Sunday, February 12, 2023

3873 - ATOMIC CLOCKS - Measuring time precisely!

 

-  3873  -    ATOMIC  CLOCKS  -  Measuring time precisely!  Ultraprecise atomic optical clocks may redefine the length of a second. The length of a second hasn't been updated in 70 years.    A strontium lattice optical atomic clock is an ultraprecise optical clock that may redefine the most fundamental unit of time in the next decade.

------------  3873  -   ATOMIC  CLOCKS  -  Measuring time precisely!

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-   The definition of a second, the most fundamental unit of time in our current measurement system, hasn't been updated in more than 70 years , give or take some billionths of a second.  But,  in the next decade that could change: Ultraprecise atomic optical clocks that rely on visible light are on track to set the new definition of a second.

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-    These newer versions of the atomic clock are, in theory at least, much more precise than the gold-standard cesium clock, which measures a second based on the oscillation of cesium atoms when exposed to microwaves. ( Review on this clesium clock is available upon request)

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-    The new type of optical clock could help unmask dark matter, the invisible substance that exerts gravitational pull; or find remnants of the Big Bang called gravitational waves, the ripples in space-time predicted by Einstein's theory of relativity.

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-    The current standard second is based on a 1957 experiment with an isotope, or variant, of cesium. When pulsed with a specific wavelength of microwave energy, the cesium atoms are at their most "excited" and release the largest possible number of photons, or units of light.

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-  That wavelength,  the natural resonance frequency of cesium, causes the cesium atoms to "tick" 9,192,631,770 times every second. That initial definition of a second was tied to the length of a day in 1957 , and that, in turn, was linked to variable things, such as the rotation of Earth and the position of other celestial objects at that time.

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-    In contrast, optical atomic clocks measure the oscillation of atoms that "tick" much faster than cesium atoms when pulsed with light in the visible range of the electromagnetic spectrum. Because they can tick much faster, they can define a second with much finer resolution.

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-    There are multiple contenders to supplant cesium as the reigning timekeeper, including strontium, ytterbium and aluminum. Each has its pluses and minuses.

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-   To achieve such clocks, researchers must suspend and then chill atoms to within a hair's breadth of absolute zero, then pulse them with the precisely tuned color of visible light needed to maximally excite the atoms. One part of the system shines the light on the atoms, and the other counts up the oscillations.

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-   The biggest challenges is in making sure the laser is emitting the exact right color of light, a certain shade of blue or red, needed to kick the atoms into their resonant frequency. The second step is to count the oscillations. This requires a femtosecond laser frequency comb, which sends pulses of light spaced at tiny intervals.

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-  Time does not simply march to its own drum; Einstein's theory of relativity says it is warped by mass and gravity. As a result, time may tick infinitesimally more slowly at sea level, where Earth's gravitational field is stronger, than at the top of Mount Everest, where it is ever-so-slightly weaker.

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-    Detecting these minute changes in the flow of time could also reveal evidence of new physics. For instance, dark matter's influence has so far been detected only in the distant dance of galaxies circling one another, from the bending of light around planets and stars, and from the leftover light from the Big Bang.

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-     If clumps of dark matter lurk closer to home, then ultraprecise clocks that detect the tiny slowing of time could find them.   Similarly, as gravitational waves rock the fabric of space-time, they squish and stretch time.

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-     Some of the biggest gravitational waves are detected by the Laser Interferometer Gravitational-Wave Observatory, a several-thousand-mile relay race for light that measures blips in space-time created by cataclysmic events such as black hole collisions.

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-    A battalion of atomic clocks in space could detect these time dilation effects for much slower gravitational waves, such as those from the cosmic microwave background.

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-   These ultraprecise atomic clock experiments confirm Einstein's predictions about time.   To create the optical atomic clocks, researchers cooled strontium atoms to near absolute zero inside a vacuum chamber. The chilling caused the atoms to appear as a glowing blue ball floating in the chamber.

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-   Using these precise atomic clocks, physicists have shown that time runs a tiny bit slower if you change your height above the Earth's surface by a minuscule 0.008 inch (0.2 millimeters) — roughly twice the width of a piece of paper. The finding is yet another confirmation of Albert Einstein's theory of relativity, which predicts that massive objects, like our planet, warp the passage of time and cause it to slow down.

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-    In 1915, Einstein showed that anything with mass will distort the fabric of space-time, an effect we experience as the force of gravity. You can think of gravity as putting the brakes on the flow of time. This mind-bending idea means that clocks nearer to Earth run slow compared with those farther from it, a phenomenon called “time dilation”.

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-    Super-precise atomic clocks flown on airplanes run appreciably faster than those on the ground.   In 2010, scientists set a new record by measuring the passage of time with two aluminum-based atomic clocks separated in height by about 1 foot (33 centimeters), finding that the higher one ran slightly faster.

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-   This latest measurement is about a factor of 1,000 better.  This experiment used a collection of roughly 100,000 atoms of the isotope strontium 87, which is often used in atomic clocks, cooled to a fraction of a degree above absolute zero and placed in a structure known as an optical lattice.

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-   An optical lattice uses intersecting beams of laser light to create a landscape of peaks and valleys resembling an egg carton, where each atom is cradled in one of the valleys.

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-    Each strontium atom oscillates back and forth, ticking by itself inside its valley 500 trillion times per second, like the pendulum of a microscopic grandfather clock, allowing the team to measure fractions of a second to an incredible 19 decimal places.

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-    The strontium atoms in the optical lattice were arranged in many layers, kind of like a stack of pancakes. By shining a laser on the layers they could measure how quickly the atoms in each layer ticked.   As you go from top to bottom, you see each layer dancing a little differently thanks to gravity.

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-    The strontium atoms are capable of being placed in what's known as a superposition of states, meaning two states at once.    According to quantum mechanics, particles can exist in two locations (or states) at once, so future experiments might place a strontium atom in a superposition where it is located in two different "pancakes" at the same time.

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-   With the particle in both places at once, the team could then measure the passage of time at different points along the superpositioned strontium atom, which would change thanks to the different gravitational force it feels. This should show that "at one end of the particle, time is running at one speed. And,  at the other end, it's running at a different speed.

 

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-    This incredibly bizarre possibility gets at the heart of the difference between the quantum and classical worlds. Classical objects, like tennis balls and people, can't exist in superpositions where they are located in two places at once. But where the switchover between quantum and classical happens is unclear. By increasing the distance between the pancakes, researchers could essentially make the particle grow larger and larger and potentially see when it stops behaving like a quantum particle and more like a classical one.

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-    Such experiments may allow physicists to get closer to a long-sought dream, a theory of everything that would unify Einstein's theory of relativity, which describes the very large, with quantum mechanics, which describes the very small.

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-   The current experiment has helped the team envision ways to produce atomic clocks that are even more precise. Future instruments could be used to measure tiny differences in the mass of the Earth beneath them, potentially making the clocks useful for detecting the flow of magma inside volcanoes, changes in meltwater inside glaciers or the movement of our planet's crustal plates.

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            February 12, 2023        ATOMIC  CLOCKS  -  Measuring time precisely!    3873                                                                                                                       

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--------------------- ---  Monday, February 13, 2023  ---------------------------

 

 

 

 

         

 

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