The Equivalence Principle

Einstein’s happiest thought

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• Gravity and acceleration are locally indistinguishable: the equivalence principle.
• So gravity bends light, confirmed in 1919.
• Freely falling observers feel weightless.

Steps

1. A windowless room
2. Light in a rocket
3. Weightless astronauts
4. Light climbing a tower
5. Light in an accelerating room
6. Where the principle breaks down