Time is an abstract concept, which shows the continuity and sequence of material movement and change. The concept of time includes two concepts: time and time period. Time is a parameter used by human beings to describe the process of material movement or the occurrence of events. The determination of time depends on the law of material circulation that is not affected by the outside world.

For example, the cycle of the moon around the earth, the cycle of the earth around the sun, the cycle of the earth's rotation, and the cycle of atomic oscillation. Einstein said that time and space are an illusion of people's cognition. According to the big bang theory, the universe starts from a singularity, which is also the singularity of time.

Einstein proposed in the theory of relativity that time, space and matter cannot be explained separately.

Time and space together constitute four-dimensional space-time and constitute the basic structure of the universe. Time and space are not absolute in measurement, and the passage of time measured by observers with different relative speeds or different space-time structures is different.

General relativity predicts that the gravitational field generated by mass will distort the structure of time and space, and the clock time near a large mass (such as a black hole) will pass more slowly than that away from it.

The existing instruments have confirmed the accurate prediction of time by these theories of relativity, and the results have been applied to the global positioning system. In addition, there is a "time dilation" effect in special relativity: in the observer's view, the time of a clock with relative motion is slower than that of a (stationary) clock in its own reference system.

Time is the quantity formed by the three-dimensional motion (displacement) of all other objects (substances) on the earth (other celestial bodies can also be theoretically) in human senses.

As far as today's physical theory is concerned, time is continuous, uninterrupted and has no quantum characteristics. Some unproven theories, such as quantum gravity theory, string theory and M theory, try to combine relativity with quantum mechanics and predict that time is discontinuous and has quantum characteristics.

Some theories speculate that Planck time may be the smallest unit of time.

According to Einstein's equation in general relativity solved by stephen william hawking, it shows that the time of the universe has a starting point, and there is no singularity before the Big Bang, so it is meaningless to discuss the time before that.

Matter and time and space coexist. As long as matter exists, time is meaningful.

Einstein thought: "The difference between the present, the past and the future is only an illusion." Going back in time is actually based on a non-existent logic.

(Note: There is no concept of time in the basic theorem of physics, and time does not participate in calculation. That doesn't mean it doesn't exist. The aging of the species and the alternation of day and night have proved its real existence, so don't misunderstand its meaning; In the theory of relativity, many motions of particles, such as fission and fusion, are related to each other, and they are reversed. )

Extended data:

Time is one of the seven basic physical quantities in physics (length m, time s, mass kg, thermodynamic temperature k (K), current unit a (A), light intensity cd (Candeira), and quantity of matter mol (mol)), and its symbol is T.

In the International System of Units (SI), the basic unit of time is seconds, and the symbol is S, 13, which was held in 1967. The definition of seconds is: 9,192,63 of radiation corresponding to two hyperfine energy level transitions of cesium-133. The cesium atom mentioned in this definition must be at rest at absolute zero, and the environment on the ground is zero magnetic field.

The second defined in this case is equivalent to the second defined in the astronomical almanac. Common time units in life are: millisecond ms, minute min, hour h, day) d, month m, year y, etc.

According to modern cosmological theory, BIGBANG had no time before.

"Forever forward" means that the increment of time is always positive.

Time expresses the birth and death arrangement of objects. "Time" is short for "Time".

Time is the movement of matter and the transmission of energy.

References:

Baidu encyclopedia time