Einstein Was Right (Again): NIST and MIT Confirm that E= mc2 (Forwarded)

News Office
Massachussetts Institute of Technology
Cambridge, Massachussetts

December 21, 2005

E=mc2 passes tough MIT test
By Elizabeth A. Thomson, News Office

In a fitting cap to the World Year of Physics 2005, MIT physicists and
colleagues from the National Institute of Standards and Technology (NIST)
report the most precise direct test yet of Einstein's most famous
equation, E=mc2.

And, yes, Einstein still rules.

The team found that the formula predicting that energy and mass are
equivalent is correct to an incredible accuracy of better than one part in
a million. That's 55 times more precise than the best previous test.

Why undertake the exercise? "In spite of widespread acceptance of this
equation as gospel, we should remember that it is a theory. It can be
trusted only to the extent that it is tested with experiments," said team
member David E. Pritchard, the Cecil and Ida Green Professor of Physics at
MIT, associate director of MIT's Research Laboratory for Electronics (RLE)
and a principal investigator in the MIT-Harvard Center for Ultracold
Atoms.

As he and colleagues report their results in the Dec. 22 issue of Nature:
"If this equation were found to be even slightly incorrect, the impact
would be enormous -- given the degree to which [it] is woven into the
theoretical fabric of modern physics and everyday applications such as
global positioning systems."

In the famous equation, E stands for energy, m for mass, and c for the
speed of light. "In the test, we at MIT measured m, or rather the change
in m associated with the energy released by a nucleus when it captures a
neutron," said former MIT graduate student Simon Rainville.

The NIST scientists, led by Scott Dewey, measured E. (The speed of light
is a defined and therefore exactly known quantity, so it was simply
plugged into the equation.)

Specifically, the NIST team determined the energy of the particles of
light, or gamma rays, emitted by the nucleus when it captures a neutron.
They did so using a special spectrometer to detect the small deflection of
the gamma rays after they passed through a very pure crystal of silicon.

The mass loss was obtained at MIT by measuring the difference between the
mass of the nucleus before the emission of a gamma ray and after. The mass
difference was measured by comparing the cyclotron orbit frequencies of
two single molecules trapped in a strong magnetic field for several weeks.

Pritchard notes that the mass of the nucleus is about 4,000 times larger
than the much smaller mass difference. As a result, "determining the mass
difference requires the individual masses to be measured with the
incredible accuracy of one part in 100 billion -- equivalent to measuring
the distance from Boston to Los Angeles to within the width of a human
hair!"

Despite the results of the current test of E=mc2, Pritchard said, "This
doesn't mean it has been proven to be completely correct. Future
physicists will undoubtedly subject it to even more precise tests because
more accurate checks imply that our theory of the world is in fact more
and more complete."

Pritchard's MIT colleagues are Rainville (now at Université Laval, Quebec)
and James K. Thompson (now an RLE postdoctoral associate in the
MIT-Harvard Center for Ultracold Atoms). Rainville and Thompson are
co-lead authors of the Nature paper.

This work was funded by the National Science Foundation and by a Precision
Measurement Grant from NIST.

Einstein on E=mc2

"It followed from the special theory of relativity that mass and energy
are both but different manifestations of the same thing -- a somewhat
unfamiliar conception for the average mind. Furthermore, the equation E is
equal to m c-squared, in which energy is put equal to mass, multiplied by
the square of the velocity of light, showed that very small amounts of
mass may be converted into a very large amount of energy and vice versa.
The mass and energy were in fact equivalent, according to the formula
mentioned above. This was demonstrated by Cockcroft and Walton in 1932,
experimentally."

To hear an audio clip of Einstein explaining this, go to
http://www.aip.org/history/einstein/voice1.htm

*****

National Institute of Standards and Technology
Gaithersburg, Maryland

CONTACT:
Laura Ost, (301) 975-4034

FOR IMMEDIATE RELEASE: Dec. 21, 2005

Einstein Was Right (Again): NIST and MIT Confirm that E= mc2

GAITHERSBURG -- Albert Einstein was correct in his prediction that E=mc2,
according to scientists at the Commerce Department's National Institute of
Standards and Technology (NIST) and the Massachusetts Institute of
Technology (MIT) 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 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 NIST/MIT 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 NIST by a group led by the late physicist Richard Deslattes, and
another done at MIT by a group led by David Pritchard. Deslattes 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.

According to the basic laws of physics, every wavelength of
electromagnetic radiation corresponds to a specific amount of energy. The
NIST 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 ***'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 NIST/MIT 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 scientists measured the angle
at which gamma rays are diffracted by crystals with known lattice spacings
at the Institut Laue Langevin (ILL) in Grenoble, France. 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.

IMAGE CAPTION:
[http://www.nist.gov/public_affairs/images/05PHY023_E=mc2_HR_CR.jpg
(1.9MB)]
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


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