Talk:General relativity/Archive 2

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Contents

GR SR connection

The best way so far as I know to grasp the connection is to understand that a GR spacetime consists of a patching-together of a continuum of inifintesimal local Lorentzian (SR) pieces. Think of a soccer ball with those little pentagons. Make them smaller and smaller, and as they are so small that they approach flatness, extend each patch as a small section of an infinite plane. When you apply the Equivalence Principle all you are doing is saying that, in principle, if you could live in that (tangent) plane, you would be in Minkowski (SR) spacetime and everything would be as usual in SR. (Be careful on notation - "tangent space" is a mathematical construct that includes, as I understand it, lots of other vectors besided displacements and velocities - I think it has E&M vectors in it, too.) Of course, all these vectors can be imagined to lie in that extended plane I just described, too.

How is this conceptualization applied to some poor chap standing in Dallas, TX and feeling the force of gravity pulling him downwards? Well, this discussion is all in four dimensions, and those little soccer ball patches all exist in a sequence of spacetime sections falling, in free fall, past this person. The patches have an inclination of their time axes to his that can be zero, if we make them by dropping them just like a lead sinker that he drops from rest, because if at rest when they meet him, the time axes are (instantaneously) aligned. If you want to extend these little patches above his head, I think you have to use ones with the time axes slanted, as if he threw up a sinker (or a clock) and then it passed by him again on the way down. But the point is that he is accelerated in regards to any of these coordinate patches that slip past him, and that is why he feels gravity. If you could extend the little patches to mate smoothly over a region larger than first order in the differentials, you would have found a local Minkowski space - hard to do in GR. It would exist inside a massive spherical shell, but not in very many other places I can think of, if matter is present. OK, nested shells, hollow at the center, that works. The patches can be just bent into a curved surface, or they can be more like vanes on a windmill, not mating smoothly. That is what happens in rotating systems; the patches that represent, for example, small regions on the Earth's surface are set up like vanes on a centrifugal fan. see: "Cylindrical Cardboard Model for a Rotating System in Special Relativity," Amer. J. Phys., 47, pp. 218‑223 . That's for special relativity, not GR, but it carries over to GR for rotating systems. Pdn 16:19, 23 Apr 2005 (UTC)

Complain!

This article is much too unwieldy and unorganized. I think this (and perhaps some related articles) need to be rewritten from scratch as as set of more focused articles, following the Wikiprinciple of passing from the general to the specific.

Suggestion: I see some stuff on "the meaning of curvature". That's important, but its really about geometry, not gtr per se, and should be incorporated into pages on Riemannian geometry (if it's not already there).

Another suggestion: there's much too much historical material here. Much of this could be moved to a separate article on the historical development of gtr.

Hillman 29 May 2005

My advice is to go for it. I looked into doing this myself, thinking that another project of mine was winding down, but it has not and I can't do much for now.
I agree with your complaint about the history section even though I wrote it. What little was there before was pathetic and misleading. So I put that section together, knowing that it almost is a first draft of the seperate article. My own feeling is that the math and the talk about the final development of the EFE should be off-loaded ASAP. I just want it to be done in the context of a fuller article on that issue.
Wikipedia is something that is going to grow in a step-like fashion as more and more people who care about these subjects and have the requisite expertise step up to the plate and add to it. Choose your battles here, realizing that you will need to build a foundation on which your GR article improvements are to rest. I myself am still grappling with the fact that the supporting articles that an improved GR article needs to reference (or which this one does) are often in need of work themselves. Often it is better to make the incremental imporvements than to do the big project, although that day must come.
--EMS | Talk 04:24, 30 May 2005 (UTC)
P.S. If you are who I now think you are (and I don't know of many entities named Hillman who can't figure out if they are human or not), then I very much look forward to your contributions. My advice is to start with the tensor and mathematics of general relativity pages. If you don't want to work on the math-of-GR article at first then at least look at my comments on it. I may work on that article soon and the comments include my intended outline.
--EMS | Talk 04:45, 30 May 2005 (UTC)

History of GR

Easier to move the history chunk to the article The development of general relativity (which I've done) and make modifications to that article (and the GR article) as appropriate. Mpatel 15:46, 30 May 2005 (UTC)

