A Brief History of Time

Publication

Early in 1983, Hawking first approached Simon Mitton, the editor in charge of astronomy books at Cambridge University Press, with his ideas for a popular book on cosmology. Mitton was doubtful about all the equations in the draft manuscript, which he felt would put off the buyers in airport bookshops that Hawking wished to reach. With some difficulty, he persuaded Hawking to drop all but one equation. The author himself notes in the book's acknowledgements that he was warned that for every equation in the book, the readership would be halved, hence it includes only a single equation: E = mc². The book does employ a number of complex models, diagrams, and other illustrations to detail some of the concepts it explores.

Contents

In A Brief History of Time, Stephen Hawking attempts to explain a range of subjects in cosmology, including the Big Bang, black holes and light cones, to the nonspecialist reader. His main goal is to give an overview of the subject, but he also attempts to explain some complex mathematics. In the 1996 edition of the book and subsequent editions, Hawking discusses the possibility of time travel and wormholes and explores the possibility of having a universe without a quantum singularity at the beginning of time.

Chapter 1: Our Picture of the Universe

In the first chapter, Hawking discusses the history of astronomical studies, including the ideas of Aristotle and Ptolemy. Aristotle, unlike many other people of his time, thought that the Earth was round. He came to this conclusion by observing lunar eclipses, which he thought were caused by the earth's round shadow, and also by observing an increase in altitude of the North Star from the perspective of observers situated further to the north. Aristotle also thought that the sun and stars went around the Earth in perfect circles, because of "mystical reasons". Second-century Greek astronomer Ptolemy also pondered the positions of the sun and stars in the universe and made a planetary model that described Aristotle's thinking in more detail.

Today, it is known that the opposite is true: the earth goes around the sun. The Aristotelian and Ptolemaic ideas about the position of the stars and sun were disproved in 1609. The first person to present a detailed argument that the earth revolves around the sun was the Polish priest Nicholas Copernicus, in 1514. Nearly a century later, Galileo Galilei, an Italian scientist and Johannes Kepler, a German scientist, studied how the moons of some planets moved in the sky, and used their observations to validate Copernicus's thinking. To fit the observations, Kepler proposed an elliptical orbit model instead of a circular one. In his 1687 book on gravity, Principia Mathematica, Isaac Newton used complex mathematics to further support Copernicus's idea. Newton's model also meant that stars, like the sun, were not fixed but, rather, faraway moving objects. Nevertheless, Newton believed that the universe was made up of an infinite number of stars which were more or less static. Many of his contemporaries, including German philosopher Heinrich Olbers, disagreed.

The origin of the universe represented another great topic of study and debate over the centuries. Early philosophers like Aristotle thought that the universe has existed forever, while theologians such as St. Augustine believed it was created at a specific time. St. Augustine also believed that time was a concept that was born with the creation of the universe. More than 1000 years later, German philosopher Immanuel Kant thought that time goes back forever.

In 1929, astronomer Edwin Hubble discovered that galaxies are moving away from each other. Consequently, there was a time, between ten and twenty billion years ago, when they were all together in one singular extremely dense place. This discovery brought the concept of the beginning of the universe within the province of science. Today, scientists use two partial theories, Einstein's general theory of relativity and quantum mechanics, to describe the workings of the universe. Scientists are still looking for a complete unified theory that would describe everything in the universe. Hawking believes that the search for such a universal theory, even though motivated by the essential human need for logic, order and understanding, might affect the survival of the human species.

Chapter 2: Space and Time

Before Galileo and Newton, it was assumed that Aristotle was right in saying that objects are essentially at rest until a force acts to set them in motion. Newton proved this to be wrong, using Galileo's experiments to devise his laws of motion. Newton also developed his law of gravitation, which accurately described the motions of space objects. According to the Aristotelian tradition, events stay in the same place over a period of time. Newton's laws proved that to be false, positing that each object in the universe is moving relative to others, and that it is impossible to assign an absolute resting position.

Both Aristotle and Newton believed in absolute time, a concept independent of space. But this belief does not work for objects moving at or near the speed of light. The speed of light was first measured in 1676 by the Danish astronomer Ole Christensen Roemer, who observed that the time it took light to come from Jupiter's moons varied depending on their distance from the earth. The speed of light was found to be very fast but finite. However, scientists found a problem when they tried to say that light always traveled at the same speed. The scientists imagined a substance called the ether to explain light's absolute speed.

But the assumption of ether did not properly explain the speed of light in many other phenomena. In 1905, Albert Einstein said the idea of the ether was not needed if another idea, the idea of absolute time (or time that is always the same) was dropped. French mathematician Henri Poincare also had the same idea, but from a mathematical perspective. Einstein's idea is called the theory of relativity.

