From a Newtonian perspective, action at a distance can be regarded as: "a phenomenon in which a change in intrinsic properties of one system induces a change in the intrinsic properties of a distant system, independently of the influence of any other systems on the distant system, and independently of any intermediary system to process and carry this influence contiguously in space and time" .
According to Albert Einstein’s theory of special relativity, instantaneous action at a distance was seen to violate the relativistic upper limit on speed of propagation of information: the velocity of light. If one of the interacting objects were to suddenly be displaced from its position, the other object would feel its influence instantaneously; meaning information had been transmitted faster than the speed of light. So, there is an evident fundamental conflict between these two visions. On one hand, Newton's theory of gravity offered no prospect of identifying any mediator of ‘gravitational interaction’. At the other hand, Einstein proposed a relativistic theory of gravitation ‘restricted’ by conditions to submit any ‘mediation among the gravitationally interacting systems’ do not exceed the speed limit c, the speed of light in a vacuum.
Kepler´s observations gave strong evidence that in planetary motion angular momentum is conserved in which Newton agreed and fostered in his works by his Euclidean approach to the universe. Due to these conceptions Gravity is also known as a force of attraction between two objects because of their mass. A related question, raised by Ernst Mach, was how rotating bodies know how much to bulge at the equator. 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 term Mach's principle for this question.
Another strikingly opposite conception raised by Einstein's idea was that ‘there is no such thing as a "force" of gravity which pulls things to the Earth´; rather, the curved paths that falling objects appear to take are an illusion brought on by our inability to perceive the underlying curvature of the space we live in. The objects themselves are just moving in straight lines.
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. 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 of special relativity.
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 to mediate the action at a distance. 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. 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, but, as we said, in Einstein's theory of motion, matter acts upon space-time geometry, deforming it, and space-time geometry acts upon matter.
REFERENCES:
- http://en.wikipedia.org/wiki/Action_at_a_distance_%28physics%29
- http://curious.astro.cornell.edu/question.php?number=649
- http://theory.uwinnipeg.ca/mod_tech/node60.html
- Illustration: http://www.ws5.com/spacetime/
