The aberration constant for QSOs.

Special relativity's stipulated independence of light velocity from source velocity has seldom been tested in the macroscopic scale, due to the difficulties encountered in the devising of a method. Since QSOs are the most extreme entity known in both velocity and scale, they constitute the ideal tool for such a test. The experiment can be conducted by taking a series of plates of QSOs, spread over the course of a year, and then looking for an elliptical displacement of the QSO relative to the stellar foreground. If a displacement is found, and it is oriented in the expected sense, it may be concluded that c has varied by a proportional amount. Direct images of four QSOs have confirmed the constancy of c to within 0.4 percent, for relativistic source velocities over a scale approaching the size of the universe.