better such definitions can be applied.
The metrology of time is not holding
still. In the April-June issue of Reviews
of Modern Physics, experimental physicist Hidetoshi Katori of the University of
Tokyo and theorist Andrei Derevianko
of the University of Nevada, Reno
declared dramatic ambitions for a
record-breaking atomic clock based on
emissions from mercury atoms.
“If someone built such a clock at the
Big Bang and if such a timepiece survived
the 14 billion years, then the clock would
be off by no more than a mere second,”
they note in the paper. That is actually
conservative. The goal formally is to lose
or gain no more than one out of every billion billion seconds. That is one second
in about 32 billion years, and is 10 to
100 times better than any existing clocks.
In scientific shorthand, the proposed mercury clock would reach
a fractional uncertainty of at most
one part in 1018—it would run for
1,000,000,000,000,000,000 seconds
before being one second awry.
Already, atomic clocks have come a
long way. While experimental clocks are
moving ahead, a device called the NIST-
F1 is the official U.S. timekeeper. It’s accurate to a few parts in 1016. It occupies a
large first floor room in Building One at
NIST’s Boulder campus. The dominant
feature is a shiny steel vacuum chamber
8 feet high. Inside is a laser-controlled
fountain of cesium atoms chilled to near
absolute zero. The atoms rise in clumps
about as large as a man’s thumb and,
responding to gravity, fall back through a
cavity in a tunable microwave generator.
Oscillations within the cesium atoms
are akin to the to-and-fro of the balance
wheel in an old wristwatch, but it is the
microwave generator that communicates with the outside world. Just as the
ticking of a watch arises from the escapement mechanism connected to the gears
and hands, oscillations within the cavity
are recorded electronically.
By itself, the microwave generator
would drift off time. So with each passage of the atoms, the generator checks to
be sure its pulsations exactly match the
signal from a chosen energy transition in
the atoms’ electron clouds — an electromagnetic wave that beats 9,192,631,770
times a second.
NIST is now working on a successor,
called F2. With an improved cooling
system and superior way of moving the
atoms through the microwave chamber,
it will be about four times better and will
beat out the current record for a long-term timekeeper, a clock in the United
Kingdom that is accurate to about two
parts in 1016.
Such astonishing accuracy is no mere
intellectual exercise. Recent advances
in timekeeping have brought practi-
cal payoffs in the design of better global
positioning systems that triangulate loca-
tions on Earth by measuring distance via
radio-signal travel time, as measured by
satellite-borne atomic clocks. Further
progress should lead to instruments able,
from the slowing or speeding of time’s
passage due to shifts in gravity, to improve
maps of the planet’s interior and to find
mineral deposits or detect the move-
ments of deep magma. Pure research on
Earth and in space may gauge to almost
unimaginable exactness the stability (or
drift) of supposed constants of physics
that not only affect nuclear decay, but
also, some astronomers say, may have
worked differently in distant eras.
Ticking gears
By historic standards, clock progress is
now frenzied.
People have long kept track of time by
monitoring processes that change measurably in a steady way. Early peoples
monitored the seasons by the motion
of the sun and moon. An 11th century
Chinese water clock, its gears driven
by a steady stream, might lose or gain
10 minutes a day — an accuracy of about
a part in 100. Large, stable swinging pendulums in the 1600s were good to a few
seconds a day. Eighteenth century navigation clocks that were the pride of the
British Navy weren’t much better. They
lost or gained a minute or less per month,
an accuracy of about one in 10,000, but
they did it while tossing about in ships at
sea. Quartz clocks and watches, paced by
electrically stimulated crystals vibrating
at about 32,768 times per second, were
developed in the late 1920s. They keep
time to within a second per day, better
than a part in 105.
Then along came atomic clocks, following the beat of quantum mechanics, the
laws that govern the energies of electrons
bound to nuclei. Every 10 years since the
first one debuted in 1949, based on oscillations in the ammonia molecule, the
accuracy has increased by about 10 times.
Recently, things have gone even faster.
Another red laser, the “clock”
laser, bathes the atoms, causing
many of them to become excited.
A blue laser probes the cloud;
emitted light reveals how many
atoms became excited.
The clock laser adjusts to properly keep pace based on the
atoms’ excitations. Light from the clock laser is passed through
a “frequency comb,” allowing the light’s ticks to be read.
