| 释义 |
time
Physics
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, Henri 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. It is thought that in a true quantum theory of gravity, time might well be an emergent concept, rather like temperature in the kinetic theory of gases.
Mathematics
In the real world, the passage of time is a universal experience which clocks of various kinds have been designed to measure. In a mathematical model, time is represented by a real variable, usually denoted by t, with t = 0 corresponding to some suitable starting point. An observer associated with a frame of reference has the capability to measure the duration of time intervals between events occurring in the problem being investigated. Time has the dimension T, and the SI unit of measurement is the second. See simultaneity, time dilation.
Astronomy
The dimension that allows events occurring at the same place to be distinguished. In the classical physics of Galileo and Newton, time had an absolute significance and a time-scale could in principle be adopted so that all observers would agree on the time at which any event occurred. Different observers would see the event occurring at different times, but these differences were explained by the travel time of light from the event to the observer. Moreover, in classical physics this common time-scale was in step with each observer’s own local measure of time, the proper time. All observers would agree that the time of any event is simply the time recorded by the local clock. Newton’s theory of gravity gives a very accurate description of orbital motion within the Solar System, allowing the calculation of each body’s position at any given instant of time. Ephemeris Time (ET) was intended to represent this concept of time, and for many years the positions of planets and other bodies were tabulated at fixed intervals of ET in The Astronomical Almanac and elsewhere. ET contrasts with Universal Time (UT) which, like sidereal time, is based on the rotation of the Earth and hence is susceptible to irregularities in the Earth’s rotation. ET could be checked only from the detailed reduction of many astronomical observations, whereas UT has a strict link to sidereal time, which can be obtained directly from observations of stars. Departures of UT from uniformity are detected by comparison with atomic clocks. Atomic clocks provide atomic time, which depends on the constants of atomic physics but is independent of the constant of gravitation. Unlike ET, atomic time is not a form of dynamical time but, if all the constants of physics are genuinely in fixed mutual ratios, the two time-scales should be strictly and uniformly related. Atomic time is the most accurate time-scale available today. In the theory of relativity, time appears in the equations in much the same way as the space dimensions. Even in Newtonian physics, these space dimensions are relative and have different meanings for different observers. In relativity, time is relative as well, so different observers measure their own proper time and time loses its absolute significance. It is still necessary, however, to have a global version of time as a means of labelling events throughout space and time. This is provided by coordinate time, which can be thought of as the proper time for one specially selected observer. To allow consistently for relativistic effects such as gravitational redshift, ET was replaced in 1984 by two new dynamical time-scales. The first of these is Terrestrial Time (TT, originally known as Terrestrial Dynamical Time). It is used for calculating geocentric positions of Solar System bodies, as published in The Astronomical Almanac. It is effectively the proper time for any observer at sea level. For calculating the orbits of Solar System bodies, ET has been replaced by Barycentric Dynamical Time (TDB). This is a form of coordinate time free from the influences on terrestrial timekeeping resulting from the Earth’s motion and the masses of the Sun and planets, but rescaled so that it has only periodic differences from TT. Both TDB and TT use the same fundamental unit as International Atomic Time (TAI), namely the SI second.
Philosophy
The nature of time has been one of the major problems of philosophy since antiquity. Is time well thought of as flowing? If so, does it flow from future to past with us stuck like boats in the middle of the river, or does it flow from past to future, bearing us with it? And might it flow faster or slower? These questions seem hard (or absurd) enough to encourage us to reject the metaphor of time’s flow. But if we do not think of time as flowing, how do we conceive of its passage? What distinguishes the present from the past and future, or is there no objective distinction (see a-series, presentism)? What gives time its direction—what accounts for the asymmetry between past and future? Can we make sense of timeless existence, or can we only make sense of existence in time? Is time infinitely divisible, or might it have a granular structure, with there being a smallest quantum or chunk of time? Many of these problems are first posed in Aristotle’s Physics, in the form of paradoxes or problems about the very existence of time. One problem is that time cannot exist, for none of its parts exist (the present instant, having no duration, cannot count as a part of time). Again, if we ask when the present instant ceases to exist, every answer involves a contradiction: not at the present, for while it exists it exists; not at the next moment, for in the continuum there is no next moment (any more than there is such a thing as the next fraction to any given fraction); not at any subsequent moment, for then it is already gone. But we cannot think of the present instant as continuously existing, for then things that happened ten thousand years ago would be simultaneous with things that have happened today. Aristotle’s puzzles, and Zeno’s paradoxes of time and space, encouraged atomistic solutions, in which the structure of time is made granular. Partisans of atomism included Diodorus Cronus (fl.c.300 bc) and Epicurus, but they were opposed by the Stoics; the countervailing arguments on each side were marshalled by Sextus Empiricus as grist to the sceptical mill. A fundamentally idealist solution, allowing different times to exist in the sense of being simultaneous objects of contemplation, is propounded by Augustine, in the Confessions, Bk. 11, and is visible in Leibniz, Berkeley, Kant, and Bergson. Other perplexing problems include the question of whether time may have a beginning, and whether there can be eventless time. See also space-time, relativity theory.
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