Ladder paradox

A special-relativistic thought experiment in which a moving ladder appears shorter than a garage in the garage’s frame, though it is longer than the garage at rest.

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Length contraction: The reduction in an object's measured length along its direction of motion relative to an observer. In the garage frame, contraction lets the moving ladder fit between the doors.
Bell's spaceship paradox: A special-relativistic thought experiment in which accelerating spaceships remain separated by a distance that causes a connecting string to break. Like the ladder setup, it tests how length contraction relates to objects and endpoints.
Special relativity: A theory of space and time in which the laws of physics are the same in all inertial frames and light has invariant speed. The paradox follows from special relativity’s rules for space and time.
Doppler effect: The change in observed wave frequency caused by relative motion between a source and an observer. Unlike length contraction, it changes observed frequency rather than spatial length measurements.
Relativity of simultaneity: The principle that observers in relative motion can disagree about which spatially separated events occur at the same time. The garage doors close simultaneously in one frame, but not in the ladder’s frame.
Pole-barn paradox: A special-relativistic thought experiment in which a moving pole fits inside a shorter barn when its doors close simultaneously in the barn's frame. This is another name commonly used for the ladder-and-garage scenario.
Inertial frame: A reference frame in which an object unaffected by net force moves at constant velocity. The garage and ladder descriptions use different inertial frames.
Terrell rotation: The apparent rotation of a rapidly moving object caused by differences in light-travel time across its surface. It concerns visual appearance, whereas the ladder paradox compares simultaneous positions in a frame.
Lorentz transformation: The equations relating space and time coordinates measured in inertial frames moving uniformly relative to one another. It converts the door-closing events between the garage and ladder frames.
Train-and-tunnel paradox: A special-relativistic thought experiment about a train longer than a tunnel that appears short enough to fit inside it. It presents the same frame-dependent length and simultaneity puzzle with a train.