54 pages • 1 hour read
Stephen HawkingBrief Answers to the Big Questions
Nonfiction | Book | Adult | Published in 2018A modern alternative to SparkNotes and CliffsNotes, SuperSummary offers high-quality Study Guides with detailed chapter summaries and analysis of major themes, characters, and more.
Chapter 5Chapter Summaries & Analyses
Chapter 5 Summary: “What Is Inside a Black Hole?”
In 1783, John Michell of England imagined stars so big that light cannot escape their gravity. He called them “dark stars,” and today they’re called black holes. Einstein and others believed that stars wouldn’t collapse beyond a certain point that bends space and time out of existence, but during the 1950s and 1960s, John Wheeler proved that they could. With enough density, such stars shrink down to a single, infinitely dense point called a singularity.
The first singularities were discovered during the early 1960s. They were called quasars because of the bright radiation released around them. Black holes otherwise all look the same, in that the contents within them are invisible since even particles of light can’t escape them within a certain distance from the singularity, a distance called the event horizon.
A black hole that’s a few times the sun’s mass would tear apart anything approaching its event horizon. A human would be stretched violently and “made into spaghetti” (106). A huge black hole with the volume of a million suns, though, would absorb a person gently. Outsiders would see that explorer slow down near the event horizon until fading from view. Once inside the black hole, the explorer would soon be crushed down into the singularity.
As objects are drawn into a black hole, its event horizon grows. It can’t radiate heat since energy gets sucked back into the hole. Surprisingly, the author’s calculations showed that black holes release matter and energy anyway because, according to the laws of quantum mechanics, black holes slowly evaporate. A small one with the mass of a mountain would evaporate quickly, however, giving off enough energy to power human civilization.
Other than mass, rotation, and charge, a black hole that evaporates would seem to emit nothing of the information contained in all the mass that originally made it up. Unfortunately, this violates the rule that all of the universe is determined by the laws of physics. Recent studies, however, suggest that black holes may contain many properties called symmetries. Some symmetries, called “supertranslations” and “superrotation charges,” might permit some or all of the information absorbed by a black hole to return to space as the singularity evaporates. Thus, black holes do retain information about the things that fall into them.
Chapter 5 Analysis
The chapter on black holes is central to the author’s life work. His answer to the question about what’s inside a black hole is fairly simple: It’s all the stuff that fell into its intense gravity well and condensed down to a single point—the singularity, where density and gravity are infinite and the laws of physics break down. Surrounding that point is a region, defined by an “event horizon,” inside which nothing that enters can escape. Everything in the event horizon is drawn into the singularity. The singularity, Hawking points out elsewhere in the book, is remarkably similar to the condition of the universe when it first began: It has infinite density and has no space or time. The difference is that the universe exploded outward, while a black hole contracts inward.
Hawking mentions the first black holes ever discovered, which were called quasars because of their quasi-stellar brightness. Black holes may be invisible, but any matter caught in their gravity fields will swirl faster and faster around the hole until torn apart by accelerating tidal forces. The shredded matter emits high-energy photons—x-rays and gamma rays—along with bright visible light. These are the last signals sent by matter on its way to the grave of the black hole.
The author mentions John Michell, an English scientist-turned-cleric who did important work in astronomy and the study of earthquakes, among other things. Michell reasoned that some stars might be so massive that even light could not escape them. He made this argument in 1783, when it was read before the Royal Society. (Pierre-Simon Laplace, another scientist that Hawking mentions in the book, separately made the same argument in the late 1790s.) Hawking’s point is that, through experiment and reasoning, scientists can make astounding predictions that later generations develop into complete theories. This supports the book’s theme of Knowing the Universe Through Science (Montgomery, Colin, et al. “Michell, Laplace and the Origin of the Black Hole Concept.” Journal of Astronomical History and Heritage, Vol. 12, No. 2, p 95, 2009).
Neither Michell nor Laplace imagined the true nature of the hyper-dense black holes their theories predicted. At the time, people knew next to nothing about the interiors of stars, much less the singularities within black holes. The predictions nonetheless demonstrate the remarkable ability of the human mind to reason out the existence of amazing things that derive logically from things already discovered. It’s part of the strength of the scientific method, which itself is one of humanity’s superpowers.
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