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. That's a definition. Atomic clocks are based on this.

Time direction is something else. It is based on information, which has long sat uneasily in the world of mathematics, because it exists only as a collective property. One quantum by itself can not have a time direction. But, any multi-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 later 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 black:

Interchanging balls as before now results in patterns like

and, finally, like

So, with quantization and its consequent limitation of information, a closed 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 how to do it. StarTrek-inspired teletransport of people will face the same limitation.

This, of course, is the 2nd law of thermodynamics - entropy is information with the sign wrong. Space-time is also dissipative with respect to information: the information obtainable from an object moving away from us is progressively reduced as its speed approaches the speed of light; anything heading away from us at the speed of light is no longer observable.

A direction of time is implicit in general relativity. Gravity is asymmetrical and self-reinforcing, and the appearance of a black hole (at this point in time, anyway) is irreversible. So, contrary to the view of many physicists whose formative years were spent solely with classical mechanics, time direction seems to be inherent in all aspects of our universe.

In time, this view may change. (String theorists are trying.) In the meantime, we make do with counting cycles from an arbitrary 'Start of Everything'.

What cycles do we count?

Our earliest measures of time were the duration of one rotation of the earth relative to the sun, a rotation of the moon about a point on the earth, and of the earth around the sun in inertial space, measures based on general relativity. 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 artifact 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. Otherwise, event time was used: when the river floods, when the rains come, when the cows come home, when people feel it's suitable. Much of the world still operates this way.

The earliest clocks, things that measured time by means of physics, were sun dials. They of course only operate in sunny daytime, so events still held sway for the most part. The first true timepieces were water clocks, the time required for a container to empty through an aperture; they were in use in Egypt by 1500 BC. In northern climates where water would freeze, sand was used, or candles. It was not until 1656 that the first pendulum clocks provided time measurement to the minute, and it was not until then that the word 'speed' appeared in the English language. By 1700, pendulum clocks showed that there were variations of about 50 seconds in the duration of a solar day. For the first time, mechanically measured time took precedence over astronomical time; remember, however that both are based upon general relativity.

We now measure seconds by counting cycles derived from quantum mechanics, international atomic clock time (TAI), which is more constant than any mechanical device. The newest clocks, based upon the strontium ions that I pioneered at Canada's National Research Council labs, can be constant to 17 figures. The number of seconds in a day is occasionally fiddled to keep our civil clock time (UTC) in step with the solar day, which is increasing in length due to tidal coupling with our moon.

We could also measure time by the stars, but we never seem to have done that except for marking seasons of the year. Some birds, however, use them for navigation; perhaps other animals do too.

Away from our earth, the rate of time depends on position and movement in space-time.

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 (16x10⁹) 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 can be modelled as being due to a loss of energy of light with time or space, not solely to physical movement. It is even possible that the universe will prove to have indistinguishable 'big bang', 'tired light' and nested black hole models, just as it has several indistinguishable models of quantum mechanics, and that time thus has no true beginning. It's best to keep an open mind on this subject for some time to come.
Our biological time began about 4 billion years ago, with the appearance of permanent liquid water on earth.
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, along with writing, 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 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 Google the Explanatory Supplement to the Astronomical Almanac.
A more recent calendar, containing only computer ticks, was created by Microsoft to begin 01-01-1980.

I am often asked, given my study of time, if time travel is possible. I have 2 comments to offer based on causality:

there is no room in our current understanding for modification of the factual past by a time traveller, and
there is no room in our current understanding for absolute knowledge of the future: we can set alarm clocks, but can never know that something we don't know of today might turn them off.

I am also often asked if 'warp drive' is possible, travel of information in excess of the speed of light:

Difference in speed of transmission in and of itself does not create contradictions. For example, a century ago a traveller from America to Europe on a steamship could bring information from one side of the Atlantic to the other, then find that Marconi's new wireless had beaten him to it. The world of Europe was changed on reception of the wireless; the slow-poke steamship information causes no problems (except to the dumbfounded traveller).
Our current understanding is that changes in the topological structure of the universe would be necessary in order for information to travel in excess of the speed of light.

However, with regard to all such predictions, I note that my grandfather Wallace was born while Faraday was still trying to figure out what electricity was, but lived to hear the Soviet Sputnik thanks to an electronic hearing aid. Paradigm changes of this order render most prediction moot.

John Sankey (1990)
other notes on physics