There are many processes in our lives that are irreversible. For example, the water can't be collected when it is spoiled on the floor, this process is irreversible. It is not absolutely impossible for you to withdraw water, but you have to do a lot of work. For another example, after the bomb explode, you couldn't get the shrapnel back together and return to the state before. So these processes are irreversible. Entropy is a measure of these irreversible course.

If there are two cups A and B, A is filled with solution containing ingredients such as protein, sugar, fat, mineral salts, etc. There is a fish in B, and the water around the fish is pure water. Assume that the content of protein, sugar, fat, mineral salt, etc. in the fish's body is equal to that in solution of A, that is, if the two cups A and B are filled with exactly the same ingredients, what is the difference? The structure is different. The structure in A is symmetrical, the various components are evenly distributed in the cup, B is in symmetry broken, the structure of A is relatively simple, and the structure of B is very complicated. After the death of a living organism, the organ tissue in the body will rot and decompose, which is equivalent to changing from B to A. This is a kind of degradation, changes from a complex structure to a simple structure. The changes from simple to complex are evolution.

The second law of thermodynamics tells us that in an isolated system, no change can reduce the total value of entropy. In other words, the world can only degenerate and cannot evolve. From this theorem, it can be inferred that the universe is inevitably going to heat death. However, the fact we see is the evolution of the world. In the course of biological evolution, biology has evolved from a simple single cell to a variety of complex and diverse organisms. The non-living and human society also has a evolutionary process.

In fact, at the beginning of the universe, the state after the Big Bang was a state of heat death. Today's Universe is a result that comes step-by-step out of heat death and a result of constant entropy reduction. Clausius said that when the universe enters a state of heat death, no further change will occur. At this time, the universe will enter a state of eternal death. But in fact, the universe easily walked out of heat death and did not introduce any negative entropy from outside. Not only has the universe created its own negative entropy, but it has also been able to produce new particles. It has continuously created new symmetry broken of matter and energy, and the structure of the universe has become increasingly complex.

Hawking's explanation is that the entropy reduction comes from outside the system and cannot come from inside. But if this is the case, then the universe as a system can only increase the entropy, it is impossible to reduce the entropy, and the universe must move toward the most balanced state. If you say that in some places entropy decreases, while in some other places, the entropy increases, but after the Big Bang, the universe has been reached the maximum entropy, it is completely uniform, and how can it increase.