Science > Physics > Physics News Update -- Number 674, February 23, 2004
| Topic: |
Science > Physics |
| User: |
"Sam Wormley" |
| Date: |
23 Feb 2004 01:14:49 PM |
| Object: |
Physics News Update -- Number 674, February 23, 2004 |
http://www.aip.org/enews/physnews/2004/674.html
Physics News Update -- Number 674, February 23, 2004
by Phil Schewe and Ben Stein
Making Decoherence Visible
Anton Zeilinger and his colleagues at the University of Vienna are
experts at making largish objects, such as carbon-70 molecules, appear
wavelike rather than particulate. They can, for instance, send a gentle
C-70 beam toward a grating where, behaving as if they were waves
analogous to light waves, the molecules scatter in such a way as to
register in detectors farther downstream in a characteristic
interference pattern (see Update 579). Now these physicists have used
the same basic setup to study how decoherence comes about.
Decoherence, a hot topic in physics, is the process by which quantum
objects (in this case C-70 molecules, acting as waves) lose their
wavelike integrity by interacting with the surrounding environment.
Decoherence is what stands between the classical (bowling ball) world
and the quantum (wave interference) world, and understanding how it
arises will be valuable if we are every going to exploit quantum
weirdness to perform future feats of quantum computation or convey
secure pieces of quantum information.
In their new experiment the Vienna researchers recorded the
interference pattern several times with a variety of C-70 beams. Each
of the beams differed in "temperature," corresponding to the amount of
laser light used to impart an internal agitation to the molecule's
atomic constituents.
One would expect that the warmer molecules, radiating away their
thermal excess in the form of photons, would be in closer contact with
their environment than the cooler molecules, and would thus be more
vulnerable to losing the precious isolation needed for retaining
quantum coherence.
A consequence of this would be for the cool beams to show a sharper
interference pattern than the warmer beams, and this proved to be the
case. The succession of patterns, corresponding to beams from cold to
hot, exhibited a steady shedding of their quantal persona.
In effect, decoherence was being made visible through the emission of
heat. This demonstration speaks to the fact that in our warm world we
don't generally observe quantum interference effects in commonplace
events. (Hackermuller et al., Nature, 19 February 2004.)
Negative Normal Stress
Negative normal sterss, discovered by physicists a quarter-century ago,
is a poorly understood attraction between two parallel plates that
occurs when certain complex fluids flow between them. First
demonstrated in liquid crystalline polymers---the main ingredients of
such technologically important materials as Kevlar and Zylon---this
rare phenomenon has now been observed in two very different systems.
The two new reports (Lin-Gibson et al., Physical Review Letters, 30
January 2004, Montesi et al., Physical Review Letters, 6 February 2004)
offer insight into solving the puzzle of what creates these "negative"
stresses in nature. Such a solution would increase our understanding
and control of these technologically important materials.
To visualize this phenomenon, imagine a liquid in between a pair of
parallel plates. For classical "Newtonian" liquids like water,
glycerin, and oil, "shearing" the plates, or sliding one with respect
to the other, will only create a tangential stress, or a force parallel
to the plates. However, some liquids, such as polymer melts and
solutions, behave differently when sheared. In addition to the
tangential stress, they react with a positive normal stress--a force
that acts perpendicular to the plates. The net effect is to push the
plates apart.
About 25 years ago, researchers studying liquid crystalline polymers
(very stiff linear molecules dissolved in a low-molecular weight
solvent) observed that subjecting these fluids to shear made the plates
want to pull together. Such negative normal stress has proven rare and
somewhat controversial. Occasionally reported in a few complex soft
materials since then, rheologists are now starting to accept and study
them.
In one new example of this phenomenon (contact Erik Hobbie, NIST,
erik.hobbie@nist.gov), a semi-dilute suspension of carbon nanotubes is
dispersed in a Newtonian polymer melt. When these suspensions are
subjected to weak-to-modest shearing flows (the parallel plate
experiment described above), the tubes entangle with each other and
form diffuse aggregates. Because these aggregates are composed of
incredibly strong carbon nanotubes, they have remarkable elasticity.
This elasticity causes the aggregates to grow and roll like logs.
The other new system (Matteo Pasquali, Rice University, mp@rice.edu) is
a concentrated suspension of soft water droplets in a Newtonian oil
called an emulsion. In this case, there is an attractive force between
the droplets. When this suspension is put in the same type of modest
shearing flows, the droplets come together to form exactly the same
type of log-rolling aggregates, and similarly exhibit negative normal
stresses.
The striking difference between these two systems is that one is a
fairly dilute suspension of nanofibers while the second is a
concentrated suspension of soft spheres, and yet they show the same
response, suggesting some underlying universal phenomenon which is now
being further investigated.
Physics News Update is a digest of physics news items arising from
physics meetings, physics journals, newspapers and magazines, and other
news sources. Subscriptions are free as a way of broadly disseminating
information about physics and physicists. Feel free to post it where
others can read it; please credit the American Institute of Physics.
Physics News Update appears approximately once a week. Questions?
Contact the editors at physnews@aip.org.
.
|
|
|
Related Articles |
PHYSICS NEWS UPDATE ---- Number 764 February 6, 2006 by PhillipF. Schewe, Ben Stein, and Davide Castelvecchi
PHYSICS NEWS UPDATE -- Number 721 February 24, 2005 by Phillip F.Schewe, Ben Stein
PHYSICS NEWS UPDATE -- Number 766 February 21, 2006 by Phillip F.Schewe, Ben Stein, and Davide Castelvecchi
PHYSICS NEWS UPDATE -- Number 813 February 27, 2007 by PhillipF. Schewe, Ben Stein and Davide Castelvecchi
PHYSICS NEWS UPDATE -- Number 812 February 20, 2007 by Phillip F.Schewe, Ben Stein and Davide Castelvecchi
PHYSICS NEWS UPDATE -- Number 718 February 2, 2005 by Phillip F.Schewe, Ben Stein
PHYSICS NEWS UPDATE -- Number 719 February 10, 2005 by Phillip F.Schewe, Ben Stein
PHYSICS NEWS UPDATE -- Number 767 February 28, 2006 by PhillipF. Schewe, Ben Stein, and Davide Castelvecchi
| Physics News Update -- Number 673, February 18, 2004
PHYSICS NEWS UPDATE -- Number 720 February 17, 2005 by Phillip F.Schewe, Ben Stein
Physics News Update - Number 672, February 2, 2004
PHYSICS NEWS UPDATE -- Number 855 February 4, 2008 www.aip.org/pnu
PHYSICS NEWS UPDATE -- Number 811 7 February 2007 by PhillipF. Schewe, Ben Stein Turner Brinton and Davide Castelvecchi
PHYSICAL REVIEW FOCUS 17 February 2006 http://focus.aps.org/
What's New -- Friday, February 20, 2004
|
|
|
