A Brief History of Time | Study Guide

Stephen Hawking

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A Brief History of Time | Chapter 6 : Black Holes | Summary



The description of light as being composed of particles (a theory favored by Sir Isaac Newton) or as waves (based on the behavior of light in certain circumstances) brings Stephen Hawking to his discussion of black holes in order to examine how light as waves might respond to gravity. Because a black hole is an object that is very dense (its gravitational pull would make it impossible for anything—including light—to escape it), the idea was long held that light behaved like a particle upon which the force of gravity could exert attractive pressure. But because light (being a massless wave) has a fixed speed, it was unclear how gravity could affect it. A theory was not put forth to make sense of this question until Albert Einstein brought forward the theory of general relativity in 1915.

Hawking proceeds to describe the life cycle of a star, beginning when a massive amount of gas collapses in on itself because of its own gravity. The atoms of the gas collide, generating so much heat they coalesce by nuclear fusion, forming larger atoms and creating tremendous amounts of energy. This process causes a star to shine, emitting light and heat until it runs out of fuel. As it cools off, the star begins to contract. At this stage, stars with low masses become white dwarfs or neutron stars. Some stars explode into space as supernovas, and the material from the explosion then becomes available for the creation of new stars. Those stars that have greater masses may completely collapse into black holes.

A physicist named Subrahmanyan Chandrasekhar "worked out how big a star could be and still support itself against its own gravity after it had used up all its fuel." This value is called the Chandrasekhar limit. If a star has a mass below 1.4 solar masses (or 1.4 times the mass of our sun) when it runs out of fuel, it becomes a stable cold star (such as a white dwarf or neutron star). But if a star has a mass above 1.4 solar masses, it collapses into a black hole.

By definition, a black hole is an object in space that can only be detected by its intense gravitational pull, which allows nothing that comes near it, not even light, to escape. Much speculation abounds regarding what a black hole formation would look like, both on the surface and from a distance. Roger Penrose's cosmic censorship hypothesis proposed that it is not possible to observe a singularity (speculated to be within a black hole) because it will always be tucked behind an event horizon (the outer "rim" of the black hole). This hypothesis was later disproven, at least in very constrained circumstances, causing Hawking to lose a bet.

Black holes are among a few phenomena scientists had predicted by using mathematical models before observations were recorded. Robert Oppenheimer predicted in 1939 that contemporary instruments would not be able to observe a black hole. In the 1960s, better instruments and strategies enabled observation of the phenomenon.

The only way to detect a black hole is by observing how stars near one behave. Thus far, black holes have been implicated by studying their interactions with several types of celestial features, including stars in orbit with seemingly empty spaces, star matter being pulled off and into a black hole, as well as observing pulsars, galactic centers, and quasars near black holes.

The event of a star collapsing into a black hole suggests the exact opposite of what physicists observe as an expanding universe, in a sense providing a model of what the earliest form of the universe might have been. Hawking's approach to understanding the comparable concentration of mass in a black hole is his approach to making guided speculations about what happened at the very beginning of the universe. Together with Roger Penrose, Hawking has used this information on his hunt for primordial black holes, or black holes still existing in the universe that had been formed at the moment of the big bang.


Resistance to revolutionary findings in science rises yet again in this chapter. Stephen Hawking cites several scientists who had a difficult time believing that a star of high mass would collapse inwards into itself to form a black hole—especially because the existence of such an object can only be inferred by the behavior of large objects (such as other stars) in its vicinity.

This chapter also gives us some insights into Hawking's personality. He describes two bets he made with other scientists. In one instance, he bet against his own hypothesis, revealing a degree of lightheartedness in his approach to his professional life and legacy.

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