Go to to sign up for free. And, the first 200 people will get 20% off their annual premium membership. The common interpretation of entropy is that it is a measure of disorder in the universe. The universe is on path to more and more disorder. And this is the reason time is thought to flow forward.
Is entropy a fundamental property of nature? Or is there something deeper?
Scottish physicist, James Clerk Maxwell came up with a thought experiment, called Maxwell’s demon, that showed a mechanism where entropy could be reversed. It would violate the 2nd law of thermodynamics. No one could resolve it for over 100 years. But when researchers at IBM solved this paradox, they showed that entropy was related to something even more fundamental - information.
All physical laws appear to be time reversible. The only law of physics which appears to be time irreversible is the law of increasing entropy – the 2nd law of thermodynamics.
Let’s take a container with gas isolated on one side with a barrier. If we remove the barrier, the gas will expand into the full volume of the container. We have just increased the entropy of the gas. The reason there is more disorder is that in a greater volume there are more places that individual gas molecules could be. This greater possible number of arrangements is a better description of entropy.
What if we pushed the gas back to half the volume of the container. Wouldn’t this reduce the entropy back to what it was? No, because compressing the gas requires putting work into the system. It would raise the temperature of the gas. It would heat up and the entropy would increase making up for any decrease in entropy due to the smaller volume.
Could the gas spontaneously arrange itself so that it fills only half the volume? Yes, and that would violate the 2nd law, but there is nothing in the laws of physics that prevents this from happening, except that it is very unlikely. The possibility that all the multiple trillions of molecules in the gas could arrange themselves in one side of the container is about one in 10^150,000,000,000,000,000,000,000. This number is so big, that it is statistically impossible.
Maxwell's demon: a chamber with a divider in the center. we start out with a gas in one chamber, this gas will distribute itself evenly between the two chambers. We put a door between the two chambers controlled by a little demon. The door is frictionless so adds no energy in the enclosed system. The demon controls the door such that he only lets the molecules travel in one direction. This demon makes it so all the gas is collected on the left chamber so that no molecules remain in the right chamber. The entropy decreased. This seems to violate the 2nd law.
Rolf Landaur and Charles Bennett showed that In order for the demon to reduce the entropy, he has to gather information about the movement of various molecules. He is building a memory record. The demon is increasing the amount of information in the system. This increase in information is an increase in entropy. The decrease in entropy due to the volume decrease is exactly counterbalanced by an increase in information in the demon’s brain. The 2nd law is not violated. They also showed that erasing information is not free. It produces heat to increase entropy.
So entropy is really a measure of the information required to express the state of a system. A system with Higher entropy requires more information to describe its microstates.
How is time related to entropy and information? All the laws of physics we know appear to be symmetrical with respect to time, except Entropy is asymmetrical. It flows only in one direction - higher. This inspired British Scientist Arthur Eddington to postulate that it is the increase in entropy that is responsible for the flow of time. It is not a good theory because the prediction it would make is that time should go backward when entropy decreases. The inside of your refrigerator is a place where entropy is decreasing. Your food is getting colder, but time doesn’t flow backward inside your refrigerator.
The low entropy start of the universe is thought to be the primary reason why time flows forward. But why was entropy so low at the beginning? Given all the different configurations it could have been in, why the lowest one? No one really knows.
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Maybe It is just a fact. Another idea is from physicists like as Alan Guth and Sean Carroll. if there is no upper limit to entropy, then no matter where the universe started, it would have been in the lowest entropy state. It has nowhere else to go except a higher entropy state.
If Time flows forward as entropy becomes higher, then we are a consequence of the increasing entropy of the universe. If the universe was in thermal equilibrium, we would not exist. The reason we have causality, evolution, biological processes, thoughts, memories and even consciousness is entropy.
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