48 The last word
THE WEEK 7 May 2022
Modern civilisation, it
is said, would be
impossible without
measurement.
And measurement
would be pointless if
we weren’t all using
the same units. So, for
nearly 150 years, the
world’s metrologists
have agreed on strict
definitions for units
of measurement through
the International Bureau
of Weights and
Measures, known by its
French acronym, BIPM,
and based outside Paris.
Nowadays, the bureau
regulates the seven base
units that govern time,
length, mass, electrical
current, temperature,
the intensity of light and
the amount of a substance. Together, these units are the language
of science, commerce and technology. Scientists are constantly
refining these standards. In 2018, they approved new definitions
for the kilogram (mass), ampere (current), kelvin (temperature)
and mole (amount of substance). Now, with the exception of
the mole, all of the standards are subservient to one: time.
The metre, for example, is
defined as the distance light
travels in a vacuum during
one-299,792,458th of a second.
That means that conceptually,
if clumsily, you could express
other units, such as weight or
length, in seconds. “You go to the grocery and say, ‘I would like
not one kilogram of potatoes, but an amount of seconds of
potatoes,’” said Noël C. Dimarcq, a physicist and the president
of the BIPM’s consultative committee for time and frequency.
Yet now, for the first time in more than half a century, scientists
are in the throes of changing the definition of the second, because
a new generation of clocks is capable of measuring it more
precisely. In June, metrologists with the BIPM will have a final list
of criteria that must be met to set the new definition. Dr Dimarcq
said he expected that most would be fulfilled by 2026, and that
formal approval would happen by 2030.
It must be done carefully. The architecture of global measurement
depends on the second, so when the unit’s definition changes, its
duration must not. “It’s like a once-in-every-50-year thing,” said
Elizabeth A. Donley, chief of the time and frequency division of
the National Institute of Standards and Technology, or NIST, in
Boulder, Colorado. She is on the BIPM’s international consultative
committee with Dimarcq. “It’s exciting to work on, for sure.”
Once, humans told time by looking at the heavens. But since
1967, metrologists have defined time instead by measuring what’s
going on inside an atom
– clocking, as it were,
the eternal heartbeat of
the universe. But time
still has its roots and
even its nomenclature
in astronomical
timekeeping. Originally,
it was based on the
path of Earth in its
daily spin. Eventually,
ancient Egyptian
astronomers who
used the duodecimal
counting system, based
on 12, divided the day
and night into 12 hours
each, which varied
according to season
and latitude.
A little more than
2,000 years ago, Greek
astronomers developed
the revolutionary idea
that a single day ought to be divided into 24 hours of the same
length. That thinking led them to combine the ancient Babylonian
method of counting by 60, the sexagesimal system, with the hour.
Just as they divided the 360 degrees of a circle or the sphere of
Earth into 60 parts, or minutes, they then divided each minute
into 60 seconds. The first division of the day’s 24 hours gave them
the length of the minute, which was one-1,440th of an average
solar day. The second division
provided them with the duration
of the second, which was
one-86,400th of a day.
That definition stood, in effect,
until 1967. But the definition
had problems. Earth is gradually slowing in its rotation; days are
growing slightly longer and so the astronomical second is too.
The differences add up: Earth as a clock has lost more than three
hours over the past 2,000 years. Therefore, the standard unit of
time isn’t constant, a reality that became increasingly intolerable
for metrologists during the first decades of the 20th century as
they discovered just how irregular Earth’s spin was. Science
demands constancy, as does time – and by the late 1960s, society
was becoming increasingly reliant on the frequencies of radio
signals, which demanded extremely precise timings.
Metrologists turned to the more predictable movement of
atomic particles. Atoms never wear out or slow down. Their
properties do not change over time. They are the perfect
timepieces. By the middle of the 20th century, scientists had
coaxed atoms of caesium 133 into divulging their secret inner
ticks. Caesium, a silvery-gold metal that is liquid at about room
temperature, has heavy, slow atoms. Scientists put caesium atoms
in a vacuum and exposed them to the energy of microwaves. The
task was to figure out which wavelength, or frequency, would
excite as many caesium atoms as possible into emitting a packet
of light, or photon. The photons were picked up by a detector and
counted. The wavelength that won the contest was designated
Redefining time: get ready for
the new, improved second
The caesium resonator: formed the basis of the standard second
Human beings once decided what time it was by gazing at the heavens. Now, for the first time in more than half a century,
scientists around the world are preparing to redefine the fundamental unit of time: the second. Alanna Mitchell reports
“The planet is slowing in its rotation. The
differences add up: Earth as a clock has lost
more than three hours in the past 2,000 years”
