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2Physics Quote:
"Can photons in vacuum interact? The answer is not, since the vacuum is a linear medium where electromagnetic excitations and waves simply sum up, crossing themselves with no interaction. There exist a plenty of nonlinear media where the propagation features depend on the concentration of the waves or particles themselves. For example travelling photons in a nonlinear optical medium modify their structures during the propagation, attracting or repelling each other depending on the focusing or defocusing properties of the medium, and giving rise to self-sustained preserving profiles such as space and time solitons or rapidly rising fronts such as shock waves." -- Lorenzo Dominici, Mikhail Petrov, Michal Matuszewski, Dario Ballarini, Milena De Giorgi, David Colas, Emiliano Cancellieri, Blanca Silva Fernández, Alberto Bramati, Giuseppe Gigli, Alexei Kavokin, Fabrice Laussy, Daniele Sanvitto. (Read Full Article: "The Real-Space Collapse of a Two Dimensional Polariton Gas" )

Sunday, September 25, 2005

Force & Matter Wave

Background of this report:
What is matter wave? The idea that atoms behave as waves as well as particles
goes back to 1924. They're called "de Broglie waves" for early 20th-century French
quantum physicist Prince Louis-Victor de Brogli, who first proposed the concept of
atom waves. Physicists have grappled with the dual wave-particle nature of atoms
for decades and, in the 1990s, they began chilling atoms to near absolute zero and
studying the wave properties of atoms in detail. The de Broglie wavelength is Planck
Constant [6.626X10^(-34) Joule-sec] divided by the momentum (mass X velocity)
of a particle.
What is van der Waals force? All atoms and molecules exhibit weak, short-
range interactions for one another. These forces are responsible for the condensation
of gases into liquids and the freezing of liquids into solids despite the absense of ionic,
covalent or metallic bonding mechanisms. The familiar aspects of behavior of matter
in bulk such as friction, surface tension, viscosity, adhesion, cohesion and so on also
arise from vander Waals forces. The van der Waals attraction between 2 molecules
at distance 'd' apart is proportional to 1/d^7 (^ stands for 'to the power of') and so is
significant only for molecules very close together.

Report: University of Arizona physicists
have directly measured how close
speeding atoms can come to a surface
before the atoms' wavelengths change.
This is a first, fundamental measurement
that confirms the idea that the wave of
a fast-moving atom shortens and
lengthens depending on its distance
from a surface, an idea first proposed by
pioneering quantum physicists in the late

UA optical sciences doctoral candidate
John D. Perreault (right in picture) and
UA assistant professor of physics
Alexander D. Cronin (left in picture)
report the experiment in the Sept. 23
Physical Review Letters. You can read
the paper here.

Perreault and Cronin found that atoms
closer than 25 nanometers to a surface
are very strongly attracted to the
surface because of the van der Waals
interaction -- so strongly that the atoms are accelerated with the force of a million g's.
A "g" is a term for acceleration any object on earth feels due to gravity (about 9.8 meter/second^2). A roller coaster rider might feel 3 to 4 g's for brief moments during
a ride. Fighter pilots can experience accelerations of up to 8 g for brief periods during
tactical maneuvers, but can black out if subjected to 4 to 6 g's for more than a few

The measurement tells nanotechnologists how small they can make extremely tiny
devices before a microscopic force between atoms and surfaces, called van der Waals
interaction, becomes a concern. The result is important both for nanotechnology,
where the goal is to make devices as small as a few tens of billionths of a meter, and
for atom optics, where the goal is to use the wave nature of atoms to make more
precise sensors and study quantum mechanics.

Source of report: Univ of Arizona Original news release

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