What is time

1
What is time? Albert Einstein defined time as
that which a clock measures. The definition
of time is traditionally considered on the
fence between physics and philosophy. How can
we define what it is? Time quantifies or
measures the interval between events, or the
duration of events. Time has long been perceived
as a dimension in which each event has a definite
(but not necessarily unique) position in a linear
sequence, but as differing from spatial
dimensions in that "motion" through time appears
restricted to having only a forward
direction. For everyday purposes, and even for
quite accurate measurements, this view is
sufficient. However, the
scientific understanding of time underwent a
revolution in the early part of the twentieth
century with the development of relativity
theory. Modern physics treats time as a feature
of spacetime, a notion which challenges intuitive
conceptions of simultaneity and the flow of time
in a linear fashion. Also, from ubiquitous
practical, and some key theoretical evidence, we
can say that there is some sort of orientation of
time that qualitatively distinguishes future
from past.   So, much like many other
quantities, we may try to define time by
giving 1) a basis for an orientation, or arrow
of time, and 2) an ability to measure the
magnitude of a time interval.
John Tracey Simon Hall Michael Stewart Stephen
Hardiman
supervised by Dr. Stefan Hutzler
Oscillators a basis for time To measure the
length of a physics poster, we place a meter
stick against it and compare its endpoints with
the marks of the meter stick. To measure a
length of time, we need a corresponding time
yardstick. Ancient humans used the dawn and
sunset as markers of time later, the cycle of
seasons and the phases of the moon. These, of
course, are all examples of oscillations, and
oscillations are still the source of human
timekeeping, in every form from quartz crystals
to cesium clocks.
Classical Physics and Time Asymmetry It is a
common statement that Newtonian physics is
invariant under time reversal. This symmetry
essentially comes from the fact that the
fundamental Newtonian equation, F
dp/dt remains unchanged when subjected to the map
x - x t - -t v - -v This symmetry would
seem to be at odds with thermodynamics, which
violates time-symmetry via the total increase in
entropy of a system with increasing time. In a
world governed entirely by time-symmetrical laws,
could the second law of thermodynamics be valid,
or is it the product of an implicit assumption of
the arrow of time?
Time Reversible
Not exactly Time reversible
In a simulation under classical laws, it is found
that by reversing the velocities of all of the
particles in a simple gaseous system
simultaneously, a net decrease in entropy did
indeed result. However, any small random
deviation from these backward path velocities
put the system onto an upward entropy climb once
more. It is postulated that random influences
are the source of the appearance of the arrow of
time from statistical mechanics. This idea poses
some interesting questions if true for a closed
system, such as the universe as a whole may be,
would classical mechanics then produce a
direction of increasing entropy? And, if
randomness is the key, where does the asymmetry
lie that causes randomness to be generated in one
direction of time, and not the other?
The steady swing of an oscillating pendulum,
provides a basic yardstick for measuring time
Keeping Time In the light of Einstein's theory
of relativity, it is a truism to say that
humanity had previously seen time as uniform,
regardless of position or velocity. However, in
practical terms time was more relative five
hundred years ago than today. Although
mechanical clocks had been in use since at least
12831, primarily for the purpose of religious
observance, time standards were still dependent
on the location at which they were measured.
Towns used the position of the sun to directly
determine the time since noon differed from
place to place, so did the time. It was not
until the development of railroad that there grew
a need for a standardised time, the first such
standard being laid down in Harvard University
Observatory in 1851, followed closely by the
Royal Observatory's standard time for Britain.
This increasingly arbitrary interpretation of
time dissociated it from the sun and stars,
culminating in the worldwide standardisation to
Greenwich Mean Time, at the 1884 International
Meridian Conference.
Time
What is Time?
Ice Low Entropy
Puddle High Entropy
Arrow of Time A property one can look at with
time is something called the Arrow Of Time.
When real time is discussed, it has a forward and
backward direction. The laws of science do not
distinguish between the past and the future but
you can see the sharp contrast in everyday life.
Imagine recording a glass falling off a table,
you could easily tell, while watching it, when
the tape is being played backward or when been
played forward. This is because it would be very
strange to see a glass reforming while defying
gravity!
Measurement of Time Before the dawn of
civilisation a natural time divide existed
Night and Day. Upon the arrival of organised
civilisation and government, a time measurement
system was developed unique to each distinct
culture. The Babylonian and Roman systems
certainly have had an effect on the system used
almost worldwide today. The Romans, for instance,
re-named the months July and August after Julius
Caesar and Augustus respectively. In 1967, at
the Thirteenth General Conference on Weights and
Measures, it was agreed to define the second
within the SI (Systeme International dUnites)
System as the duration of 9,192,631,770 periods
of the radiation corresponding to the transition
between two hyperfine levels of the ground state
of the cesium-133 atom. It follows that there are
then 60 seconds in a minute, 60 minutes in an
hour and a day is the time it takes for the earth
to spin on its axis, or 24 hours. Seven days
make a week.
