The 10 Best Atomic Clocks
The Invention Of The Atomic Clock
The concept of measuring time by atomic transition was first proposed in 1879 by Lord Kelvin, but the technology wasn't yet on par with philosophy. It wasn't until over 50 years later when magnetic resonance was developed by Isidor Rabi, that an actual method for doing this emerged.
In 1945, he first publicly proposed that atomic beam magnetic resonance could be used for timekeeping. Just four years later in 1949, a functioning atomic clock was built at the U.S. National Bureau of Standards, which is currently known as the National Institute of Standards and Technology. It was an ammonia maser device and it was actually less accurate that existing quartz time clocks. It did demonstrate the feasibility of the concept though and promoted further research.
The first atomic clock that tracked the passage of time accurately, was built in 1955 by Jack Parry and Louis Essen. It was constructed at the National Physical Laboratory in the U.K. and used a caesium standard, which was based on a transition of the caesium-133 atom. The astronomical time known as ephemeris time (ET) was used to calibrate the caesium standard atomic clock because, at the time, it was the most accurate fundamental unit of time.
Basing the atomic clock on the caesium-133 atom led to the internationally agreed definition of the SI second. It being the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two energy levels of the caesium-133 atom.
The Science Behind An Atomic Clock
The trick to counting time is by tracking the intervening span of time between something that occurs repeatedly with little, or preferably no, variation. Unfortunately, even the most precise quartz-crystal based clocks and mechanical pendulums will have slight discrepancies.
For this reason, the best method of keeping time is tracking the naturally occurring and exact vibrations in an energized atom. If exposed to specific radiation frequencies, electrons that orbit an atom's nucleus oscillate back and forth between different energy states. This is the basis for how atomic clocks track the passage of time.
Following the laws of quantum physics, all atoms emit or absorb electromagnetic energy when changing states. For every atom of a given element, all resonant emission frequencies are identical. Since caesium-133 atoms radiate energy at a fixed known frequency, their magnetic resonance can be used as a reference for tracking time in an incredibly accurate manner.
Inside of an atomic clock, caesium-133 atoms are sent down a tube and pass through radio waves transmitting at 9,192,631,770 cps. This causes them to resonate or vibrate and change to a new energy state. A detector at the other end of the tube is then used to track the number of atoms that have changed their energy state. The more tuned the frequency is to 9,192,631,770 cps, the more resonating caesium-133 atoms reach the detector. This information is then fed back into the radio wave generator and it synchronizes the radio waves frequency to match the one that created the highest number of caesium atoms striking the detector. This frequency is then counted and a second will be ticked off when the correct frequency count is met.
Though we have different types of atomic clocks these days, it doesn't matter whether they are based on hydrogen atoms, caesium atoms, or rubidium gas, the basic principal of how they work remains the same.
The Most Accurate Atomic Clock In The World
Atomic clocks have been incredibly accurate since they were first perfected in 1955, but scientist have worked hard since then to produce more and more accurate versions. The most recent record-breaking atomic clock doesn't lose even lose or gain one second in 15 billion years - which is the estimated age of the universe.
This new clock was developed at the Joint Institute for Laboratory Astrophysics (JILA) and is based on the oscillation of strontium atoms. It roughly three times more precise than the previous record holder, which wouldn't have lost or gained a second over a 5 billion year period. It is expected that this strontium-based atomic clock may one day become the standard on which the world's official time is based.
There have been previous strontium-based atomic clocks, but the newest one has improved on them by eliminating measurement errors resulting from external sources of electromagnetic radiation by placing radiation shields around the device. They have also placed platinum thermometers inside of the vacuum tube to account for the extra heat generated. Unlike previous versions that could only be operated at cryogenic temperatures, this new one functions at room temperature. The JILA clock is also precise enough to reveal the tiny shifts in time that were predicted by Einstein's theory of relativity.