Interesting concept. Now tell me what happens if a random EM pulse happens
anywhere within a hundred miles of this "electronic kilogram". Never mind, I
will. NO "electronic kilogram". Sometimes we scientists can get ahead of
ourselves in the cause of science.
Amy Harlib <[EMAIL PROTECTED]> wrote:
[EMAIL PROTECTED]
Just plain fascinating.
--------------------------------------------------------------------------------
October 16, 2005
Measuring the World: From Material to Ethereal
By KENNETH CHANG
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.
a.. Copyright 2005 The New York Times Company
b..
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