Black holes, chapter 6

 

Chapter 6

Black holes

·         Two hundred years ago, there were two theories about light. One which Newton favored was that light was composed of particles. The other one that it was made of waves. Now by the wave/particle duality of quantum mechanics, light can be regarded as both a wave and a particle.

·         A star that was sufficiently massive and compact would have such a strong gravitational field that light could not escape: any light emitted from the surface of the star would be dragged back by the star’s gravitational attraction before it could get very far. Although we would not be able to see these stars because the light from them would never reach us, we would still feel their gravitational attraction.

Such objects are what we now call black holes.

A consistent theory of how gravity affects light did not come along until Einstein proposed general relativity in 1915.

·         To understand how a black hole might be formed, we first need an understanding of the life cycle of a star: A star is formed when a large amount of gas (mostly hydrogen) starts to collapse in on itself due to its gravitational attraction. As it contracts the atoms of the gas collide with each other and the gas heats up. Eventually the gas will be so hot that when the hydrogen atoms collide they no longer bounce off each other but they coalesce to form helium. The heat released in this reaction, which is like controlled hydrogen bomb explosion, is what makes the star shine. This additional heat also increases the pressure of the gas until it is sufficient to balance the gravitational attraction, and the gas stops contracting.

·         When a star runs out of fuel, it starts to cool off and so to contract. If its mass is under a certain limit, it stops contracting and settles down to a possible final state as a “white dwarf”.

But stars with masses above the Chandrasekhar limit, when they come to the end of their fuel, in some cases they may explode or manage –rarely – to throw off enough matter to reduce the mass below the limit.

·         The gravitational field of a star changes the paths of light rays in space-time and eventually when the gravitational field at the surface of the star becomes very strong, the light cones are bent inward so much that light can no longer escape. So, if light cannot escape, neither can anything else. Everything is dragged back by the gravitational field. The light rays are trapped in and no light can be seen.

 A black hole is a region of space-time (where a star collapsed) from which nothing, not even light, can escape, because gravity is so strong.

·         In the long history of the universe, many stars must have burned all their nuclear fuel and have had to collapse. The number of black holes may well be greater than the number of visible stars, which totals about one hundred thousand million in our galaxy alone.

 Black holes are not really black after all: they glow like a hot body, and the smaller they are, the more they glow!

(continued in chapter 7)