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→Action at a distance: My humble reply.
So which is it? Can either of you enlighten me? Or fix the article? (By the way, you both write very well.) [[User:PatrickD|PatrickD]] 22:28, 12 November 2009 (EST)
So which is it? Can either of you enlighten me? Or fix the article? (By the way, you both write very well.) [[User:PatrickD|PatrickD]] 22:28, 12 November 2009 (EST)
I've seen what KSorenson wrote, and her explanation of the locality and causality aspects are far superior to anything I could do. But I would like to point out that, historically, it meant something different. One thing that the old and new definitions have in common is an element of "spookiness". Of course, what was spooky in the 1600's is perfectly sensible now.
The term is considered rather quaint now; I think it should properly be used only in the old context. As KS points out, in the modern world of field theories, "locality" and "non-locality" are better terms. If something (like an electric charge) locally affects the field (electric field or Faraday tensor) here, and there are equations (Maxwell's equations) relating the behavior of the field here with the behavior there, and the field there causes a particle there to feel a force, that is not really "action at a distance". It's perfectly acceptable. In modern terms, it's a local field theory. The field "mediated" the force. The way the equations relate the field at one point to the field at another point (divergences and such) that carry the force from "here" to "there".
But the carrying of force from one point to another wasn't so easy to accept in the 1600's. It was considered spooky, just as Schrodinger's cat and quantum entanglement are considered spooky today. In the 1600's, people generally believed that one object could only exert a force on another object if the two were in physical contact. There was plenty of practical experience underlying this belief.
:But there were two effects that I can think of that violated this: There were magnets--natural lodestones; and their ability to point North had been known since antiquity. And there was gravity. The apple fell to the ground without anyone pushing it. How did they reconcile this? My guess is that they just didn't think as hard about such issues as perhaps they might have.
I think that Newton's discovery that gravity makes the planets move forced people to think about what they had been ignoring. You can overlook an apple falling to the ground, but you can't overlook a force making Mars move. Was it the fact that the force operated through a vacuum that made people uneasy? Or was it the fact that the distances were so vast? I suspect it was a little of each. That which had been overlooked before could no longer be denied. There was action at a distance. Across hundreds of millions of miles of empty space. This bothered people. It even bothered Newton. He wrote about not believing in action at a distance, even though he had just devised the world's first field theory. (He was not the only person to have a hard time coming to grips with his own discoveries.)
Action at a distance has also been used, classically, to refer to action traveling faster than the speed of light. But this did not come up in Newton's time. That there should be a finite speed of propagation wasn't an issue. In fact, Newton proved that angular momentum would not be preserved unless gravity propagated instantaneously.
:How do we reconcile that with the fact that it is propagated at the finite speed of light? Classical mechanics is completely consistent with special relativity under the assumption that the speed of light is infinite. Newton was doing a relativistically correct proof, as long as c = infinity.
Later on, as the finiteness of the speed of light became known, the questions arose whether the effects of fields (electric, magnetic, and gravitational) could travel faster than the speed of light. Such an effect might have been called "action at a distance", and would have been spooky. But the issue was pretty much settled in the 19th and early 20th centuries.
The issue arose again in the mid 20th century, referring to the possibility that fields (this time the quantum field) might mediate actions faster than light. The problem is often posited in terms of transmission of information faster than light, which would violate common assumptions about causality.
And that's where we stand now, though "non-locality" is a much better modern term. I hope this helps.
[[User:SaraT|SaraT]] 14:44, 14 November 2009 (EST)