The term black hole is of very recent origin. It was coined in 1969 by the American scientist John Wheeler as a graphic
description of an idea that goes back at least two hundred years, to a time when there were two theories about light:
one, which Newton favored, was that it was composed of particles; the other was that it was made of waves. We now
know that really both theories are correct. By the wave/particle duality of quantum mechanics, light can be regarded as
both a wave and a particle. Under the theory that light is made up of waves, it was not clear how it would respond to
gravity. But if light is composed of particles, one might expect them to be affected by gravity in the ...view middle of the document...
(Also, the particle theory of light went out of favor during the nineteenth century; it seemed that everything could
be explained by the wave theory, and according to the wave theory, it was not clear that light would be affected by
gravity at all.)
In fact, it is not really consistent to treat light like cannonballs in Newton’s theory of gravity because the speed of light is
fixed. (A cannonball fired upward from the earth will be slowed down by gravity and will eventually stop and fall back; a
photon, however, must continue upward at a constant speed. How then can Newtonian grav-ity affect light?) A
consistent theory of how gravity affects light did not come along until Einstein proposed general relativity in 1915. And
even then it was a long time before the implications of the theory for massive stars were understood.
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 more and more frequently and at greater and greater speeds –
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...