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A Brief History of Time by Stephen Hawking.
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Science has profound implications for philosophy and everyday life because science describes and predicts how the universe works. To make the discoveries and implications of modern science more accessible to everyone, renowned physicist Stephen Hawking described the principles of modern physics for a general audience in A Brief History of Time.

In this guide, we’ll present Hawking’s exposition of modern physics through the lens of five big questions that the book answers: Is reality relative or absolute? Is the future predetermined? How did the universe begin? What is the nature of a black hole? And can you build a time machine?

Question 1: Is Reality Relative or Absolute?

Science is founded on the assumption that reality works the same for everyone: If two scientists conduct exactly the same experiment, they should get the same results. Ironically, this principle has driven scientists to the conclusion that many things—including motion, position, time, and weight—are relative to the observer.

Motion Is Relative

Hawking explains that motion is relative to the observer, because there is no absolute reference frame.

To illustrate this concept, imagine you’re sitting in a boat on a body of water. Looking into the water, you see a fish swim by. Relative to your frame of reference, the fish is moving at a certain speed. But an observer on land might disagree. Let’s say the body of water is a river, and the fish is holding a constant position, relative to the river bottom, while your boat drifts by. So is the fish moving or staying in the same place? In an absolute sense, we can’t tell. Relative to your boat, the fish is moving. Relative to the earth, the fish is stationary. But of course the earth is also moving, relative to the sun, so the fish is moving relative to the sun. And the sun is moving, relative to the galaxy, and so on.

(Shortform note: One familiar scenario where the relativity of speed becomes important is driving a car. To avoid collisions, you have to keep track of the motion of other cars relative to your own. You also have to keep track of your speed relative to the road, since posted speed limits are implicitly specified relative to the road. To maintain a constant following distance behind another car, you have to adjust your speed to be zero relative to the other car.)

Time Is Relative

In the same way that the motion of objects is relative to the observer, Hawking argues that different observers may measure time differently. Hawking explains that, in particular, time appears to slow down for entities that are moving very fast. This is because of the relationship between time, speed, and distance, and the fact that everyone who accurately measures the speed of light gets the same result. He notes that this is true regardless of their particular reference frame, because the laws of physics are consistent for all observers.

(Shortform note: When Hawking refers to “the speed of light,” technically he’s referring to the speed of light in a vacuum. Light slows down slightly when it travels through materials like water or glass. This is what causes refraction of light, where the change in speed at the boundary between two materials causes the light to bend.)

For example, imagine two kids playing laser tag on a starship that is traveling at the speed of light. As they fire pulses of light back and forth, from opposite sides of the ship, an observer on the ship measures the distance that the light travels and finds it equal to the width of the ship. But if you’re outside the ship, you would see the light traveling a greater distance. Specifically, the distance would be the hypotenuse of a right triangle, with one leg equal to the width of the ship, and the other equal to the distance the ship traveled while the light was moving across it. Since you perceive the light traveling a greater distance (at the same speed) than the people on the starship, you must also perceive more time elapsing during the event than they do.

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Relativity

When time progresses at a different rate for one person than another, scientists call this “time dilation.” As Hawking points out, Albert Einstein developed his theories of relativity as a mathematical model for predicting the motion of objects, even at speeds where time dilation becomes significant. (Earlier theories of motion didn’t account for time dilation.)

As Hawking recounts, Einstein introduced his theory of relativity in two phases: first, special relativity and then general relativity. Let’s discuss each of these theories in turn.

(Shortform note: “Special relativity” is “special” because it only works in situations where there is no gravity or acceleration. Physics problems with no gravity or acceleration are simpler, and constitute a “special case,” where a relatively simple theory of relativity can be applied.)

As Hawking recounts, when Einstein developed his theory of special relativity he ignored gravity and acceleration to make the mathematical derivation simpler. This allowed him to publish his theory quickly and gain support for it. Then he developed his theory of general relativity as a more generalized version of the theory, which could account for gravity and acceleration as well.

Hawking notes that the key idea that allowed Einstein to complete the theory of general relativity was his realization that gravity could be viewed as the curvature of space itself. In other words, general relativity is based on the principle that mass actually causes space to warp, such that the shortest distance between two points is an arc segment, rather than a straight line.

Testing General Relativity

Initially, Einstein’s theory of general relativity was somewhat controversial, and so it was subjected to even more testing...

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A Brief History of Time Summary Shortform Introduction

Science has profound implications for philosophy and everyday life because science describes and predicts how the universe works. For example, the discovery that time itself had a finite beginning point implies that the physical universe is not eternal, and allows us to infer certain things about its origins.

To make the discoveries and implications of modern science more accessible to everyone, renowned physicist Stephen Hawking described the principles of modern physics for a general audience in A Brief History of Time.

