Text 208, 78 rader
Skriven 2004-10-09 17:51:58 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 703
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 703 October 5, 2004
by Phillip F. Schewe and Ben Stein

THE 2004 PHYSICS NOBEL PRIZE goes to David J. Gross (Kavli Institute,
University of California, Santa Barbara), H. David Politzer (Caltech), and
Frank Wilczek (MIT) for their discovery of asymptotic freedom, according to
which the interaction between quarks inside nuclear particles such as protons
and neutrons actually gets weaker the closer the quarks are to each other and
stronger the farther they are apart.  This hypothesis helped lead to the
establishment of quantum chromodynamics (QCD) as a firm theory of the strong
nuclear force, somewhat, but not exactly, in analogy with the quantum
electrodynamics (QED), the theory of the electromagnetic force. The Work of
Gross/Politzer/Wilczek explained why individual quarks could never be observed
in the lab.  In their picture, quarks are connected by lines of force embodied
in the form of particles called gluons.  The quarks themselves possess a "color
charge" analogous to electrical charge.  That is why the strong force among
quarks is referred to also as the color force (whence the name "chromo"
dynamics).  The energy that could be used to free quarks from each other's
embrace---energy in the form, say, of a fast-moving incoming beam
particle---would indeed force the quarks farther apart for a while, but this
energy (imagine a rubber band being stretched) would eventually be converted
into the creation of a new quark-antiquark pair. One or the other of these
newly made quarks would immediate ally itself with one of the two separating
quarks, resulting not in any free quarks but only in two quark pairs.  (This
process has been compared to trying to saw a bar magnet in half attempting to
create two isolated magnetic poles; you only succeed in creating two new bar
magnets.)  Conversely, quarks very close together are practically free of each
other's influence. QCD has passed every confirmed experimental test so far, but
physicists continue to look for oddities that might signify a departure from
this theory.
(Background: Physics News Update has carried many items relating to QCD---for
example, see PNU's 533, 585, 549, 642, 600, 666, 216, 699, 554, 526 at
www.aip.org/pnu.  Some pertinent magazine articles: Physics Today (PT), April
88, Georgi, flavor symmetries; Scientific American (SA), Oct 75, Glashow, color
and flavor; SA, Dec 80, Wilczek, matter-antimatter asymmetry; PT, Oct 04,
Wilczek, essay on forces of nature; SA, Jun 03, beyond the standard model;
Nature, 28 Jan 99, Wilczek, lattice gauge theory; PT Aug 00, QCD made easy; PT
Feb 04, lattice QCD; PT, Mar 95, Witten, confinement; CERN Courier, free quarks
in nuclear collisions; CERN Courier, tests of QCD; PT, Aug 00, Wilczek; SA, Apr
81, grand unified theories; SA, Dec 99, Weinberg, theory of everything; SA, Apr
85, Quigg, standard model; SA, Jun 80, 't Hooft, fields; CERN Courier, Jun 04,
Wilczek; SA, Feb 83, lattice theory; SA, Jul 93, calculation of masses; PT, Feb
04, QCD; Physics World, May 03, QCD and string theory; SA, Nov 76, quark
confinement; SA, Nov 98, glueballs.  Websites: Nobel Prize website:
www.nobel.se/physics/laureates/2004; germane Physics Today articles will be
posted at this site: physicstoday.org/vol-57/iss-10/nobel.html;
webphysics.davidson.edu/mjb/qcd.html, QCD website; www.cpt.dur.ac.uk/qcdnet/,
QCD website; fafnir.phyast.pitt.edu/exotica/, QCD website;
http://www-cdf.fnal.gov/physics/new/qcd/QCD.html, QCD website.)
                        
AN MRI WIND TUNNEL.  The subject of last year's Nobel prize for physiology or
medicine, magnetic resonance imaging (MRI) is well known for making beautiful
images of the inside of the body. MRI is less recognized for its ability to
track movement, such as clinical studies of blood flow. Now, researchers in
Canada (Benedict Newling, University of New Brunswick, bnewling@unb.ca) have
invented a new MRI method  suitable for measuring much faster fluid speeds, ten
times more rapid than the fastest human blood flow. Their approach uses a
constant, shorter-than-usual measurement interval (six milliseconds).  In
effect, the MRI scanner becomes a new type of practical wind tunnel, one that's
non-invasive too. An obstruction of any shape can be placed in the flow at the
center of the scanner. The resulting flow of liquid or gas around the object is
readily measured. As an example, the researchers measure gas flow past a wing
at realistic speeds (corresponding to a stalled aircraft) and compare them with
computer-based calculations of the expected flow (see figure at
www.aip.org/png). The flows they measure are highly turbulent, which means the
fluid velocity at every position varies rapidly around some average value. The
MRI measurement contains information about both the average velocity and
fluctuations. MRI is naturally three-dimensional and works just as well in
opaque or transparent fluids.  Furthermore, MRI can measure several positions
simultaneously, unlike most conventional wind-tunnel measurements, and
therefore has the potential to deliver measurements in substantially reduced
times. (Newling et al., Physical Review Letters, 8 October 2004; text at
www.aip.org/physnews/select)

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 * Origin: Big Bang (1:106/2000.7)