MATTER & ENERGY
It’s official: Kilogram will be redefined
Measurement system will soon rely on fundamental constants
BY EMILY CONOVER
Out with the old — kilogram, that is.
Scientists will soon ditch a specialized
hunk of metal that defines the mass of
a kilogram. For years, every measurement of mass made anywhere on Earth
has been tied back to this one cylindrical object. Known as “Le Grand K,” the
cylinder, cast in 1879, is kept carefully
sequestered in a secure, controlled environment outside of Paris.
On November 16, at a session of the
26th General Conference on Weights
and Measures in Versailles, France, rep-
resentatives of countries from around
the world voted to kick that convoluted
system to the curb, enacting a plan to
redefine several units of measurement
(SN: 11/12/16, p. 24).
For a small cadre of scientists called
metrologists — researchers who specialize in the science of measurement — the
news is big. “It’s about as
excited as you’re going
to see metrologists get,”
says David Newell of the
National Institute of
Standards and Technology
in Gaithersburg, Md. He
has spent much of his
career working toward the change. “I
can’t believe we’re finally getting it
done,” he says.
On May 20, 2019, Le Grand K will lose
its special status. The mass of a kilogram will be defined by a fundamental
constant of nature known as the Planck
constant. At the same time, other mainstays of the metric system will also
be revamped: the ampere (the unit of
electric current), the kelvin (the unit of
temperature) and the mole (the unit for
an amount of substance).
Now, instead of being based on arbitrary quantities or physical artifacts that
might change over time, all definitions
will be based on fundamental constants,
says metrologist Estefanía de Mirandés
of the International Bureau of Weights
and Measures in Sèvres, France.
Those unchanging numbers, which
include the speed of light and the charge
of the electron, are the same everywhere
in the universe, making them useful
pegs upon which to hang the metric
system’s hat. Out of seven basic units
in the metric system, three already met
this criterion. Now the remaining four
will conform to this ideal. “It’s a very big
change of paradigm, and now it’s complete,” says de Mirandés.
Most people won’t notice the switcheroo: A kilogram of ground beef will still
make the same number of burgers. But
metrologists say the change will put
precision measurements on a firmer
Carefully secured under several bell jars, a
metal cylinder known as Le Grand K will no
longer define the kilogram starting next year.
foundation. For example, it will be easier to measure masses that are much
smaller than a kilogram, something that
could be useful for tasks like doling out
tiny quantities of pharmaceuticals.
In preparation for the kilogram’s
update, several teams of scientists
carefully measured the Planck constant, quantifying it to an accuracy of
about 10 parts per billion. After May 20,
As a result, Le Grand K
will no longer be a perfect kilogram — its mass
will have a fudge factor of plus or minus
10 micrograms. Despite Le Grand K’s loss
of stature, metrologists will keep studying the object to understand how stable
its mass is over time. Scratches or gunk
on the surface of the cylinder may cause
its mass to change slightly, for example.
The kilogram’s history can be traced
back to 1795, when France adopted
a standardized system of units, the
metric system. The kilogram was originally designed to be equal to the mass of
a liter of water. Soon, the mass came to
be represented by a cylinder, and other
countries adopted the units.
A key idea behind the development of
the metric system, known formally as the
International System of Units, was that
the units should be accessible to everyone and last forever. “When they defined
the kilogram, they fell short of this,” says
NIST physicist Stephan Schlamminger.
Only a select few people have access to
Le Grand K while countries have to rely on
imperfect copies of the official kilogram.
Soon anyone with the right expertise
will be able to use the fixed value of the
Planck constant to measure mass, using
a device known as a Kibble balance.
In celebration of the new, more accessible kilogram, Schlamminger and Newell
had the Planck constant tattooed on
their arms, along with the French phrase,
“A tous les temps, à tous les peuples” — for
all times and for all people — an ideal that
the new kilogram will now meet. s
“It’s about as
excited as you’re
going to see