The book both puppy-walks the non-scientist through a number of complicated cosmological concepts, and introduces some advanced mathematics which Hawking then seeks to unpack and explain. Hawking's primary goal in the book is to explain the existence of wormholes in the universe which he believes will give us the ability to time travel.
In Chapter One, Hawking talks about astronomy and some of the first philosophers to study it. He also points out that Aristotle was the first mainstream philosopher to believe that the Earth was round - most people before him had believed it to be flat. However, his grasp of planetary orbit was counter to what we know to be true today - he believed that the Sun went around the Earth, a belief that was debunked in 1609. Thanks to Nicolas Copernicus, the suggestion that it was the Earth that was actually in the Sun's orbit first caught the attention of philosophers and astronomers, and was validated by Galileo some one hundred years later.
He also talks about the origin of the Universe, a subject that Aristotle also considered, believing time to go back forever. Ten centuries later, this was also espoused by Immanuel Kant. In 1929, Edwin Hubble, after whom the Hubble telescope was named, discovered that galaxies are moving away from each other, much like the Earth's plates, and that there was therefore a time when they were all together in one place. This is considered to be the beginning of the Universe by modern scientists.
In Chapter Two, Hawking goes back to Aristotle again, this time with reference to his theory of absolute space. Newton disproved this with his realizations about rest and motion, supported by Galileo who disproved Aristotle's theory that heavy things fall more quickly than light ones. He also brings in James Clerk Maxwell who was the first to conclude that light travels at a fixed speed. The Special Theory of Relativity states that light travels with a finite speed no matter what the speed of the observer is. Speed of light is also the fastest speed that there is; if someone says that something was faster than the speed of light then they are guilty of exaggeration.
The Expanding Universe is the subject of Chapter Three. Physicists and astronomers worked out the distance of stars from the Earth which Hubble translated using luminosity and brightness tests. The brighter the star, the nearer to Earth they must be. Using this method he calculated the distances of each galaxy from the Earth.
Chapter Four talks about the Uncertainty Principle. This says that the speed and position of a particle cannot be discovered at the same time. This is because when a light is shone on a particular particle, that light will change the speed at which it is moving. If a lower level of light is used then the speed that a particle is travelling at can be far more accurately pinpointed, but this makes it much more difficult to ascertain the position of the particle. Light and sound behave in a similar way; light is actually a series of waves, which act like particles. This is the foundation stone of Heisenberg's uncertainty principle.
Elementary Particles and Forces of Nature are the focus of Chapter Five. Quarks are very small things that make up everything that we see. There are six different kinds, in three different colors, making up all forms of matter. Quarks are the Lego bricks of matter that when put together correctly make up everything. This chapter also talks about the forces of gravity, and other electromagnetic forces that can be either weak or strong depending on the degree of spin that they have.
Black Holes are next; Chapters Six and Seven talk about these collapsed stars that suck things into their center because of an exceptionally strong gravitational force at their core. Black holes are the stuff of science fiction space travel, but the cinematic image of spacecraft spinning off into the center of the unknown is not actually too far from the truth. A star has to be over twice the size of the Sun to become a black hole. A star that is small than this is still able to collapse in on itself, but it becomes a different kind of star.
Entropy, otherwise known as disorder, is related to black holes. This is the Second Law of Thermodynamics and says basically that disorder will always increase in the Universe.
Chapter Eight deals with the Origin and the Fate of the Universe. How was it started? How will it come to an end - if at all? Chapter Eight seeks to reveal this. Scientists believe the Universe began with a Big Bang (hence, the Big Bang Theory.) This happened when everything in the Universe, which was previously very hot, clumped together until it had to separate, and smaller galaxies within the Universe were born.
Chapter Nine, The Arrow of Time, talks about why human-interpreted time is different from actual scientific time, but both seem to move in a forward direction, from the past to the future. This is possible due to the arrows of time. These are again part of a theory of thermodynamics that states that whilst everything begins in an orderly state, as time progresses everything becomes more disorderly. The second arrow of time is a psychological one, which is our need for time and events to only travel in a certain direction. We remember the past, not the future, because the future has not happened yet.
The Unification of Physics is the title and subject of Chapter Eleven. Physicists have a range of theories to explain a limited number of things, but there are no complex theories that take the unknown into account at all. In other words, all theories have to prove something. Hawking believes that one day a unified theory that takes into account the things that cannot be known or proven yet will be accepted. This will lead to a unified theory of the Universe. This must include both the uncertainty principle and the things of which scientists are certain, such as gravity. So far, theories that have sought to unify like this have produced unfeasibly small universes or a kind of "super-gravity" binding the galaxies together. Neither was an accepted nor workable theory and both were abandoned.
To conclude, as he does in Chapter Twelve, Hawking postulates that humans have always had a need to understand everything and this is even more true when related to the Universe and the place of the human within it. He would like to see everyone able to understand simple theories of evolution and the Big Bang theory so that every human would be able to follow with reason the way in which the Universe and life on earth was formed, and the reason for it.
