- You may not be able to squeeze blood
out of a stone, but you can squeeze energy out of a vacuum. That is what
the Dutch physicist Hendrik Casimir theorized in 1948, predicting that
two metal plates would be attracted by a quantum force. Last year, the
"Casimir effect" was finally confirmed in the laboratory. Now,
physicists have shown that the strength of the force matches predictions.
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- In the 1930s, the English physicist Paul
Dirac proposed that a vacuum actually teems with electromagnetic waves
called "zero point energy." This energy would be contained in
"virtual photons," or light particles that are constantly winking
in and out of existence. Fluctuations in the energy--like ripples on the
surface of an infinite ocean--can be observed in laboratory experiments.
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- Casimir proposed an ingenious way to
observe the energy directly. If two perfectly reflective metal plates were
placed a micrometer apart, they would, in effect, form a narrow channel
in the electromagnetic ocean that allowed only certain wavelengths of light,
and their respective virtual photons, to exist there. But the ocean outside
the channel would have virtual photons of all wavelengths. This would create
an ever-so-slight discrepancy between the energy density inside and that
outside the channel, causing a tiny force pushing the plates together,
roughly equivalent to a speck of dust landing on the top plate.
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- Physicists cannot yet align two plates
precisely enough to test Casimir's original idea, but they can do it with
a single plate and a ball. Umar Mohideen and Anushree Roy of the University
of California, Riverside, used an atomic force microscope to position an
aluminum-coated sphere less than a micrometer away from a plate and measured
the resulting force. After correcting for experimental inaccuracies--the
fact that the plate was not perfectly reflective, the roughness of the
surfaces, and the temperature of the room--they found that the Casimir
force was within 1% of the predicted value. Moreover, Mohideen says, "we
are confident that we can improve the accuracy by a factor of 1000"
by enlarging and cooling the ball.
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- The work is an "experimental tour
de force," says Alex Maradudin, a condensed-matter physicist at the
University of California, Irvine, although some mathematical details remain
to be ironed out. For example, Steven Lamoreaux, the Los Alamos National
Laboratories physicist who first detected the Casimir force using a different
method, worries that the small distances may invalidate Mohideen and Roy's
corrections--which could seriously skew the results.
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