four equations precisely describe all of electromagnetism and the nature of light everywhere in the universe from the stars and the galaxies down to the inside of an atom. In fact, Maxwell’s equations are so fundamental that when pitted against the venerable and well-tested theory of mechanics laid out by Sir Isaac Newton, it was Newtonian mechanics that had to be altered as Albert Einstein ushered in the theory of special relativity. Yet almost any physicist will readily agree that Sir Eddington is absolutely justified in saying that the second law of thermodynamics is even more fundamental.
The origin of the second law is actually quite easy to understand in common sense terms. You see, it is all about available possibilities and probabilities. Ask any teenager trying to pass the driving test, “which part of the test do you dread the most?” and the answer would almost inevitably be, “parallel parking.” Parallel parking is never trivial, and even some of us with aging licenses might still be challenged when parking a car in the streets of New York City or the narrow alleys of old European cities, where cars are literally parked bumper to bumper on the street side. But no one ever finds it very challenging to pull out of parallel parking. Why not? Why is it so much more difficult to parallel park than to pull out of parallel parking? In a way, this has also got to do with the basic idea of entropy. There are many, many more ways you can be outside of the parking spot than you can be in the parking spot, or in the language of physics, there are more possible “states” outside than there are inside. That is exactly at the heart of the second law: There are zillions of ways to be disorderly, but sometimes only one way to be orderly. Thus, any system naturally has a much higher probability to be in one of the many disorderly states than in the orderly one you might prefer. For example, if you fill up a box with black and white marbles as shown in , you are unlikely to get all the black ones on one side and the white ones on the other, because there are many more ways for them to become mixed up together. And, therefore, with time, disorder prevails and entropy inevitably increases.
. But we can never go back in time. It has nothing to do with paradoxes, as in the parents not hooking up à la Back to the Future. In fact, about all the equations in classical and quantum mechanics and electrodynamics are invariant with respect to the direction of time; they work just as well with time going backward. As far as they are concerned, we should be able to go back in time as well forward. It is the second law of thermodynamics that sets the “arrow of time.” The universe as we know it can only evolve toward the direction of increasing entropy, and that is toward the future. Going backward in time would imply reducing the overall entropy of the universe, violating the second law of thermodynamics. That is forbidden—there is no going back in time, I am sorry to say, not in this universe. Perhaps we could jump to some parallel universe some day in the distant future where the second law runs backward, but that is all speculation and science fiction really. Don’t count on it!
Really, technology has made so much happen already that would have been things of magic not so long ago, that today a general belief pervades the worldview of most educated people that given enough time, anything is possible. Therefore, it is worth reminding ourselves that certain things will never be possible, because anything that violates the second law of thermodynamics, anything that reduces the net disorder in the universe, is unlikely ever to happen. How to reduce the net entropy in the universe will always remain “The Last Question”—the title of my favorite Isaac Asimov story (and apparently his own favorite self-penned one, as well), which makes that very point beautifully by following eons of human evolution and technology always faced with that one unanswerable question that can be phrased, “Can the workings of the second law of thermodynamics be reversed?” It ends with a surprising final answer that makes it well worth the read!
James Clerk Maxwell was a Scottish physicist from the mid-nineteenth century who demonstrated that electricity and magnetism, previously thought of as two distinct phenomena, are actually intimately related and are describable by four compact equations that bear his name. These equations essentially unified electricity and magnetism into a single description that is now known as electromagnetism.
There is one exception. One of the fundamental forces of nature, the weak nuclear interaction, breaks the time-reversal symmetry. But while this is tied to specific dynamical interactions inside the atom (giving different outcomes going backwards or forwards in time), the setting of the direction of time by the second law of thermodynamics has its roots in the origin of the universe itself.