About the Author

Stephen Hawking first became famous in the physics community for his study of “singularities” (a concept we’ll explain later in this guide) in the 1960s, along with George Ellis and Roger Penrose. One of their notable conclusions was that time itself began at a finite point. In 1974, at age 32, he became the youngest scientist ever elected as a Fellow of the Royal Society. He continued to make contributions to the advancement of theoretical physics throughout his life.

However, Hawking’s popularity outside the close-knit community of theoretical physicists was due as much to...

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A Brief History of Time Summary Introduction to Formulating Scientific Theories

According to Hawking, the ultimate goal of science is to develop a theoretical model that fully explains how the universe works. Developing theories that explain the universe involves a bit of trial and error. Over time, scientists refine their theories and weed out theories that don’t fit with their observations.

Hawking highlights three characteristics that he says make a good scientific theory:

1. A good theory is consistent with past observations. It should apply to a broad range of circumstances, with as few exceptions or qualifiers as possible. General relativity and quantum mechanics are examples of theories that apply to a wide variety of situations with very few exceptions. Specifically, the theory of general relativity can model the behavior of any set of large-scale objects interacting with gravitational forces. Similarly, quantum mechanics can model the behavior of all kinds of subatomic particles interacting through electromagnetism and nuclear forces. The one exception where these theories can’t be used is when so much mass is concentrated in such a small space that you have to model gravitational interactions and nuclear interactions at the same time. (We’ll...

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A Brief History of Time Summary Question 1: Is Reality Relative or Absolute?

In this Question, we’ll discuss Hawking’s explanation of the relative nature of space, motion, time, and mass.

Relativity in the Big Picture

Philosophers have long argued about whether reality is objective or subjective: If something is true for me, does that make it true for you? Or could your truth be different and yet equally valid? Science is founded on the assumption that reality works the same for everyone: If two scientists conduct exactly the same experiment, they should get the same results.

Ironically, this principle has driven scientists to the conclusion that many things, including motion, position, time, and weight are relative to the observer (or rather to the observer’s frame of reference, which is what you are taking measurements relative to).

Space Is Relative

To illustrate how space is relative, Hawking begins by discussing how ancient theories of absolute space were disproven and then presents the more modern theories that replaced them.

Specifically, he explains that the ancient Greek philosopher Aristotle believed that space and motion were absolute. In other words, there was some frame of reference that all observers...

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A Brief History of Time Summary Question 2: Is the Future Predetermined?

As we have seen, scientific theories allow us to model the behavior of objects and make predictions about future observations or the outcomes of experiments. Hawking points out that if we could formulate a unified theory of physics that could be applied in any context, then if we knew the exact state of the entire universe at any point in time, we could use the theory to predict the state of the universe at any other time. This would make human free will an illusion, since we could calculate everyone’s future actions. Hawking refers to this as “scientific determinism.”

However, Hawking also discusses three limitations of scientific determinism:

  1. Scientists have not yet been able to find a unifying theory of physics.
  2. The theory of quantum mechanics (one of the major theories in modern physics) gives results in terms of probability, rather than deterministic results.
  3. The uncertainty principle of quantum mechanics limits our ability to measure the state of the universe at any given time, so even if you had a unified, deterministic theory, you couldn’t measure the present state of the universe accurately enough to exactly predict its future state.

In the remainder of...

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Shortform Exercise: Analyze an Unpredicted Event

As we discussed, scientific theories allow you to predict the future, given enough information about the present. However, this predictive power is limited in two ways: There’s currently no theory that can model everything, and quantum uncertainty limits how much you can know about the present. In this exercise, you’ll have a chance to analyze how these principles apply to your life.


Think about the last time you experienced something unexpected. Briefly describe what happened.

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A Brief History of Time Summary Question 3.1: How Did the Universe Begin?

Hawking points out that the origins of the universe have profound philosophical implications. He contrasts the Judeo-Christian belief that God created the universe with the atheistic view that many scientists held in the nineteenth century, namely that the universe was infinite and had always existed. Since the universe had always existed, there was no need for a divine creator to bring it into existence. Hawking refers to this view as the “static universe model.”

Hawking recounts that in the twentieth century, new scientific discoveries challenged the theory that the universe had always existed. Based on these discoveries, the “big bang” theory ultimately replaced the static universe model. The big bang theory posits that the universe is expanding outward from a point where it came into existence at a finite time in the past.

The big bang theory presented two significant philosophical problems for Hawking and other atheist scientists. First, the idea that the universe had a beginning seemed uncomfortably similar to the Judeo-Christian concept of creation. Second, the theory arguably implied that the universe was fine-tuned for humankind, because the big bang model was...

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A Brief History of Time Summary Question 3.2: The Fine-Tuning Problem

Hawking relates how, as scientists continued to refine the big bang theory, their models implied that the initial conditions of the universe (like its initial rate of expansion, density and uniformity, and so forth) would have to be exactly right for habitable planets to form. If any of the physical parameters of the big bang had been even slightly different, human life would never have been possible.

