Entropy is a thermodynamic property that measures the amount of disorder in a system. The second law of thermodynamics states that a system entropy can never decrease. If the system is an isolated system in equilibrium, meaning that there is no transfer of energy or matter to the outside, the change in the entropy will be zero. If the system is not isolated, the entropy will increase. The increase in entropy is also the reason time only moves forward, it establishes the ‘arrow of time’, in which time always goes from past to future. Everything goes from order to disorder. Think of a neat stack of papers in a desk. If I were to grab the stack and throw it into de air the papers would fly all over the room, and the stack will no longer be ordered. The papers would fall randomly on the floor. This is what the second law of thermodynamics says. If one were to violate the second law of thermodynamics, the entropy of the system would decrease. This would mean that if you took a bunch of disordered papers, not neatly stacked, and threw them into the air, they would fall perfectly stacked and neatly into the floor.

In 1867, James Clerk Maxwell, a renowned physicist who made important contributions to electromagnetism and thermodynamics, came up with a thought experiment. Imagine you have a box filled with gas that is separated into two parts: A and B.

A gas is formed by a group of molecules and/or atoms moving around at different speeds. The average speed of this molecules is a measure of the temperature of the gas.

The faster they move, the hotter the gas. If we leave the box for a while, both sides will reach a state of equilibrium and A and B will be at the same temperature. But if there was some entity that could open a door between A and B and only let through the fastest molecules from A to B, and the slowest molecules from B to A. What would happen then?

First, the temperature in B would start to increase and the temperature in A would start to decrease. This would decrease the entropy of the system, since it is becoming more ordered: fast moving, hot molecules in one side, and slow moving, cold molecules in the other. This entity is called Maxwell’s demon. Let’s return to the example of the stack of papers. If you threw the stack of papers and Maxwell’s demon was around, it would start grabbing the papers mid-air and start stacking them in an ordered pile in the middle of the room, again, violating the second law of thermodynamics. As was mentioned before, the second law of thermodynamics is also the physical law that defines the arrow of time. If the law could be altered, the arrow of time could be altered. In principle, Maxwell’s demon could change the arrow of time. If you threw the stack of ordered papers they would start in order and then fall randomly, but if you turn back the arrow of time the papers would go from being randomly disordered to being ordered, as if they would go back in time. So Maxwell’s demon could have the ability to change the arrow of time.

All the previous information begs the question: Is Maxwell’s demon real? Can we manufacture a machine that works like Maxwell’s demon, and therefore violate the second law of thermodynamics? This is a complicated question and, in principle, the answer is NO. The second law of thermodynamics cannot be broken. But there are some experiments in chemistry and nanotechnology that allow local areas where entropy can decrease. In 2006 A. Ruschhaupt, J. G. Muga, and M. G. Raizen described and experiment, called an atom diode which function similarly to Maxwell’s demon. It separates cold atoms from hot atoms, just like the though experiment of the box with sides A and B. But it has one important difference: when cooling the atoms, the energy they have must be conserved (a very important physical principle), so the energy the atom had must go somewhere. This energy is transferred to a photon, which is a particle of light, with that same energy. This means that, while this experiment works like Maxwell’s demon, the second law of thermodynamics is never broken. The entropy of the system is still increasing because of this photon that is thrown out. This photon adds randomness to the exterior, increasing the entropy. There have been many other experiments similar to this one (see Szilard’s engine), but the outcome is always the same: locally, entropy can be decreased, but if we take into account the entire system, entropy can never decrease.

The second law of thermodynamics is one of the most important laws in physics. Its effects are seen not only in thermodynamics, but in information theory, biology, gravity, and many other subjects. It is also the law that forbids perpetual motion machines. It is a law that might not be as popular as Newton’s laws or the principle of conservation of energy, but it is a law that constantly surrounds us, giving us the direction of time, and proving that all things tend to fall out of order.

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