Astronomers sense the presence of black holes throughout the universe.  These fascinating bodies sit at the centers of many galaxies, including our own Milky Way.

          A black hole is pure gravity.  The surface of a black hole, the “event horizon,” is defined solely by a fierce gravitational attraction that traps everything, including light.  For an object at the surface of a black hole the escape velocity is the speed of light, therefore, nothing can escape a black hole’s event horizon.  A black hole’s intense gravity bends light beams and slows clocks.

          Even though black holes seem mysterious or bizarre, most professional astronomers embrace the concept of a black hole.  They see black-hole formation as a natural and inevitable consequence of the deaths of massive stars, and they readily appeal to supermassive black holes to explain the extraordinary luminosities of quasars. 

          There is, however, a serious problem with black holes, one that leaves some scientists sceptical about their existence.  The mystery lies hidden in the hole’s center.  Einstein’s general theory of relativity predicts that we will find there an object more massive than a million Earths and yet smaller than an atom.  So small, in fact, that its density approaches infinity.  Physicists dislike the idea of such an infinitely dense point, which they call singularity, because no conceivable theorem can describe what occurs there. 

          Observations of quasars, the centers of nearby normal galaxies, and the nucleus of our own Milky Way suggest that black holes really exist.  The best available evidence, however, comes from studies of three X-ray binary stars, systems composed of a normal star and a supercompact, X-ray-emitting companion in close orbit. 

          The first black-hole candidate, Cygnus X-1 (the first X-ray source discovered in Cygnus), became famous the moment it was identified in 1971.  Anne Cowley and her colleagues found the second probable black hole, LMC X-3, in the Large Magellanic Cloud in 1983.  More recently, Ronald Remillard and Jeffrey McClintock discovered a third candidate: A0620 – 00.  The “A” refers to Ariel 5, the British satellite that discovered the X-ray star, and the numbers indicate the star’s position in the constellation Monoceros.  It has a faint visual counterpart known as V616 Monocerotis.

          The great outpouring of X-rays, coupled with rapid variations in X-ray brightness, proves beyond doubt that each of these three systems contains a compact object that is either a neutron star or a black hole.  Only such a tiny, massive object can produce the power of 10,000 Suns and also flicker in a split second.

          The X-rays are produced when gas captured from the normal star falls toward the collapsed companion, thereby converting its gravitational potential energy into kinetic energy.  Because the stars are in orbit about their common center of mass and angular momentum must be conserved, the gas cannot move directly from one to the other; instead, it forms an accretion disk around the compact star.  Collisions between gas atoms gradually convert the ordered energy of orbital motion into the distorted energy of heat and cause the material in the disk to spiral slowly inward.  Near the neutron star or black hole the gas temperature rises to millions of degrees, and this heat energy is radiated as X-rays.

          Because these compact stars are in binary systems, it has been possible to determine their masses by measuring the velocities and orbital periods of their visible companions.  This procedure, a cornerstone of 20th-century astronomy, has been applied to thousands of ordinary binaries and is valid regardless of the nature of the unseen star.  By this means, astronomers have convinced themselves that each of these three X-ray stars is more massive than three Suns, the accepted upper limit or a neutron star.  In fact, the most likely mass of each is closer to 10 Suns.

          In summary, the argument for black holes in Cygnus X-1, LMC X-3, and 0620 – 00 is the following: prodigious, flickering X-rays reveal the presence of an extracompact star; the rapid orbital motion of the visible companion implies that this X-ray star overweighs three Suns; and relativity theory rules that nothing can keep such a massive, compact star from collapsing to form a black hole.

          The observational evidence is of high quality, but the argument is indirect; it is not based on observations of super-strong gravity effects like extreme light bending and the freezing of time, phenomena peculiar to black holes alone.

          No theory, not even general relativity, can prove by itself that stellar black holes or any other kind exist.  General relativity does, however, allow that they may exist, and it strongly favours their formation by predicting a definite limit to the outward pressure a star can gather to prevent gravitational collapse.  Also, we have found stars that are almost certainly black holes if general relativity is valid.

          Despite all this, there is good cause for scepticism.  That’s because the argument sketched has a flaw: general relativity has been tested only in weak gravity, gravity one millionth as strong as that at the surface of a black hole.  Therefore our reasoning is circular: we presume that Einstein’s theory correctly describes strong gravity when we argue that certain X-ray stars are black holes, yet, at the same time, these alleged black holes are the test of Einstein’s theory of strong gravity.

          There are two possibilities: general relativity is right about black holes, or it is wrong.  It is entirely possible that Einstein’s theory is wrong, that it breaks down near the surface of a black hole.  For 70 years Einstein’s theory has passed all weak-gravity tests, but it is only the simplest of many possible theories of curved space-time.  This would pose quite a challenge, because it would leave physicists with the task of formulating a new gravitational theory to describe massive X-ray stars and quasars.  Even more important, cosmological models of the evolution of our universe must be founded on a correct description of strong gravity.  Of course, Einstein may have been right about black holes.  That too would leave us with a considerable difficulty: the dreaded singularity.  If he was right, it will demonstrate that purely deductive thought can lead us anywhere.










Lasota, Jean-Pierre.  “Unmasking Black Holes.”  Scientific American.  May 99, 40.
McClintock, Jeffrey.  “Do Black Holes Exist?”  Sky and Telescope.  Jan. 88, 28.