Time is the continuity and sequence of material movement. The so-called persistence means that the movement of any object must go through a long or short process. For example, the "life span" of a pion is only one billionth of a second, but there is still a continuous process. Electronic computers can perform thousands, tens of thousands, or hundreds of millions of calculations per second, but they still have a continuous time. No one can build an electronic computer that does not require calculation time. Although 1 second is short, in this second, a snail can crawl 1.5 mm, a horse can run 25 meters, light waves can travel 300,000 kilometers, the earth can rotate 460 meters, the earth can travel 30 kilometers around the sun, etc. . The so-called sequentiality means that there is a sequential relationship between the occurrence of motion processes between different things. The characteristic of time is one-dimensionality. The word "dimension" comes from Latin, which means "completely measured." There are several dimensions that must be measured with several quantities. The one-dimensionality of time has two meanings: First, it means that only one quantity is needed to measure time. At any moment, it can be completely measured and expressed by a quantity. Another meaning is that time has only one direction and is irreversible. The passage of time always moves along the straight line of "past? present? future" and is gone forever. The ancients often used wise sayings such as "when time comes, time will never come again", "opportunity cannot be missed, time will never come again", "an inch of time is worth an inch of gold, an inch of gold cannot buy an inch of time" to illustrate the irreversibility of time, and we should cherish it very much. time.