Keeping precise time on the universe’s scale By Charles Petit
Time is an ancient and contrary mystery. Augustine of Hippo, writing his Confessions in a North African monastery, asked
“Who can even in thought comprehend
it, so as to utter a word about it? But what
in discourse do we mention more famil-
iarly and knowingly, than time?”
More than 16 centuries later, many
scholars share the feeling, if not the pros-
pect of sainthood. “We don’t even know
what time is. But we can measure it really,
really well,” says Chris Oates, a physicist
at the National Institute of Standards
and Technology’s Boulder, Colo., campus.
In what’s called an “optical lattice
clock,” thousands of atoms are held in
wells made of laser light. Oscillations
of the atoms’ electrons keep the time.
Speedy metrology
Such accuracy is why time is not just one
dimension among several but a foundation for defining other fundamental
units. The meter’s definition has been
defined with increasing accuracy by
such things as one ten-millionth the distance on a circular arc from the equator
to the North Pole, and by a precision-made “prototype meter” bar of metal
alloy kept in Paris. In 1983 the meter
officially became the distance light will
travel in a vacuum in 1/299,792,458 of
a second. The better the stopwatch, the
FROM TOP: NIST; PAUL CLEMENTS
A laser jubilee
From the outside, a
lattice clock appears to
be a dizzying array of
lasers, but (as shown
in one type of strontium
clock at right) each
laser has a role in
reading the time from
atomic oscillations.
SOURCE: NIS T
Thousands of strontium atoms are
cooled by a system of blue lasers.
Red lasers further cool and shrink
the cloud of atoms.
An infrared laser system traps
the atoms, locking them into
pancake-shaped wells.
