THE STRONGEST GRAVITATIONAL FIELDS EVER MEASURED, corresponding to a spacetime warping of 30%, have been recorded by scientists using the Rossi X-Ray Timing Explorer (RXTE) satellite. By comparison, the proportional curvature of space is 100% at a black hole, but only about one part in a million near the sun's surface and one part per billion near the Earth's surface. RXTE was designed to monitor (over microsecond time intervals) the x rays coming from binary star systems in which matter from a conventional star is siphoned off into an accretion disk surrounding a nearby neutron star or black hole. In about 16 binary-star systems that contain neutron stars, blobs of gas in the disk are thought to spiral in toward the neutron star, picking up speed before they make a final plunge onto the surface. The x rays produced in this process are regularly dimmed when the hot gas is on the far side of the star. This leads to quasi-periodic oscillations (QPOs) in the x-ray brightness of the star. Also notable is the fact that the brightness variations only occur at certain well-defined rates, "pure tones" corresponding to special orbital periods for the gas going around the star. The spacetime encountered by the gas is so highly warped because the gas is able to skim within a few km of the neutron star, which itself is only about 10 km in diameter. At this week's meeting of the American Physical Society in Columbus, Ohio, Frederick Lamb of the University of Illinois (217-333-6363, f-lamb@uiuc.edu) described how the observed variations in the x-ray brightness can be used to deduce properties of the neutron star, such as its mass and size. At a press conference, Lamb and William Zhang of NASA Goddard concentrated on the binary-star system 4U1820-30, about 20,000 light years from Earth. The neutron star has a mass of 2.3 solar masses and orbits its companion star in only 11 minutes. Close observations of this system confirm a prediction made by Lamb and his colleagues Coleman Miller and Dimitrios Psaltis that the gas blobs would continue to spiral inward until they reached an "innermost stable orbit," where they would orbit before making the dive for the surface. This is a purely general relativistic (GR) effect; in Newton's mechanics, by contrast, the blob could have gotten arbitrarily close to the surface, providing it were going fast enough. The observations by Zhang and his collaborators now confirm Lamb's prediction, thus opening up a new "strong-gravitational field" era in GR studies. The measurements of the gas motion even provide hints as to the nature of the strong nuclear force sustaining the neutron star against further gravitational collapse. The new evidence indicates that the nuclear force is stiffer and more repulsive than has generally been thought.