Concept preview

What you will be able to do

1. 01Use the Lorentz factor γ = 1/$\sqrt{1 - v^2/c^2}$
2. 02Calculate time dilation (Δt = γΔt₀) and length contraction (L = L₀/γ)
3. 03Explain the evidence: muons, GPS and the twin paradox

Here's Einstein's own thought experiment. Imagine a light clock — a pulse of light bouncing straight up and down between two mirrors; each round trip is one 'tick.' Now set that clock moving sideways past you. From your view the light must travel a longer, diagonal zig-zag path to reach the moving mirror. But light can only go at $c$ — so a longer path at fixed speed takes more time. The moving clock ticks slower. This is time dilation, and it comes straight from the constancy of $c$.