Time, Relatively Speaking By the end of the 19th
century, people thought that time flowed
uniformly, independent of our intrinsic
existence. In 1905, a young patent clerk named
Albert Einstein wrote a scientific paper that
changed the way we look forever. He hypothesized
that the passage of time is relative to the
person measuring it. He stated that there exists
a time-dilation when one is moving relative to
another which is quantitavely given by the
Lorentz transformation     where t,t are the
times measured by each observer in their
respective inertial reference frames, x the
position of the unprimed observer, v is their
relative velocity and c is the speed of light. We
can see from this transformation that as a body
speeds up, the time measured in its frame dilates
in comparison to that of the other. In 1915,
Einstein published another ground breaking paper
in which he incorporated gravity into his theory
of relativity. He showed that in a strong
gravitational field, time also slows down for
relative observers. The fabric of Einsteins
universe is a 4 dimensional space-time, where
gravity is described not as a force, but as a
curvature of this space-time. A body in
gravitational orbit is simply following a
geodesic (the path of shortest distance) in 4
dimensional space-time.
The universe tends towards disorder
This is because everything must obey the second
law of thermodynamics which states that in a
closed system, disorder entropy must always
increase with time. This is one example of an
Arrow Of Time. Another is ones own personal
arrow of time. This is an arrow that everyone
feels time pass. The third arrow of time is one
associated with the expanding/contracting of our
universe. All three arrows point in the same
direction defining our life of remembering the
past, not the future Why do we see all three
arrows pointing in the same direction? One looks
to the anthropic principle, which states that We
see the universe the way it is because if it were
different, we would not be here to observe it
This answer stems from various arguments
producing the result that if one arrow were
pointing opposite to another, life would not be
possible due to uncertain results of non-standard
rules of thermodynamics in this hypothetical
universe. So whichever arrow of time you feel
most intrinsic to you, rest assured its pointing
the right way.
A month is the approximate time needed for the
moon to revolve once around the earth. The lunar
month actually takes 29 days, 12 hours, 44
minutes and 3 seconds. There is another natural
time divide in the form of the cyclic change in
weather the changing seasons. The cycle lasts
for one year - now defined as the time for one
revolution of the earth around the sun equal to
365 days, five hours, 48 minutes and 46 seconds
or approximately 365¼days. In order that we have
our nice 365 days in one calendar year we
introduce an extra day to the year every 4 years
in what has become known as a leap year. To
account for the minute difference of 11minutes
and 14 seconds less than 365¼days we make a
century year a leap year if and only if it is
divisible by 400. This gives us an extra 3,300
years before the calendar and solar year again
differ by one day. The earth is undergoing a
slight deceleration caused by breaking action of
tides and other effects. Civil time must be
adjusted by one second to ensure that the
difference between a uniform time scale defined
by atomic clocks does not differ from the earths
rotational time by more than 0.9 seconds. This
has become known as a leap second.
The Beginning and End of Time Most physicists
agree on the Big Bang theory which states that
the universe exploded out of a single point or
singularity sometime between 13 and 15 billion
years ago. It was initially suggested because it
explains why distant galaxies are receding at
such great speeds. Time, as it is, was created
in this singularity and ends in a singularity
cloaked behind the event horizon of a black hole.
When we observe very distant galaxies we are in
fact looking back in time at the early universe.
The more red shifted the spectrum of their light,
the quicker they are receding from us, the
further away they are and the further back in
time we are seeing. The theory also predicts the
cosmic background radiation, the afterglow of the
early universe. In the very earliest fractions of
a second after time t0, the basic constituents
of matter still hadn't come into existence, and
the fundamental forces of nature were unified
into one force. And the future of the universe?
Well that depends on the gravitational mass of
the matter in the universe. If it is great enough
to halt the universe's expansion, then Universe
is said to be closed, and time will end in a Big
Crunch just like it started with a Big Bang.
Otherwise the Universe is said to be open and
galaxies will continue receding until eventually
even single elementary particles will be
separated by vast distances.
The large mass of the planet curves its local
space-time
We therefore see that time can be considered as a
quantity which can be changed, manipulated and
perhaps reversed. The tests of Einsteins
theories have been thorough and very successful
over the past century showing that time is not
just a concept invented by human civilization,
but something we feel by virtue of our existence
and maybe someday, something we can touch.

• References
• "Time Reversal Symmetry Violation and the
  H-Theorem - A. Aharony, Physics Letters Vol 37A
  No1.
• "The Enigma of Time - P. T. Landsberg.
• "Physics of Time Asymmetry - PCW Davies.
• "A Chronicle of Timekeeping - W. Andrews,
  Scientific American Vol.287 No.3
• Cosmology, a Very Short Introduction Peter
  Coles

• The Road to Reality Roger Penrose
• Black Holes Time Warps - Kip S. Thorne
• A Brief History of Time Chapter 9, Arrow Of
  Time - Stephen W. Hawking
• http//www.pbs.org/deepspace/timeline/
  (for universe timeline timeline)
• http//www.tycho.usno.navy.mil/leapsec.html
• http//www.big-bang-theory.com