Atomic time boffins build better second-watcher
NPL and precision on the final frontier
See you in 3752, hopefully
UTC is currently set using a network of 300 atomic clocks around the world that are responsible for TAI, but UTC also factors in astronomical time, which is taken from the rotation of the earth. UTC has been used rather than TAI to help mariners as it was felt a system was needed that factored the earth's rotation for accurate navigation on the high seas. Problem is the earth's rotation is not constant, it's slowing down, and every so often a "leap second" is added to ensure accuracy: defined as the Sun crossing the Greenwich meridian at noon UTC to within 0.9 of a second.
Some reckon this means UTC is doomed because the process of adding leap seconds will become unworkable in 1,700 years. Steve Allen, of the Lick Observatory in Santa Cruz, California, said here this summer that based on current calculations UTC will have gone from one leap-second a year in 1981 to one leap second needing to be inserted per month by the year 3752.
This might change as more polar-ice-cap melt, altering the composition of the oceans, slowing the earth further thanks to the dragging effect of water moving around the planet's surface. Alternatively: "If a super volcano erupts or asteroid strikes, all bets are off."
Rise of the time-keeping machines
The BIPM also infavors the death of leap seconds and global convergance on a single, consistant time standard. We are in the process of the earth being surrounded by four satellite systems each with their own separate time offsets, causing confusion and possible disaster the BIPM says. The current US GPS network and Russia's Global Navigation Satellite System (GLONAS) are in the process of being joined by Europe's Galileo system and China's Beidou network.
Elisa Felicitas Arias, director of the BIPM Time Department, told The Reg: "If you have a GPS receiver and are not aware of the differences, you could make a mistake in the order of tenths of a second. If you are landing an aircraft this mistake will have dramatic consequences. We want for such a problem to be avoided if we can do it."
Equally, given so much digital equipment is already installed with in-built offsets it could be argued making any switch also contains dangers.
According to Arias, British delegates have resisted the death of the leap second because no problems have been detected in the past. UTC is, of course, synonymous with Greenwich Mean Time (GMT) so there could be some national pride involved. Disagreement was voiced during a special meeting of experts held under the aegis of the Royal Society in London earlier this month.
If UTC is de-coupled from the earth's rotation, that would mean atomic clocks become completely responsible for setting the time of all other timepieces and apps on Earth.
Unlike UTC, TAI is a constant because its readings are based on a chemical-reaction process that doesn't change. While the reaction is a constant, the process of getting an accurate measurement is a challenge and that challenge arises thanks to the presence external factors that taint the results.
Arias notes, the TAI name could get phased with UTC continuing and the atomic clocks running under the covers of UTC. That could happen because UTC is the legally and internationally accepted system of time measurement while TAI is more of a reference standard. A decision on this could happen at a September 2012 meeting of the ITU following January's vote on decoupling.
In the sidelines of this international struggle is NPL-CSF2. The clock keeps the TAI accurate through its process of exciting caesium atoms using microwaves, and then measuring their reaction. A laser is used to slow down and control the flow of the atoms to help take accurate readings.
The all-important second is measured using a process that takes place when the caesium atoms are exposed to the microwaves: spin flip. The caesium atoms are resonated using microwaves until they flip at a specific frequency – 9, 192,631,770 Hz; a flip defines a second. The atoms are bundled into batches of around 100 million and are streamed through that cavity where they are exposed to microwaves.
Accuracy is not an absolute, however, and NPL is working to make things even more precise.
Looking not unlike a water-tank sitting on polished-steel stilts, the cylindrical NPL-CSF2 lives in controlled, laboratory-like conditions that are free of outside interference. The biggest intrusion in the air-conditioned, windowless room is the quiet tick-ticking of a huge workbench of tiny mechanisms raising and lowering little arms like train track signals to control the lasers used by the clock.
Louis Essen (right) with NPL colleague John Parry and the first caesium clock