--------- #1341 - Optical Lattice Clocks Tell Time, Exactly
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- We all need to punctual in this fast and furious world we live in. But, do we need a clock that would be within one second accuracy over the age of the Universe. There is such a clock under development. This clock would not loose more than one second in 30 billion years. How?
- The new clock is called an “optical lattice clock”. It works using laser beams to capture atoms and measure their vibrations at near Absolute Zero temperatures. The accuracies attained are 1 second in 10^18 seconds.
- The first clocks invented in the 1100’s were Chinese Water Clocks. A steady stream of water was used to turn wooden gears and obtain accuracies of about 10 minutes per day. This is accuracies of 600 seconds in 86,400 seconds. Or, 1 second in 144 seconds. ( 1 second in 10^2 seconds).
- Pendulum clocks came along in the 1600”s that were accurate to (1 second in 1,000 seconds).
- The first navigational chronometers in 1700’s were accurate to ( 1 second in 10,000 seconds). ( 1 second in 10^4).
- The quartz clock in the 1920’s used the vibrations of a crystal oscillating at 32,768 times per second. That gave clock accuracies to ( 1 second in 10^5).
- The first cesium atomic clock in 1949 attained accuracies of ( 1 second in 10^10).
- A modern version cesium fountain atomic clock has accuracies of ( 1 second in 10^15).
- The Optical Lattice Clock that you will learn about has accuracies of ( 1 second in 10^18). The Universe is 13.7 billion years old.
----------------- 13.7 * 10^9 years * 365 days * 24 hours * 60 minutes * 60 seconds = 4.3 * 10^17 seconds.
--------------- This Lattice Clock if started with the Big Bang would not have lost one second in the last 13.7 billion years. Now, that is punctual. Actually, that statement is too conservative. Really the clock should be accurate to 1 second in 10^18, which is 32 billion years.
- Accurate clocks do not just tell time. They can be used to create standards for many other types of measurements. For example: The GPS in your car now measures distances with triangulation to an accuracy of 10 feet. This is done measuring the time it takes for a radio signal to get to a couple of satellites. A change in time is a change in distance. Once these satellites have 10^18 clocks installed they can measure your changes in distance down to mere inches. Not just on Earth. This could be done on the surface of Mars with satellite receivers here on Earth.
- Time is now used to define the distance of 1 meter. A meter started out as being defined as 1 ten-millionth of the distance between the equator and the North Pole. Later that distance became the length of a metal bar stored in a vault in Paris, France. In 1983 the distance of 1 meter became the distance light could travel in a vacuum in 1 / 299,792,458th of a second.
- With these accuracies science can measure the shifts in distances due to the gravity changes as a satellite circles the Earth. This data can measure the interior of the Earth to locate mineral deposits. These accuracies can test natural constants like those affecting nuclear decay. And, many more applications to follow.
- The U.S. standard time clock in Bolder, Colorado, uses a laser-controlled fountain of cesium atoms cooled to near Absolute Zero. The atoms rise in clumps and fall back through a tunable microwave cavity. The microwave signal is tuned to the oscillations of the cesium atoms. The microwave signal gets an error voltage feedback that keeps it tuned to exactly 9,192,631,770 oscillations per second. This clock is accurate to ( 1 second in 10^15).
- The U.K. has a modified version of this cesium clock that is accurate to ( 1 second in 10^16).
- The clock that will get us to ( 1 second in 10^18) uses a laser controlled lattice design that captures the oscillations of single atoms. The clock is constructed as a sphere of lasers aimed at a target of mercury, or strontium, atoms. Each set of lasers has a separate mission to accomplish. One set of lasers is used to slow the motion of atoms. This cools the atoms down to 1 millionth of a degree above Absolute Zero.
- Another set of lasers has cross beams that interfere with each other and create a lattice of standing waves. The lattice is like an egg carton with depressed nodes that trap a single atom in an energy well.
- Inside a vacuum that is only 100 micrometers thick other lasers are tuned to match the frequency at which the atom most easily absorbs and emits a photon of light. This light flicker is then locked to a laser optical comb. The comb laser has harmonics that spread over a wide frequency range. A lower harmonic in the microwave frequencies is locked to a microwave signal that is then synchronized to the clock laser. The clock will then stay accurate to ( 1 second in 10^18).
- In 2007 the first strontium based lattice clock was demonstrated to have an accuracy of ( 1 second in 10^16).
- In 2020 science plans to have a 10^18 Space Optical Clock installed in the International Space Station.
- Optical Lattice Clocks need not stop there. In the future lattice clocks could be built using ultraviolet of even X-ray light that would be more accurate than visible light lasers.
- Lattice Clocks may be designed to not just work off of electrons circling the atoms but of the vibrations of the nucleus neutrons and protons. Maybe a thorium -229 isotope nuclei that emits an ultraviolet photon can be synchronized to a laser frequency comb and fed into an electronic counter that would be the “ultimate clock“. What on Earth would we do with that accuracy? An announcement will be made shortly, stay tuned.
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