Message-ID: <Pine.NEB.4.63.0506222209440.13102@panix3.panix.com>
From: Alan Sondheim <sondheim@panix.com>
To: Cyb <cybermind@listserv.aol.com>,
"WRYTING-L : Writing and Theory across Disciplines" <WRYTING-L@LISTSERV.UTORONTO.CA>
Subject: Physics News Update 734 (fwd)
Date: Wed, 22 Jun 2005 22:09:49 -0400 (EDT)
( URLs/DVDs/CDroms/books/etc. see http://www.asondheim.org/advert.txt ) ---------- Forwarded message ---------- Date: Wed, 22 Jun 2005 14:12:41 -0400 From: physnews@aip.org To: sondheim@PANIX.COM Subject: Physics News Update 734 PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 734 June 22, 2005 by Phillip F. Schewe, Ben Stein SUPERFLUIDITY IN AN ULTRACOLD GAS OF FERMION ATOMS has been demonstrated in an experiment at MIT, where an array of vortices has been set in motion in a molecular Bose Einstein condensate (BEC) of paired lithium-6 atoms. There have been previous hints of superfluidity in Li-6, for example, (http://www.aip.org/pnu/2004/split/681-1.html) but the presence of vortices observed in the new experiment clinches the case since vortices manifest the most characteristic feature of superfluidity, namely persistent frictionless flow. Wolfgang Ketterle and his MIT colleagues use laser beams to hold the chilled atoms in place and separate laser beams to whip up the vortices. In general the quantum behavior of bosonic atoms (those whose total internal spin---the spin of the nucleus added to that of the electron retinue---is an integral number of units) and fermi atoms (those with a half-integral-valued total spin) is very different. Gaseous Li-6 represents only the second known superfluid among fermi atoms, the other being liquid helium-3. (Superconductivity is also a form of fermion superfluidity, but in this case the constituents are charged particles, electrons, unlike the neutral atoms used in the experiments described here.) There are great advantages in dealing with a neutral superfluid in dilute gas form rather than in liquid form: in the gas phase (with a material density similar to that of the interstellar medium), inter-atomic scattering is simpler; furthermore, the strength of the pairing interaction can be tuned at will using an imposed external magnetic field. According to Ketterle, one of those who won a Nobel prize for his pioneering work with boson BECs, the study of fermionic superfluidity is much richer than for bosons: control over forces will permit researchers to vary the strength and nature of the pairing (fermi atoms must pair up before falling into BEC form) and to load atoms into an optical lattice. Additional pairing mechanisms can also be explored. One further superlative: the ultracold lithium gas represents, in a narrow sense, the first "high-temperature" superfluid. Consider the ratio of the critical temperature (Tc) at which the superfluid transition takes place to the fermi temperature (Tf), the temperature (or energy, divided by Boltzmann's constant) of the most energetic particle in the ensemble. For ordinary superconductors, Tc/Tf is about 10^-4; for superfluid helium-3 it is 10^-3; for high-temp superconductors 10^-2; for the new lithium superfluid it is 0.3. (Zwierlein et al., Nature, 23 June 2005) GRAVITY IS NORMAL DOWN TO THE 100-nm LEVEL. Gravity at the level of planets is well studied, and was known accurately even in Newton's day. This is owing to the fact that the other physical forces, such as the strong and weak nuclear forces, don't operate over such great distances, and electromagnetic forces between immense far-apart, electrically-neutral objects like planets are dilute. Gravity at shorter lengths, by contrast, is harder to measure, partly because all the other forces are in full play. Furthermore, theories of particle interactions hypothesizing the existence of additional spatial dimensions suggest that the strength of gravity will depart from Newton's famous inverse-square formulation. To test these propositions, various tabletop setups have been devised to probe gravity below the micron level. One previous experiment, conducted by Eric Adelberger's group at the University of Washington, ruled out extra gravity components having a strength comparable to conventional gravity down to a size scale of about 100 microns (http://www.aip.org/pnu/2000/split/pnu483-1.htm). A new experiment, carried out by a Indiana/Purdue/Lucent/Florida/Wabash collaboration examines a shorter distance scale---100 nm---but is able to rule out only corrections to gravity that are, in fact, a trillion times larger than gravity itself. Nevertheless, such measurements help to constrain the general pursuit of unified theories of particle physics, including explanations of gravity. The sort of "Yukawa" corrections being sought are analogous to the force proposed by Hideki Yukawa in the 1930s to explain how mesons transmit the nuclear force between nucleons and would come about because of transmission of the presumed force particles associated with the hypothetical extra dimensions. The present measurements improve the exclusion of such corrections by a factor of ten. According to Ricardo Decca of Indiana University-Purdue University (rdecca@iupui.edu, 317-278-7123), the sensitivity of the apparatus should grow by a factor of a hundred over the next year. The size of the sample is smaller here than in many other tabletop gravity experiments. The flea-sized torsional apparatus must operate with such concern for forces acting over small distances that one of the chief goals here is reducing the background produced by the Casimir force---a quantum effect in which two very close objects are drawn together because of the way they exclude vacuum fluctuations (that is, the spontaneous creation of pairs of virtual particles) from occurring in a slender volume of space---between a flat plane and sphere lying only 200 nm apart. (Decca et al., Physical Review Letters, 24 June 2005) *********** PHYSICS NEWS UPDATE is a digest of physics news items arising from physics meetings, physics journals, newspapers and magazines, and other news sources. It is provided free of charge as a way of broadly disseminating information about physics and physicists. For that reason, you are free to post it, if you like, where others can read it, providing only that you credit AIP. Physics News Update appears approximately once a week. AUTO-SUBSCRIPTION OR DELETION: By using the expression "subscribe physnews" in your e-mail message, you will have automatically added the address from which your message was sent to the distribution list for Physics News Update. If you use the "signoff physnews" expression in your e-mail message, the address in your message header will be deleted from the distribution list. Please send your message to: listserv@listserv.aip.org (Leave the "Subject:" line blank.)