# Talk:Kerr–Newman metric

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I'd like the new article to be called "Kerr-Newman electrovacuum". See the new "Exact solutions in general relativity" category to see why.---CH (talk) 02:46, 27 July 2005 (UTC)

I don't see from the category or the article Exact solutions in general relativity why this should be renamed, but I also don't think I would understand your reply if you explained it. I do however know a fair bit about naming conventions, so I may be able to help. - Taxman Talk 19:25, August 3, 2005 (UTC)

Comments moved in from Kerr-Newman which has been merged here:

If I ever get around to creating the proposed infobox template for describing explicit examples of exact solutions, the surviving page should be modified to use the template. ---CH

## Defining terms

The article defines Q and J but much of the rest is left up to imagination. Could someone knowledgable define these clearly?

M is mass, Q is electrical charge (both having geometric units of length) and J is angular momentum (which has geometric units of area). If you encounter a, that is a=J/M, specific angular momentum (which has geometric units of length).
You should think of these as measured at large distances by observers who are not moving with respect to the object at the center of the coordinate system. A subtle point is that because of the nonlinearity of gtr, there is an important distinction between local notions of mass (as measured by any observer anywhere, using any of various methods) and the former notion, which is a global notion of mass. Thus for example if a static observer near the hole compares the inward rocket engine thrust he must exert to maintain his position (so that he is accelerating outward with respect to test particles he drops at any time) with Newtonian theory, he would estimate a "mass" which differs from place to place depending on his position. This feature shows up the remarkable symmetries of the Ernst vacuum solutions to which the Kerr vacuum solution belongs. The Ernst family contains all stationary axisymmetric vacuum solutions to the Einstein field equation (a generalization also includes a possible electromagnetic field).
This will no doubt be confusing. Unforunately (see next section), I won't be able to follow up.

## Students beware

This article concerns a topic dear to my heart, and I had been monitoring it for bad edits, and had planned to greatly improve it, but I am leaving the WP and am now abandoning this article to its fate.

Just wanted to provide notice that I am only responsible (in part) for the last version I edited; see User:Hillman/Archive. I emphatically do not vouch for anything you might see in more recent versions, although I hope for the best. Unfortunately, general relativity attracts many cranks and the Kerr solution is well known to some of them, so it is likely that at least some future versions of this article will contain slanted information, misinformation, or disinformation. Beware also of external links to outside websites, which may portray a wide variety of pseudoscience or fringe science topics as more respectable than they really are.

Good luck to all students in your search for information, regardless!---CH 01:26, 1 July 2006 (UTC)

Also, note that this article states that the units are geometrized, but doesn't state how. Can we get a statement saying how they are geometrized? McKay 14:45, 20 February 2007 (UTC)

## Special cases

I've been working on this article a bit lately. The special cases section looks okay, except for some of the stuff that was added here. In particular, I don't think we really need that info about "orthogonal metric for the oblate spheroidal coordinates in the non-relativistic limit." The cited source is an article about "maximal analytic extension" which seems like more of a generalization than a special case, and James Hartle has said that the maximal analytic extension “is not very important for physics”.[1] Certainly, it has not been deemed by Wikipedia editors to be important enough to be covered in articles about other metrics (e.g. Schwarzschild). So, while I'm not 100% confident about this, I'll go ahead and remove it for now.Ferrylodge (talk) 23:00, 1 January 2009 (UTC)

Hello, Does the mass has an importance in the value of the electromagnetic potential? It doesn't look like, but I don't understand well I am french. What about the synchroton radiation of a rotated body, I know they are weak for a uniform charged body at slow speed, but I don't know at high speed. What about an equivalent phenomenon with gravity, a gravitysynchrotron radiation? Does the acceleration is already in these equation or is there only rotation? thanksKlinfran (talk) 01:08, 14 February 2009 (UTC)

No, it appears that the mass does not affect the electromagnetic potential, according to the Kerr-Schild formulation provided here in this Wikipedia article. Also, the Kerr-Newman solution does not involve any synchotron radiation, since there is only rotation.Ferrylodge (talk) 20:58, 15 February 2009 (UTC)
Well, its an excellent question, though. There may not be any synchrotron radiation in this solution, but how do we know that its physically correct or physically obtainable? How do we know that the correct physical system isn't some null-dust solution with a Kerr-Newman at the center of it? (the null dust being the emitted synchrotron radiation?) That answer to this is hardly obvious. 67.198.37.16 (talk) 05:08, 17 March 2016 (UTC)