Physics News Update 703

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Number 703, October 5, 2004 by Phil Schewe and Ben Stein

The 2004 Physics Nobel Prize

The 2004 Nobel Prize in Physics 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.
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 in Physiology/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 (0.6 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 Physics News Graphics).
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)

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