What is Time?
To physicists, time is defined by quantum mechanics. A photon with energy h (Planck's constant) behaves as
though it were oscillating once per second.
Modern atomic clocks
are based on this.
Time direction is something else. It is based on information, which sits uneasily in the world of physics.
But, any quantum system must have an arrow of time.
To see this most simply, set up a row of ten numbered balls
Now, apply any process we don't understand. For example, pick pairs of balls at random and
interchange them. You will soon see patterns like
and, finally, like
Without quantization, this is the way the universe would look. You would be able to put it into, or
back into, any state you wished, any 'time' you wished. Time would not have a direction to it.
The essence of quantization is that information is limited - many 'different' particles are indistinguishable.
We can not distinguish one electron from another, we can only observe the recent history of each one as
reflected in its few quantum numbers. So, repeat the experiment, but represent balls 1-5 as white, 6-10 as
Interchanging balls as before now results in patterns like
and, finally, like
So, with quantization, and its consequent limitation of information, a universe progresses from an ordered
state to a disordered state - a direction of time. And, when the number of particles is large, even as large as
the number of molecules in a cubic millimetre of air, the disordered state is permanent, compared to the
apparent lifetime of our universe anyway. We can't put things back the way they were, because we can
never know, even in principle, how to do it.
A direction of time seems implicit in general relativity. Gravity is always positive, and the appearance of a
black hole (at this point in time, anyway) is irreversible. Maybe, someday, we'll figure out how to put time
together to figure this out. In the meantime, we simply count cycles from an arbitrary 'Start of Everything'.
What cycles do we count?
The earliest measures of time were the duration of one rotation of the earth relative to the sun, a rotation of
the moon about the earth, and of the earth around the sun. These were built into the genetic heritage of all
life long before human beings arose. We still use them today, as our day, month and year. In our Western
civil calendar, we keep count of days to mark the passage of time. The month is an artefact of history since
it no longer matches moon cycles, and the number of days in a year is fiddled to keep our day time and year
time in step.
We now measure seconds by counting cycles derived from quantum mechanics, international atomic clock
time (TAI), which is much more accurate than the rotation of heavenly bodies. The number of seconds in a
day is occasionally increased by 1 to keep our civil clock time (UTC) in step with the solar day.
We could also measure time by the stars, but this was rarely done except for marking seasons of the year.
When was the 'Start of Everything'?
- The commonest view of scientists is that the universe, and hence time, came into existence with a
'big bang' about 16 billion (thousand million) years ago. This theory is based primarily on observations of
colour patterns of stars, of the background radiation between stars, and of the distribution of elements in the
universe. It is possible, however, that the colour patterns are partly due to a loss of energy of light with
time, not solely to physical movement. It is even possible that the universe will prove to have
indistinguishable 'big bang' and 'steady state' models, just as it has indistinguishable models of
and that time thus has no true beginning. It's best to keep an open mind on this subject for some time to
- Our biological time began about 4 billion years ago, with the appearance of permanent liquid water
- The earliest human calendars (one is dated to 30,000 B.C.) kept track of the days in the moon cycle.
Later ones counted moons or years from the accession to power of the head of a cultural group.
- The early Egyptian year began with the first visible rising of Sirius above the horizon. It was
probably in use by 4200 B.C..
- The Mayan calendar of Central America began 12 August 3113 B.C. Gregorian, possibly the
religious date of foundation of the first Egyptian dynasty. It was brought to the Olmec of the Gulf of Mexico
by a fleet of Egyptian-derived reed boats headed by a senior religious person, the 'Plumed Serpent', about
2200 years ago. It is no longer in use, but is truly carved in stone.
- One Bishop Ussher considered that, according to the Bible, God created the world on 23 October
4004 B.C. We have just entered his 7th millennium. (A hundred years ago, 23 October 1997 would have
been considered by many to be a more important date than 1 January A.D. 2000.)
- A short-lived French revolutionary calendar, with 10 day weeks, began 22 September 1792.
(Napoleon canned it.)
- For details of the main calendars in current use see the
Explanatory Supplement to the Astronomical Almanac.
And, links to just about everything online about calendars are at
- Possibly the most recent calendar, containing only computer ticks, was created by Microsoft to begin
For some other views of time, see the 'Encyclopedia of Time', edited by S.L.Macey (1994).