If black holes are really objects from which no light, nor anything else, can escape, how can we detect their presence ? There are three main possibilities which we take in turn.

First, there is the discovery by the Cambridge astrophysicist Stephen Hawking that black holes are not completely black. Matter antimatter pairs of particles are continually being produced throughout space, even in a perfect ‘vacuum’. This may be considered a consequence of a variant of the Uncertainty Principle; large amounts of energy are available for proportionally brief periods of time. This energy may be sufficient to create an electron-positron pair, say, which then annihilates almost instantaneously. The net effect is nothing produced from nothing. In the extreme gravitational environment close to the Schwarzschild radius of a black hole, one of the newly-created particles may be dragged within the black hole. This time the net effect is a free particle outside the hole. We cannot get something for nothing however, and the black hole decreases in mass by an amount proportional to the mass and energy of the free particle. In this way a black hole evaporates away its rest mass energy. The rate of evaporation is proportional to the inverse square power of the mass of the hole. For this reason, this effect is only important for the less massive, smaller black holes; for example, a black hole of one solar mass would last 1067 years! On the other hand, black holes with masses of less than about 10nkg, which were formed at the origin of the Universe some 1010 years ago, would all have evaporated by now. In fact those with masses of around 10nkg would be completing their evaporation at this moment. It is possible that astronomers might detect the final stages of the evaporation as the former black hole disappears with a flash of gamma rays.

The second property of the gravitational field of a black hole that we can make use of to help us to detect such objects is its ability to bend light rays. Light passing close to any gravitating body is deflected, the amount of deflection being proportional to the strength of gravitational field encountered. The Sun bends light rays which pass close to it by a small amount and the verification of this is one of the key experiments in support of the theory of relativity. Light passing close to a black hole can be bent through a substantial angle. In this way a black hole can act as a GRAVITATIONAL LENS, magnifying objects which lie behind it and distorting them in a characteristic manner. Unfortunately the chance of a black hole and a distant object lining up in this manner is small, but if astronomers could find an unusually magnified object they could conclude that the object responsible for the magnification was a black hole.

Third, because the effective escape velocity from a black hole is the speed of light, an object which falls towards a black hole reaches velocities approaching the speed of light before it dis¬appears from view. In other words, the kinetic energy of the object before it disappears is comparable to its rest mass energy. When it disappears, however, it takes almost all of its energy with it. If we could find some means of tapping this energy, we would have at our disposal the most efficient means of transforming matter into energy that is theoretically possible. For example, if we were to lower a particle of mass one kilogram into a black hole on a string over a pulley, we could use the other end of the string to generate “power. The amount of energy we could obtain theoretically is 42 per cent of the rest mass energy of the kilogram, that is 4x 1016 joules. If the string were made of even the strongest materials we know, however, it would break under its own weight and we could only obtain a minute fraction of the rest mass energy. Fortunately astronomers do have quite a strong ‘string’ available. It is the same ‘string’ which prevents the Earth from crashing into the Sun, that is, centrifugal force.

Suppose we have a quantity of gas which is circling in orbits around a black hole. Then, for the same reason that the inner planets circle the Sun at a faster rate than the outer ones, the gas closer to the black hole orbits it at a faster rate than the gas which is further away. The gas therefore forms a disc of gas that rotates more slowly further from its centre. Friction between the parts of the disc attempts to speed the outer parts up, and to slow the inner parts down, allowing them to fall inwards. In the disc, friction transfers angular momentum to the outer parts and allows the material in the disc to spiral slowly inwards. The heat produced by the friction, which stems originally from the gravitational energy that is being released by the matter falling inwards, is radiated away. Because the inner parts of the disc are so close to the black hole, a large amount of energy can be released for every kilogram that is accreted by the hole in this way. Such an ACCRETION DISC can radiate away up to about 40 per cent of the rest mass energy of the accreted material. Because an accretion disc is such an efficient converter of matter into radiation, we might expect some of the luminous objects in the Universe to be powered by accretion discs. Among the possibilities are quasars, galactic nuclei and A -ray stars.

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