The fundamental principle of physics is all observers of an event must have identical results, especially when the recorded data are transformed to a common inertial frame. The Mickelson-Morley experiment demonstrates contradictions with length contraction when three or more independent inertial observers concurrently record the output. Length contraction has never been observed directly despite current technological precision. Special relativity does not explain the constant output of the Mickelson-Morley and Kennedy-Thorndike interferometers, since Earth’s rotation causes unequal changing velocities for both arms. Displacements, instead of time intervals, for the split beams are analyzed when the interferometer is stationary and moving at a constant velocity. This paper reveals that any interferometer with equal or unequal arms can neither prove nor disprove the existence of a hypothetical medium for light transmission. All interferometer experiments indicate light velocity obeys vector velocity addition involving moving light sources. This property of light explains how measurements of light speed in any moving laboratory are precisely identical when sources and detectors are fixed relative to each other. The universal speed of light is an excellent approximation due to high speed, but not an exact constant when light sources and observers move independently. Many physical concepts will require reexamination.