END OF GRAVITY ON THE EARTH 01

Einstein's Theory of RelativetyAccording to Einstein's 'Theory of Relativety' there is no absolute value except the speed of light (c). He established the speed of light as a universal constant:c = 3.00 x 1000000000 m/s (670 million mph). This is the ultimate speed limit of the universe. Relativity involves all other measurable parameters in the universe. Rosenfeld (1) gives, as an illustrative example, a box on a table. The box was not always a box of the given length, width and height. It began to be a box only at a certain point in time, on the day it was made. Nor will it always be a box. At any moment it may be broken or destroyed. So the three dimensions of space (length, width and height) constitute a valid description of the box only relative to a certain specified period of time. Thus physical processes, in the theory of relativity, cannot be staged by space and time taken separately but by their combination into a single geometric entity: spacetime (2). Another example of relativity is Gordon Cooper's heroic flight in 1963, orbiting around the earth 22 times.

It made him younger! In the course of each 90-minute orbit he aged a millionth of a second less than he would if he had stayed on earth in Cape Canaveral, USA (1). The slowdown in Cooper's aging process was due to his 17500-mph speed in orbit. The amount of slowdown is precisely calculable from the equations of Einstein's relativety theory. For spacemen of the future, plying the distances between the stars at speeds we can hardly imagine, their age slowdown may become astounding.Atomic Clocks and the Second (SI unit of time) In 1967 the SI unit of time, the second, was redefined using the frequency of the caesium-133 atom as the eference clock? As a result, the second was redefined as 9 192 631 770 times the period of one complete vibration of radiation from the caesium-133 atom (3). In 1997 this was further refined to specify that the second so-defined referred to a caesium atom at rest at absolute zero. More than 260 caesium atomic clocks on the earth, and 24 above the earth in satellites, currently calculate the UTC (Coordinated Universal Time) commonly used by the world (4).

The atomic clock at the NIST (National Institute of Standards and Technology, U.S. Department of Commerce) keeps time with an accuracy of about 3 millionths of a second per year. This is equivalent to an uncertainty of less than one second every 30 000 years (4). The International Bureau of Weights and Measures (BIPM) in France is responsible for international standards of measurement, and its time section administers the formula for UTC. Their officials send out two timescales: International Atomic Time (TAI) and UTC. They monitor the accuracy of the aforementioned atomic time clocks, on ground around the world, that are used for this purpose (5).The GPS, from the U.S. Defense Department's Global Positioning System satellites, broadcasts time signals from its own internal atomic clocks. These signals are compared with the atomic clocks used to determine UTC (5).