A Brief History of Time

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A Brief History of Time  
Author Stephen Hawking
Genre(s) Cosmology
Publisher Bantam Books
Release date 1988
Published in
Prequel to The Illustrated A Brief History of Time

A Brief History of Time (1988) is a book written by the scientist and mathematician Stephen Hawking. This book is about physics, or the study of laws that predict how things work in the universe. It is also about cosmology, or how we see the universe and how the universe exists.

In this book, Hawking talks about many theories in physics. Some of the things that he talks about are the history of physics, gravity, how light moves in the universe, space-time, elementary particles (very small objects that make up things in the universe), black holes, the Big Bang (the theory that the universe started from one point), and time travel (the idea that travel can be done to the past and to the future.)

There are two other versions of this book: The Illustrated A Brief History of Time and A Briefer History of Time. The Illustrated A Brief History of Time has pictures to help explain its ideas. It was also updated because new information was found. A Briefer History of Time is shorter than the first version and was also updated.

This book is very popular and well-known. This book was on the London Sunday Times bestseller list for over 4 years.[1]

Summary[change | change source]

Chapter 1: Our Picture of the Universe[change | change source]

A picture of what Ptolemy thought about the location of the planets, stars, and sun.

In the first part of the book, Hawking talks about the history of physics. He talks about the ideas of philosophers such as Aristotle and Ptolemy. Aristotle, unlike many other people of his time, thought that the Earth was round. He also thought that the sun and stars went around the Earth. Ptolemy also thought about how the sun and stars were located in the universe. He made a planetary model that described Aristotle's thinking. Today, it is known that the opposite is true; the earth goes around the sun. The Aristotle/Ptolemy ideas about the position of the stars and sun was disproved in 1609. The person who first thought of the idea about the Earth going around the sun was Nicholas Copernicus. Galileo Galilei and Johannes Kepler, two other scientists, helped to prove that Copernicus's idea was right. They looked at how the moons of some planets moved in the sky, and they used this to prove Copernicus right. Isaac Newton also wrote a book about gravity, which helped to prove Copernicus's idea right.

Chapter 2: Space and Time[change | change source]

This is a light cone

In this chapter, Hawking talks about space and time. He describes the motion of planets moving around the sun and how gravity works between the planets and the sun. He also talks about the ideas of absolute rest and absolute position. These ideas are about the thought that events stay in place over a period of time. This was found not to be true by Newton's laws of gravity. The idea of absolute rest did not work when objects move very fast (at the speed of light, or light speed).

The speed of light was first measured in 1676 by the Danish astronomer Ole Christensen Roemer. The speed of light was found to be very fast, but at a finite speed. However, scientists found a problem when they tried to say that light always traveled at the same speed. The scientists created a new idea, called the ether, which tried to explain light's speed.

Einstein said that time was not absolute, or always the same

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. Einstein's idea was also the same as Henry Poincare's idea. Einstein's idea is called the theory of relativity.

Also in this chapter, Hawking talks about light. He says that 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. The center of the light cone is the event. Besides light cones, Hawking also talks about how light can bend. When light goes past a big mass, like a star, the light changes direction slightly towards the mass.

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[change | change source]

The Big Bang is shown here. From the picture, we can tell the universe is getting bigger over time

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[change | change source]

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.

Here is a picture of a light wave.

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 interference causes many colors to appear.

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[change | change source]

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.

A particle of spin 1 needs to be turned around all the way to look the same again, like this arrow.

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".

This is a proton. It is made up of three quarks. All the quarks are different colors because of confinement.

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[change | change source]

A picture of a black hole and how it changes light around it.

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.This singularity is a point of space-time which rotates at a high speed.That is the reason that black holes have no time. Black holes suck things into their center because its gravity is very strong. 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[change | change source]

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[change | change source]

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.

Chapter 9: The Arrow of Time[change | change source]

In this chapter Hawking talks about why "real time" as humans observe and experience it (in contrast to the "imaginary time" in the laws of science) seems to have a certain direction, notably from the past towards the future. The things that give time this property are the arrows of time.

Firstly, there is the thermodynamic arrow of time. According to this, starting from any higher order organized state, the overall disorderliness in the world always increases as time passes. This is why we never see a broken pieces of a cup gather themselves together to form a whole cup. Even though human civilizations have tried to make things more orderly, the energy dissipated in this process has created more overall disorder in the universe.

The second arrow is the psychological arrow of time. Our subjective sense of time seems to flow in one direction, which is why we remember the past and not the future. Hawking claims that our brain measures time in a way where disorder increases in the direction of time. We never observe it working in the opposite direction. In other words, the psychological arrow of time is intertwined with the thermodynamic arrow of time.

