TIME

a measured or measurable period, a continuum that lacks spatial dimensions. Time is of philosophical interest and is also the subject of mathematical and scientific investigation. a measured or measurable period, a continuum that lacks spatial dimensions. Time is of philosophical interest and is also the subject of mathematical and scientific investigation. What then, is time? If no one asks me, I know what it is. If I wish to explain it to him who asks me, I do not know. In this remark St. Augustine in the 5th century AD drew attention to the fact that while time is the most familiar of concepts used in the organization of thought and action, it is also the most elusive. It cannot be given any simple, illuminating definition. In the face of this problem philosophers have sought an understanding of time by focussing on two broad questions: What is the relation between time and the physical world? And what is the relation between time and consciousness? According to those, such as Sir Isaac Newton, who adopt an absolutist theory of time, the answer to the former question is that time is like a container within which the universe exists and change takes place. Time's existence and properties are independent of the physical universe. Time would have existed even if the universe had not. Time is held to be nonending, nonbeginning, linear, and continuous. That time has these properties is established philosophically, without reference to scientific investigation. According to the rival relationist theory, time can be reduced to change. Time is nothing over and above change in the physical universe; all hypotheses about time can be translated into hypotheses about the physical universe. Consequently the question Has time a beginning? becomes Was there a first event in the history of the universe? Also, investigating the properties of time is to be done scientifically. Relationists explore the possibility that physics could show time to have structure: it might consist of discrete particles (chronons), for instance, or it might be cyclical. It has been realized in the 20th century that time cannot be treated in isolation from space. Consequently philosophers now tend to focus attention on spacetime, conceived, after Einstein, as a continuum. While the temporal aspects of spacetime remain importantly different from its spatial aspects, there is an interdependence that is shown in the case of measurement: the measure of an interval of time assigned by a clock depends on the path and speed with which it is moved. The fundamental controversy between the absolutist and the relationist remains; some philosophers argue that Einstein's theories of relativity vindicate relationist theories, others that they vindicate the absolutist theory. The primary issue concerning the relation between time and consciousness is the extent, if any, to which time or aspects of time depend on the existence of conscious beings. It is perhaps a commonsensical idea that, even in the absence of consciousness, events would still occur in an order that could be described using the relations of before and after. But perhaps it is not so. Normally events in time are thought of in terms of the notions of past, present, and future, which some philosophers treat as mind-dependent. They argue that to say that something is now happening (has happened, will happen) is to say that its happening is simultaneous with (earlier than, later than) one's current state of consciousness or the act of utterance itself. Thus, it is maintained, in the absence of conscious beings there is no past, present, or future. This view that the past, the present, and the future are mere subjective projections of the human mind has been supported by appeal to physics, for such notions were thought to play no role in physical theories. (Recent evidence from the subatomic realm has raised questions about the symmetry of time in physical processes.) Other philosophers, however, believe that time is independent of perception and hold that the past, present, and future are objective features of the world. The measurement of time involves establishing a precise system of reference for specifying when any event occursi.e., specifying the epoch and establishing a standard interval of time. Astronomy and civil affairs are concerned both with epochs and with time intervals, whereas physics deals almost entirely with time intervals. The fundamental unit of time interval is the second. A reference time scale could be based on any phenomenon that involves change with time, such as the rotation of the Earth or the motion of a pendulum. To be generally useful it should be possible to determine the same numerical value of the time, within very close limits, anywhere on the Earth. This condition is satisfied by the rotating Earth but not by the pendulum. Until recently, the rotation of the Earth about its axis furnished the only time scale in general use, mean solar time. Other independent time scales and systems have recently come into use, however. For a treatment of these different scales and systems, see in the Micropaedia: atomic time; Coordinated Universal Time; dynamical time; Ephemeris Time; Greenwich Mean Time; sidereal time; solar time; standard time; Universal Time. To indicate time and to measure time intervals, various devices have been developed. For a treatment of the different types, see atomic clock; clock; sundial; watch.