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# Light-dragging effects

In physics, there are several situations in which the motion of matter might be said to drag light.

Under special relativity's simplified model it is assumed that these light-dragging effects do not happen, and that the speed of light is independent of the speed of a 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 effects — when more realistic assumptions are made (real objects are made of particulate matter, and/or have gravitational properties), the resulting descriptions include light-dragging as an effect.

## 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 deflected 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.

## Acceleration-dependent effects

Under general relativity, the acceleration of a body in a straight line drags light. See 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, NY, 1995) - frame-dragging around black holes