LOCKED in a vault in Paris is a cylinder about the size of a plum. Its mass is exactly one kilogram. It is the kilogram.
For 116 years, this cylinder made of platinum and iridium has been
the world's defining unit of mass. It's an easy concept to understand.
Scientists at the National Institute of Standards and Technology in
Gaithersburg, Md., announced last month significant progress toward
supplanting this cylinder. Their concept is not so easy to understand.
It's a two-story-tall contraption that looks one part Star Trek, one
part Wallace and Gromit. Briefly put, it measures the power needed to
generate an electromagnetic force that balances the gravitational pull
on a kilogram of mass.
“It's such a very complicated thing that's hard to explain,” said
Richard Steiner, the physicist in charge of the project. He has been
working on this “electronic kilogram” machine for more than a decade.
“That's what everybody kind of laughs at,” Dr. Steiner said.
“They're all impressed it's such a complicated thing and then they ask,
'What do you need it for?' “
The general answer is that humans have always needed to quantify and
standardize, to make their world more certain. Without a standard
kilogram – roughly 2.2 pounds – how would scientists know their
measurements of mass were accurate? Without a standard meter, how would
a manufacturer make a ruler and know that it is precise?
More specifically, the high-tech kilogram is needed because
scientists prefer a definition based on the universal constants of
physics – something they could in principle calibrate in their own
laboratories – rather than on an artifact sitting in a distant vault.
Another problem with the kilogram cylinder is that it is not
necessarily unchanging. Over time, contamination might add smidgeons of
mass, or cleaning might scrub away some atoms, leaving a lesser
kilogram. Better, scientists say, not to have to worry about dust, dirt
or disaster striking the Paris vault.
The kilogram, in fact, is decades behind the meter, which used to be
defined as the distance between two scratches on a metal bar. In 1960,
scientists defined the meter in terms of the wavelength of a specific
orange light emitted by krypton atoms. In 1983, they redefined the
speed of light to be exactly 299,792,458 meters per second, so a meter
is now just the distance that light travels in a vacuum in
1/299,792,458th of a second.
The newer definitions hark back to the original metric definitions,
which were based on features of the natural world, not human artifacts.
A kilogram was the mass of water filling a cube that is one-tenth of a
meter on each side, or one liter of volume, and a meter was one
ten-millionth of the distance from the North Pole to the Equator, along
the path passing through Paris (since it was the French Academy of
Sciences that defined the meter).
Neither definition proved practical, and the French scientists
botched their calculation of how much the Earth is squashed by the
centrifugal force of its rotation, so the metal bar they made to
represent a meter was off by a fraction of a millimeter.
It is also not easy to measure precisely a liter of pure water,
which is complicated by impurities and gases dissolved in the water and
by how water density changes with temperature and pressure. Instead,
that platinum-iridium cylinder was established as the official
definition, in 1889.
The search for standards began with the rise of civilization.
Measures were needed, especially for commerce. At first, people simply
used parts of the body. A cubit, for example, was the distance from the
elbow to the tip of the middle finger – which differed from person to
person, until an Egyptian pharaoh declared a cubit to be the distance
from his elbow to the tip of his middle finger (and possibly the width
of his palm).
It was hardly convenient to borrow the pharaoh's arm to measure a
bolt of cloth, so a piece of granite was carved and declared the
official cubit. Other people would make their own cubit rulers, usually
out of wood, based on the granite standard.
The same idea underlay the standards for the kilogram and the meter
– a cylinder and a bar, respectively. “Those were not bad standards at
the time,” said John L. Hall, a scientist at the Institute of Standards
and Technology and a winner of this year's Nobel Prize in Physics, who
helped refine the definition of the meter two decades ago. “But they're
kind of hard to duplicate and disseminate.”
Dr. Steiner's team with its two-story contraption has now fixed the
mass of a kilogram to 99.999995 percent accuracy. To satisfy the
international body that sets measurement standards, they probably need
to raise that last “5” to an “8.”
As science measures ever tinier bits of the universe, measurement
must become more precise. If scientists can define units in terms of
constants like the speed of light and the charge of the electron, then
they can better study whether constants really are constant. “It's a
much more serious question than it appears to be,” Dr. Hall said.”
