What is the effect called when the length measured between events is shorter for an observer in motion compared to a stationary observer?

The effect where the length measured between events is shorter for an observer in motion compared to a stationary observer is known as length contraction. This phenomenon is a fundamental concept in the theory of relativity, specifically derived from Einstein's special theory of relativity.

According to this theory, as an object moves at relativistic speeds—close to the speed of light—length is measured differently by observers in different frames of reference. A moving observer will measure lengths along the direction of relative motion to be shorter than those measured by an observer at rest. This contraction occurs only in the direction of motion, and it becomes significant at speeds approaching the speed of light.

Length contraction can be mathematically described by the equation:

[ L = L_0 \sqrt{1 - \frac{v^2}{c^2}} ]

where (L) is the contracted length observed by the moving observer, (L_0) is the proper length (the length measured by the stationary observer), (v) is the relative velocity of the moving observer, and (c) is the speed of light. This clearly illustrates how motion can affect spatial measurements, leading to the conclusion that the correct term for this effect is length contraction.