Events can be described by light cones. The top of the light cone tells where the light from the event will travel, the bottom tells where the light was in the past, and the center is the event itself. Besides light cones, Hawking also talks about how light can bend. When light goes past a highly massive object, such as a star, the light changes direction slightly towards the object.

After talking about light, Hawking talks about time in Einstein's theory of relativity. One prediction that Einstein's theory makes is that time will go by slower when something is near huge masses. However, when something is farther away from the mass, time will go by faster. Hawking used the idea of two twins living at different places to describe his idea. If one of the twins went to live on a mountain, and another twin went to live near the sea, the twin who went to live on the mountain would be a little bit older than the twin who went to live at the sea.

Chapter 3: The Expanding Universe

In this chapter, Hawking talks about the expanding universe. The universe is getting bigger over time. One of the things he uses to explain his idea is the Doppler shift. The Doppler shift happens when something moves toward or away from another object. There are two types of things that happen in Doppler shift - red shifting and blue shifting. Red shifting happens when something is moving away from us. This is caused by the wavelength of the visible light reaching us increasing, and the frequency decreasing, which shifts the visible light towards the red/infra-red end of the electromagnetic spectrum. Red-shift is linked to the belief that the universe is expanding as the wavelength of the light is increasing, almost as if stretched as planets and galaxies move away from us, which shares similarities to that of the doppler effect, involving sound waves. Blue shifting happens when something is moving toward us, the opposite process of red-shift, in which the wavelength decreases and frequency increases, shifting the light towards the blue end of the spectrum. A scientist named Edwin Hubble found that many stars are red shifted and are moving away from us. Hawking uses the Doppler shift to explain that the universe is getting bigger. The beginning of the universe is thought to have happened through something called the Big Bang. The Big Bang was a very big explosion that created the universe.

Chapter 4: The Uncertainty Principle

This chapter is about the uncertainty principle. The uncertainty principle says that the speed and the position of a particle cannot be found at the same time. To find where a particle is, scientists shine light at the particle. If a high frequency light is used, the light can find the position more accurately but the particle's speed will be unknown (because the light will change the speed of the particle). If a lower frequency light is used, the light can find the speed more accurately but the particle's position will be unknown. The uncertainty principle disproved the idea of a theory that was deterministic, or something that would predict everything in the future.

How light behaves is also talked more about in this chapter. Some theories say that light acts like particles even though it really is made of waves; one theory that says this is Planck's quantum hypothesis. A different theory also says that light waves also act like particles; a theory that says this is Heisenberg's uncertainty principle.

Light waves have crests and troughs. The highest point of a wave is the crest, and the lowest part of the wave is a trough. Sometimes more than one of these waves can interfere with each other - the crests and the troughs line up. This is called light interference. When light waves interfere with each other, this can make many colors. An example of this is the colors in soap bubbles.

Chapter 5: Elementary Particles and Forces of Nature

Quarks and other elementary particles are the topic of this chapter.

Quarks are very small things that make up everything we see (matter). There are six different "flavors" of quarks: the up quark, down quark, strange quark, charmed quark, bottom quark, and top quark. Quarks also have three "colors": red, green, and blue. There are also anti-quarks, which are the opposite of the regular quarks. In total, there are 18 different types of regular quarks, and 18 different types of anti quarks. Quarks are known as the "building blocks of matter" because they are the smallest thing that make up all the matter in the universe.

All particles (for example, the quarks) have something called spin. The spin of a particle shows us what a particle looks like from different directions. For example, a particle of spin 0 looks the same from every direction. A particle of spin 1 looks different in every direction, unless the particle is spun completely around (360 degrees). Hawking's example of a particle of spin 1 is an arrow. A particle of spin two needs to be turned around halfway (or 180 degrees) to look the same. The example given in the book is of a double-headed arrow. There are two groups of particles in the universe: particles with a spin of 1/2, and particles with a spin of 0, 1, or 2. All of these particles follow Pauli's exclusion principle. Pauli's exclusion principle says that particles cannot be in the same place or have the same speed. If Pauli's exclusion principle did not exist, then everything in the universe would look the same, like a roughly uniform and dense "soup".

Particles with a spin of 0, 1, or 2 move force from one particle to another. Some examples of these particles are virtual gravitons and virtual photons. Virtual gravitons have a spin of 2 and they represent the force of gravity. This means that when gravity affects two things, gravitons move to and from the two things. Virtual photons have a spin of 1 and represent electromagnetic forces (or the force that holds atoms together).

