due to the Demon’s behaviour. Hence this occurrence decreases the entropy, thereby violating the second law of thermodynamics which states that two bodies at different temperatures in an isolated system when brought to contact with each other, will evolve to a thermodynamic equilibrium in which both bodies have an approximately equal temperature. Also in an isolated system, entropy never decreases. But here in case of Maxwell’s demon, the entropy decreases which has attracted most debates in last decades.

We often have major confusion in understanding entropy.To understand entropy ,we first got to understand that disorder only applies to isolated systems which don’t exchange energy with the surroundings.Because when it interacts with other systems its own entropy may decrease ,consequently increasing the entropy of the other system.So we get confused here that how the entropy of the first system decreased.But we have to know that whenever entropy decreases somewhere ,it ultimately increases somewhere else by at least as much.So we say that energy of a system is either increasing or zero;it can’t be less than zero.So whenever we talk or think about entropy,first of all we have to take the system into considerations.What is the system? How much is the total entropy of the system ?etc.etc.

## LANDAUER’S ARGUMENT:

Rolf Landauer takes the help of information theory to argue about the thought experiment. His argument was that the Demon must have collected information about the molecules; which ones are fast or which ones are slow, to get them through that door. So the demon must either discard the information or record it. Well discarding the information will eventually lead to an increase in entropy which ultimately doesn’t violate the Second Law of Thermodynamics.
Charles Bennett also had a similar kind of argument where the Demon will have a shortage of information storage space for which he’ll start deleting the information he had collected, eventually increasing the entropy of the system, which consequently doesn’t violate the 2nd law of thermodynamics.
So what do you think? Is Maxwell’s Demon a villain or just a character in thermodynamics we haven’t quite understood yet?

Author:SOM ABHISEK.