Feynman teaching note

Time Dilation

SubjectPhysics

Area tags

Physics

Einstein started from one stubborn fact: the speed of light is the same for everyone, no matter how fast they move. Hold onto that and something strange follows: moving clocks tick slower. Imagine a clock made of a light beam bouncing between two mirrors, one tick per round trip. If that clock flies past you, from your view the light has to travel a longer diagonal path (up-and-forward, then down-and-forward). But light can't speed up to cover the extra distance. So each tick takes longer as seen by you. The moving clock runs slow. This isn't an illusion or a mechanical fault; time itself stretches. It's tiny at everyday speeds but real: GPS satellites must correct for it or your maps would drift by kilometres a day.

Same topic, fresh practice—does not count as an adaptation.

Practice building on this note. If you publish, it can show “Based on” and add an adaptation.

Feedback & gaps

Clarity score: 78/100How well you know it: Can explain it simply

Explain why the constant speed of light forces moving clocks to slow down, using the light-clock picture.

Doesn't mention that each observer sees the OTHER's clock as slow (symmetry).
Skips length contraction, the partner effect.

How I worked through the gaps

Why must the moving clock slow down?

Connecting constant light speed to slower ticks.

Q1If the light path gets longer but light can't go faster, what has to give?

YouThe time per tick gets longer, so the clock ticks slower.

CoachExactly. Fixed speed + longer path = more time per tick. That's time dilation.

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