WHY TIME WILL STOP
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Clocks are machines of whimsy. Every timepiece has a personality and a character of its own—and we are not talking about mother-of-pearl Rolexes and titanium Tissots here. Synchronise all the clocks in your house today and a year later, some will have fallen behind and some zipped ahead by a few seconds. Unless, of course, they happen to be cesium atomic clocks.
"Once synchronised, a cesium atomic clock—which harnesses the transition of electron energy levels in cesium atoms to measure time accurately—won't miss a second in 10,000 years. It is far superior to a quartz clock, which needs tuning every few months," says Dr P. Banerjee, head of the Time and Frequency Section at the National Physical Laboratory (NPL) in Delhi. The keeper of Indian Standard Time (IST), NPL has five such atomic clocks, "just to be sure". Ironically, for a machine that seeks to tame the cosmic continuity of time—something that cannot be apprehended by physical senses—by gauging it down to the nanosecond, a cesium atomic clock typically has a life span of less than a decade. "It is an electronic instrument; it may fail any time," says Dr Banerjee. In fact, one of the five clocks at NPL has just been returned to the grid after being shipped to the US for repairs.
The clocks are nowhere near exotic. They are regular electronic boxes with little digital displays, stacked together in a sanitised chamber—you must leave your shoes outside—where the temperature and humidity are clinically constant at around 23 degrees and 55 per cent respectively. "Back in the mid-1970s, when we got our first atomic clock, people flocked to NPL to take a look, but went home disappointed by its boxy appearance," Dr Banerjee says. But these clocks are the keepers of time, smoothening out the kinks in our measured knowledge of this most dynamic of parameters. While Dr Banerjee trusts NPL-7—the current master clock that was installed three or four years back—the most, he has tasked research scholars working under him with developing algorithms to calculate an average of the five clocks that would have a higher degree of accuracy than individual measures.
Even a cesium atomic clock—cesium is preferred because it is a stable element that is also easy to handle—cannot be 100 per cent correct, since it "probes" the cesium atom and disturbs it. "Let me give you an example. To a scientist, a thermometer tucked under the armpit can never reveal the exact temperature of the body, since it absorbs some of the body heat while measuring it," Dr Banerjee says. For all practical purposes, though, this discrepancy is negligible. Just as the micro-inaccuracies of atomic clocks may be.
What is not negligible is the difference between the time taken by the earth to rotate on its own axis with respect to the sun—which accounts for what is known as a mean solar day—and the time kept by the exactest of clocks. "Man has traditionally identified time with the movement of heavenly bodies. Twenty-four hours as measured by our atomic clocks must never be too different from the time the earth actually takes to complete one rotation around its own axis," says Dr Banerjee. To the meticulous mavericks of scientific persuasion, "too different" is a variation of over 0.9 seconds, stemming from slight anomalies in the earth's rotation due to geological fluctuations. And to take them into account, a 'leap second' is introduced in atomic clocks around the world. (In keeping with the exigencies of accuracy, a second is defined as "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 cesium 133 atom".) The International Earth Rotation and Reference Systems Service (IERS), which monitors the earth's rotation through observatories across continents, adjusts Coordinated Universal Time (UTC)—determined by the Bureau International des Poids et Mesures (BIPM) in Paris—accordingly whenever necessary.
As per the latest IERS directive, time will stop for one second on December 31, 2008, at UTC 23:59:60, or 5:30 a.m. on New Year morning as per IST. "This is the 34th time such a 'leap' of a second—a positive one, which involves adding a second—is going to be carried out. Some years, there are two leaps, and some don't have any, depending on the rotational correlation," says Dr Banerjee, whose romance with timekeeping spans three decades.
"Knowing the exact time may not be important to you and me, but those who use the Global Positioning System (GPS), for example, need an accurate measure, down to the nanosecond. (In one nanosecond, a billionth of a second, light travels 30 cm.) The accuracy of GPS coordinates depends on the accuracy of the time measure given by the atomic clock on the GPS satellite," Dr Banerjee explains. Over 350 atomic clocks around the world compare their time with GPS time and send the data to the BIPM, which calculates UTC as an average of the global atomic clock measures. "The US has about 150 atomic clocks. In India, apart from NPL, ISRO has a few. I want all the clocks to be hooked up to a network so a nationwide average can be calculated," he says.
Accuracy has never been so much in vogue. Space research stations and power grids require a highly specific time measure. And with a host of radio stations jostling for frequency, there is demand for a more accurate frequency source for broadcasting—unlike pendulum frequencies, atomic frequencies used in cesium clocks are constant and universal. The most accurate clock in the world, the cesium fountain clock developed in France and the US, makes the Rs 35-lakh cesium atomic clock seem antique. It can run for 20 million years without derailing the time measure by even a second. NPL is trying to develop one of these, but has so far met with little success.
NPL uses INSAT to transmit the correct time, down to the nanosecond, to high-accuracy users. For low-accuracy users who are content watching the milliseconds roll, Dr Banerjee developed a much cheaper alternative that could receive electronic signals from NPL through a phone line. The Tele Clock was launched in 2000 and is now ticking away at the Parliament, airports and railway stations across the country, and even in Nepal and Saudi Arabia. The digital clock automatically dials NPL at regular intervals—maybe once a day—for the correct time, updates itself and disconnects to run on its own quartz mechanism.
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