WHAT  IS  time

Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects.  The temporal position of events with respect to the transitory present is continually changing; future events become present, then pass further and further into the past. 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. A simple definition states that "time is what clocks measure".

Time is one of the seven fundamental physical quantities in the International System of Units. Time is used to define other quantities — such as velocity — so defining time in terms of such quantities would result in circularity of definition.  An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and everyday affairs of life. The operational definition leaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be measured. Investigations of a single continuum called spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy.

Two contrasting viewpoints on time divide many prominent philosophers. One view is that time is part of the fundamental structure of the universe, a dimension in which events occur in sequence. Sir Isaac Newton subscribed to this realist view, and hence it is sometimes referred to as Newtonian time.  Time travel, in this view, becomes a possibility as other "times" persist like frames of a film strip, spread out across the time line. The opposing view is that time does not refer to any kind of "container" that events and objects "move through", nor to any entity that "flows", but that it is instead part of a fundamental intellectual structure (together with space and number) within which humans sequence and compare events. This second view, in the tradition of Gottfried Leibniz  and Immanuel Kant, holds that time is neither an event nor a thing, and thus is not itself measurable nor can it be travelled.

Temporal measurement has occupied scientists and technologists, and was a prime motivation in navigation and astronomy. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart. Currently, the international unit of time, the second, is defined in terms of radiation emitted by caesium atoms (see below). Time is also 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 life spans.

Ray Cummings, an early writer of science fiction, wrote in 1922, "Time... is what keeps everything from happening at once", a sentence repeated by scientists such as C. J. Overbeck, and John Archibald Wheeler

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Writer and physicist Paul Davies has called "time" Einstein’s unfinished revolution. There are many questions about the nature of time. What
is time? What causes time?  Why time slows in gravity? Why time slows in motion? Is time a dimension? Aristotle had speculated that time
may be motion. He however added that motion could be slower or faster but not time. Aristotle did not have the privilege of knowing about
Einstein’s relativity in which time also becomes amenable to change. Similarly when Einstein was working to develop theory of general
relativity and proposed the revolutionary idea that mass curves space he did not know that the universe was expanding. This discovery by
astronomer Edwin Hubble came 13 years after Einstein had published  his theory of General Relativity.  Had Einstein known of this great
discovery he may have incorporated these ideas into his theories.  Conceptually it is easier to derive space curvature in an expanding
universe as an area of slower expansion under the influence of gravitons.  One of the most dramatic aspects of the universe is that it is
expanding and the presence of motion, forces and curved space-time happens in the expanding space.  

In the next few paragraphs I will show that time is an emergent concept. There is an underlaying process of motion and forces from which
time emerges, however what we perceive as time is mostly an illusion. Our memory creates the illusion of the past. Conscious perception of
events gives the feeling of present. Future is a mental construct patterned on the memory experience of the past.  Concept of time emerges
as our mind tries to make sense of the world around us which is filled with change.

 Mc TAGGART  ON THE ILLUSION OF TIME

John Ellis McTaggart (1908)  and many other philosophers have proposed that the passage of time is an illusion suggesting that only the
present is real.  McTaggart is famous for his A, B and C series analysis of time. A brief review is as follows:

The earlier and later aspect of time is basically the same as the arrow of time. The birth of a person always comes before their death even as
these events become part of the distant past. This is a fixed relationship so McTaggart asserts it must be more fundamental to time.

The past the present and the future aspect of time is constantly changing, future events are moving to the present and then into the past and
then further back into the past.  This aspect deals with the feeling of flow of time.  This constantly changing relationship is also essential to the
description of time.  McTaggart felt that time is unreal because distinction of past present and future (a changing relationship) is more
essential to time then the fixed relationship of earlier and later.

 TIME AS MEMORY

McTaggart's most interesting observation however is that historical events have same time characteristic as made up stories. For
example made up stories, as well as past historical events have in them, the earlier and the later as well as the past the present and the
future, thus suggesting that past really is more like memory of events and does not exist any more than imagination of a writer. The above
point becomes more clear when we compare past present and future to other recording devices like laser discs CDs and hard drives.  PRESENT IS AN INFINITESIMAL?

Present is the most real perception of time however almost all of what we perceive as
the  present is already past. The present is a fleeting moment, whatever is
happening now (present) is confined to an infinitesimally narrow point on the time
line which is being encroached upon by what we think of as the past and the future.

Present resembles the sharp point of a recording laser or needle while past
being a duration or extension resembles the recorded material  like tape or CD.

Present may be mental awareness of the recording of memory into our brain. A
person can go to an event but fall asleep and miss the event completely.  So that
event basically does not exist in his past. Unless we are consciously aware of an
event it does not seem to enter our past memory. PAST AND FUTURE ARE DURATIONS IN TIME

Unlike the present we see past and future as measurable
durations of hour days months and years.

Past historical events, an upcoming meeting, wedding, or other
events are all measurable durations or extensions in time, just
like a recorded material on tape, video recordings, and tracks of
CD or DVD.

This similarity suggests that past is just a recorded memory,
while future is like an unrecorded tape. Future is a projection
created by our past experiences which are stored in our memory MEASURING TIME

Be it money, weight, a piece of estate, speed, distance or resistance. We  measure by comparison to a standard that we have defined. When
we measure a mass we use a standard mass like KG or LB for comparison.  Distances we measure using a standard of length like a meter
yard or feet. Keeping the above in mind let us think how we measure motion. We use time to measure motion i.e. Feet per second or miles
per hour. This provides a hint that when dealing with time we are actually dealing with some standard of motion.

We may have made the concept of time more complicated then what it really is. Measurement of time started early on in human
development. There are plenty of clues in every language in the greetings and the meetings. Time of the day is related to the position of sun
in the sky or its absence thereof. There is dawn, sunrise, early morning, morning, mid morning, noon, afternoon, late afternoon, evening,
sunset, dusk, night and mid night. Then there are years, months, weeks, based on earth’s yearly orbit around the sun and the changing
seasons. The use of units like seconds and minutes which are radial angle measurements in geometry points toward the original
connection of time measurements to radial motion of astronomical objects across the sky.  Once we started using clocks, watches, and then
digital time we got completely disconnected from the original method of measurement and time developed a life of its own.

The problem of time may be easy to solve if we go back to the original concept of sun moving across the sky. When we measure the speed of
a car, we are just comparing its motion to the motion of the hands of the clock and also indirectly to the fractional motion of sun across the sky.
We are not measuring speed with something abstract called time we are just comparing a known motion (of the sun) with an unknown motion
of the car.

 TIME AS UNIT OF MOTION

In a recent article in Scientific American Craig Callender elegantly compares time with a unit of motion. Time is a way to compare or describe
different kinds of motions like speed of light, how fast heart beats or how frequently earth spins around its axis. But these processes could be
compared directly without making reference to time. Time may just be common currency or unit of motion  making the world easier to
describe but having no independent existence. Measuring processes (of motion) with time is like using money rather than barter transactions.
This again suggests that underlying mechanism of time is just presence of motion. WHAT REALLY IS TIME?

Time is an emergent concept. There is a real phenomenon a continuous change through which we live. To understand time we have to
understand the mechanism which brings about this continuous change from which our mind creates the illusion of flow of time.

Time becomes evident through motion and is measured by comparison with other motions. Sunrise sunsets, night and day, the changing
seasons, the movement of the celestial bodies are all indicative of continuous change.  The aging process is a reminder that molecular
motion and interactions are also at work and are a part of time. Other very important aspect of time is presence of motion of particles like
photon and motion at atomic level.