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Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
, there are several situations in which the motion of matter might be said to drag light
Under special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
'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 gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...
al effects, or to provide a complete relativistic description of acceleration
In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...
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.
- 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
In the 19th century, the theory of the luminiferous aether as the hypothetical medium for the propagation of light was widely discussed. An important part of this discussion was the question concerning the state of motion of Earth with respect to this medium. The aether drag hypothesis dealt with...
(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
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
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
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
, the acceleration of a body in a straight line causes light to drag, in an effect known as frame-dragging
Einstein's general theory of relativity predicts that non-static, stationary mass-energy distributions affect spacetime in a peculiar way giving rise to a phenomenon usually known as frame-dragging...
Under general relativity, the rotation of a body gives it an additional gravitational attraction due to its kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
, and light is also pulled around (to some degree) by the rotation (Lense-Thirring effect).