MEASURE OF TIME

 The fear of the time which passes does it find a money, does it dilute in the concern of a more and more fine measure of duration? Our enquiry is going to cover in some lines five thousand years of history.

Babylonians Pyramids, so called "ziggurats", would have allowed to observe celestial bodies and to establish a first calendar. In the neighborhood of 1500 before J.-C. The Egyptians finalized(worked out) the gnomon,

A simple planted stick is in hiding it which allows to estimate hour according to the shadow which it throws(plans). The gnomon establishes(constitutes) the ancestor of the sundial.

The Egyptians also invented the water clock,

Sort of clock with water always used in Moyen-Age and in the Renaissance, notably by Galilee to make its measures of speed of motives.

 The egg timer dates also this time.

Julius Caesar invented the calendar which carries(wears) its name, the Julian calendar,

By the introduction of leap years. But in 1582, the pope Grégoire XIII puts in the point/invente the Gregorian calendar

- That does not invent-; it is the calendar that we even use today.

In the XVIè century appears the mechanical watch, with the physicist Huygens.

But the first really reliable individual watch in the second near will be H-1 created by the carpenter John Harrison in 1785.

Clocks placed at the top of churches and bell towers become carillons which give rhythm to social life until first industrial revolution. A lot closer of us, the quartz watch considerably increased the precision of the measure of time with its hundred thousand vibrations by second. But it's nothing next to the precision supplied with an atomic clock. The standard of measure of time is at present supplied with the atom of cesium 133.

To reach(affect) a precision of the order of the picoseconde as far as time, it was necessary to redefine what is conventional unit of time: the second. Previously, second was 86400-th day part. Today, it is the duration of 9192631770 periods of radiation corresponding to the transition between the two hyperfine levels of the fundamental state of the atom of cesium 133. As for the unit of length, the metre, she(it) depends on this unit of time: it is the length the route crossed(gone through) in the space by the light during a duration of 1/299792458-th of second. These definitions look like maybe complicated well but they make reference to something simple: frequency. Frequency, it is the number of vibrations by unit of time in a periodic phenomenon, which repeats regularly. The 9192631770 periods of radiation represent 9192631770 vibrations during one second.

One will point out that it was necessary to know what was worth 1 " before being able to correspond her(it) to these 9192631770 vibrations!

Indeed, one left of the 86400-th day part, and one noticed that it corresponded to the 9192631770 periods of radiation. The interest in this new way of measuring second, it is that she(it) is very stable, much more stable than what indicated clocks to there.

But the essential point of this chapter is the notion of " time of Planck ", or a limit absolved in the possibility of dividing time. Result of a formula in which intervene the three fundamental constants of the physics, G, a hour and c, the time of Planck means that one can not divide time beyond 10^-43s.

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