Number 556, September 13, 2001
by Phil Schewe, James Riordon, and Ben Stein
Empty Spaces Can Exert Forces on Each Other
Empty spaces can exert forces on each other through the action of intervening
matter, a US-Germany team has proposed (Aurel Bulgac, University of
Washington, Seattle and Andreas Wirzba, Forschungszentrum Juelich).
If experimentally confirmed, this effect would constitute a new kind
of force, akin to the traditional "Casimir force," the slight
attraction between two metallic plates in a vacuum. The traditional
Casimir attraction (Update
300-3) occurs because of the fleeting electromagnetic fields that
exist in the vacuum. These fields exert forces on the plates. In between
the plates, however, certain electromagnetic waves cannot reside, namely
those with wavelengths larger than the plate separation. This imbalance
of electromagnetic forces serves to push the plates together.
In the newly proposed force, two or more cavities (empty regions of
space) alter the waves associated with surrounding matter in the form
of non-interacting fermions, such as a gas of electrons. For a simple
example, consider two hollow spheres separated by a sea of electrons
which, according to quantum mechanics, can be considered as rippling
waves. If the wavelengths of the electrons are comparable to the dimensions
of the spheres, then forces between the empty spheres could result.
The spheres, even though they're separated, can effectively interact
because the electron waves bounce back and forth between them. Whether
the spheres attract or repel each other depends on the overall effect
of all matter waves between them.
Demonstrating this effect is likely to be very challenging. One approach
might be to immerse C60 molecules (buckyballs) in liquid mercury. The
buckyballs, effectively hollow spheres, could bind to each other through
the action of conducting electrons in the liquid mercury. This new effect
could act over an even longer range than the weakly attractive "van
der Waals force" between molecules. (Bulgac
and Wirzba, Physical Review Letters, 17 September 2001.)
A Novel Interferometer
Interferometers are sensitive measuring devices that use the wave properties
of light to detect tiny changes in length. Physicists Michelson and
Morley ushered in the age of modern physics when their interferometer
helped show that there is no aether filling the voids between the stars
and planets. Countless other experiments have relied on interferometers
of various designs.
Now, researchers at the Universität Stuttgart have developed a
new type of interferometer that may exceed the measurement sensitivity
of older designs by as much as five hundred times. In most interferometers,
a beam of light is allowed to travel over some distance that a physicist
would like to measure. The measurement beam is then combined with a
reference beam that always travels a fixed distance. The two beams create
an interference pattern that consists of a number of bright and dark
bands, or fringes, that shift as the distance traveled by the measurement
beam changes.
The new multimode waveguide interferometer (MWI), on the other hand,
does not have a fixed reference beam. Instead, a single beam of light
enters a waveguide formed by two movable parallel mirrors. The beam
propagates as a combination of many modes, effectively following numerous
paths simultaneously through the waveguide. Each mode interferes with
every other mode, leading to a modulation in the light transmitted through
the waveguide, and a sensitive measurement of the distance between the
waveguide mirrors.
A laboratory test of the MWI resulted in detection of motion as little
as a ninth as large as the wavelength of the light that entered the
interferometer, and theoretical calculations suggest that a more refined
version could detect motion a thousand times smaller than the light
wavelength used. Conventional interferometers, by contrast, are capable
of accurately measuring distance changes only half the wavelength of
the input light or larger.
In addition to opening the door to new, high precision measurements,
MWIs may serve as fast optical switches and other communication-related
devices. But perhaps most importantly, the researchers point out, the
MWI shows that significant innovations can be surprisingly simple even
in refined and fundamental fields like classical optics. (Ovchinnikov
and Pfau, Physical Review Letters, 17 September 2001.)
"The Italian Navigator Has Landed"
"The Italian navigator has landed" was the wartime coded
message announcing the successful first operation of a nuclear reactor,
on December 2, 1942. The expression refers to Columbus's exploration
of continents previously unknown to Euroeans, but also could apply to
the exploration of another unknown continent, the atomic nucleus. This
exploration was exemplified in the work of Enrico Fermi, the man who
oversaw that first reactor. September 29, 2001 is the centenary of Fermi's
birth, and celebrations are planned at a number of institutions, such
as Fermilab,
the University of Chicago,
and the University
of Pisa. A US
Department of Energy website summarizes some of the accomplishments
of this great experimentalist and theorist. Many objects in the world
of physics bear his name: an element (100), a national lab (Fermilab),
a Presidential award, an institute
(at the University of Chicago), a unit of distance (10-15
m), one of the two broad categories of particle (fermion), an energy
level (condensed matter physics), a type of interaction, a constant,
a temperature, a gas, and now a brand new US postage stamp.
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