Journal 39

This article on Black Holes is a condensed version of the talk I gave to the Society earlier this year. Most people are familiar with the term 'Black Hole' and have at least a vague idea of what they are. The very term conjures up the notion of a bottomless pit or abyss. Furthermore science fiction films on the subject such as Walt Disney's "The Black Hole" and "Event Horizon" have to a certain extent contributed popular general interest in this area.

A Black Hole is, in simple terms, just a region of space-time where the gravitational field is so strong that nothing, not even light, can escape. So what made them such a topic of interest and why all the fuss about them during the second half of this century?

What often surprises many people is that the idea of a 'Black Hole' was first contemplated more than two hundred years ago in the late 1700s.

A paper written by the Rev John Michell in 1783 was discovered in the 1970s. This is the first known discussion of the concept of a black hole. John Michell (1724-1793) was born three years before the death of Isaac Newton. He became a well-known British geologist and astronomer and was later regarded as the 'Father of Seismology' in his study of Earthquakes. He is also credited with the idea of Binary Stars, the demonstration of an inverse square law in magnetism, and was the inventor of the torsion balance before instigating the experiment, later completed by Cavendish, to weigh the Earth.

At the time the 'corpuscular' theory of light was the vogue. This regarded light as being made up of 'corpuscles' or particles similar in some respects to the modern idea of the photon. It was therefore considered a possibility that light could be affected by gravity in the same way as ordinary matter. Over one hundred years prior to this in 1676 Olaus Roemer had discovered that the speed of light was finite through observed variations in the period of Jupiter's moon Io. Observations of stellar aberration by James Bradley in 1728 produced further confirmation and a more accurate value for the speed of light of 295,000 kilometers per second compared to today's figure of 300,000 km per second. The Newtonian concept of escape velocity as being the minimum velocity needed to escape from a planet's surface to infinity was well understood. For a spherical mass M of radius R it is simply: sqrt(2GM/R) where G is the Gravitational constant. The escape velocity thus increases as the object's mass increases and also increases if the mass remains the same but the radius gets smaller.

Michell pondered a body so massive that the escape velocity at its surface was equal to the speed of light. In his 1783 paper to the Royal Society Michell wrote:

If the semi-diameter of a sphere of the same density as the Sun in the proportion of five hundred to one, and by supposing light to be attracted by the same force in proportion to its [mass] with other bodies, all light emitted from such a body would be made to return towards it, by its own proper gravity.

In the above Michell contemplated the existence of a star 500 times the radius of the Sun and of the same density. For such an object he calculated that the gravitational field would be so strong at its surface that the escape velocity would exceed the speed of light. From this hypothetical star not even light could escape and the star would be invisible. Although he thought it unlikely, he considered the possibility that many such objects could be present in the cosmos without us being able to see them.

In 1796, thirteen years later the great French mathematician, astronomer and physicist Pierre Laplace proposed similar ideas to those of Michell in his famous paper 'Exposition du Systeme du Monde'.

In the early 1800's experiments on optical interference led to the predominance of the wave theory of light and the end of the corpuscular theory. Since light waves were thought to be unaffected by gravitation interest in the hypothetical "dark stars" ceased.

In 1905 Albert Einstein published his Special Theory of Relativity and in 1915 his General Theory of relativity. The General Theory was a new theory of gravitation and one of its fundamental predictions was the effect of gravity on light. According to the theory matter causes space-time to curve. The paths followed by light rays or matter is determined by the curvature of the space-time and allowed a modern scientific proof of Mitchell's hypothesis.

Further researchers followed on from Einstein and this will be described in Part 2.