Stephen Hawking (1942–2018) was a physicist at the University of Cambridge who studied black holes and the beginning of the universe. At 21 he was found to have motor neurone disease (ALS), which slowly paralyzed him; doctors expected him to live only a few years. He lived another 55, worked for most of them, and from 1985 spoke through a computer voice. From 1979 to 2009 he held the Lucasian chair of mathematics at Cambridge, the chair Isaac Newton once held.
Key idea
Borrow the book
A Brief History of Time by Stephen Hawking is still under copyright, so we do not reproduce it. Find it at a library: Open Library or WorldCat. Look for the updated and expanded tenth anniversary edition (1998) or the illustrated edition (1996); they add a chapter on wormholes and time travel.
How Hawking came to it
Hawking's best-known discovery, in 1974, was that black holes are not completely black: quantum effects near the edge should make them give off a faint glow, now called Hawking radiation, and very slowly shrink. It joined general relativity (the science of gravity) with quantum theory (the science of the very small) in one result. It has not yet been observed, because for real black holes the glow is far too faint. He wanted ordinary readers to share these ideas, and wrote a book for them.
Reading guide, chapter by chapter
Chapter titles are from the updated edition. Summaries are our own.
| Chapter | What it is about, in our words |
|---|---|
| 1. Our Picture of the Universe | How ideas about the universe changed from Aristotle and Ptolemy to Copernicus, Galileo and Newton, and what makes a good scientific theory: it explains what we see and makes predictions we can test. Think about: Can a theory ever be proved right, or only not yet proved wrong? |
| 2. Space and Time | Newton's absolute time gives way to Einstein: the speed of light is the same for everyone, and gravity is the bending of space and time by mass. Think about: Why can't anything go faster than light? |
| 3. The Expanding Universe | Galaxies are moving apart, as Hubble found in the 1920s; run the film backward and everything was once packed together, the big bang. Think about: If the universe expands, what is it expanding into? (Hawking's answer: nothing; space itself stretches.) |
| 4. The Uncertainty Principle | In quantum theory you cannot know both where a particle is and how fast it moves exactly; nature is a matter of chances. Think about: How can something be in a definite place if we can't measure it exactly? |
| 5. Elementary Particles and the Forces of Nature | Quarks, electrons and the four forces: gravity, electromagnetism, and the strong and weak nuclear forces. Think about: Why might physicists want the four forces to be one? |
| 6. Black Holes | When a big star runs out of fuel it can collapse until not even light escapes. The edge is the event horizon. Think about: Would you notice crossing an event horizon? |
| 7. Black Holes Ain't So Black | Hawking's own discovery: black holes glow faintly and slowly evaporate. Think about: Why is it so hard to test this one? |
| 8. The Origin and Fate of the Universe | What the early universe was like, inflation, and Hawking's idea that the universe might have no edge or boundary in time. Think about: Does a question like "what came before the big bang" have an answer? |
| 9. The Arrow of Time | Why we remember the past but not the future: disorder (entropy) grows, and the universe's expansion points the same way. Think about: Why can't a broken cup put itself back together? |
| 10. Wormholes and Time Travel | Whether shortcuts through space or trips to the past are allowed by physics (added in later editions). Think about: What is the grandfather paradox? |
| 11. The Unification of Physics | Attempts to join gravity with quantum theory, including string theory. Think about: Is a theory of everything the end of physics? |
| Conclusion | Why people ask why the universe exists, and why science and philosophy both matter to that question. Think about: What would knowing the final theory change for you? |
Numbers to try
Light travels about 300,000 km each second. The Sun is about 150 million km away, so its light takes about 500 seconds, a little over 8 minutes, to reach us. When you look at the Sun, you see it as it was 8 minutes ago. Look far enough and you see the young universe.
Worked example: Light from the Sun
- $150{,}000{,}000 \div 300{,}000 = 500$ seconds.
- $500 \div 60 \approx 8.3$ minutes.
Answer: 500 seconds, about 8.3 minutes.
Worked example: How big is a black hole?
- $3 \times 10 = 30$ km.
Answer: About 30 km: a city-sized object with ten times the Sun's mass.
Common mistake
| When | What happened |
|---|---|
| 1942 | Hawking born in Oxford, England, on January 8 |
| 1963 | Diagnosed with motor neurone disease, age 21 |
| 1974 | Shows that black holes should give off radiation |
| 1979 | Becomes Lucasian Professor of Mathematics at Cambridge |
| 1988 | A Brief History of Time published |
| 2001 | The Universe in a Nutshell published |
| 2018 | Dies in Cambridge |
Read more: Stephen Hawking and A Brief History of Time.