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An instrument called GAMS4,
originally designed and built at NIST and now located
at Institut Laue Langevin in France, was used in experiments
that helped to confirm Einstein’s famous equation
E=mc². GAMS4 measured the angle at which gamma
rays are diffracted by two identical crystals made of
atoms separated by a known distance. The two crystals
are the dark gray rectangles on circular platforms in
the foreground and background of the photo.
Photo
by Artechnique, Courtesy of ILL
View
a high resolution version of this image.
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GAITHERSBURG-Albert
Einstein was correct in his prediction that E=mc2,
according to scientists at the Massachusetts Institute of Technology (MIT), the Commerce Department's National
Institute of Standards and Technology (NIST), and the Institute Laue Langevin, Genoble, France (ILL) who conducted the most precise
direct test ever of what is perhaps the most famous formula
in science.
In experiments
described in the Dec. 22, 2005, issue of Nature,*
the researchers added to a catalog of confirmations that matter
and energy are related in a precise way. Specifically, energy
(E) equals mass (m) times the square of the speed of light
(c2), a prediction of Einstein's theory of special
relativity. By comparing NIST/ILL measurements of energy emitted
by silicon and sulfur atoms and MIT measurements of the mass
of the same atoms, the scientists found that E differs from
mc2 by at most 0.0000004, or four-tenths of 1 part
in 1 million. This result is "consistent with equality" and
is 55 times more accurate than the previous best direct test
of Einstein's formula, according to the paper.
Such
tests are important because special relativity is a central
principle of modern physics and the basis for many scientific
experiments as well as common instruments like the global
positioning system. Other researchers have performed more
complicated tests of special relativity that imply closer
agreement between E and mc2 than the MIT/NIST/ILL work,
but additional assumptions are required to interpret their
results, making these previous tests arguably less direct.
The Nature paper describes two very different precision measurements,
one done at MIT by a group led by David
Pritchard and another done at the ILL by a NIST/ILL collaboration led by the late physicist Richard
Deslattes (NIST) and Hans Börner (ILL). Deslattes and his collaborators developed methods for using optical and
X-ray interferometry-the study of interference patterns created
by electromagnetic waves-to precisely determine the spacing
of atoms in a silicon crystal, and for using such calibrated
crystals to measure and establish more accurate standards
for the very short wavelengths characteristic of highly energetic
X-ray and gamma ray radiation. Börner and his collaborators were responsible for a highly successful gamma-ray measurement program at the ILL.
According
to the basic laws of physics, every wavelength of electromagnetic
radiation corresponds to a specific amount of energy. The
NIST/ILL team determined the value for energy in the Einstein
equation, E = mc2, by carefully measuring the wavelength
of gamma rays emitted by silicon and sulfur atoms.
"This
was Dick's original vision, that a comparison like this would
someday be made," said Scott Dewey, a NIST physicist who is
a co-author of the Nature paper. "The idea when he
started working on silicon was to use it as a yardstick to
measure the wavelengths of gamma rays, and use this in a test
of special relativity. It took 30 years to realize his idea."
The MIT/NIST/ILL
tests focused on a well-known process: When the nucleus of
an atom captures a neutron, energy is released as gamma ray
radiation. The mass of the atom, which now has one extra neutron,
is predicted to equal the mass of the original atom, plus
the mass of a solitary neutron, minus a value called the neutron
binding energy. The neutron binding energy is equal to the
energy given off as gamma ray radiation, plus a small amount
of energy released in the recoil motion of the nucleus.
The gamma
rays in this process have wavelengths of less than a picometer,
a million times smaller than visible light, and are diffracted
or bent by the atoms in the calibrated crystals at a particular
energy-dependent angle. Using a well-known mathematical formula,
scientists can combine these angles with values for the crystal
lattice spacing to determine the energy contained in individual
gamma ray particles.
In the
experiments described in Nature, NIST/ILL scientists
measured the angle at which gamma rays are diffracted by
crystals
with known lattice spacings at the ILL high flux reactor. The ILL has the world's premier
facility for colliding nuclei and neutrons and capturing the
resulting gamma rays at the same instant. Accurate gamma-ray
measurements are particularly challenging because the diffraction
angles are less than 0.1 degree. The measurements were done
using an instrument that was originally designed and built
at NIST.
The MIT
team measured the mass numbers used in the tests of Einstein's
formula by placing two ions (electrically charged atoms) of
the same element, one with an extra neutron, in a small electromagnetic
trap. Scientists counted the revolutions per second made by
each ion around the magnetic field lines within the trap.
The difference between these frequencies can be used to determine
the masses of the ions. The experiment was performed with
both silicon and sulfur ions. The novel two-ion technique
virtually eliminates the effect of many sources of "noise,"
such as magnetic field fluctuations, that reduce measurement
accuracy. This work led to greatly improved values for the
atomic masses of silicon and sulfur.
The work
was supported by NIST and the National Science Foundation.
As a
non-regulatory agency of the Commerce Department's Technology
Administration, NIST promotes U.S. innovation and industrial
competitiveness by advancing measurement science, standards
and technology in ways that enhance economic security and
improve our quality of life.
* S.
Rainville, J.K. Thompson, E.G. Myers, J.M. Brown, M.S. Dewey,
E.G. Kessler Jr., R.D. Deslattes, H.G. Börner, M. Jentschel,
P. Mutti, D.E. Pritchard. 2005. A direct test of E = mc2.
Nature. Dec. 22, 2005.
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