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A Brief History of Time by Stephen Hawking.
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Since the dawn of our species, mankind’s relentless search for knowledge has led us to ponder the universe. How did it come into being, and did it even have a beginning? Does it have an end? Why is it the way it is? What is our role in it?

With leaps in our knowledge of quantum physics, we now know more about the true nature of the universe than ever. Although the theoretical physicist Stephen Hawking, author of A Brief History of Time, was confined to a wheelchair and deprived of the powers of speech by the motor neurone disease ALS, he was able to advance our understanding of the universe, the nature of time, and our role within existence itself more than almost any other person who ever lived.

This summary will cover a wide range of topics, from Newton’s laws of gravity to special and general relativity, the Big Bang, black holes, the possibilities of time travel, and quantum theory. These are all complicated and highly counterintuitive topics that push the limits of what humans can conceive. Accordingly, we will discuss each of them at a high level, exploring just the main concepts and theoretical frameworks for each.

The Nature of Theories

We try to make sense of our universe by testing different theories about why we observe the phenomena we observe. The ultimate goal of all scientific endeavors is to produce a unified theory that completely explains the universe—a detailed roadmap of existence. A good theory accurately describes events, with few exceptions or changes; predicts future observations; and has the ability to be disproven in the future. Even one observation that contradicts the theory can negate it. In this sense, we never “prove” theories—we only grow more confident in them with every accurate prediction they make.

One theory of the universe that once had wide acceptance was the geocentric model of the universe, in which celestial bodies revolve around the Earth in fixed circular orbits. But this theory failed to predict the movement of the Moon accurately. Thus, it was discarded in favor of a heliocentric model, in which the Earth and the planets revolve around the sun—which lined up far better with observed reality. Subsequent theories and discoveries have enhanced our understanding.

Everything Is in Motion

Other theories that have enhanced our understanding of the universe include Newton’s Laws of Gravity. Newton’s Laws of Gravity state that force doesn’t set things in motion—everything is already in motion. There is no absolute standard of rest, because objects are always moving in relation to one another. The concepts of “rest” and “motion” are entirely relative. A stationary train may appear to be at rest, but it’s not—because it is sitting upon a moving Earth.

This relationship between the two objects (the train and the Earth) would be the same if the Earth was at rest and the train was moving. Because there’s no absolute standard of motion or rest, there’s also no absolute standard of space. It’s impossible to determine whether two events occurring at different times occurred in the same space, because everything is moving relative to everything else.

The Relativity of Time

It’s not just space—time is also relative. We know that light moves at a constant and finite speed to all observers, regardless of their position in space. But this could only be reconciled with Newton’s laws if time, too, was relative. This was the basis of Albert Einstein’s special theory of relativity, which argued that the laws of science should apply universally, regardless of the differing speed of the observers. Thus, if the speed of light is constant for everyone looking at it, then time must also be relative. ** This was expressed in Einstein’s famous equation e=mc2, with “e” standing for energy, “m” standing for mass, and “c” standing for the speed of light.

Thus, time is not a dimension separate from space: it is inherently interwoven with it, in the fabric of space-time . Einstein’s general theory of relativity, proposed in 1916, incorporated gravity into this framework. Under this theory, gravity is a special force that exists because of the curvature of space-time itself.

Space-time, according to this theory, is not flat. It is analogous to placing an object on a stretched-out piece of fabric. The weight of this object will cause the fabric to sink—this is the same mechanism by which gravity warps the curvature of space-time.

Universal Gravitation

In addition to his laws of motion, Newton’s laws describing gravity have also changed the way we understand the universe. Newton’s theory of universal gravitation states that bodies are attracted to one another by a force (gravity) that grows stronger in proportion to their mass and inversely proportional to their distance . But gravitational attraction, left unchecked, would eventually cause all the matter to collapse in on itself—the only way this wouldn’t happen would be if the universe was dynamic and expanding, a notion that was totally at odds with religious and theological concepts of the time.

The Big Bang

In 1823, the German astronomer Heinrich Wilhelm Olbers observed that if the universe were truly infinite, static, and eternal, the sky would be blinding white light—because in an infinite universe, there would be an infinite number of stars. Every line of sight would end at a star. This paradox pointed the way toward an understanding of the universe as finite, expanding, and with a definite beginning point in time. The reason we aren’t blinded by light when we gaze at the sky is because the light from some stars hasn’t reached Earth yet. And if this is the case, then the universe must be finite and there must have been some point at which the stars “turned on” and the universe began. This beginning is what we now know as the Big Bang.

In 1929, the American astronomer Edwin Hubble confirmed through observation...

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A Brief History of Time Summary A Brief History of Time Guide Chapter 1: The Quest for Understanding

Since the dawn of our species, mankind’s relentless search for knowledge has led us to ponder the universe. How did it come into being, and did it even have a beginning? Does it have an end? Why is it the way it is? What is our role in it? With leaps in our knowledge of quantum physics, we now know more about the true nature of the universe than ever.

