Scottish physicist James Clerk Maxwell (1831 – 1879) proposed a thought experiment that would violate the second law of thermodynamics. This law establishes that the entropy of every system must increase in every physical process.

In turn, entropy is a measure of the disorder of a system. A teenager’s room is a good place to exemplify this. No matter how tidy the room is, eventually the room will be a mess. We know from experience! The clothes initially neatly folded and tidy will now be thrown and dirty everywhere, the impeccably made bed will now be unmade with the sheets and blankets thrown away, etc.

This is a natural, everyday manifestation of increased entropy in the universe. In fact, a more elegant and scientifically founded way to draw attention to a teenager for his messy room is not to say, “Get your room tidied up!” but: “Lower it to the entropy level of your room!” … With this wake-up call, maybe we even managed to get him to go into Google to search Wikipedia for that mysterious word, “entropy”. The fact is that in every physical process in the universe there is an increase in entropy.

Let’s think that initially we have a gas, let’s call it A, on one side of a container, separated by a membrane from the other side of the container where another gas is found, let’s call it B. If the membrane is removed the molecules of the two gases, A and B will inevitably begin to mix, increasing their disorder and therefore their entropy.

Finally, after a sufficient time, you will have a homogeneous mixture of molecules A and B. Imagine the reverse process, in which a mixture of molecules A and B spontaneously separates to finally have the A molecules on one side of the container and the B molecules on the other side is something impossible because it implies decreasing and not increasing the entropy of the system. This would be as impossible as seeing in reverse (with inverted time) the film of a plate that falls from a table and shatters a thousand pieces, we would then see a thousand pieces on the ground that spontaneously conglomerate forming a beautiful plate that flies off to reach the edge of a table.

We could also imagine the film, also in reversed time, of a diver who jumps out of the water of a pool and flies to reach the edge of the diving board. These processes do not occur in nature because they violate the second law of thermodynamics, which precisely establishes the increase in entropy in all physical processes.

In fact, a consequence of this is that this elusive term, such as “arrow of time”, can be defined precisely on the basis of the concept of entropy: In every physical process the arrow of time advances in the direction of the increase of entropy.

Maxwell proposed to imagine a container with mixed A and B molecules, but in the middle of the container there is a membrane with a tiny door in which is a small, but very intelligent demon which will be in charge of leaving the A molecules aside. of the container and to the B molecules on the other side. It only lets some on one side pass and others on the other side.

The result of this is as unreal and impossible as watching the aforementioned movies, the shattered plate that is hit again, or the diver who jumps out of the water to reach the diving board.

While in high school I asked my teacher if it would be possible to build an iron (the kind used for ironing clothes) that would take its energy and heat from the user’s hand to operate.

This impossible iron would require making the iron temperature higher than the user’s hand temperature, which is impossible as it would violate the second law of thermodynamics. This is how exciting my physics classes were that I now remember with nostalgia and joy.

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