What is time?
Time is a useful concept for humans, which enables them to define points in time, periods of time and duration to their advantage.
"Time is what the clock shows."
Nature itself does not need time. It is a consequence of the second law of thermodynamics: "There is no change of state whose only result is the transfer of heat from a body with a lower temperature to a body with a higher temperature." (Rudolf Julius Emanuel Clausius, German physicist 1822 - 1888) . This means that processes that take place on their own lead from a higher to a lower energy state (entropy). Since this transition, e.g. B. from hot to cold, does not happen suddenly, it makes sense to speak of a "passing of time" during such a process.
From this follows the irreversibility of time. The film excerpt is from "The stuff the cosmos is made of - DVD 1 - The illusion of time" (2011) by Brian Greene. Published by daylimotion:
Anyone who would like detailed information about the physical background of time can watch the film "Wie geht Zeitreisen?" (in German). It has a length of 29:58 minutes.
In the case of a glass that falls on the floor, one would therefore have to apply an impulse that is reversed to its movement to each shard, down to the atomic level, in order to put the glass back together. Theoretically conceivable, but practically impossible.
However, this results in the method of "measuring" time:
Galileo Galilei (1564 - 1642) built a device to count pendulum oscillations and also designed a plan for building a wheel clock, which Christiaan Huygens realized in 1673.
A weight pulls on a gear, the rotation of which is released by an escapement in the rhythm of a pendulum. Several gears slow down a shaft so that its pointer correctly counts the minutes on a scale (dial). This is the description of a wheel clock that is driven by weight.
Time is measured by observing and counting a periodic process.
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In addition to the oscillation movement, the circular movement is in principle also suitable for time measurement. Theoretically, the amounts of the kinetic and potential energy do not change here. In practice, however, energy is continuously withdrawn from the rotation due to frictional losses, which would have to be immediately supplemented by the same amount, which is technically very difficult, if not even impossible.
Nevertheless, the rotation of the earth (the day) and the movement of the earth around the sun (the year) was one of the first ways of measuring time because of their importance in everyday life. However, there are difficulties for an exact time measurement:
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With one revolution of the earth until the sun is at the same horizontal angle again, this observation can be used to define the length of a day (true solar day), but this only corresponds to approximately 24 hours. One is forced to define a mean solar day with the help of a uniformly running clock in order to arrive at the 24 hours of a day. These deviations will even out after a year.
Since the rotation speed of the earth decreases in principle - it is slowed down by the moon - you occasionally have to add a second to the day in order to keep the brightness of the day in the range between 6 a.m. and 6 p.m. in the long term. The abolition of leap seconds is currently being discussed in order to solve problems of adaptation, e.g. in computer systems to avoid. To compensate, one would need a leap hour around the year 2600.
A "tropical year" is defined so that the seasons do not shift in the calendar. It is defined as the period of time during which the earth's axis again assumes the same angle of inclination when the earth revolves around the sun. It has a length of 365.24219052 days (365 days, 5 hours, 48 minutes and 46 seconds). As a result, you need an additional day every four years, which fails every 100 years, but is still necessary every 400 years - so the year 2000 was a leap year.
The fact that the seasons change meteorologically due to global warming has nothing to do with the definition of the year as the earth's orbit around the sun.