The Riddle of Black Holes

The most mysterious objects in the universe must certainly be the invisible and inconceivably dense objects known as black holes. Nothing can escape the grip of a black hole. But how can we be certain of their existence if we cannot see them?

To imagine a black hole, think of an abyss - small in size and yet unimaginably dense - that swallows everything that comes within its reach. The formal definition of a black hole is a star that has collapsed under its own weight several times, becoming more compact each time. Because of its extremely high density, this bottomless pit attracts even the smallest speck of dust that comes near. Nothing, absolutely nothing, can excape the pull of a black hole - not even light. In fact, this is why it is known as a black hole.

Ever since the name 'black hole' was coined, by the American astrophysicist John Weeler, astronomers have been trying to uncover the mysteries of this cosmic riddle. Even Albert Einstein (1897-1955), whose theory of relativity predicted the existence of black holes, was not quite sure if such an absurdity could really exist.

Physical forces of attraction.

Ever since the revolution in physics initiated by Sir Isaac Newton(1643-1727), we know that there is a shared force of attraction between two bodies, and that this becomes stronger with the decrease in distance between them and with the increase in their mass. However, it is a huge leap from this basic principle to the idea that there might be stars so compact that notyhing can resist the pull of their gravity.

Fortunately, we can turn to theoretical calculations to assist our imagination. These rell us, for example, that if our sun were to become a black hole, its mass would have to compact to 10³⁰ kg - a one followed by 30 zeros - which would turn it tinto a ball 2 km in diameter. To put this into perspective, remember that the sun has a diameter of 1,392,000 km (measured at the equator). And if the earth were to mutate into a black hole, it would have to find room for its present mass of 10²⁵ kg in a ball just 2 cm in diameter - from its present diameter of 12,756 km (measured at the equator). But it is almost impossible for us to conceive of such immensely dense matter, and of the forces that are capable of compacting matter in this way.

Celestial furnaces

It is likely that we will begin to understand black holes once astronomers have learned more about the general processes at work in the universe, and particularly about the processes that produce and sustain stars. Like living beings, stars go through several stages. We know that stars are huge balls of gas, consisting primarily of hydrogen, and derive their energy from the conversion of hydrogen into helium. This takes place through nuclear fusion, which occurs only at extremely high temperatures and under immense pressures. Once the fusion process becomes self-sustaining, the energy that is produced blances the weight of the star's external surface. As a result, the star maintains a state of equillibrium as long as it has enough fuel to keep its nuclear furnace burning, it will not collapse.

The next stage, however, begins when the fuel begins to dwindle. When this happens, the weight of the outside mantle begins to press in on the star, and it implodes (collapses in on itself) in stages. Each time this happens, it causes the temperature and pressure inside the star to rise to the point where its reserves of helium are used up. After several collapses, the fiery celestial body becomes an old, compact star. If its radius then falls below a certain size - the so called Schwatrzschild tadius - the pull of its gravitational force increases to the extent that it captures all matter and radiation. At this point, the star has become a black hole.

Indirect traces.

In theoretical terms, this is what happens to stars which have at least three times the mass of our sun - as many in fact have, or more complex. One problem is the difficulty of discovering black holes. We cannot observe them, since they are invisible. So astrophysicists are forced to look for indirect indications of their existence. Their search is made easier by the phenomenon of so-called binary stars - pairs of stars which rotate around one another. If one of these becomes a black hole, the other begins a slow struggle to the death. Its matter is attracted by its partner until it is completely absorbed. Before it is oaborbed, however, the flow of dust forms a so-called accretion disc around the black hole. Within the disc, extremely hot matter is strongly accelerated, releasing radio waves. The disc rotates with a velocity directly proportional to the mass of the black bole, which can then be calculated.

This technique has already been used to determine the location of black holes - for example, in the galaxy M106, located 21 light-years from earth. Some astronomers even believe that there is a black hole in the centre of every galaxy.

Perhaps black holes will provide the key to another cosmic mystery, that of the hidden mass of the universe. Since galaxies rotate like giant centrifuges, the objects at their outside edges are only held in place by the total mass of the objects at the centre. Think of a carousel or merry-go-round at a fair; the faster it turns, the tighter you have to hold on. Similarly, the force that holds the stars at the edge of the galaxy has to increase as their velocity increases. This means that a greater mass is needed to exert this force.

According to astrophysicists calculations, the mass of all matter in space is not sufficient to hold the distant stars. This has given rise to the idea that there is some invisible mass of matter that is capable of exerting sufficient force on all existing stars. It is possible that this force could be exerted by one or two gigantic black holes at the centre of the universe.