- Action at a distance (physics)
In
physics , action at a distance is theinteraction of two objects which are separated inspace with no known mediator of the interaction. This term was used most often with early theories ofgravity andelectromagnetism to describe how an object could "know" the mass (in the case of gravity) or charge (in electromagnetism) of another distant object.Electricity
Coulomb's law inelectrostatics appears to be a theory with action-at-a-distance - Coulomb's law deals with charges which have always beenstatic . Efforts to develop a theory of interaction between moving charges,electrodynamics , led to the necessity to introduce the concept of a field with physical properties. In the theory of electrodynamics as formulated inMaxwell's equations , interactions between moving charges are mediated by propagating deformations of an electromagnetic field. These deformations propagate with the speed of light and Maxwell's wave theory was later extended to cover Coulomb's law by theLorenz gauge . The deformations of the field can carry momentum independently, thus facilitating conservation of angular momentum.Gravity
: "Main article:
Speed of gravity "Newton
Newton's theory of gravity offered no prospect of identifying any mediator of gravitational interaction. His theory assumed that gravitation acts instantaneously, regardless of distance. Newton had shown mathematically that if the gravitational interaction is not instantaneous,
angular momentum is not conserved, and Kepler's observations gave strong evidence that in planetary motion angular momentum is conserved. (The mathematical proof is only valid in the case of anEuclidean geometry .)A related question, raised by
Ernst Mach , was how rotating bodies know how much to bulge at the equator. How do they know their rate of rotation? This, it seems, requires an action-at-a-distance from distant matter, informing the rotating object about the state of the universe. Einstein coined the termMach's principle for this question.Einstein
According to
Albert Einstein 's theory ofspecial relativity , instantaneous action-at-a-distance was seen to violate the relativistic upper limit on speed of propagation of information. If one of the interacting objects were suddenly displaced from its position, the other object would feel its influence instantaneously, meaning information had been transmitted faster than thespeed of light .One of the conditions that a relativistic theory of gravitation must meet is to be mediated with a speed that does not exceed lightspeed. It could be seen from the previous success of electrodynamics that the relativistic theory of gravitation would have to use the concept of a field or something similar.
This problem has been resolved by Einstein's theory of
general relativity in which gravitational interaction is mediated by deformation of space-time geometry. Matter warps the geometry of space-time and these effects are, as with electric and magnetic fields, propagated at the speed of light. Thus, in the presence of matter, space-time becomes non-Euclidean, resolving the apparent conflict between Newton's proof of the conservation of angular momentum and Einstein's theory ofspecial relativity . Mach's question regarding the bulging of rotating bodies is resolved because local space-time geometry is informing a rotating body about the rest of the universe. In Newton's theory of motion, space acts on objects, but is not acted upon. In Einstein's theory of motion, matter acts upon space-time geometry, deforming it, and space-time geometry acts upon matter.Quantum mechanics
Current physical theories incorporate the upper limit on propagation of interaction as one of their basic building blocks, hence ruling out instantaneous action-at-a-distance. However, the correlations between separated particles in
quantum entanglement proved difficult to understand in terms of a classical picture that obeyed locality, with Einstein coining the term "spooky action at a distance" to describe these situations. Relativisticquantum field theory requires interactions to propagate at speeds less than or equal to the speed of light, so "quantum entanglement" cannot be used for faster-than-light-speed propagation of matter, energy, or information. Measurements of one particle will be correlated with measurements on the other particle, but this is only known after the experiment is performed and notes are compared, therefore there is no way to actually send "information" faster than the speed of light. Einstein could not believe this, and therefore he proposed, along withBoris Podolsky andNathan Rosen , a thought experiment called theEPR paradox . John Bell derived an inequality that showed a testable difference between the predictions of quantum mechanics and localhidden variables theories. Experiments testing Bell-type inequalities in situations analogous to EPR's thought experiments have been consistent with the predictions of quantum mechanics, showing that local hidden variables theories can be ruled out. Whether or not this is interpreted as evidence fornonlocality depends on one'sinterpretation of quantum mechanics ; for example, theBohm interpretation does give a non-local explanation for the correlations seen in entanglement, but many advocates of themany-worlds interpretation argue that it can explain these correlations in a way that does not require a violation of locality, [http://arxiv.org/abs/quant-ph/0103079] by allowing measurements to have non-unique outcomes.ee also
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quantum teleportation References
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