A Brief History of Time | Study Guide

Stephen Hawking

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A Brief History of Time | Chapter 7 : Black Holes Ain't So Black | Summary



Stephen Hawking opens this chapter with a revelation that came to him in 1970 about how light and matter behave inside a black hole versus how it would have to behave at its boundary (its event horizon). Instead of automatically falling into the black hole, both matter and light in a region at the cusp of a black hole neither fall in nor escape.

This means that as matter and light come into the black hole, the event horizon can increase but never decrease. Furthermore, two black holes can collide and merge into a single larger one. This behavior of never decreasing also invokes the property of entropy, which is the measurement of the degree to which an ordered system breaks down into disorder. This is in keeping with the second law of thermodynamics, which states that entropy (the progression of disorder in an ordered system over time) will only increase—not decrease—in an isolated system. This led to the idea that the area of a black hole's event horizon could be used to measure its entropy.

But if black holes have entropy, they must also have temperature. This presents a problem, because for something to have a temperature, it must emit energy. To this point black holes were defined as not allowing energy to escape. During a visit to Moscow, Hawking discussed this problem with other researchers, who argued that black holes must emit radiation.

Although it seemed impossible, Hawking began to agree with their proposal, having found a solution in the uncertainty principle and the behavior of virtual particles. Because of the behavior of particles and their paired antiparticles around a black hole, it is possible for one of the pair to alter its properties and either fall into the black hole or leave its proximity. Hawking makes it clear that "the particles do not come from within the black hole, but from the 'empty' space just outside the black hole's event horizon." So, while mass does not escape the black hole, energy does.

Based on this notion, it is possible that very small black holes are actually white hot, emitting high levels of gamma radiation. What happens when the mass of a black hole is depleted is still unclear; it may explode, or it may simply disappear.

Hawking concludes the chapter by saying that the emissions of radiation from black holes suggest that its gravitational collapse is not an absolute certainty, as originally thought. The loss of energy from black holes has informed Hawking's future efforts to discover primordial black holes and learn more about the big bang.


Once again, Stephen Hawking discusses the resistance to new scientific ideas, which appears to be a recurring theme in the history of science, especially in the 20th century. He points out the fact that he wrote a research paper in part because of his own annoyance at a student whom he believed had "misused [Hawking's] discovery of the increase of the area of the event horizon." Similarly, Hawking reports being "greeted with general incredulity" after his presentation of his calculations that predicted radiation escaping black holes. In both cases, the scientists had eventually agreed in general with the new realizations, given enough time to digest it and offer enough additional scrutiny.

The prediction that black holes emit radiation is notably the first such prediction that relies heavily on both general relativity and quantum mechanics. This breakthrough in understanding would shape Hawking's career going forward. From 1975 onward, his approach to finding a quantum theory of gravity took on different tactics, as will be explained in later chapters.

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