An object moving at relativistic speed appears shorter in the direction of motion. That is length contraction, a prediction of special relativity. The faster the object moves, the more it contracts. At 87 percent of the speed of light, an object is half its rest length. At 99.99 percent, it is a tiny fraction. The effect is not an illusion. It is a real change in the measured length of the object, as observed from a different frame of reference.

Length contraction is closely related to time dilation. Both come from the fact that the speed of light is the same for all observers, regardless of their relative motion. To keep the speed of light constant, space and time must adjust. A moving clock runs slower. A moving ruler is shorter. The effect is only noticeable at speeds close to the speed of light. At everyday speeds, the contraction is negligible. A jet flying at 500 mph is shorter by about the width of an atom. But in particle accelerators, where particles reach 99.999 percent of the speed of light, length contraction is significant and must be accounted for.

Length contraction facts

• Definition. Moving objects appear shorter.
• Direction. Only in the direction of motion.
• Formula. L = L₀√(1 - v²/c²).
• Frame-dependent. Observed from a different frame.
• Related to. Time dilation.

Length contraction is not the same as optical distortion. It is a real effect predicted by relativity and confirmed by experiment.