Please can you (briefly) tell me how the concept of 'non locality', or what Einstein referred to as 'spooky action at a distance' was arrived at?
Asked by: George Jackson


While I'm not a historian of science, the concept of action-at-a-distance probably originates with Newton's law of Universal Gravitation. Prior to Newton all of physics (indeed all of science generally) was conceived of in purely mechanical terms. Everything from the motion of the planets to visual perception was described in terms of particles bouncing off of one another, translating their kinetic motion from one body to another. Thus, all action was inherently 'local' (two objects have to be at the same point in space and time to collide).

Newton's law of gravity says that a given mass influences every other mass with a strength inversely proportional to the square of the distance separating them. There is no intercession of particle collisions or anything of the sort. In fact, the influence was to be taken as an instantaneous effect. Thus the sun influences the earth's motion though the two are physically and mechanically separated.

With relativity theory and quantum physics together we now know that something does in fact propagate between the two bodies -- namely, gravitational waves whose quanta are called gravitons. In fact we now know that all forces are mediated by such quanta called gauge bosons (such as photons) that can be thought of as being emitted by one body and absorbed by the other in a constant process of 'exchange.' Thus quantum field theory brings us back to 'local' forces.

Modern preoccupation with action-at-a-distance (to which Einstein objected) revolves around a purely quantum phenomenon. This is the celebrated thought experiment in which an 'entangled' system of two objects is created. The wavefunction of object #1 is correlated with the wavefunction of object #2. By the axioms of quantum mechanics a measurement of the properties of object #1 forces its wavefunction to collapse, instantly implying a correlated collapse in object #2 though it be arbitrarily far away. Though no information can be passed using such a method (thus not coming into conflict with special relativity) it does seem to imply some action-at-a-distance that is NOT accounted for by the exchange of force-mediating quanta such as gravitons or photons. This continues to be an unsettling state of affairs and a focus of philosophical and physical research, as it was for Einstein and as it was for the philosophers of Newton's day who were equally troubled by action-at-a-distance.
Answered by: Brent Nelson, M.A. Physics, Ph.D. Student, UC Berkeley

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