If you feel the world is breaking down into chaos, you’re right. You may or may not be correct if that feeling is in regard to politics and the state of society, but on the timescale of the universe there is no doubt, order is breaking down. That is what it always does. But why?
Physicists call the amount of disorder in a system entropy. Technically it is defined as the “measurement of the energy in a system or process that is not available to do work”. Others refer to it as the amount of randomness or disorder in a system. Either way, it comes to the same thing.
The second law of thermodynamics states that the entropy of an isolated system cannot decrease. Since it also cannot stay the same when any work is being done, this means entropy is constantly rising in a closed system. Since the universe, almost by definition, is a closed system, that means its entropy must be rising.
Answering why this is can lead to questions about which features of the universe are inevitable, and which might have been different had things gone a little otherwise. That’s not something that is currently answerable, and maybe it never will be. Nevertheless, it is fair to say that while we can imagine a universe in which certain laws of physics were a little different, and sometimes even find out they’re not as we thought, the tendency to disorder appears inevitable.
The physicist Arthur Eddington, perhaps most famous for providing the first confirmation of general relativity, advised his students: “The law that entropy always increases—the Second Law of Thermodynamics—holds, I think, the supreme position among the laws of Nature.”
“If someone points out to you that your pet theory of the universe is in disagreement with Maxwell’s equations – then so much the worse for Maxwell’s equations. If it is found to be contradicted by observation – well these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.”
The quote is remembered more than a century later because while other pillars of physics from Eddington’s era have tumbled, this one stands firm.
What the second law says
A newcomer to physics encountering the second law of thermodynamics won’t always grasp its significance, because it’s often stated in ways that don’t mention entropy at all.
A common way of presenting the law is the seemingly obvious statement that heat naturally flows from a hotter region to a colder one. It’s possible to reverse this – air conditioners operate on the basis of cooling an indoor space that is usually colder to start off with than the outside where the heat is dumped. But to do that you need to put in a great deal of energy, as is obvious to anyone receiving their electricity bill after a summer where they ran the air con.
The connection of this observation to entropy isn’t obvious, but starts to become clearer when we add the law’s other side; that not all the heat in a system can be converted to useful work in a cyclic process. No engine can be 100 percent efficient in converting heat to other forms of energy, let alone exceeding that.
The inefficiency means more waste heat, which is the same thing as more disordered molecules, and more total entropy. An engine can increase the order within a system, but only at the cost of creating more disorder around it.
Although talk of heat flowing and the efficiency of engines may seem abstract, the second law is a way of translating a concept familiar in other fields: there is no such thing as a free lunch.
In a universe where the second law did not apply, free lunches would be universal. One could get out more energy from an operation than one put in. It might be nice to imagine such a circumstance, but it makes a kind of intuitive sense to most of us that the universe does not owe us a living, at least not an easy one.
Not everyone accepts Eddington’s warning. Every year patent offices and physics departments around the world receive messages from people who claim to have invented a perpetual motion machine. Some of these work by harvesting the closest thing we have to a free lunch, the energy emitted by the Sun or another external source. These do not violate the second law because they rely on the fact the Earth is not a closed system; it receives sunlight and cosmic rays from space and radiates back out again.
The incoming energy can be used to create more order on Earth: plants and photosynthesizing algae have been doing it for a very long time, and our solar panels are only just catching up. From a broader perspective, however, any local progress made in the battle against disorder is more than overcome by the increased entropy the Sun produces as it fuses molecules to make heat.
A true perpetual motion machine, one that does not rely on external energy, violates the second law because if we built enough of them, we could make the universe as a whole more ordered, reversing the rise of entropy. Countless people have tried, including some of the greatest minds in physics.
James Clerk Maxwell, he of the equations Eddington referred to, proposed the idea of a miniscule being, later nicknamed Maxwell’s demon, that could create a perpetual motion machine by sorting molecules. It took decades to show why this was impossible, although quantum physics continues to complicate the question.
Many lesser minds have claimed to have succeeded where Maxwell failed, but none actually have. The second law stands undefeated.
There’s still a lot of uncertainty about the eventual fate of the universe. Some models propose the second law may not rule over us forever. However, at this stage, probably the most likely way for everything to end remains the very depressing one known as the “heat death of the universe”, where all energy is evenly distributed and entropy comes to overwhelm us all.
Source Link: Why Does Everything Eventually Become Disordered?