Complexity_ A Guided Tour - Melanie Mitchell [21]
By the end of the nineteenth century two fundamental laws concerning energy had been discovered, the so-called laws of thermodynamics. These laws apply to “isolated systems”—ones that do not exchange energy with any outside entity.
First law: Energy is conserved. The total amount of energy in the universe is constant. Energy can be transformed from one form to another, such as the transformation of stored body energy to kinetic energy of a pushed car plus the heat generated by this action. However, energy can never be created or destroyed. Thus it is said to be “conserved.”
Second law: Entropy always increases until it reaches a maximum value. The total entropy of a system will always increase until it reaches its maximum possible value; it will never decrease on its own unless an outside agent works to decrease it.
As you’ve probably noticed, a room does not clean itself up, and Cheerios spilled on the floor, left to their own devices, will never find their way back into the cereal box. Someone or something has to do work to turn disorder into order.
Furthermore, transformations of energy, such as the car-pushing example above, will always produce some heat that cannot be put to work. This is why, for example, no one has found a way to take the heat generated by the back of your refrigerator and use it to produce new power for cooling the inside of the refrigerator so that it will be able to power itself. This explains why the proverbial “perpetual motion machine” is a myth.
The second law of thermodynamics is said to define the “arrow of time,” in that it proves there are processes that cannot be reversed in time (e.g., heat spontaneously returning to your refrigerator and converting to electrical energy to cool the inside). The “future” is defined as the direction of time in which entropy increases. Interestingly, the second law is the only fundamental law of physics that distinguishes between past and future. All other laws are reversible in time. For example, consider filming an interaction between elementary particles such as electrons, and then showing this movie to a physicist. Now run the movie backward, and ask the physicist which version was the “real” version. The physicist won’t be able to guess, since the forward and backward interactions both obey the laws of physics. This is what reversible means. In contrast, if you make an infrared film of heat being produced by your refrigerator, and show it forward and backward, any physicist will identify the forward direction as “correct” since it obeys the second law, whereas the backward version does not. This is what irreversible means. Why is the second law different from all other physical laws? This is a profound question. As the physicist Tony Rothman points out, “Why the second law should distinguish between past and future while all the other laws of nature do not is perhaps the greatest mystery in physics.”
Maxwell’s Demon
The British physicist James Clerk Maxwell is most famous for his discovery of what are now called Maxwell’s Equations: compact expressions of Maxwell’s theory that unified electricity and magnetism. During his lifetime, he was one of the world’s most highly regarded scientists, and today would be on any top fifty list of all-time greats of science.
In his 1871 book, Theory of Heat, Maxwell posed a puzzle under the heading “Limitation of the Second Law of Thermodynamics.” Maxwell proposed a box that is divided into two halves by a wall with a hinged door, as illustrated in figure 3.1. The door is controlled by a “demon,” a very small being who measures the velocity