Thirdly there is the cosmological arrow of time, the direction of time in which our universe is expanding and not contracting. Hawking believes that in order for us to observe and experience the first two arrows of time, the universe would have to begin in a very smooth and orderly state. And then as it expanded, it became more disorderly. So the thermodynamic arrow agrees with the cosmological arrow.

However, because of the "no boundary" proposal for the universe, after a period of expansion, the universe will probably start to contract. But it will probably not go backwards in time to a more smooth, orderly state. The thermodynamic arrow in the contracting phase will not be as strong.

As for why humans experience these three arrows of time going in the same direction, Hawking postulates that humans have been living in the expanding phase of the universe. He thinks that intelligent life couldn't exist in the contracting phase of the universe. Only the expanding phase of the universe is suitable for intelligent beings like humans to exist, because it contains a strong thermodynamic arrow. Hawking calls this the "weak anthropic principle".

Chapter 10: Wormholes and Time Travel[change | change source]

Chapter 11: The Unification of Physics[change | change source]

Physicists have come up with partial theories to describe a limited range of things, but a complete, unified and consistent theory which can take into account all of these partial theories remain unknown. Hawking is cautiously optimistic that such a unified theory of the universe may be found soon. Such a theory must combine the classical theory of gravity with the uncertainty principle found in quantum mechanics. Attempts to do that have led to the occurrence of absurd infinitely massed particles or an infinitely small universe. In 1976, the theory of "supergravity" was suggested as a solution. But the calculations to verify the theory was deemed time-consuming and thus abandoned. In 1984, another kind of theories called the "string theories", where basic objects are not particles but two-dimensional strings, became popular among physicists. They were claimed to explain the existence of certain particles better than supergravity and other theories. However, according to string theories, instead of the usual four space-time dimensions, the universe could have dozens of them. It is imagined that humans do not experience the other dimensions because these are too tightly curled up. This is due to the "weak anthropic principle", according to which intelligent beings like humans cannot exist in any other way. Sting theories appear to allow this situation for certain regions of the universe, but there may be other regions of the universe where more than four dimensions are prominent. Furthermore, supergravity, p-brane and string theories all describe different situations with similar results, as if using different approximations of the same theory.

Hawking thus proposes three possibilities: 1) there exists a complete unified theory that we will eventually find; 2) there are an infinite number of theories that overlap and describe the universe more and more accurately and 3) there is no ultimate theory. The third possibility has been sidestepped by acknowledging the limits set by the uncertainty principle. The second possibility describes what has been happening in physical sciences so far, with increasingly accurate partial theories. Hawking believes that such refinement has a limit and that by studying the very early stages of the universe in a laboratory setting, it is possible to finally find a complete unified theory in the 21st century. Such a theory might not be proven but would be mathematically consistent. The predictions of such a basic set of laws would match our observations. However, given the complicated nature of realistic situations, it would only be a first step to a complete understanding of the events around us.

Chapter 12: Conclusion[change | change source]

Humans have always wanted to make sense of the universe and their place in it. At first, events were considered random and controlled by human-like emotional spirits. But in astronomy and in some other situations, regularities were observed. With the advancement of the human civilization in the modern age, more regularities and laws were discovered. Laplace suggested at the beginning of the nineteenth century that the universe’s structure and evolution could eventually be precisely explained by a set of laws. However, the origin of these laws was left in God’s domain. In the twentieth century, quantum theory introduced the uncertainty principle, which set limits to the predictive accuracy of laws to be discovered. The big bang implied by the general theory of relativity indicates that a creator of the universe or God has the freedom to choose the origin and the laws of the universe. When one combines theory of gravity with quantum mechanics, however, a unified and completely self-contained theory may emerge, in which God has little or no role to play. So the search of a unified theory may shed light on the nature of God. However, most scientists today are working on the theories themselves than asking such philosophical questions. On the other hand, these physical theories are so mathematical and technical that philosophers are not discussing them like they used to do, let alone ordinary people. Hawking would like to see that eventually everybody would one day talk about these theories in order to understand the true origin and nature of the universe, accomplishing the ultimate triumph of human reasoning.

Other editions[change | change source]

  • 1988 — The first edition is published. This edition had an introduction by Carl Sagan.
  • 1990 - Similar to the 1996 but with an introduction by Carl Sagan, uncolored pictures, and it was printed in paperback
  • 1996 — An illustrated, updated and expanded edition is published, called The Illustrated A Brief History of Time. This hardcover edition contains full color illustrations and photographs to help explain the text. It also has topics not in the original book, including a new chapter on wormholes and time travel.
  • 1998 — The Tenth Anniversary Edition is published. It has the same text as the one published in 1996, but was also released in paperback and has less diagrams.
  • 2005 — The release of A Briefer History of Time (written with Leonard Mlodinow), which is a shorter version of the original book. It was updated again to include new scientific developments..

Notes[change | change source]

References[change | change source]