PHYSICS NEWS UPDATE

The American Institute of Physics Bulletin of Physics News

Number 804 December 5, 2006 by Phillip F. Schewe, Ben Stein,
and Davide Castelvecchi www.aip.org/pnu

THE PHYSICS STORY OF THE YEAR FOR 2006 was, we believe, the new high
precision (0.76 parts per trillion uncertainty) measurement of the
electron’s magnetic moment by Gerald Gabrielse and his colleagues at
Harvard. Then in a second paper the same experimenters used the new
moment in tandem with a fresh formulation of quantum electrodynamics
(QED) provided by theoretical colleagues to formulate a new value
for the fine structure constant (denoted by the letter alpha), the
pivotal parameter which sets the overall strength of the
electromagnetic force. The new value has an uncertainty of 0.7
parts per billion, the first major revision of alpha in 20 years. A
comparison between this new value and values determined by other
methods provides the best test yet of quantum electrodynamics (QED)
(http://www.aip.org/pnu/2006/split/783-1.html; also see Physics
Today, Aug 2006).

Other top physics stories for the year, in no particular order, are
listed below with links to pertinent PNU items (and sometimes
figures) from the past year.

These top stories include the
observation of many more supernovas at redshifts of 1, thus
establishing the idea that dark energy was around even in the early
universe (http://www.aip.org/pnu/2006/split/802-1.html); the first
direct measurement of turbulence in space
(http://www.aip.org/pnu/2006/split/802-2.html); the best direct test
of Einstein’s E=mc^2 formula
(http://www.aip.org/pnu/2006/split/761-1.html); new WMAP
measurements of the cosmic microwave background, including
polarization information, help to sharpen cosmological numbers such
as the age or the flatness of the universe
(http://www.aip.org/pnu/2006/split/769-1.html);
(http://www.aip.org/pnu/2006/split/794-1.html); first
matter-antimatter chemistry
(http://www.aip.org/pnu/2006/split/796-1.html); elements 116 and
118; 2006 Nobel prize in physics for Smoot and Mather
(http://www.aip.org/pnu/2006/split/795-1.html); advances in
plasmonics, or “two-dimensional light”
(http://www.aip.org/pnu/2006/split/770-1.html); advances in the
study of graphene, including the discovery of a new form of the Hall
effect (http://www.aip.org/pnu/2006/split/769-2.html); progress at
several labs in modeling gravity wave transmissions from black hole
mergers, the kinds of events which LIGO or LISA would possibly
detect (http://www.aip.org/pnu/2006/split/771-1.html); measuring the
presence of virtual strange quarks inside protons
(http://www.aip.org/pnu/2006/split/776-1.html); acoustic lasers
(http://www.aip.org/pnu/2006/split/779-1.html); evidence for
negative electrical resistance
(http://www.aip.org/pnu/2006/split/780-1.html); a particle laser or
“PASER” (http://www.aip.org/pnu/2006/split/792-1.html); hypersound
(http://www.aip.org/pnu/2006/split/797-1.html); heaviest baryons
discovered (http://www.aip.org/pnu/2006/split/798-1.html);
investigating whether the electron/proton mass ratio changed over
time (http://www.aip.org/pnu/2006/split/774-1.html); optical
“cloaking” (Science, 8 Sept); telecloning,
http://www.aip.org/pnu/2006/split/765-1.html; rare positronium ion,
http://www.aip.org/pnu/2006/split/763-1.html; wireless energy
transfer (http://www.aip.org/pnu/2006/split/801-1.html); the
sharpest object ever made
(http://www.aip.org/pnu/2006/split/788-2.html); chemical transistor,
(http://www.aip.org/pnu/2006/split/786-1.html); radioactive scorpion
venom for brain cancer therapy
(http://www.aip.org/pnu/2006/split/782-1.html); liquid flowing
uphill (http://www.aip.org/pnu/2006/split/772-1.html); and stock
market criticality (http://www.aip.org/pnu/2006/split/765-2.html).

POLARIZED AND UNPOLARIZED SUPERFLUID STATES. In many areas of
science, such as the study of chemical reactions among atoms or the
nuclear interactions among protons in collisions at an accelerator,
the strength of the interaction between species is imposed by
nature. In some cases, however, the researcher has some control
over the interaction strength and can contrive exotic states of
matter thereby. An important example of this dexterity is the study
of nanokelvin fermionic atoms (atoms with a half-integral total spin
value, such as 1/2). The Pauli exclusion principle forbids Fermi
particles from distilling into a monolithic quantum fluid like a
Bose-Einstein condensate (BEC). Paired up, however, fermions become
bosons (integral-spin objects) and can condense. Fermi atoms, such
as lithium-6, can marry in a variety of ways and this is what over
the past few years has made them valuable to physicists in their
effort to intervene in the basic interactions among particles.
Typically the pairing is induced by adjusting an external magnetic
field. The result can be a chemical bond: lithium atoms become
tight diatomic molecules which then condense into a molecular BEC.
Alternatively, the atoms might form weakly bound Cooper pairs of
large size (many times the average interatomic spacing). Or the
pairing can be some kind of in-between state. This in-between
pairing regime is poorly understood but intensely interesting since
discoveries there may offer great insights into basic condensed
matter interactions. Theorists say that one potential route to
discovering of exotic new atomic condensates is the use of
unbalanced clouds with an excess of spin-up or spin-down atoms.
Such systems are relevant to the study of magnetized superconductors
and possibly even to pairing in cold quark matter at the centers of
neutron stars. Recently, these systems have moved within reach of
experiments, since in addition to being able to vary interaction
strength, experimenters using cold atoms can vary the relative
number of spin-up and spin-down atoms. One result of such an
imbalance can be a separation into a two-phase gas consisting of a
superfluid core of paired atoms surrounded by a normal-fluid mantle
consisting of unpaired atoms.

Randy Hulet (randy@rice.edu) and his colleagues at Rice University
and Utrecht University have now, for the first time, found evidence
for two distinct superfluid regimes in an imbalanced gas of
fermionic lithium-6 atoms. At lower temperatures, a sharp boundary
between the fully-paired superfluid core and the excess unpaired
atoms is observed, as expected for a first-order phase transition
(the kind of transition---such as water changing to ice---in which
the internal energy of the substance takes a discontinuous jump). At
a slightly higher temperature, the fully paired core and the normal
fluid mantle are separated by a diffuse mixed-phase which is also
superfluid. Moreover, while the higher-temperature gas maintains
the long cigar shape (aspect ratio of 30) imposed by the fields of
the atom trap, the superfluid core of the lower-temperature gas,
under the action of surface tension between the superfluid and
normal phases, deforms towards a more spherical shape (aspect ratio
as small as 2). (Partridge et al., Physical Review Letters, 10
November 2006; text at www.aip.org/physnews/select)

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