For example, if the initial expansion rate of the universe was too high, all the matter would have dispersed too quickly for stars and planets to form. If it was too low, gravity would have caused it to collapse again before life-supporting planets could form. Hawking points out that we observe the universe expanding at approximately the “critical rate,” that is, the maximum rate at which it could eventually re-collapse.

(Shortform note: We infer that this is important because it would give stars and planets the greatest window of opportunity to form, since any less would cause the universe to collapse in on itself sooner, and any more would remove the materials that are needed for star formation more quickly.)

To Hawking, **the need for fine-tuning of the initial conditions...

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Shortform Exercise: Turn Problems Into Solutions

In science, new discoveries often overturn prior theories, and problems with current theories sometimes lead to new discoveries. (For example, Penzias and Wilson thought there was a problem with their microwave antenna, but eventually figured out that they had discovered the cosmic microwave background.) In this exercise, you’ll generalize this principle to consider what you can learn from problems, whether in science or in other areas of life, like work, business, or philosophy.


Briefly describe a problem or apparent contradiction that you’ve recently encountered. (For example, maybe your coworker says she emailed you an important document last week, but it never showed up in your inbox. Or maybe your houseplants suddenly start turning yellow or showing other signs of stress.)

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A Brief History of Time Summary Question 4: What Is a Black Hole?

Hawking explains that a “black hole” is an object with such strong gravity that its gravity can trap light. Moreover, according to Hawking, the theory of general relativity implies that nothing can travel faster than light, so if light can’t escape from a black hole, then nothing can.

Since the black hole would trap light instead of emitting or reflecting it, it would be completely black. That said, Hawking predicts that black holes would appear to emit a small amount of radiation that’s actually produced outside the black hole, just far enough away that it can escape. To facilitate our discussion of black holes, we’ll start by introducing some terminology. Then we’ll discuss where black holes come from in the first place, after which we’ll explain “Hawking radiation,” the light that Hawking expects to be generated just outside a black hole.

Black Hole Nomenclature: Singularities and Event Horizons

General relativity predicts that there is a singularity at the center of a black hole, where all of the object’s mass is concentrated in an infinitely small, infinitely dense point. Furthermore, since there is a correlation between density and gravity, and between gravity...

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A Brief History of Time Summary Question 5: Can You Build a Time Machine?

Writers of science fiction have long contemplated the idea of a time machine: a device that allows you to travel forward or backward in time to any point in history or the future. Hawking asserts that this possibility will probably always be relegated to the realm of fiction, based on his analysis of relevant scientific theories. His analysis focuses on two possibilities: moving backwards through time directly, or passing through a “wormhole” in spacetime that connects the present to the past.

Moving Backwards Through Time

Hawking begins his analysis of time travel by observing that time is reversible in all the laws and theories of physics. The math would be just as valid if time was running backwards as it is when time is running forwards. And yet, you never observe time running backwards (except in a certain interpretation of quantum fluctuations, which we’ll discuss later). To explain why we perceive time moving forwards and not backwards, Hawking first provides three ways to define the direction of time and shows how they relate to each other. This sets the stage for his assessment of the possibility of backwards time travel based on relativity and quantum...

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Shortform Exercise: Consider Your Approach to Time Travel

Hawking views time travel (at least backwards or arbitrary time travel) as unlikely, but he discusses how quantum particles can travel back in time and how wormholes could connect two different points in spacetime that might be located in different time periods.


As technology progresses, do you think developing time travel is technically feasible? Why or why not?

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Shortform Exercise: Use Science to Write Science Fiction

Imagine that the writers of a new science fiction series have hired you as a consultant. They want their series to be reasonably scientifically accurate.


Season 1 of the series is about people leaving earth and traveling to a distant planet to build a new colony. They’ll make the trip on spaceships that travel very close to the speed of light. Brainstorm a few ideas for interpersonal drama or other plot elements that could arise from the relativity of space, time, or motion, as people travel to the new colony. (For example, because the ships are traveling close to the speed of light, the people on board will age much slower than their loved ones back home, which could cause problems.)

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Table of Contents

  • 1-Page Summary
  • Shortform Introduction
  • Introduction to Formulating Scientific Theories
  • Question 1: Is Reality Relative or Absolute?
  • Question 2: Is the Future Predetermined?
  • Exercise: Analyze an Unpredicted Event
  • Question 3.1: How Did the Universe Begin?
  • Question 3.2: The Fine-Tuning Problem
  • Exercise: Turn Problems Into Solutions
  • Question 4: What Is a Black Hole?
  • Question 5: Can You Build a Time Machine?
  • Exercise: Consider Your Approach to Time Travel
  • Exercise: Use Science to Write Science Fiction