Mousehound
01-02-2009, 08:39 AM
Atomic clocks measure time by tracking the magnetic spin of caesium atoms. They spin 9,192,631,770 times per second rather than once a minute for the fastest hand on most mechanical clocks.
Since 1967, the International System of Measurements has based its unit of time, the second, on the spin properties of caesium atoms.
Atomic clocks provide the primary frequency standard used to define Coordinated Universal Time (UTC), the official world time which is computed in Paris at the International Bureau of Weights and Measures.
In 1970, an international agreement established two timescales - one based on the rotation of the Earth and the other on atomic time (UTC).
As the Earth's rotation is slowing to reconcile this timescale with atomic time, scientists invented the 'leap second' to keep the two systems within one second of each other. The next leap second will be added to the world's clocks at 23 hours, 59 minutes and 59 seconds UTC on December 31, 2008.
This will be the 24th leap second added to UTC since 1972. As the Earth slows further, early in the 22nd century two leap seconds will be needed every year.
--Snip--
However some scientists are once again looking to the heavens for ever more accurate timekeepers.
Dr George Hobbs, a member of the Parkes Pulsar Timing Array project, says the nature of pulsars make them perfect clocks with which to calibrate terrestrial time.
Just 20 kilometres across, with a mass 1.4 times that of our sun, these rapidly spinning, highly magnetised stars are the remnants of stars (five to ten times bigger than our Sun) that went supernova, or exploded, millions of years ago. The radio waves they emit as they spin in space a few hundred times a second is observed on Earth as 'pulses', whose arrival times are so regular they can be predicted precisely, making them excellent timekeepers.
While atomic clocks were extremely stable over short time scales, they needed "little tweaks" every few weeks or months to keep them in line with world time standards, says Hobbs.
Pulsars, however, are extremely stable over long periods of time: "But I agree, every now and again you get a slight irregularity in its spin. They do have slightly different spin-down rates which we can model."
"We can use the pulsar signals to see if there are any problems or irregularities in the terrestrial atomic clock timescale," Hobbs says.
"We don't want to replace it with pulsar time. For very short periods, such as a few minutes between two events, we will still use atomic clocks."
But he says there is a growing demand for more reliable timescales over longer periods, particularly in space. The ability to send and track probes over vast distances required extremely accurate time standards.
The Parkes project has been timing 20 millisecond pulsars (pulsars with rotational rates of milliseconds) every few weeks since 1994 and is focused on using them to find gravitational waves predicted by Albert Einstein.
The theory is if a gravity wave passes by the Earth, it will change space and time around it. This means the pulses from some pulsars in a given direction will appear to arrive early whereas those from a different direction will seem to arrive late.
"If we do see this as predicted by general relativity, then this would be the first direct detection of gravitational waves," he says. "To do this of course we need to time these pulsars extremely precisely."
Hobbs points out that five years ago the technology was not advanced enough to time pulsar signals to the same level as atomic clocks.
"We can now measure these pulses to 100 nanoseconds or so," he says. "Atomic clocks are more stable on a short time span but over months or years, they are not as stable as pulsars."
University of Texas pulsar scientist Fredrick Jenet, who also is using precision timing of pulsars to detect gravity waves says, "Pulsars are not affected by the same things that affect clocks on our planet. It is only by comparison that we understand the strengths and weaknesses of our own time".
Warrington believes pulsars may make a useful contribution to atomic clock timescales over long periods of time, as even the best clocks drift.
"However atomic clocks are still needed on short timescales that people are used to."
Full article here:
http://www.abc.net.au/science/articles/2008/12/18/2450349.htm?site=science
Since 1967, the International System of Measurements has based its unit of time, the second, on the spin properties of caesium atoms.
Atomic clocks provide the primary frequency standard used to define Coordinated Universal Time (UTC), the official world time which is computed in Paris at the International Bureau of Weights and Measures.
In 1970, an international agreement established two timescales - one based on the rotation of the Earth and the other on atomic time (UTC).
As the Earth's rotation is slowing to reconcile this timescale with atomic time, scientists invented the 'leap second' to keep the two systems within one second of each other. The next leap second will be added to the world's clocks at 23 hours, 59 minutes and 59 seconds UTC on December 31, 2008.
This will be the 24th leap second added to UTC since 1972. As the Earth slows further, early in the 22nd century two leap seconds will be needed every year.
--Snip--
However some scientists are once again looking to the heavens for ever more accurate timekeepers.
Dr George Hobbs, a member of the Parkes Pulsar Timing Array project, says the nature of pulsars make them perfect clocks with which to calibrate terrestrial time.
Just 20 kilometres across, with a mass 1.4 times that of our sun, these rapidly spinning, highly magnetised stars are the remnants of stars (five to ten times bigger than our Sun) that went supernova, or exploded, millions of years ago. The radio waves they emit as they spin in space a few hundred times a second is observed on Earth as 'pulses', whose arrival times are so regular they can be predicted precisely, making them excellent timekeepers.
While atomic clocks were extremely stable over short time scales, they needed "little tweaks" every few weeks or months to keep them in line with world time standards, says Hobbs.
Pulsars, however, are extremely stable over long periods of time: "But I agree, every now and again you get a slight irregularity in its spin. They do have slightly different spin-down rates which we can model."
"We can use the pulsar signals to see if there are any problems or irregularities in the terrestrial atomic clock timescale," Hobbs says.
"We don't want to replace it with pulsar time. For very short periods, such as a few minutes between two events, we will still use atomic clocks."
But he says there is a growing demand for more reliable timescales over longer periods, particularly in space. The ability to send and track probes over vast distances required extremely accurate time standards.
The Parkes project has been timing 20 millisecond pulsars (pulsars with rotational rates of milliseconds) every few weeks since 1994 and is focused on using them to find gravitational waves predicted by Albert Einstein.
The theory is if a gravity wave passes by the Earth, it will change space and time around it. This means the pulses from some pulsars in a given direction will appear to arrive early whereas those from a different direction will seem to arrive late.
"If we do see this as predicted by general relativity, then this would be the first direct detection of gravitational waves," he says. "To do this of course we need to time these pulsars extremely precisely."
Hobbs points out that five years ago the technology was not advanced enough to time pulsar signals to the same level as atomic clocks.
"We can now measure these pulses to 100 nanoseconds or so," he says. "Atomic clocks are more stable on a short time span but over months or years, they are not as stable as pulsars."
University of Texas pulsar scientist Fredrick Jenet, who also is using precision timing of pulsars to detect gravity waves says, "Pulsars are not affected by the same things that affect clocks on our planet. It is only by comparison that we understand the strengths and weaknesses of our own time".
Warrington believes pulsars may make a useful contribution to atomic clock timescales over long periods of time, as even the best clocks drift.
"However atomic clocks are still needed on short timescales that people are used to."
Full article here:
http://www.abc.net.au/science/articles/2008/12/18/2450349.htm?site=science