You didn't do the second half of that job, namely cutting the fat out of the GR article's history section. I have now done that. The new article also could use some elaboration and expansion, but that job I will leave mostly to others. --EMS | Talk 00:05, 31 May 2005 (UTC)

Suggestion on cutting down on 'EP'

The Equivalence Principle (EP) section appears to be larger than necessary, as there is a separate page for the EP. I suggest that any valuable bits in the EP section of the GR article be moved to the equivalence principle page (if they're not already there) and the current section be reduced accordingly. I think that only the gist of the EP needs to be mentioned in the GR article. Responses appreciated. Mpatel 15:52, 2 Jun 2005 (UTC)

No objection here. All that I ask is that the 1907 Einstein quote be retained. Beyond that I agree that it can be a lot shorter. The first sentence of the last paragraph should also apprear in the final product, but the rest of that last paragraph needs to be ditched.
Do go ahead and look at the equivalence principle page, but myself and Joke137 have put a lot of work into it, and I doubt that you will find any need to transfer data from this page to that one. Instead it may be more of an issue of making the GR section on the EP small and consistent with the EP page.
--EMS | Talk 19:29, 2 Jun 2005 (UTC)

Resolved

Hi, EMS and MP, thanks for the kind words. I am very glad to see you are both doing your bit (or more) to upgrade the gr articles here! I already have compiled a long list of articles I want to (re)-write here on gr topics. While, I haven't yet found the energy/time/courage to start in on the gr article itself, I -did- rewrite the article on Brans/Dicke theory! :-/

Looks like we all agree that ideally the main gr article should feature the briefest possible summaries of various broad topics, including

  • the dramatic players (gravitation, matter) and the stage (spacetime) as a player,
  • Einstein's motivation, desiderata, and heroic search for GR,
  • role of diffeomorphism covariance,
  • equivalence principle,
  • Newtonian limit,
  • geometric/physical meaning of the EFE, nonlinearity,
  • global versus local distinction,
  • conformal/causal structure, horizons,
  • role of boundary conditions, smoothness,
  • role of approximations (linearization/weak-field, far-field, slow motion),
  • gravitational radiation,
  • reformulations of GTR (ADM, Regge),
  • relationship to other gravitation theories, PPN parameters,
  • tests of GTR (past, present, future).

At the end of each brief summary, a link could point interested readers to a longer article devoted to this particular aspect, which can be written from scratch, perhaps using the best of what is already present in various Wikipedia articles. Right now, the only one of these topics which appeals to me is the meaning of the EFE.

As things stand, I think that even the list of references which I entered is fairly unwieldy. Is there any wikiprecedent for putting a list of printed refrences in a separate page? Maybe only the two on-line tutorial articles should be in the gr article itself--- maybe a very brief reference section could include these two and then link to a longer page containing all the other references?

FWIW, some much easier projects I'd like to carry out include:

  • add suitable references to all the longer articles, and put any existing references into a uniform (wikitemplated) format,
  • improve various of the less unwieldy articles,
  • add suitable figures (e.g. to the Regge calculus and Penrose diagram articles).

In the near future, I'd like to rewrite the weak-field theory article, add an article on multipole moments, add more pp-wave related articles, add an article on the Neugebauer/Meinel disk solution, and write articles on the Weyl and Ernst families of vacuum solutions. We'll see.

--Chris Hillman

Chris -
First of all, thanks for the outline. This is something that people can and may well work towards. I myself a few months ago mentioned a set of principles that the article of the time did not address. Now they are addressed, mostly due to the work of others. If you feel comfortable with addressing the physical meaning of the EFE, then go ahead and write it up. Just be aware that the EFE has its own article, and that much the same (but in more detail) should also be there.
As for moving the full list of references to another page: I know of no precedent, but an article on the available printed and web resources on GR is not inappropriate. Normally there is not such a wealth of resources available on a topic such that a special page is needed. I don't know that I would remove all references from the GR page itself. Instead I would keep 4-6 (including the web tutorials) and then have a prominent link ("For addition references ... " perhaps) to the full bibliography/references page.
Beyond that I repeat my prior advice: Choose your battles. There is so much that can and needs to be done, including fixing up many of the existing articles. As you already see, Wikipedia is living growing resource that over time is constantly improving due to the efforts of its contributors. That does not mean that you are not needed. In fact part of what is happenning is that as Wikipedia improves so does the quality of the contributors. So hopefully in a few more years the GR part of Wikipedia will be high quality text guarded and maintained by some of the better people in the field. For now we are a band of mostly dedicated amateurs with a few professionals in the mix, but your presense is a sign that the mix is changing.
--EMS | Talk 15:42, 8 Jun 2005 (UTC)

