Definition: Time is the relationship between the different states of the world as a whole resulting from all structural and functional changes in matter.

We sometimes say, "Let time freeze", "lock this moment", in fact, what we want to freeze or lock is not time, but structure. The so-called "time reversal" actually means that the structure changes of various substances back to the previous state. Physicists tell us that there was no time before the universe came into being. The so-called non-existence of time means that the structure of the universe did not exist. There is no motion or structural change without matter, and there is no time without structural changes.

(2)Calendar and Clock

The changes we usually talk about, such as driving cars, biological growth, social development, etc., are functional changes or structural changes. Functional changes includes progress and degeneration, both of which need to be measured and therefore both need a frame of reference. Time is a frame of reference to measure all functional and structural changes.

Figure 1.1 System b is the time parameter of system a

If the world has only one system, when it changes from structure 1 to structure 2 and then from structure 2 back to structure 1, we can say that the time has gone backwards. In fact, time itself has no direction, the direction of time comes from the interrelationship of different structures. If there are more than two systems in the world, when system A changes from structure 1 to structure 2, some changes also occur in system B. then when system A changes from structure 2 back to structure 1, system B does not change back to structure 1 of system B, then we cannot say that the time has gone back. At this time, the state of system B can be used as the reference frame for the structural change of system A, on which the change process of system A can be recorded. This is the function of the change of system A regarding system B, that is the history of A, and B is the time parameter of A. The order in which the states of system B occur in this history is the direction of time. Conversely, A can also be the time parameter of B.

The direction of time is not necessarily one-way. When system B is taken as the time parameter of system A, if system B changes from state 2 to state 1, it means that time flows backwards. Such a graph is easy to draw. But when we analyze functional relationships, we don't like this kind of graph, we like single-valued functions. So we need to set a fixed time direction. If we think that the sun rises in the east and sets in the west every day, we can go back to the past and cannot calibrate the direction of time, then we can use the change of seasons as a reference to mark the direction of time. If we feel that the change of seasons is not enough to determine the direction of time, we can use slower changes as the coordinate system, such as human growth and aging. If that was not enough, some long-period systems could be artificially programmed, and various calendars were produced. Therefore, time reversal is not impossible, but is not conducive to human understanding of the world.