61The lutetium-based atomic clock at the National University of Singaporeradiation. So if the number of cycles
of radiation necessary to stimulate an
atom can be counted during the best
existing non-atomic definition of a sec-
ond, the world would have a new defi-
nition of the second grounded in the
unchanging atom. In effect, it’d have an
atomic clock.
Over a period of almost two decades,
scientists determined that cesium,
whose vibrations are comparatively
easy to observe and which exists in
only one stable form, and thus wouldn’t
need to be purified, was the best ele-
ment to use. In 1967, the General
Conference on Weights and Measures,
an intergovernmental organization,
replaced the old celestial definition
of a second with the amount of time it
takes to measure 9,192,631,770 oscilla-
tions of the microwave required to stim-
ulate vibration of cesium-133.
Even before the atomic second was
defined, there were scientific, military,and commercial ventures deeply inter-
ested in taking advantage of more pre-
cise timing. The U.S. military was one
of the earliest patrons of atomic clocks,
and in 1973 the armed forces intro-
duced the global positioning system,
a super-accurate navigation network
that uses satellites outfitted with atomic
clocks to measure how long it takes a
signal to reach and bounce back from
a receiver on Earth.
Missiles and other military tech-
nologies were the immediate bene-
ficiaries of GPS. The private sector
benefited, too, from Google Maps to
modern telecommunication networks
that synchronize using GPS. According
to a June 2019 study sponsored by the
National Institute of Standards and
Technology (NIST) in Colorado, GPS
was responsible for $1.35 trillion in eco-
nomic benefits to the U.S. from 1984 to- China, Europe, and Japan have
their own satellite navigation systems
relying on atomic clocks; a collective
outage, as unlikely as that is, would
cripple the global economy.
For nearly two decades scientists
around the world have been working
on improved atomic clocks that stim-
ulate atoms using visible light, which
oscillates roughly 100,000 times faster
than the microwaves in a cesium clock.
Creating these optical clocks isn’t just a
matter of switching out microwaves for
lasers and cesium for another element.
Instead, scientists must overcome a
range of scientific and technical chal-
lenges, including the fundamental ques-
tion of which element is best suited for
the work of telling time.
There’s no lack of candidates, includ-
ing aluminum, mercury, and strontium.
For now, the front-runner appears to
be ytterbium, which is being investi-
gated by large, well-funded teams at
NIST and labs in Europe and Asia. In late
2018, NIST announced that a pair ofBloomberg Businessweek / SEPTEMBER 2, 2019 THE ELEMENTS