Portal:Physics/Selected article/August 2010

The flow of sand in an hourglass can be used to keep track of elapsed time.

Time is the theme for this month. A description of time, crafted by a Wikipedia editor, which is derived from reliable sources, is as follows:

Time is a one-dimensional quantity used to sequence events, to quantify the durations of events and the intervals between them, and (used together with space) to quantify and measure the motions of objects. Time is quantified in comparative terms (such as longer, shorter, faster, quicker, slower) or in numerical terms using units (such as seconds, minutes, hours, days). Time has been a major subject of religion, philosophy, and science, but defining it in a non-controversial manner applicable to all fields of study has consistently eluded the greatest scholars.

2007 description of time

Previously, in January of 2007, the opening lead for the article entitled Time read as follows:

There are two distinct views on the meaning of time. One view is that time is part of the fundamental structure of the universe, a dimension in which events occur in sequence. This is the realist view, to which Sir Isaac Newton subscribed, in which time itself is something that can be measured.

A contrasting view is that time is part of the fundamental intellectual structure (together with space and number) within which we sequence events, quantify the duration of events and the intervals between them, and compare the motions of objects. In this view, time does not refer to any kind of entity that "flows", that objects "move through", or that is a "container" for events. This view is in the tradition of Gottfried Leibniz and Immanuel Kant, in which time, rather than being an objective thing to be measured, is part of the mental measuring system. The question, perhaps overly simplified and allowing for no middle ground, is thus: is time a "real thing" that is "all around us", or is it nothing more than a way of speaking about and measuring events?

Many fields avoid the problem of defining time itself by using operational definitions that specify the units of measurement that quantify time. Regularly recurring events and objects with apparent periodic motion have long served as standards for units of time. Examples are the apparent motion of the sun across the sky, the phases of the moon, and the swing of a pendulum.

Time has long been a major subject of science, philosophy and art. The measurement of time has also occupied scientists and technologists, and was a prime motivation in astronomy. Time is also a matter of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in human lifespans. This article looks at some of the main philosophical and scientific issues relating to time.

For further reading see the main article: Time. Below are other articles related to time:

Other articles

Main article: Time in physics

Time in physics is largely defined by its measurement: time is what a clock reads. One can measure time and treat it as a geometrical dimension, such as length, and perform mathematical operations on it. It is a scalar quantity and, like length, mass, and charge, is usually listed in most physics books as a fundamental quantity. Time can be combined mathematically with other fundamental quantities to derive other concepts such as motion, energy and fields. Timekeeping is a complex of technological and scientific issues, and part of the foundation of recordkeeping.

Main article: Orders of magnitude (time)

Time occurs in orders of magnitude from less than one billionth of a second (nanosecond), to billions of years (age of the universe) and beyond.

Main article: Spacetime

According to certain Euclidean space perceptions, the universe has three dimensions of space and one dimension of time. By combining space and time into a single manifold, physicists have significantly simplified a large number of physical theories, as well as described in a more uniform way the workings of the universe at both the supergalactic and subatomic levels.

Main article: Planck epoch

In physical cosmology, the Planck epoch (or Planck era), named after Max Planck, is the earliest period of time in the history of the universe. At this point approximately, 13.7 billion years ago one could also say that it is the earliest moment in time, as the Planck time is perhaps the shortest possible interval of time, and the Planck epoch lasted only this brief instant.

Main article: System time

In computer science and computer programming, system time represents a computer system's notion of the passing of time. In this sense, time also includes the passing of days on the calendar. System time is measured by a system clock, which is typically implemented as a simple count of the number of ticks that have transpired since some arbitrary starting date, called the epoch. For example, Unix and POSIX-compliant systems encode system time as the number of seconds elapsed since the start of the epoch at 1 January 1970 00:00:00 UT. Windows NT counts the number of 100-nanosecond ticks since 1 January 1601 00:00:00 UT as reckoned in the proleptic Gregorian calendar, but returns the current time to the nearest millisecond.

Main article: Atomic clock

Atomic clocks are the most accurate time and frequency standards known, and are used as primary standards for international time distribution services, to control the frequency of television broadcasts, and in global navigation satellite systems such as GPS. The clocks maintain a continuous and stable time scale, International Atomic Time (TAI). For civil time, another time scale is disseminated, Coordinated Universal Time (UTC). UTC is derived from TAI, but synchronized, by using leap seconds, to UT1, which is based on actual rotations of the earth with respect to the solar time.

Template: Time measurement and standards

Wikipedia has a number of articles related to "Time". See our template entitled Time measurement and standards , and click on the link entitled "show".