Geometric physical theory

What is this supposed to mean? Why is it important to non-logged-in editor 81.218.238.113 to have this particular language? This editor keeps reverting to this with no explanation. WCFrancis 17:09, 10 Jun 2005 (UTC)

  • This user is a vandal. They're modifying this and a handful of other pages to reflect their own unorthodox views of relativity and cosmology. I added the user to Wikipedia:Vandalism in progress a day or two ago, so hopefully their crusade will be curtailed in the not too distant future. --Christopher Thomas 20:22, 10 Jun 2005 (UTC)

The spacetime myth

Ok i see this is some kind of religion. Well i respect your religion but don't be surprised when someone will finally contradict your bizzare space-time theory. Space-time is a joke. It tells us nothing about motion, nor does it tell us anything about weight and gravitation. -- 81.218.238.113 11:39, 11 Jun 2005 (UTC)

That comment is so full of ignorance, it doesn't even deserve a response --- Mpatel 11:41, 11 Jun 2005 (UTC)

Sorry but you are the ignorant. Gravity has nothing to do with spacetime curvature[1]. Spacetime is a geometric construct with no counterpart in nature (see physical space). -- 81.218.238.113 13:42, 11 Jun 2005 (UTC)

Ok, you've asked for it now.
  • 'spacetime is a geometric construct with no counterpart in nature' - you sure about that? - if it were true, then you can use the same argument to conclude that space doesn't exist (same for time) - because we use geometric constructs to model space (3-D euclidean, usually) and time (1-D euclidean) individually. The constructs are mathematical and we can visualise them.
  • 'spacetime tells us nothing about motion' - that's not true, as by drawing a spacetime diagram (STD) you can conclude a lot about motion. A straight worldline in a STD can represent, for example, a particle travelling at a constant speed (relative to a given reference frame) - if you've done any physics or maths at high school level, you'll realise that what you said is incorrect, as you draw a STD every time you draw simple space-time graphs of an accelerating car (it's a 2-D STD) - curved lines represent acceleration, straight lines mean zero acceleration (sound familiar ????).
  • I think you're perhaps confused about the idea of spacetime curvature. In a given physical situation, the spacetime is given. Only if there is any change in the dynamical state of that system does spacetime change in any way. Modelling the motion of a planet around the Sun (and treating the planet as a point particle), for example, doesn't mean that spacetime changes (as spacetime is fixed) - but if you were to draw a STD for that planet, then the worldline would look like a spiral (see the diagram in spacetime). I repeat, the spacetime is fixed and nothing moves in spacetime (in a STD nothing moves - that's obvious, but you can still talk about a car moving along a road during a certain time interval and represent that on a STD). In GR, the geometry of spacetime is not flat, but it is curved (if you were to take timelike slices, then you would not have 3D euclidean hypersurfaces). If my interpretation is correct, the geometry of spacetime only changes when the physical system in question changes it's dynamical state (for example, if a star were to suddenly supernova, then clearly the local geometry would change - any planets orbiting the star would get flung out or destroyed). In oversimplified language, when we say that spacetime is curved, we mean: space is curved, therefore this thing that we call spacetime (= space + time) must also be curved.
  • The idea of gravity as a geometric phenomena is better (meaning experimentally and aesthetically) than any other idea at present - just take a look at the results of experimental tests of GR.

I hope that clears up any misunderstandings you have about gravity being curved spacetime and motion in spacetime. --- Mpatel 15:02, 11 Jun 2005 (UTC)

  • RE: The problem with differential geometry is that it deals with abstract metric spaces. Structures are easier to visualize but what does it tell us about physical processes (such as nuclear fusion) or the energy levels of the electron? Also if particles are absolute, why the hell do they need a reference frame to be relative to?

