What Is “Now”?

The relativity of simultaneity

An interactive lesson with 7 steps, about 6 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• Events that are simultaneous in one frame need not be simultaneous in another.
• This resolves the apparent paradoxes of relativity.
• Cause and effect are protected because nothing travels faster than light.

Steps

1. Lightning strikes
2. Two flashes, two passengers
3. No universal “now”
4. Cause and effect
5. Tilting “now”
6. How much disagreement?
7. Now on Andromeda