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as the natural resonance frequency of the
atom. Think of it as a pendulum operating
in a rhythm unique to that type of atom.
In the case of caesium 133, the frequency
is nearly 9.2 billion ticks per second. The
length of the second used in the experiment
was based on the length of the day in
1957 when the experiments were taking
place, and was derived from measurements
of Earth, the Moon and stars. By 1967,
metrologists at the BIPM had set the
natural resonance frequency of caesium
133 as the official length of the second.
Despite that caesium-based definition,
astronomical time and atomic time are
still inextricably conjoined. For one thing,
atomic time occasionally needs to be adjusted to match
astronomical time, because Earth continues to change its pace at
an irregular rate, whereas atomic time remains constant. When
atomic time gets nearly one second faster than astronomical time,
the timekeepers stop it for a moment, allowing Earth to catch up
- they insert a leap second in the year. So while the duration of the
second doesn’t change, the duration of a minute occasionally does.
After an initial insertion of ten leap seconds in 1972, timekeepers
now add a leap second to atomic time roughly every year and a
half. In addition, as weird as it may seem, we still tick through
1957-era seconds, even with our modern atomic clocks. That’s
because the natural resonance frequency of caesium 133 was
measured in 1957 and locked to the duration of the astronomical
second in that year, a fact that will not change even when the
second is redefined once more.
The redefinition is in the works
because scientists have developed
new instruments called optical
atomic clocks. These operate
on similar principles to caesium
clocks, but measure atoms that
have a much faster natural resonance frequency, or tick. There
are several species of optical clock, each counting the ticks of a
different atom or ion – ytterbium, strontium, mercury, aluminium
and more. So far, no species has emerged as the clear favourite for
the upcoming redefinition. “Optical clocks are very definitely not
ready for prime time,” said Judah Levine, a physicist at NIST.
“They are laboratory projects.” For one thing, although they are
built to examine such tiny atoms, most are massive. Some fill
a laboratory. They are also difficult to operate.
“It requires a whole bunch of specialists who are chained to the
table, if you know what I mean,” Dr Levine said. “It’s not, just,
push a button and walk away.” In all, about 20 or 30 optical
atomic clocks of all species exist today, Dr Donley said. Three
are in Boulder. A typical one is settled on a steel slab to isolate it
from floor vibrations. It is shielded from disturbances in Earth’s
magnetic field. At its heart is a vacuum chamber about a foot in
diameter, containing whichever atom or ion is under scrutiny.
Lasers are mounted on the sides of the table. They chill the atoms
or ions to near absolute zero. Then the lasers probe the atoms or
ions, beaming a nearly pure colour of light on them that scientists
tune to find the precise wavelength that will elicit the desired tiny
shift in energy. “Just as a child only achieves great height on a
playground swing if her parent’s pushes arrive at the right rhythm,
the atoms become detectably excited only if the laser colour is
tuned perfectly,” Jeffrey A. Sherman, a physicist in NIST’s time
realisation and distribution group, told me by email.
The trick is then to be able to read the laser’s colour in order to
determine the precise frequency of the wave that elicits the shift in
energy. And this is where the optical atomic clock’s secret weapon
kicks in. A key component of the clock is a second type of laser
called a femtosecond-laser frequency comb, the discovery of
which led to a Nobel Prize in Physics in- It is a pulsed laser, equivalent to a
series of spikes of light spaced by precisely
the same amount, like the teeth of a hair
comb. This comb of light can read the
wavelengths of the lasers that are exciting
the atoms or ions. The waves are fast,
moving at rhythms, or frequencies, some
100,000 times that of the microwave
energy that excites caesium. This enables
optical atomic clocks to measure time
far more precisely than caesium clocks.
Why do we need such precision? Partly
because time is not just time; it is tied
to, and influenced by, gravity and mass.
Nor is time constant, despite what the
existence of an international standard
might suggest. Albert Einstein’s theory of general relativity, for
example, predicts that time moves more slowly when it is near a
massive body, such as a planet, because it is slowed by gravity’s
pull. That means that if the tick of a clock changes, even slightly,
the physical conditions in which the clock is situated may have
changed, too. Being able to read these changes opens the
possibility that the clocks could detect such entities as dark
matter or gravitational waves, Dr Donley said.“They’re very exquisite tests of fundamental physics, which is
one of the exciting things about optical clocks,” she said. One
experiment has already taken place. In 2015, physicists at NIST
were in the early days of developing their optical atomic clocks.
They were puzzled by the fact that the seconds were measuring
slightly differently across the clocks, which were in labs spread
throughout Boulder. Then they
thought about the theory of
general relativity. Could these
optical clocks be responding
to slight changes in gravity?They asked Derek van Westrum,
a physicist at the National Geodetic Survey, to investigate. In 2015
and 2018, Dr van Westrum measured height differences among
the labs where the clocks were stationed. Like time, height is
linked to gravity and mass. He found that the clocks were at
different heights, and that their slightly different measurements of
time were capturing minuscule changes in the gravitational field.
A clock just one centimetre higher than another ran faster. “That
Einstein’s crazy prediction of what mass and gravity do to time
would actually have a practical application, to me is just
incredible,” Dr van Westrum said, chuckling.If several optical atomic clocks could be placed in different
parts of the world, geodesists could measure ticking differences
between them, and therefore differences in height and the
gravitational field, he said. For example, a network set up near
a flooding river could explain where the water would flow and
identify escape routes for residents.Such possibilities lie in the future. Today, physicists are still trying
to make optical clocks talk to one another over distances. A recent
experiment published in the journal Nature last year linked the
three clocks in Boulder through both optic fibre and air. And
scientists are looking once more to the heavens for help. Now,
though, it’s not to track the movements of planets or stars, but to
use information from far beyond our galaxy. Researchers in Italy
and Japan recently tried to link two optical atomic clocks about
5,500 miles apart. The experiment involved several antennas
reading radio signals from distant outer space, and then linking
the information to atomic clocks. It worked, and for a moment
time and space merged, mediated by the stars.A longer version of this article appeared in The New York Times
© The New York Times CompanyThe last word
7 May 2022 THE WEEKThe world’s most precise and stable atomic clock“The redefinition of the second is in the
works because scientists have developed new
instruments called optical atomic clocks”