Length Contraction and Relativistic Momentum Interactive Widget
Moving objects shrink, but only in the direction they move
At high speeds near the speed of light, objects contract along their direction of motion while staying unchanged in perpendicular directions. A spaceship rushing past you would appear squashed front-to-back but maintain its full height and width. Drag the slider to increase velocity and watch the spaceship contract.
Relativistic Observation
v = 0.00c
Spaceship proper length: 100 m (at rest)
Observed Length: 100.0 m · γ = 1.00
Velocity (fraction of c)
0.00c0.50c0.99c
Contraction Factor
1.00
Momentum Boost
1.00×
Length contraction reveals that space itself is relative. The proper length L₀ is measured in the object's rest frame and is always the longest measurement. An observer watching the object move at velocity v measures a contracted length L = L₀/γ, where γ (gamma) is the Lorentz factor. Critically, only the dimension parallel to motion contracts—perpendicular dimensions remain unchanged. A meter stick moving lengthwise shrinks, but if it moves sideways, its length stays constant. This directional nature proves that spacetime mixing occurs only along the motion axis. At everyday speeds where v ≪ c, gamma approaches 1 and the contraction becomes unmeasurably small, which is why we never notice it in daily life.
Know This
Only lengths parallel to the direction of motion contract while perpendicular dimensions remain unchanged, so a moving spaceship appears squashed but maintains its full height and width.