From an apple to a black hole

Heavy things bend spacetime, and time runs slower where you move fast or where gravity is strong. Follow the clock through 14 scenes: Earth and the Sun, a light clock, the twins’ trip, GPS satellites, neutron stars and black holes.

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Chapter 1 · Gravity is curved spacetime

Heavy things bend the space around them, and other things move along those bent paths.

1 Flat spacetime
Drawing empty space as a net
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2 Earth makes a dent
Heavy things bend the spacetime around them
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3 Newton and Einstein
A pulling force, or a curved path?
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4 The Sun and Earth
Trying to go straight, but turning along the curve
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Chapter 2 · Moving fast slows time

Everyone measures the same speed of light. Because of that, a fast-moving clock runs slow.

5 The speed of light
The fastest thing in the universe
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6 The light clock
Why a moving clock runs slow
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7 Speeding up
The closer to light speed, the slower time runs
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8 The twins’ trip
The twin who travels comes back younger
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Chapter 3 · Strong gravity slows time

Clocks deep in curved spacetime also run slow. And light follows the curve too.

9 The GPS satellite’s clock
A clock higher up runs faster
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10 Light bends too
What the 1919 eclipse showed
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Chapter 4 · Black holes

Where a star collapses and spacetime becomes endlessly deep. Neither light nor time gets out.

11 A star dies
Three endings, depending on mass
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12 The event horizon
The boundary even light cannot cross back
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13 What a black hole looks like
A glowing disk and a dark shadow
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14 Spaghetti and the singularity
Get close, and you get stretched
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