Number 447, September 9, 1999 by Phillip F. Schewe and Ben Stein
A "FERMI-DEGENERATE" ATOMIC GAS, a gas of fermion atoms (atoms composed of an odd total number of constituents--electrons, protons, and neutrons, each of which has half-integer spin) which essentially overlap with one another, has been created for the first time, promising tabletop insights into the basic properties of neutron stars, superfluid helium and all forms of superconductivity. Preparing this gas of fermions requires the exact same conditions as for preparing a Bose-Einstein condensate (BEC) of boson atoms, atoms composed of an even number of constituents with half-integer spin. One must cool a gas of atoms to the point that they exhibit wavelike properties and pack them densely enough so that the average distance between atoms is comparable to their "deBroglie wavelength." At this point, individual atoms become impossible to distinguish. If the atoms are bosons, they fall collectively into the lowest-energy (ground) state to form a BEC (Update 233). If the atoms are fermions, however, this cannot happen. The Pauli exclusion principle prohibits two fermions from occupying the same state. Instead, the fermions dutifully occupy different quantum states on the lowest available energy levels, just as water fills a bottle from the bottom up to some top level. (See figures at Physics News Graphics.) This ensemble of atoms is called a "quantum degenerate gas" owing to the fact that the differences between bosons and fermions only become important in this low-temperature, high-density regime. A Fermi degenerate gas has more energy than predicted by classical physics, because fermions have to occupy higher and higher energy levels once the lower ones get filled up. Achieving this state has been difficult because cooling fermions is more difficult than cooling bosons: placed in a trap made with magnetic fields, fermions in similar states tend to repel each other and avoid the energy-transferring collisions required for "evaporative cooling." To combat this, researchers in Colorado (Deborah Jin, 303-492-5735, NIST/University of Colorado) prepared potassium-40 atoms in two different states of spin, a quantity which describes how the atoms respond to an external magnetic field. The two species could collide with one another and this enabled evaporative cooling to occur. Then, one spin species was removed by a radio-frequency field, leaving about a million of atoms in the other spin species for study. The Colorado group deduced their temperature to be approximately 290 nanokelvins--the lowest ever recorded for a gas of fermions. They witnessed that the fermion nature of the atoms dramatically inhibited evaporative cooling. This is due in part to the Fermi pressure--the repulsion of atoms in the trap--which resists the compression necessary for effective evaporative cooling. (Therefore, this system can provide insights into how the fermions that make up white dwarfs and neutron stars remain buoyant instead of collapsing by the force of gravity.) In the future, researchers hope to study superconductivity by forming Cooper pairs with the fermions, at even lower temperatures than presently achieved. Creating such a "Fermi superfluid" will enable investigations into all forms of superfluidity and superconductivity. (DeMarco and Jin, Science, 10 September 1999.) Other groups are pursuing these and similar states with other fermion atoms (Phys. Rev. Focus, 24 May 1999).
ANOMALOUS ACCELERATION UPDATE. Last year a team of scientists published an assessment of the longterm trajectories of certain spacecraft, including Pioneer 10 and 11, showing that even after all known sources of gravity (sun, planets, comets, etc.) and other forces were taken into account, an extra acceleration seemed to be present (Anderson et al., Physical Review Letters, 5 Oct 98; Update 391). Now a series of letters concerning this assessment appears in the same journal (30 August 1999). Edward Murphy of Johns Hopkins (410-516-4521) argues that the explanation lies in the asymmetric radiation given off by the crafts' electronics as waste heat. Jonathan Katz of Washington University (314-935-6202) implicates the recoil of radiation (from radioactive thermal generators, or RTG, powering the craft) off the rear of the high-gain antenna. The authors of the original paper (contact John Anderson, JPL, 818-354-3956) basically assert (Reply 1, Reply 2) that these two particular explanations fall short of accounting for the anomalous acceleration by roughly a factor of five or more. (All PRL papers are available to journalists from AIP Public Information.)