What is a Maxwell’s Demon?

Did you ever notice how things go? Things fall apart, people grow old, memories fade out, good things abound, but invariably have a cost, and meanwhile the inner energy of isolated systems stays the same. This is Thermodynamics in three lines. In particular its First and Second Laws (that’s basically all there is to it). And the Second Law seems to be the sad part. However it only looks bad, while really it is the motor of all that happens around us.

J. C. Maxwell
J.C. Maxwell

You, and all that surrounds you, are made of minuscule grains of matters, let’s say atoms and molecules (not to mention hadrons (which are made of quarks) and leptons [who said strings?]), and these are restlessly moving, vibrating, rotating. We are very lucky that they somehow assumed a very interesting and surprising configuration here on Earth. But this is only a step of their descent into chaos, and sooner or later they go their own way dismantling this probabilistic lunapark. What could we do to fix this? Very little…as we know almost nothing of the fate of each one of them, and you can’t control what you don’t know!

But J.C. Maxwell, one of the founders of Thermodynamics once had a vision: a minuscule creature, with very sharp senses, a microscopic demon, capable of sorting and exploiting atoms and molecules, with their chaotic motion, because he knows where each one is, and where it is headed. Like a sheperd he guides its swirling herd of particles by just knowing each by name.

 Its favourite trick is that of sorting slow (that’s to say cold) gas molecules  from fast (hot) ones, in a box. It seems nothing special, but it is very special indeed. One of the ways to express the Second Law, the most humble way, is to say that no body  can spontaneously heat a hotter one (without other transformations occurring around – a fridge is in your kitchen after all, but somewhere a power station is working hard for it to keep your beer cold!).  It is interesting that such a simple, obvious, and apparently harmless statement dictates the direction of time, separating what is possible as we move towards the future, from all the funny,  absurd things we see in a reversed movie. However our demon would break this law with its trick, opening the door to all sorts of extravagancies…if only he existed!

What J.C. wanted to highlight is that the strongest law of nature, the merciless Second Law, is only a statistical one. It is overwhelmingly improbable for those molecules to sort themselves into two separated groups at different temperature, but nothing prevents it from happening. Also the fragments of a broken cup on the floor can spontaneously jump back, and rebuild it from time to time. Only we should wait some google of years before seeing this scene.

After some 150 years of thinking on this famous, apparent paradox, the jury is still out, but scientists came to think of Maxwell’s demons in a new way. They should be information processing beings, capable of converting information into useful energy. Like the unbelievably complex molecular machines operating in our cells, ‘feeding’ on the huge amount of information encoded in DNA, and exploiting thermal energy for all sorts of biological activities, making up the amazing out of equilibrium self-sustaining chemical reaction we call life.

These may be the closest things to a real Maxwell’s demon, and if this is true they are inside of us – they would be in a sense the machinery of life and consciousness!

For a more detailed account of how the Maxwell’s demon accomplish its trick, entropy, information and much more, you can watch these videos: