The key ideas

A Brief History of Time: From the Big Bang to Black Holes

Stephen Hawking, 1988

Did the universe have a beginning? What happens at the edge of a black hole? Why do we remember the past but not the future? Stephen Hawking tried to answer these questions with almost no equations and to bring the answers together into one picture of the world. Here you will find his key thought experiments and an honest line between what is observed and what is a hypothesis.

Who it is for

For anyone who has ever looked up at the starry sky and wondered where it all came from, even if school physics is long forgotten.

Most people would find the picture of our universe as an infinite tower of tortoises rather ridiculous, but why do we think we know better?

What the book is about

The book opens with a joke. After a lecture on how the universe works, an elderly lady in the audience tells the scientist that it is all nonsense: the world is a flat plate resting on the back of a giant tortoise. “And what is the tortoise standing on?” “On another tortoise. And so on, all the way down.”

Stephen Hawking asks an awkward question: what makes our theories any better than tortoises? Where did the universe come from, did it have a beginning, and why does time only move forward?

The physicist from Cambridge set out to explain all this without equations. He was told that every equation would halve the number of readers, so he kept just one: E = mc².

The book works like a single investigation. Again and again, Hawking looks for the place where our familiar picture of the universe stops working: at the very beginning, inside a black hole, in the direction of time. And each time he asks which model will turn out to be more complete. Every clue leads to one point, and by the end you will see which one. Along the way, it is worth keeping apart what is observed, what follows from a theory and what remains Hawking's own hypothesis.

Idea 1. A theory is a model in your head

Hawking starts simply. A theory is a model of the universe that we build in our heads. A good model describes many observations with a few rules and makes predictions that can be tested.

Then comes the twist. A theory can never be proved once and for all: however many experiments support it, the next one could disprove it. Newton's theory worked for centuries, until precise observations called for Einstein's.

And here is the setup for the whole book. Physics has two great theories. General relativity describes the large: stars, galaxies, the universe. Quantum mechanics describes the very small: atoms and particles. Both have been tested superbly well, yet, in Hawking's words, they are inconsistent with each other. As long as each works on its own ground, we can live with that. The investigation begins where the two grounds meet.

Tomorrow's weather forecast is a model too: we trust it while it comes true, and we change it when it keeps getting things wrong. This is a Livria analogy, not an example from the book.

If a theory is a model, the most interesting places are where the model breaks. That is where you can see what picture is needed next.

Read in Livria

About the author

Stephen Hawking (1942 to 2018) was a British theoretical physicist and a professor at the University of Cambridge. He is known for his work on black holes and the origin of the universe, including his prediction that black holes give off radiation. He lived most of his life with amyotrophic lateral sclerosis (ALS).

All books by this author: Stephen Hawking

Terms from the book

  • Einstein
  • Newton
  • Space-time
  • Quantum mechanics
  • Cambridge
  • Big Bang
  • General relativity
  • E = mc²: Einstein's famous formula: energy (E) equals mass (m) multiplied by the speed of light (c) squared. Even a tiny mass carries an enormous amount of energy.
  • Hubble: Edwin Hubble was an American astronomer. In 1929 he showed that distant galaxies are moving away from us, and the farther they are, the faster they move: the universe is expanding.
  • Penrose: Roger Penrose is a British mathematician and physicist. Together with Hawking, he proved that according to Einstein's theory there must have been a singularity at the beginning of the universe. He won the 2020 Nobel Prize in Physics.
  • Singularity: A state of infinite density that general relativity predicts, where the known laws of physics stop working.
  • Black hole: A region of space where gravity is so strong that not even light can escape. One can form, for example, when a large star collapses.
  • Event horizon: The boundary of a black hole: from inside it, not even light can get out.
  • Thorne: Kip Thorne is an American theoretical physicist who specialises in gravity and black holes. He won the 2017 Nobel Prize in Physics for the detection of gravitational waves.
  • Cygnus X-1: A bright source of X-rays in the constellation Cygnus, the Swan, and one of the first candidates for a black hole. Today it is considered a black hole.
  • Gravitational waves: Ripples in space-time created by the motion of very massive bodies, such as two black holes merging. They were first detected in 2015.
  • Hawking radiation: A faint radiation from black holes, predicted in theory, that could make them slowly lose mass. It has not yet been observed directly.
  • Hartle: James Hartle was an American theoretical physicist. In 1983, together with Hawking, he proposed a model of a universe with no boundary.
  • Imaginary time: A mathematical device Hawking uses in his hypothesis of a universe with no boundary. It is a tool for calculation, not a time you could feel.
  • Thermodynamic: The thermodynamic arrow of time is the direction in which disorder increases. Thermodynamics is the branch of physics that deals with heat and energy.
  • Arrow of time: The direction in which time runs: from the past to the future. Hawking names three such arrows.
  • Unified theory: A hypothetical theory that would bring general relativity and quantum mechanics together. It does not exist yet.

All terms

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