In 19th century

physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...

, there were several situations in which the motion of matter might be said to ''drag light''. This

aether drag hypothesis In the 19th century, the theory of the luminiferous aether as the hypothetical medium for the propagation of light waves was widely discussed. The aether hypothesis arose because physicists of that era could not conceive of light waves propagating ...

was an attempt by classical physics to explain

stellar aberration In astronomy, aberration (also referred to as astronomical aberration, stellar aberration, or velocity aberration) is a phenomenon which produces an apparent motion of celestial objects about their true positions, dependent on the velocity of t ...

and the

Fizeau experiment The Fizeau experiment was carried out by Hippolyte Fizeau in 1851 to measure the relative speeds of light in moving water. Fizeau used a special interferometer arrangement to measure the effect of movement of a medium upon the speed of light. A ...

, but was discarded when

Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...

introduced his theory of relativity. Despite this, the expression ''light-dragging'' has remained in use somewhat, as discussed on this page. Under

special relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates: # The laws o ...

'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

gravitation In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stron ...

al effects, nor does it provide a complete relativistic description of

acceleration In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by the ...

. When more realistic assumptions are made (that real objects are composed of particulate matter, and have gravitational properties), under

general relativity General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...

's more sophisticated model the resulting descriptions include light-dragging effects. Einstein's theory of

provides the solution to the

Fizeau Experiment The Fizeau experiment was carried out by Hippolyte Fizeau in 1851 to measure the relative speeds of light in moving water. Fizeau used a special interferometer arrangement to measure the effect of movement of a medium upon the speed of light. A ...

, 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 In relativistic physics, a velocity-addition formula is a three-dimensional equation that relates the velocities of objects in different reference frames. Such formulas apply to successive Lorentz transformations, so they also relate different fra ...

of special relativity. Einstein's theory of

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 Frame-dragging is an effect on spacetime, predicted by Albert Einstein's general theory of relativity, that is due to non-static stationary distributions of mass–energy. A stationary field is one that is in a steady state, but the masses cau ...

) or rotational motion (causing rotation-dragging).

Velocity-dependent effects

Special relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates: # The laws o ...

predicts that the velocity of light is modified by travelling through a ''moving'' medium. * 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 Armand Hippolyte Louis Fizeau Royal Society of London, FRS FRSE MIF (; 23 September 181918 September 1896) was a French physicist, best known for measuring the speed of light in the namesake Fizeau experiment. Biography Fizeau was born in Paris t ...

experiment). This effect was originally predicted by dragged-aether theories (''see: e.g.

Fresnel Augustin-Jean Fresnel (10 May 1788 – 14 July 1827) was a French civil engineer and physicist whose research in optics led to the almost unanimous acceptance of the wave theory of light, excluding any remnant of Newton's corpuscular theo ...

''). 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) ).

General relativity General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...

predicts that the ''acceleration'' of a body in a straight line will cause light to drag, an effect known as Frame dragging (or

gravitoelectromagnetism Gravitoelectromagnetism, abbreviated GEM, refers to a set of formal analogies between the equations for electromagnetism and relativistic gravitation; specifically: between Maxwell's field equations and an approximation, valid under certain c ...

). * 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 (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

to accelerate space probes, using the gravitational

slingshot effect In orbital mechanics and aerospace engineering, a gravitational slingshot, gravity assist maneuver, or swing-by is the use of the relative movement (e.g. orbit around the Sun) and gravity of a planet or other astronomical object to alter the C ...

Rotation-dragging effects

Under

, the rotation of a body gives it an additional gravitational attraction due to its

kinetic energy In physics, the kinetic energy of an object is the energy that 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 accele ...

; and light is pulled around (to some degree) by the rotation ( Lense–Thirring effect). * In the case of rotation, under general relativity we observe a velocity-dependent dragging effect, since, for a rotating body, the tendency of the object to pull things around with it can be accounted for by the fact that the receding part of the object is pulling more strongly than the approaching part.

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'' {{Reflist

Velocity-dependent effects

Rotation-dragging effects

References

Further reading

See also