Einstein’s Second Postulate

Speed of light in vacuum is the same for all observers

Observer Speed: 0.50c Start / Restart Pause / Resume Information Home

Observation:
A light pulse spreads outward from its source.
Both observers track how far the light travels and how long it takes.

Stationary frame:
Distance = x, Time = t
c = x / t = 1.00
(Distance and time are scaled in this simulation)

Moving frame:
Distance = x′, Time = t′ (both change due to motion)
c′ = x′ / t′ = [γ(x - vt)]/[γ(t - vx/c²)] = c = 1.00

Although x' and t' change (via Lorentz transformation),
their ratio remains the same.

Conclusion:
All observers measure the same speed of light: c