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<title>Light-dragging effects</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Light-dragging effects</span></span>

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<div id="mw-content-text" class="mw-body-content mw-content-ltr" lang="en" dir="ltr"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><p>In 19th century <a href="Physics" title="Physics">physics</a>, there were several situations in which the motion of matter might be said to <i><b>drag light</b></i>. This <a href="Aether_drag_hypothesis" title="Aether drag hypothesis">aether drag hypothesis</a> was an attempt by classical physics to explain <a href="Stellar_aberration" class="mw-redirect" title="Stellar aberration">stellar aberration</a> and the <a href="Fizeau_experiment" title="Fizeau experiment">Fizeau experiment</a>, but was discarded when <a href="Albert_Einstein" title="Albert Einstein">Albert Einstein</a> introduced his theory of relativity. Despite this, the expression <i>light-dragging</i> has remained in use somewhat, as discussed on this page.

</p><p>Under <a href="Special_relativity" title="Special relativity">special relativity</a>'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 <i>particulate matter</i> effects or <a href="Gravitation" class="mw-redirect" title="Gravitation">gravitational</a> effects, nor does it provide a complete relativistic description of <a href="Acceleration" title="Acceleration">acceleration</a>. When more realistic assumptions are made (that real objects are composed of particulate matter, and have gravitational properties), under <a href="General_relativity" title="General relativity">general relativity</a>'s more sophisticated model the resulting descriptions include light-dragging effects.

</p><p>Einstein's theory of special relativity provides the solution to the <a href="Fizeau_experiment" title="Fizeau experiment">Fizeau Experiment</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 <a href="Velocity-addition_formula" title="Velocity-addition formula">velocity-addition formula</a> of special relativity.

</p><p>Einstein's theory of <a href="General_relativity" title="General relativity">general relativity</a> 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 <a href="Frame-dragging" title="Frame-dragging">frame-dragging</a>) or rotational motion (causing <a href="Lense%E2%80%93Thirring_effect" class="mw-redirect" title="Lense–Thirring effect">rotation-dragging</a>).

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<div class="mw-heading mw-heading2"><h2 id="Velocity-dependent_effects">Velocity-dependent effects</h2></div>

<p><a href="Special_relativity" title="Special relativity">Special relativity</a> predicts that the velocity of light is modified by travelling through a <i>moving</i> medium.

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<ul><li><b>For a moving particulate body</b>, 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 (<a href="Fizeau" class="mw-redirect" title="Fizeau">Fizeau</a> experiment). This effect was originally predicted by <a href="Aether_drag_hypothesis" title="Aether drag hypothesis">dragged-aether theories</a> (<i>see: e.g. <a href="Fresnel" class="mw-redirect" title="Fresnel">Fresnel</a></i>). 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) ).</li></ul>

<p>General relativity predicts that the <i>acceleration</i> of a body in a straight line will cause light to drag, an effect known as <a href="Frame_dragging" class="mw-redirect" title="Frame dragging">Frame dragging</a> (or <a href="Gravitoelectromagnetism" title="Gravitoelectromagnetism">gravitoelectromagnetism</a>).

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<ul><li><b>For a moving gravity-source</b> 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, <b>a moving gravitational field drags light</b> and matter. This general effect is used by <a href="NASA" title="NASA">NASA</a> to accelerate space probes, using the gravitational <a href="Slingshot_effect" class="mw-redirect" title="Slingshot effect">slingshot effect</a>.</li></ul>

<div class="mw-heading mw-heading2"><h2 id="Rotation-dragging_effects">Rotation-dragging effects</h2></div>

<p>Under general relativity, the rotation of a body gives it an additional gravitational attraction due to its <a href="Kinetic_energy" title="Kinetic energy">kinetic energy</a>; and light is pulled around (to some degree) by the rotation (<a href="Lense%E2%80%93Thirring_effect" class="mw-redirect" title="Lense–Thirring effect">Lense–Thirring effect</a>).

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<ul><li><b>In the case of rotation</b>, 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.</li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

<ul><li>R.W. Ditchburn, <b>Light, (3rd ed.), Vol.2</b> (Academic Press, London, 1976) <i>- light and the motion of particulate media</i></li>

<li>Kip Thorne, <b>Black holes and timewarps: Einstein's outrageous legacy</b> (Norton, New York, 1995) <i>- frame-dragging around black holes</i></li></ul>

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<ul><li><a href="Aether_drag" class="mw-redirect" title="Aether drag">Aether drag</a></li>

<li>Democratic principle</li>

<li><a href="Einstein%E2%80%93Infeld%E2%80%93Hoffmann_equations" title="Einstein–Infeld–Hoffmann equations">Einstein–Infeld–Hoffmann equations</a></li></ul></div><div><div class="zim-footer">

This article is issued from <a class="external text" title="Last edited on 2024-12-19" href="https://en.wikipedia.org/wiki/?title=Light-dragging_effects&oldid=1263958722">Wikipedia</a>. The text is available under <a class="external text" href="https://creativecommons.org/licenses/by-sa/4.0/deed.en">Creative Commons Attribution-Share Alike 4.0</a> unless otherwise noted. Additional terms may apply for the media files.

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