Science
by Stephen Hawking
Did time have a beginning?
Stephen Hawking wrote this for readers with no physics. He was warned that each equation would halve his sales, so he kept only one, E=mc². He takes on the big questions, like whether the universe had a beginning and why we remember the past but not the future.
Hear a sample · read by Laura
Einstein joined space and time into one thing, spacetime. His general relativity says that mass and energy curve it, and we feel that curving as gravity. The Earth goes round the Sun because it follows the straightest path it can through the spacetime the Sun has bent.
In 1929 Edwin Hubble showed that distant galaxies are moving away from us, and the further away they are, the faster they're going. Run that backwards and everything was once packed together, unimaginably dense and hot. Hawking calls that starting point the big bang.
Hawking finds three ways time has a direction. Disorder grows, which is why a broken cup never leaps back onto the table. We remember the past but not the future. And the universe is expanding rather than shrinking. He argues memory's arrow has to point the same way as disorder's.
With Jim Hartle, Hawking proposed that space and time together might be finite but have no boundary, just as the Earth's surface has no edge. Then the universe wouldn't need a special moment of creation with its own rules. He's careful to call this a proposal, which still needs testing.
Expansion
The universe is expanding, so long ago everything must have been packed into one point.
Curved space
Mass bends space and time around it. Gravity is simply that bending.
Uncertainty
You can never know exactly where a particle is and how fast it's moving at the same time.
Black holes
A black hole pulls so hard that not even light can get out.
Hawking radiation
Hawking showed black holes slowly leak energy and could one day vanish.
A story from the book
Jacob Bekenstein suggested a black hole's horizon area measured its entropy, or disorder. Hawking admits this irritated him. Anything with entropy has a temperature and should glow, but black holes were thought to let nothing out. Then his quantum calculations showed they do glow faintly. We call it Hawking radiation.
Hot take · true or myth?
A clock at the top of a tower runs slightly faster than one at the bottom.
True. General relativity says a clock nearer a large mass, like the Earth, runs slower. The difference is tiny, but precise experiments have measured it, just as the theory predicts.
One thing to try
Hawking points out that if the Sun went out, we wouldn't know for about eight minutes, the time its light takes to reach us. On the next clear night, find a bright star. Its light left it years ago, so you're seeing the star as it was then.
Knotd has the key ideas from 250+ non-fiction books, each in a few minutes, in plain words. A Brief History of Time comes with a Hot Take round and a Name That Idea round to make it stick.
Get the whole book on KnotdOpen in the appDid time have a beginning? Stephen Hawking wrote this for readers with no physics. He was warned that each equation would halve his sales, so he kept only one, E=mc². He takes on the big questions, like whether the universe had a beginning and why we remember the past but not the future.
Gravity is curved spacetime; The universe is getting bigger; Three arrows of time; Finite, but with no edge.
Hawking points out that if the Sun went out, we wouldn't know for about eight minutes, the time its light takes to reach us. On the next clear night, find a bright star. Its light left it years ago, so you're seeing the star as it was then.
Yes. Knotd has the key ideas from A Brief History of Time in a few minutes, in plain words, plus a Hot Take round and a Name That Idea round to test them. It's one of 250+ non-fiction books in the app.