While it may seem vast and unknowable, far beyond the limits of human comprehension, the universe is actually governed by rational laws that we can observe, predict, and understand.Throughout our existence, rational observation has been the tool we’ve used to change and update our knowledge. We’ll see how humankind revised its early, primitive understanding of the Earth and the universe by making observations, testing those observations against existing theoretical frameworks, and adjusting or discarding those frameworks based on how well observation lined up with prediction.

What Is a Theory?

Before we dive into our exploration of the universe and the forces that govern it, we need to answer a simple question: What is a theory?. A good theory satisfies three criteria. It must:

  1. Accurately...

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A Brief History of Time Summary A Brief History of Time Guide Chapter 2: It’s All Relative

This lack of absolute space was distressing to European thought in the 17th century (including for Newton himself) because it did not square with then-dominant Judeo-Christian ideas of an absolute God. But an even more revolutionary discovery was in the offing: There was also no absolute concept of time. Time, too, was relative.That is, time moves at different speeds for different observers.

Abandoning Absolute Time

Newton’s contemporary, the Danish astronomer Ole Rømer, discovered that light moved at a constant and finite speed. He was the first person to calculate a finite speed for light, though his initial numbers were quite far off the mark (he believed it moved at 140,000 miles/second, when modern calculations have it at 186,000 miles/second). Still, his observation about the uniformity of the speed of light held true—no matter the positions of different observers, and no matter how fast those observers were travelling, light would always appear to be travelling at 186,000 miles/second.

But if this was the case, how did that reconcile with Newton’s law stipulating that there was no state of absolute rest? Light traveled at a constant speed, but...

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Shortform Exercise: Understand the Dynamic Universe

Test your understanding of the expanding universe.


Briefly explain the phenomenon of red-shift and what it tells us about the universe’s expansion.

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A Brief History of Time Summary A Brief History of Time Guide Chapter 3: Black Holes

In 1783, the English scientist John Michell wrote a paper arguing that if a star were of sufficient mass, its gravity field would be so strong that not even light could escape from it. These massive stars would be invisible, detectable only through their gravitational effects on other objects. This was the earliest sketch of what we now know of as a black hole.

How do stars become black holes? Let’s trace the life cycle of a star. Stars are first formed when free-floating hydrogen atoms begin to cluster together due to gravitational attraction. Eventually, the heat and energy from these interactions causes nuclear reactions within those atoms. Those reactions then form other elements, mostly helium. Eventually, these atoms cluster together through the force of gravitational attraction to form stars. The “outward” energy produced by the nuclear reactions that are occurring at the core of the star balances out the “inward” gravitational pressure and the star reaches a state of stability.

Eventually, however, the star exhausts its supply of hydrogen and nuclear fuel. The star begins to cool, causing it to contract. **At this point, the attractive force of gravity becomes...

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Shortform Exercise: Understanding Black Holes

Test your understanding of black holes and singularities.


In a few sentences, describe how black holes form.

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A Brief History of Time Summary A Brief History of Time Guide Chapter 4: The Building Blocks of the Universe

As we explored, general relativity tells us that all physical theories break down at the Big Bang, including general relativity itself. The time immediately after though, during the infancy of the universe, requires us to study quantum mechanics—the study of extraordinarily small particles.

The German physicist Max Planck suggested that waves (like light, radio, and microwaves) emit energy not randomly or arbitrarily, but instead, in discrete quantities which he called “quanta.” The amount of energy released in a quantum is proportional to the wave frequency of the radiation type in question. Wave frequency is simply the number of waves that pass through a fixed point in a given unit of time.

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In quantum theory, the higher the frequency, the more energy that is released. This means that, at very high frequencies, the release of the quantum requires more energy than is actually available. In such a scenario, bodies lose energy at a finite rate. This theory had the advantage of accurately predicting and describing the observed radiation of extremely hot...

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A Brief History of Time Summary A Brief History of Time Guide Chapter 5: Past, Present, and Future

For most of the 20th century, the “Hot Big Bang” model has been used to tell the story of the origin and development of the universe. The Hot Big Bang theory states that, after the initial Big Bang explosion, the universe was incredibly hot (hence the name), meaning that particles were moving too quickly to merge together to form protons, neutrons, atoms, and molecules. But as the universe expanded, it began to cool, and the particles slowed down. This led to a series of nuclear fusions, in which stars were formed, as well as more complex elements like hydrogen and helium (this all would have taken place within mere milliseconds of the Big Bang). After just a few hours, most of the hydrogen and helium in our universe today were created, concentrated in enormous clouds.

Eventually, as the universe continued expanding and cooling over the next few million years, these massive clouds collapsed under their gravity, catalyzing a new series of nuclear reactions. Moreover, the first generation of stars began to suffer gravitational collapse within roughly 100 million years after their creation, throwing off mass and energy as they died. Through these combined effects, some of...

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Table of Contents

  • 1-Page Summary
  • Chapter 1: The Quest for Understanding
  • Chapter 2: It’s All Relative
  • Exercise: Understand the Dynamic Universe
  • Chapter 3: Black Holes
  • Exercise: Understanding Black Holes
  • Chapter 4: The Building Blocks of the Universe
  • Chapter 5: Past, Present, and Future