Light-dragging effects
In 19th century physics, there were several situations in which the motion of matter might be said to drag light. This aether drag hypothesis came about to explain stellar aberration and the Fizeau Experiment, but was discarded when Einstein introduced his theory of special relativity. Despite this, the term has remained in use somewhat, as discussed on this page.
Fresnel drag occurs when the velocity of light is modified while traveling through a moving medium. The velocity of light in a moving medium is given by the velocity-addition formula of special relativity.
Frame dragging (gravitoelectromagnetism) occurs when the velocity of light is modified by the motion and rotation of nearby masses.
Under special relativity's simplified model it is assumed 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 claim to deal with "particulate matter" effects or gravitational effects, or to provide a complete relativistic description of acceleration — when more realistic assumptions are made (real objects are made of particulate matter, and have gravitational properties), the resulting descriptions include light-dragging effects.
Velocity-dependent effects
- For a moving particulate body, light moving through the body's structure is known to move faster in the direction of the body's motion than it does in the opposite direction (Fizeau experiment). This effect was originally predicted by dragged-aether theories (see: e.g., Fresnel). Light aimed transversely through a moving transparent body is also seen to be translated in the direction of the body's motion (R.V. Jones, J.Phys A 4 L1-L3 (1971) ).
- For a moving gravity-source the gravitational field can be considered as an extension of the object, and carries inertia and momentum - since a direct collision with the moving object can impart momentum to an external particle, interaction with the object's gravitational field should allow "momentum exchange" too. Consequently, a moving gravitational field drags light and matter. This general effect is used by NASA to accelerate space probes, using the gravitational slingshot effect.
- In the case of rotation under general relativity (see below), we also have an apparent velocity-dependent dragging effect, since for a rotating body, the tendency of the object to pull things around with it can be described by saying that the receding part of the object pulls more strongly than the approaching part.
Under general relativity, the acceleration of a body in a straight line causes light to drag, in an effect known as frame-dragging.
Rotation-dragging effects
Under general relativity, the rotation of a body gives it an additional gravitational attraction due to its kinetic energy, and light is also pulled around (to some degree) by the rotation (Lense–Thirring effect).
References
- R.W. Ditchburn, Light, (3rd ed.), Vol.2 (Academic Press, London, 1976) - light and the motion of particulate media
- Kip Thorne, Black holes and timewarps: Einstein's outrageous legacy (Norton, New York, 1995) - frame-dragging around black holes