Where Space Is Sharply Curved

Abstract

As soon as the theory of general relativity was propounded, astronomers and physicists made ready to test its observational predictions. There was of course the excess motion of the perihelion of the orbit of Mercury, which the theory had been designed in part to explain. There was the gravitational bending of light beams (verified at the eclipse of 1919), the gravitational redshifting of light and the gravitational retarding of light (both verified in recent years). Light rays and the planet Mercury would exist without the theory of general relativity. Its effects on their behavior are small. But there is a class of objects whose existence is predicted by general relativity and which do not exist if the theory in some form is not true. These are the black holes, objects that have collapsed under the influence of their own gravitation until they are so dense and have such strong gravitational fields that neither matter nor radiation can escape from their surfaces. To a general relativist a black hole is an example of extremely curved space. In and around it the effects of general relativity are not minor; they are dominant, and they determine the nature of the phenomena that occur. The discovery of an actual black hole would provide an astrophysical working place for general relativity, a region where the effects of space curvature could be studied readily rather than having to be sorted out as tiny corrections to the nearly flat space that surrounds ordinary objects. As Remo J. Ruffini of Princeton University puts it, the discovery of a black hole would be for general relativity what the discovery of the photon (also, by the way, associated with Einstein) was for quantum theory and the physics of the microcosm. Ruffini believes, and has believed for more than a year, that at least one black hole is currently under observation (SN: 1/13/73, p. 28). Controversy rages, and to the discussion Ruffini now brings theoretical arguments and observational evidence, some of which was barely analyzed in time for him to use it at the meeting of the American Physical Society two weeks ago in Chicago. Ruffini's candidate for a black hole is Cygnus X-1, one of a newly discovered class of celestial X-ray sources, X-ray binaries. Up to, a point there is general agreement as to what these binary sources are: They are systems in which a more or less normal star is gravitationally bound to a highly condensed dark companion. The usual explanation as to how the companion got condensed is that it is the last stage of stellar evolution, what is left over after a supernova explosion. Matter streams out from the normal star and falls onto the dark companion. The streaming matter forms a rotating disk around the dark companion in which pieces of matter gradually spiral inward until they fall on the dark companion. As they do so they convert energy they gain through their fall to X-rays. (The matter in the disks is electrically charged ionized plasma, not neutral gas, so it has means to do this.) The argument starts over the nature of the condensed body. The main candidates are black holes and neutron stars-though a few observers may hold out for white dwarfs. To make the case for black holes or for a black hole one needs a way of distinguishing observationally between the two. Ruffini provides a way. It has to do with magnetic fields. Stars have magnetic fields. When they collapse, even through the violence of a supernova explosion, they should retain those fields. Thus neutron stars and black holes can have magnetic fields. But in the configuration of those fields lies the difference. A neutron star