Light-dragging effects
Pre-relativistic physical theory
In 19th century physics, there were several situations in which the motion of matter might be said to drag light. This aether drag hypothesis was an attempt by classical physics to explain stellar aberration and the Fizeau experiment, but was discarded when Albert Einstein introduced his theory of relativity. Despite this, the expression light-dragging has remained in use somewhat, as discussed on this page.
Under special relativity's simplified model Einstein assumes that light-dragging effects do not occur, and that the speed of light is independent of the speed of the emitting body's motion. However, the special theory of relativity does not deal with particulate matter effects or gravitational effects, nor does it provide a complete relativistic description of acceleration. When more realistic assumptions are made (that real objects are composed of particulate matter, and have gravitational properties), under general relativity's more sophisticated model the resulting descriptions include light-dragging effects.
Einstein's theory of special relativity provides the solution to the Fizeau Experiment, which demonstrates the effect termed Fresnel drag whereby the velocity of light is modified by travelling through a moving medium. Einstein showed how the velocity of light in a moving medium is calculated, in the velocity-addition formula of special relativity.
Einstein's theory of general relativity provides the solution to the other light-dragging effects, whereby the velocity of light is modified by the motion or the rotation of nearby masses. These effects all have one property in common: they are all velocity-dependent effects, whether that velocity be straight-line motion (causing frame-dragging) or rotational motion (causing rotation-dragging).
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