A Brief History of Time

Key takeaways

• Space and time are woven together, and mass bends them like a weight on a sheet
• The universe is expanding, which means it had a beginning in a hot, dense state
• The very large is ruled by relativity and the very small by quantum mechanics
• Black holes are not fully black; they leak radiation and can slowly evaporate
• Time has a direction because disorder grows, and that is why we remember the past

Inside this bit

1. The universe in plain language

   A scientist gives a public lecture on astronomy and an elderly listener insists the world rests on the back of a giant turtle. Asked what the turtle stands on, she replies that it is turtles all the way down.

2. Space and time bend

   For centuries, people thought of time as a universal clock ticking the same everywhere. Einstein overturned that.

3. Everything is flying apart

   In the 1920s, Edwin Hubble noticed that distant galaxies are moving away from us, and the farther away they are, the faster they recede. This is what an expanding universe looks like from the inside.

4. The strange rules of the very small

   Relativity governs stars and galaxies, but a different set of rules runs the world of atoms. Quantum mechanics says that you cannot know both the exact position and exact speed of a particle at once; nature has a built-in fuzziness.

5. Black holes and the arrow of time

   A black hole forms when a massive star collapses so far that not even light can escape its gravity. For decades it was assumed nothing could ever come out.

6. Put it to work

   This week, step outside on a clear night and pick one star, remembering that the light entering your eye left it years or centuries ago, so you are looking at the past. Watch a short explainer on how GPS satellites correct for relativity, and you will see Einstein's ideas at work in your phone.