Physics Help
Time
Time
One can say that one event occurs after another
event. Furthermore one can measure how much one event occurs after
another. The answer to how much is the amount of time
between the those two events.
One way of defining the idea of 'after' is based on the
assumption of
causality. The work humanity has done to increasingly understand the nature
and measurement of time, through the work of making and improving
calendars and
clocks, has been a major engine of scientific discovery.
The standard
unit
for time is the
SI
second, from which larger units are defined like the
minute,
hour,
day,
week,
month,
year,
decade, and
century. Time can be
measured, just like other physical
dimensions. Measuring devices for time are
clocks. Very accurate clocks are often called
chronometers. The best available
clocks are
atomic clocks.
There are several continuous
time scales in current use:
Universal Time,
International Atomic Time (TAI), which is the basis for other time scales,
Coordinated Universal Time (UTC), which is the standard for civil time,
Terrestrial Time (TT), etc. Mankind has invented
calendars to track the passages of days, weeks, months, and years.
In
physics, time is defined as the
distance between
events along the fourth axis of the
spacetime manifold.
Special relativity showed that time cannot be understood except as part of
spacetime, a combination of space and time. The distance between events now
depends on the relative speed of the observers of the events.
General relativity further changed the notion of time by introducing the
idea of curved spacetime. An important unit of time in theoretical physics is
the Planck time – see
Planck units for more details.
Important questions in the philosophy of time include: Is
time absolute or merely relational? Is time without change conceptually
impossible or is there more to the idea? Does time "pass" or are the ideas of
past, present and future entirely subjective, descriptions only of our deception
by the senses?
Zeno's paradoxes fundamentally challenged the ancient conception of time,
and thereby helped motivate the development of the
calculus. A point of contention between
Newton and
Leibniz concerned the question of absolute time: the former believed time
was, like
space, a container for events, while the latter believed time was, like
space, a conceptual apparatus describing the interrelations between events.
McTaggart believed, rather eccentrically and on the basis of a very shaky
argument, that time and change are illusions.
Parmenides (of whom
Zeno was a follower) held a similar belief based on a similarly shaky, but
rather
more interesting argument.
Einstein's
theory of relativity linked time and space into
spacetime in a way that also had philosophical consequences, making the idea
of
block time more credible, and thus affecting ideas of
free will and
causality.
The engineer
J. W. Dunne developed a theory of time whereby he considered our perception
of time like notes being played on piano. Having had a number of prescient
dreams, he monitored his dreams and found that they generally included as many
past as future events. From this he concluded that in dreams we escape linear
time. He published his ideas in An Experiment with Time in
1927
and followed this with other books.
One may perceive time to go fast ("time flies"), meaning that
a duration seems less than it is;
this may be considered an advantage:
it may be considered a disadvantage:
-
(on the other hand, that the time has flown is considered a
sign that it has been enjoyable)
Time also seems to go fast when
sleeping, some of the above applies, e.g. it may be an advantage to sleep as
train or car passenger, and sleep long in the case of boredom, while it may
be wasteful to sleep long on holidays.
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