In The Universe in a Nutshell, Stephen Hawking summarizes what astronomers know about the universe. The book contains far more illustrations, and less detailed text, than its popular predecessor, A Brief History of Time.

Toward the end of the 19th century scientists believed they were close to a complete description of the universe. They imagined that space was filled by a continuous medium called the Ether. Light rays and radio signals were waves in this ether just as sound is pressure waves in air. All that was needed for a complete theory were careful measurements of the elastic properties of the ether.

Hawking describes the history of astrophysics. As scientists continued to gain more knowledge, their picture of the universe became more solid. This drove numerous scientists to the opinion that humans would soon have a thorough understanding of their environment. Such an understanding would allow scientists to calculate any phenomenon. But soon thereafter, new discoveries perplexed human understanding, revealing a far more mysterious universe.

Not only does a black hole have a temperature, it also behaves as if it has a quantity called entropy. The entropy is a measure of the number of internal states (ways it could be configured on the inside) that the black hole could have without looking any different to an outside observer, who can only observe its mass, rotation, and charge. This black hole entropy is given by a very simple formula I discovered in 1974. It equals the area of the horizon of the black hole: there is one bit of information about the internal state of the black hole for each fundamental unit of area of the horizon.

Hawking relates his discovery of a surprising feature of black holes. Whereas previously astronomers had expected black holes to swallow all information, instead Hawking discovered that black holes have certain measurable properties. These include temperature and entropy. Hawking radiation is a form of emission from black holes, named after author Stephen Hawking. The knowledge that a black hole has entropy proportional to its area reveals that gravity, a fundamental physical force, has much to do with heat, another important topic in physics. Furthermore, Hawking argues that this knowledge supports a universe that stores information in fewer dimensions than appear.

Despite this cautionary tale, I believe we can and should try to understand the universe. We've already made a remarkable progress in understanding the cosmos, particularly in the last few years. We don't yet have a complete picture, but this may not be far off.

Hawking defends the merit of investigating the universe scientifically. Despite the failures of previous scientists to understand the environment in its entirety, Hawking holds out hope that humans may soon have such an understanding. More recent technological advances show a far deeper and more detailed picture of what the universe contains than ever before. Hawking writes that the universe contains billions of galaxies. This suggests that the universe may contain far more material than humans can fully comprehend, despite Hawking's defense of the scientific pursuit. However, astrophysicists and others do gain a better understanding of their surroundings from such investigations.