Talk:Action at a distance (physics)

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Contents 1 Deleted CS Stuff 2 Gravity - instantaneous or not? 3 Not straightforward? 4 Quantum mechanics 5 Naive? 6 Mach's principle

[edit] Deleted CS Stuff

Note that User:TakuyaMurata removed the computer science information for this article, which is actually how it began browsing through the history. I'm not really sure what to make of this. At least two CS articles are linking to here. - RedWordSmith 05:03, 2 Aug 2004 (UTC)

Never mind, it was spun off into Action at distance (computer science). There's a disambig. page at Action at distance, and I think I fixed the comp. sci. articles that were linking to this page. - RedWordSmith 23:18, 2 Aug 2004 (UTC)

[edit] Gravity - instantaneous or not?

Maybe I am getting confused, but the statement "Also, gravitation is assumed to act instantaneously, regardless of distance" seems to be contradicted later in this article when it is stated that Einstein's theories put a limit (of the lightspeed) on the speed of action of gravity.

It is not worded perfectly, but there is an implicit "...in Newton's theory..." in that statement. Einstein came along later and suggested that this (among other things) was wrong. Jmeppley 15:31, 26 May 2005 (UTC)

[edit] Not straightforward?

I have removed the text:

The question of whether this 'spooky action' at a distance constitutes a violation of the relativistic upper limit on the propagation of interaction is not straightforward.

I think it's misleading the reader to suggest that this is not a solved problem in quantum mechanics (even if it is not straightforward for the lay-person). I have made the statement more precise. Specificaly, quantum mechanics is clear that energy and information cannot travel faster than light speed, but that there can nevertheless be very high correlations between spatially seperated quantum systems. (Quantum mechanics may be 'weird', but for encyclopedia purposes, I think it's best to just stick to the facts.)

[edit] Quantum mechanics

I removed this misleading statement:

"At the same time however, instantaneous action at a distance appears to be an essential feature of some very fundamental quantum mechanical effects like entanglement and quantum nonlocality."

This is false, since relativistic quantum field theory is purely local, implying that interactions must propagate at less than the speed of light.

I also removed this poorly worded statement:

"In the modern treatment of relativistic quantum mechanics (quantum field theory), entanglement leads to high correlation between spatially separated systems, yet the laws of quantum mechanics clearly show that entanglement cannot be employed for relaying energy or information from one place to another."

"The laws of quantum mechanics" is a poorly defined concept. It's much better to state that locality is axiomatic to quantum field theory.

I then removed redundant statements and put the section into logical order. Dave Kielpinski 22:47, 17 December 2005 (UTC)

[edit] Naive?

While a naive interpretation of quantum mechanics appears to imply instantaneous action at a distance, careful reasoning about these cases shows that no such effects are actually present. Is the term "naive" necessary here? Wouldn't it be better to simply state the name of the interpretation in question, and leave it to the reader to form a POV? --Smithfarm 21:02, 22 January 2006 (UTC)

The enormously successful theory of relativistic quantum mechanics, i.e., quantum field theory, rules out the possibility of instantaneous action at a distance. People with a naive, i.e., not expert, understanding of quantum mechanics often infer incorrectly that QM implies action at a distance. In line with common scientific usage, I am not using "naive" in a pejorative way, which would indeed imply a POV, but just as a descriptive term characterizing a common but incorrect interpretation. If you have a better way of clearly stating the situation, please go ahead and change the wording. Dave Kielpinski 12:44, 25 January 2006 (UTC)

The revisions of 64.17.80.128 are incorrect. That user also has no previous edits on quantum mechanics. I have therefore reverted. Perhaps that user would like to discuss the proposed changes on the talk page. Dave Kielpinski 13:08, 19 February 2006 (UTC)

I thought the EPR paradox proved the nonlocality of QM, hence, action-at-a-distance. Are you saying QFT resolves the EPR paradox on local terms?

The concept of action-at-a-distance, or causality-at-a-distance, is interlinked with the ability to relay information from one point to another. It can be shown that QFT theory predicts that information (for example series of digits in a particular order) can only be relayed at lightspeed from one point to antother.

So on one hand there is this tantalizing glimpse of non-local connections, but those connections cannot be utilized to relay information. It is very peculiar: why would nature go through the trouble of having those non-local connections, while at the same time barring any 'use' of those connections? The situation reminds me a bit of what physics looked like in the years just before 1905, the years just before Einstein's introduction of relativity. Why would nature go through all the trouble of hiding the ether, if the ether is necessary anyway?

We can only hope that in the future a deeper theory will address those issues. --Cleonis | Talk 21:33, 25 February 2006 (UTC)

I think that Einstein's "spooky action at a distance" is really a misnomer, and a mistake on his part because of the already mentioned fact that there is no faster-than-light information transfer here. He really ought to have called it a "spooky correlation at a distance" (I certainly agree with Einstein that "spooky" is the right word here). (August 1, 2006 MvH).

PS. Repeating Einstein's error seems to be very common in popular science media. I hope that introductory texts on QM alert the reader that when they hear about "spooky action at a distance" that there is in fact no action here, and that what's going on is that distant measurements can display a spooky correlation but no spooky action. (August 1, 2006 MvH).

[edit] Mach's principle

The rotation of a spherical object causing a bulge at the equator has been related to Mach's principle though I dont see what is the connection. The only way Mach's principle is involved here is in the inherent relativity of the inertial frame about which the rotation rate is determined. And, in any case, as far as I can see, this phenomena dosen't demonstrate action at a distance.

I am just waiting for some feedback & confirmation before deleting that paragraph.

Mach's principle originated with Einstein. If you would grant the possibility of instantaneous action at a distance, then it is possible to speculate about a theory in which the inertia of all individual objects is due to some instantaneous interaction with all of the matter in the universe together.

Mach described such a speculation, adding that proper science should only concern itself with finding economic formulations of the laws of physics, and not indulge in speculations that cannot be confirmed anyway. So, historically, Mach rejected that which Einstein later coined as "Mach's principle".

Before the introduction of special relativity, the reigning view was that gravity acts instantaneously. (I think it was Lagrange who had proved the following: if the speed of gravity would be finite, then the force of gravity exerted for example by the Sun on Jupiter would not point exactly towards the Sun. The solar system has long term stability if and only if the force of gravity points exactly in the direction of the primary; the Sun. On that basis, Lagrange had calculated a minimum velocity for the speed of gravity, and that gave a velocity many, many times the speed of light.)

If you have to assume that gravity acts instantaneously anyway, there is room for speculation that inertia is due to action at a distance from distant matter.

After the introduction of special relativity, the whole landscape has changed, of course. It is not clear how a version of Mach's principle can be formulated at all.

In the first years after introducing the general theory of relativity, Einstein claimed that one of the foundations of general relavity was a principle that he called 'Mach's principle'. As Einsteins thinking evolved, he lost enthusiasm for even this version of Mach's principle and in the early 1920s Einstein abandoned Mach's principle altogether.

So, at the end of this elaborate story, I think I agree that it's better to remove mention of Mach's principle. --Cleonis | Talk 20:04, 3 August 2006 (UTC)