Maxwell’s Demon
James Clerk Maxwell was one of the greatest minds of the nineteenth century. His discovery that light is a form of an electromagnetic wave was groundbreaking. This discovery affirmed Faraday's beliefs that electricity and magnetism are closely related and later helped Einstein develop Special Relativity, as there was an apparent mismatch between the theories of Maxwell and Newton. Maxwell (along with Ludwig Boltzmann) also played a big part in developing our understanding of thermodynamics. Maxwell's demon is a thought experiment that he came up with, which I am writing about here.
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⏱️The Arrow of Time
The second law of thermodynamics states that the entropy of a closed system must either increase or remain the same; it cannot be decreased. Entropy is a measure of the dispersion of energy or the measure of disorder. If we leave a system as it is for some time, it will become more and more disordered (my room is a great example of this law).
In his amazing book called "A Brief History of Time," Stephen Hawking wrote that entropy can also be thought of as the arrow of time. As time moves forward, the usable energy in the universe becomes unusable (unusable energy is defined as energy that cannot be used to do any work). For example, when a cup falls and breaks, the potential energy of the cup is converted into heat, sound, and the energy of the shattered pieces. This is wasted energy; it cannot be used to do any work.
As our universe moves to the future, entropy increases. All spontaneous processes increase entropy. Heat flows from hot to cold bodies. A hot cup of tea cooling down to room temperature is an example of increasing entropy. The second law of thermodynamics is sacred.
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👹 Maxwell’s Demon
James Clerk Maxwell thought of an experiment that could violate this law. Imagine a box with a bunch of hot and cold particles. The box is partitioned in the middle with a wall, and there is a door in the wall. There is an entity, Maxwell's demon (he is up to no good), who knows the position and velocity of each of these particles. He watches them and rearranges them using the door in such a way that all the hot particles end up on the right side of the box, and all the cold particles end up on the left side of the box. This is not the result of a spontaneous process. The system becomes more ordered, and the entropy decreases. How is Maxwell's demon able to violate the second law of thermodynamics?
Information is physical. The demon has to use memory to store the information about the particles (is the particle hot or cold? On which side of the room is it?). If the demon has a small memory, it will have to erase it once he runs out of space. The erasure of information releases heat into the surroundings. So, by erasing his memory so he can keep moving more and more particles, he still ends up increasing the net entropy of the universe.
What if he has a lot of memory and doesn't need to erase it to order the box? Then you can say that the demon has succeeded for the time being. But the universe can wait. There will be a time in the future when its memory is full, and in order to cause more mischief, it must erase it. It is just a matter of time.