Besides the force of gravity and the electromagnetic forces, there are weak and strong nuclear forces. Weak nuclear forces are the forces that cause radioactivity, or when matter emits energy. Weak nuclear force works on particles with a spin of 1/2. Strong nuclear forces are the forces that keep the quarks in a neutron and a proton together, and keeps the protons and neutrons together in an atom. The particle that carries the strong nuclear force is thought to be a gluon. The gluon is a particle with a spin of 1. The gluon holds together quarks to form protons and neutrons. However, the gluon only holds together quarks that are three different colors. This makes the end product have no color. This is called confinement.

Some scientists have tried to make a theory that combines the electromagnetic force, the weak nuclear force, and the strong nuclear force. This theory is called a grand unified theory (or a GUT). This theory tries to explain these forces in one big unified way or theory.

Chapter 6: Black Holes

Black holes are talked about in this chapter. Black holes are stars that have collapsed into one very small point. This small point is called a singularity. Black holes suck things into their center because they have very strong gravity. Some of the things it can suck in are light and stars. Only very large stars, called super-giants, are big enough to become a black hole. The star must be one and a half times the mass of the sun or larger to turn into a black hole. This number is called the Chandrasekhar limit. If the mass of a star is less than the Chandrasekhar limit, it will not turn into a black hole; instead, it will turn into a different, smaller type of star. The boundary of the black hole is called the event horizon. If something is in the event horizon, it will never get out of the black hole.

Black holes can be shaped differently. Some black holes are perfectly spherical - like a ball. Other black holes bulge in the middle. Black holes will be spherical if they do not rotate. Black holes will bulge in the middle if they rotate.

Black holes are difficult to find because they do not let out any light. They can be found when black holes suck in other stars. When black holes suck in other stars, the black hole lets out X-rays, which can be seen by telescopes.

In this chapter, Hawking talks about his bet with another scientist, Kip Thorne. Hawking bet that black holes did not exist, because he did not want his work on black holes to be wasted. He lost the bet.

Chapter 7: Black Holes Ain't So Black

This chapter explains more about black holes.

Hawking realized that the event horizon of a black hole could only get bigger, not smaller. The area of the event horizon of a black hole gets bigger whenever something falls into the black hole. He also realized that when two black holes combine, the size of the new event horizon is greater than or equal to the sum of the event horizons of the two other black holes. This means that a black hole's event horizon can never get smaller.

Disorder, also known as entropy, is related to black holes. There is a scientific law that has to do with entropy. This law is called the second law of thermodynamics, and it says that entropy (or disorder) will always increase in an isolated system (for example, the universe). The relation between the amount of entropy in a black hole and the size of the black hole's event horizon was first thought of by a research student (Jacob Bekenstein) and proven by Hawking, whose calculations said that black holes emit radiation. This was strange, because it was already said that nothing can escape from a black hole's event horizon.

This problem was solved when the idea of pairs of "virtual particles" was thought of. One of the pair of particles would fall into the black hole, and the other would escape. This would look like the black hole was emitting particles. This idea seemed strange at first, but many people accepted it after a while.

Chapter 8: The Origin and Fate of the Universe

How the universe started and how it might end is talked about in this chapter.

Most scientists believe that the universe started in an explosion called the Big Bang. The model for this is called the "hot big bang model". When the universe starts getting bigger, the things inside of it also begin to get cooler. When the universe was first beginning, it was infinitely hot. The temperature of the universe cooled and the things inside the universe began to clump together.

Hawking also talks about how the universe could have been. For example, if the universe formed and then collapsed quickly, there would not be enough time for life to form. Another example would be a universe that expanded too quickly. If a universe expanded too quickly, it would become almost empty. The idea of many universes is called the many-worlds interpretation.

Inflationary models are also discussed in this chapter, and so is the idea of a theory that unifies quantum mechanics and gravity.

Each particle has many histories. This idea is known as Feynman's theory of sum over histories. A theory that unifies quantum mechanics and gravity should have Feynman's theory in it. To find the chance that a particle will pass through a point, the waves of each particle needs to be added up. These waves happen in imaginary time. Imaginary numbers, when multiplied by themselves, make a negative number. For example, 2i X 2i = -4.

Editions

The introduction was removed after the first edition, as it was copyrighted by Sagan, rather than by Hawking or the publisher, and the publisher did not have the right to reprint it in perpetuity. Hawking wrote his own introduction for later editions.

Film

In 1991, Errol Morris directed a documentary film about Hawking, but although they share a title, the film is a biographical study of Hawking, and not a filmed version of the book.

Opera

The New York's Metropolitan Opera has commissioned an opera to premiere in 2015–16 based on Hawking's book. It will be composed by Osvaldo Golijov with a libretto by Alberto Manguel in a production by Robert Lepage.