Relativity Lab

One speed slider, four experiments. Set the relative speed v, then explore what "moving clocks run slow" actually looks like.

Speed v / c 0.600 c Lorentz factor γ1.25 moving clocks tick at80% moving lengths shrink to80%

Clock at rest in your frame Identical clock moving at v — its light pulse must travel the diagonal, so each tick takes longer (by exactly γ)

Both clocks are identical: a light pulse bouncing between two mirrors. Watch the tick counters drift apart — after enough ticks the ratio approaches 1/γ.

Proper length L₀ 100 m

The ship's crew measures their ship at its proper length L₀. In your frame the moving ship is contracted to L = L₀/γ along its direction of motion (the height doesn't change). The gridlines are 20 m apart.

Creation altitude 15.0 km

Cosmic-ray muons (mean lifetime τ = 2.2 μs at rest) are created high in the atmosphere. Without time dilation almost none should reach the ground — yet detectors see them. Survival fraction = e^−t/(γτ) where t is the trip time in your frame. Collect data at several speeds and altitudes, then plot ln(survivors) — the classic evidence for time dilation.

A lamp at the center of a moving train car flashes once. Top: in the train frame, light reaches both ends at the same moment. Bottom: in the platform frame the rear end runs into the light while the front end runs away from it — the two arrivals are not simultaneous. Neither frame is wrong; simultaneity itself is relative.