7 OCTOBER 1972, Page 11


Tampering with time

Oliver Stewart

Time is short before the date by which all member countries of the European Economic Community must go over to the international system of measuring units, the SI. As stated in the Metrication white paper of February, this date is January 1 1978.

But what is time? What, in particular, is SI time?

In the SI there is one and only one unit of time, the caesium second. The system reluctantly admits the non-decimalised minute and hour as a gracious gesture to established custom. But the higher metrology recognises only the second as it is counted out by the caesium clock. That is the SI second and it has the lower case symbol 's'.

The awkward fact is that the caesium second is not the same as the second which is obtained by dividing up the months and clays and hours and minutes of the tropical year. It is not the same, in short, as the second of time we use to count out our seasons and our life span.

That is the reason for the ludicrous juggling that now goes on with the radio time signals. There is the 500 millisecond "lengthened pip" (the BBC's phrase) which ends the signal and there are the even sillier "positive leap seconds" and "negative leap seconds." These have been introduced in a desperate effort to adjust those incompatibles, the caesium second and the natural second. They are splendid examples of how unscientific scientists can become when they find their lovely laboratory ideas at variance with the facts of life.

Time is something people have always liked to tamper with in one way or another. Clock confidence tricks include the twiddling of the hands so as to dupe people into starting work earlier without knowing it or into thinking that their days are being made longer by some brilliantly clever horological legerdemain.

But dishonesty reaches its peak with the present dodges and deceits. There appears to be a firm case for keeping the caesium second and the natural second (for at any rate it is the second which dominates human nature) apart. Admittedly the caesium second can be said to be based upon natural phenomena; but the point that must be made is that it is not a unit of time. As the French physicist, DanlouxDumesnils, has repeatedly observed it is a unit of duration and that is not exactly the same thing.

In spite of its resemblance to scientific jargon I must quote the official definition of the caesium second. It is: "The duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom." These are the terms of the 1967 resolution of the Conference Generale des Poids et Mesures.

Before this resolution the SI second was, what the second had always been, a frac tion of the tropical year. So it is that damned laboratory oscillator that is causing all the trouble. It attains a precision in the measurement of duration far beyond anything previously known; but it is aloof from the sun-earth relations'hip.

The extravagant trickery which has had to be used in the efforts to fit together the SI unit of duration and the second unit of time is in itself a clear indication that the two are incompatible.

This is not the only field where the SI has gone off the rails. It is in most res pects a splendid achievement, a system of measuring units enabling a higher precision to be attained than in any other system. But its nomenclature has aroused criticism, some of it from scientific workers.

If, from its seven base units, it were to drop what it calls its unit of " time " and were to substitute a unit of "duration " a great many difficulties would be removed. At first sight this seems a rather trivial al teration; but it would ease the way towards making the SI — in Lord OrrEwing's words when he was appointed chairman of the Metrication Board this year — " part of the fabric of everyday life."