- 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.
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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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