Number 94, September 16, 1992 by Phillip F. Schewe and Ben Stein GALILEO'S EXPERIMENT AT THE LEANING TOWER OF PISA has been vindicated anew. Italian scientists at the University of Pisa (appropriately enough) have verified the universality of the free fall acceleration, g, at the Earth's surface to an accuracy of a few parts in 10**-10. (S. Carusotto et al., 21 Sept. 1992 issue of Physical Review Letters). They did so by measuring the angular acceleration of a falling disk in vacuum. The falling disk, with mirrors at its rim, acted as a sort of Michelson interferometer. A NEUTRON IMAGE OF THE SUN has been produced using the Compton Telescope on board the Gamma Ray Observatory. Comptel usually records gamma rays, but on this occasion---a solar flare occurring on 15 June 1992---the influx of neutrons was measured. The resultant picture represents the first time a celestial object has been imaged by particles rather than electromagnetic radiation. (Sky & Telescope, October 1992.) SOFT X-RAY LASERS ARE NOW OPERATING AT SATURATION , a condition in which the laser's output intensity increases linearly with the length of the lasing medium, in this case a column of high-temperature, high-density plasma. The availability of such lasers would be important in such fields as biological microscopy and holography, but progress has been slow because it is difficult to achieve a high electron temperature and density in a plasma larger than about 1 cm. Two groups have succeeded in reaching saturation. A Rutherford/Oxford/York/Essex/Hull/Belfast/Paris collaboration has used a germanium plasma to produce light at a wavelength of 23.6 nm (A. Carillon et al., Phys. Rev. Lett., 11 May 1992). A Livermore/LBL team has produced 20.6 nm light in a selenium plasma (J.A. Koch et al., Phys. Rev. Lett., 1 June). ELECTRONS HAVE BEEN TRAPPED ABOVE POTENTIAL ENERGY WELLS , confirming a prediction made in the early days of quantum mechanics by Eugene Wigner and Max von Neumann. Usually, electrons are trapped inside potential wells; that is, they are trapped because they do not have enough energy to escape the attractive forces binding them. But if the potential wells are specially designed, Wigner and von Neumann argued, the electrons can interfere in such a way as to be in a bound state--but have more than enough energy to escape the well. Because these setups require a specific, rigorous geometry, the proposal was regarded at first as an abstract theoretical scenario instead of a physical possibility. But the advent of molecular beam epitaxy (MBE) techniques has changed that. Using MBE, Federico Capasso (908-582-7737) and his co-workers at AT&T Bell Labs constructed a semiconductor device made of alternating GaInAs and AlInAs layers and impurity-doped GaInAs layers. The alternating layers create a row of narrow wells which serve as mirrors, and they surround a wide well constructed with the doped layers. The mirrors reflect the electron waves, which interfere so that a bound state is created above the wide well. The researchers shone infrared light on the device and the absorption patterns reveal a transition between a bound state inside the potential well and a bound state above the energy well. (Nature, 13 August 1992.)