Who decides how long a second is John Kitching
In 1967, researchers from around the world
gathered to answer
a long-running scientific question—
just how long is a second?
It might seem obvious at first.
A second is the tick of a clock,
the swing of a pendulum,
the time it takes to count to one.
But how precise are those measurements?
What is that length based on?
And how can we scientifically define
this fundamental unit of time?
For most of human history,
ancient civilizations
measured time with unique calendars
that tracked the steady march
of the night sky.
In fact, the second as we know it wasn’t
introduced until the late 1500’s,
when the Gregorian calendar
began to spread across the globe
alongside British colonialism.
The Gregorian calendar defined a day
as a single revolution of the Earth
about its axis.
Each day could be divided into 24 hours,
each hour into 60 minutes,
and each minute into 60 seconds.
However, when it was first defined,
the second was more of a mathematical
idea than a useful unit of time.
Measuring days and hours was sufficient
for most tasks in pastoral communities.
It wasn’t until society became
interconnected
through fast-moving railways that cities
needed to agree on exact timekeeping.
By the 1950’s, numerous global systems
required every second
to be perfectly accounted for,
with as much precision as possible.
And what could be more precise
than the atomic scale?
As early as 1955, researchers began
to develop atomic clocks,
which relied on the unchanging laws
of physics
to establish a new foundation
for timekeeping.
An atom consists of negatively charged
electrons
orbiting a positively charged nucleus
at a consistent frequency.
The laws of quantum mechanics
keep these electrons in place,
but if you expose an atom
to an electromagnetic field
such as light or radio waves,
you can slightly disturb an electron’s
orientation.
And if you briefly tweak an electron
at just the right frequency,
you can create a vibration
that resembles a ticking pendulum.
Unlike regular pendulums that quickly lose
energy, electrons can tick for centuries.
To maintain consistency and make ticks
easier to measure,
researchers vaporize the atoms,
converting them to a less interactive
and volatile state.
But this process doesn’t slow down
the atom’s remarkably fast ticking.
Some atoms can oscillate
over nine billion times per second,
giving atomic clocks an unparalleled
resolution for measuring time.
And since every atom of a given
elemental isotope is identical,
two researchers using the same element
and the same electromagnetic wave
should produce perfectly
consistent clocks.
But before timekeeping could
go fully atomic,
countries had to decide which atom
would work best.
This was the discussion in 1967,
at the Thirteenth General Conference
of the International Committee
for Weights and Measures.
There are 118 elements
on the periodic table,
each with their own unique properties.
For this task, the researchers were
looking for several things.
The element needed to have long-lived
and high frequency electron oscillation
for precise, long-term timekeeping.
To easily track this oscillation,
it also needed to have a reliably
measurable quantum spin—
meaning the orientation of the axis
about which the electron rotates—
as well as a simple energy
level structure—
meaning the active electrons are few
and their state is simple to identify.
Finally, it needed to be easy to vaporize.
The winning atom? Cesium-133.
Cesium was already a popular
element for atomic clock research,
and by 1968, some cesium clocks
were even commercially available.
All that was left was to determine
how many ticks of a cesium atom
were in a second.
The conference used the most
precise astronomical measurement
of a second available at the time—
beginning with the number of days
in a year and dividing down.
When compared to the atom’s ticking rate,
the results formally defined one second
as exactly 9,192,631,770 ticks
of a cesium-133 atom.
Today, atomic clocks are used all over
the Earth— and beyond it.
From radio signal transmitters
to satellites
for global positioning systems,
these devices have been synchronized
to help us maintain a globally
consistent time—
with precision that’s second to none.