| 单词 | time |
| 释义 | time A dimension that enables two otherwise identical events that occur at the same point in space to be distinguished (see space—time). The interval between two such events forms the basis of time measurement. For general purposes, the earth's rotation on its axis provides the units of the clock (see day) and the earth's orbit round the sun (see year) provides the units of the calendar. For scientific purposes, intervals of time are now defined in terms of the frequency of a specified electromagnetic radiation (see second). See also time dilation; time reversal. In physics, since the publication of the special theory of relativity in 1905, Einstein has frequently been said to have abandoned the concept of absolute time. In this context absolute time is taken to mean “time that flows equably and independently of the state of motion of the observer”. Time dilation effects and the collapse of absolute simultaneity mean that absolute time in this sense cannot be applied to the measurement of an interval of time. Although philosophers tend to describe Einstein's work on relativity as the beginning of a 20th century revolution in science, many of these ‘revolutionary’ concepts were not entirely original. In 1898, for example, Jules Poincaré (1854–1912), the French mathematician, questioned the concept of absolute simultaneity commenting that “we have no direct intuition about the equality of two time intervals”. Poincaré was also aware of the need to consider local time for a given observer. In 1904, he observed that clocks synchronized by light signals sent between observers in uniform relative motion “will not mark the true time”, but, rather, “what one might call the local time”. A frequent misconception is that the theory of relativity removes absolute time from mechanics. This is true for the measurement of time as discussed above, but not for time itself. Newton's definition of absolute time is essentially a philosophical concept. Indeed, challenges to this concept in Newton's lifetime were usually made on philosophical, rather than experimental, grounds. Newton never claimed that one could measure absolute time; this absolute quantity had to be distinguished from the “sensible measures” used in “ordinary affairs”. In Einstein's view of the universe, descriptions of a physical phenomenon need to be fully relativistic, requiring Lorentz transformations between the coordinates of systems in uniform relative motion. Contrary to popular belief, Newtonian mechanics was not based on absolute space and time and was fully relativistic, but in the Galilean sense; that is, Galilean transformations were required between the coordinates of systems in uniform relative motion. In considering simultaneity Einstein made use of a thought experiment (see relativity). As a result of this experiment in Einstein's view, the concept of absolute simultaneity has to be abandoned. His universe is causal, and in a causal universe, there is no such thing as simultaneity as there are no simultaneous events. Events have a definite order based on their causal sequence, which cannot be changed. This is what Newton meant by absolute time. Without making a direct statement, Einstein effectively introduced a third postulate in his theory of relativity; that no information can be transmitted faster than the speed of light. For both Newton and Einstein absolute time is really the absolute order of events, determined by causality, and not the measurement of time, which is the subject of ordinary observation |
| 随便看 |
|
物理辞典收录了3346条物理词条，基本涵盖了常用物理知识及物理英语单词词组的翻译及用法，是物理学习的有利工具。