When large group of hydrogen molecules in space attract each other to produce stars, it seems more of a quantum interaction than molecules warping spacetime. Molecules in intergalactic space attracting other molecules by exchanging graviton particles just seems right.

My conclusion is that physics cannot be done using abstract geometry. We can only use elements, particles and the interaction processes between them.

I wish I could be more enlightening, but it seems that current knowledge levels are too low to produce anything more than just postulates. --81.218.238.113 15:45, 11 Jun 2005 (UTC)

Wikipedia doesn't do original research, so your conclusions are out of scope here. You can publish or promote them at other places, your own WWW page or the USENET, I suggest. Wikipedia has to report the current scientific consensus, including relevant minority viewpoints, if verfieable sources can be produced. --Pjacobi 20:07, 2005 Jun 11 (UTC)
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I'm sorry if this sounds rude, but one whose knowledge level is low is yours. GR, though a combination of spacetime curvature and the principle of geodesic motion does explain the observed motion of objects.
Think that you cannot see GR in action? Then drop something. Nothing is pushing on it, yet it accelerates downward, the same as it would from your point of view if you in a giant centrifuge being pushed against the walls.
GR easily explains the equivalence of inertial and gravitational mass. Does your graviton exchange model do that? If so, then what is the math be which you prove it?
In addition, GR
  • predicts the observed non-Newtonian perihelion precession Mercury,
  • predicts the observed bending of light passing close to the Sun being twice that of the Newtonian prediction.
  • predicts the time delay of signals going deeper into a gravitaional field,
  • predicts the time dilation effects that the GPS needed to account for in order to function properly,
  • and corresponds to Newtonian physics in the weak field limit.
Does your model do all that, and if so the what is the math?
In your defense, I will note the GR is a highly non-intuitive theory which many people find hard to grasp. The spacetime of GR is a curved, four-dimensional manifold in which one of the dimensions involved in the curvature is time. When Einstein started to work on the thery Max Plank told him "you will never succeed, and if you do noone will believe you". He was wrong on both counts, but to this day there are a lot of people who would rather not believe him than either deal with the math or admit their own failings.
--EMS | Talk 02:53, 12 Jun 2005 (UTC)

Max Planck was ineed wrong but not because he rejected relativity. Quantum mechanics fails to describe the gravitational interaction. Both quantum mechanics and relativity are wrong in ignoring very weak interactions[2]. -- 81.218.230.86 01:53, 13 Jun 2005 (UTC)

I have looked at your link, and all that I can do is hold my nose at what I read. Perhaps the worst offense is that statment that "GR is not fully compatible with Newtonian theory". Indeed it is not, and is not intended to be (except in the weak field limit). However, Einstein's GR works when Newton's theory does not!
Be advised that I have walked in your shoes. So I will warn you now:
  1. You do not have a theory at this point. Instead all that you have is a speculation. It is an unfortunate attribute of -uh- "independent researchers" that they often do not realize this at first.
  2. This is not the place to discuss your non-thoery.
I am now done with this thread. I do wish you luck, but you will not find it here. --EMS | Talk 03:42, 13 Jun 2005 (UTC)
P.S. I just felt that you might like to know that I liked your last edit, and agree with it's contents. However, it is totally lacking in NPOV. Therefore, I refuse to place anything like that in the GR page, and will revert any such content that I am the first to find there. A page dedicated to objections to the black hole, dark matter, and dark energy may actually be quite useful. There actually is a lot of objection to those in the field. However, before you get started do realize that it is the objections and misgivings of prominent scientists such as Einstein and Hawking that need to be documented. Your feelings have no place here, nor on this issue do mine (which along with the level of research required are the reasons that I will not do that article).
Do be advised that inappropriate articles are deleted here, sometimes rapidly. So don't try anything. Your work can be tracked, and will be dealt with. I may sympathize with your view, but I am not your friend. --EMS | Talk 05:13, 13 Jun 2005 (UTC)

I am not a friend of yours either and i don't need/want any permission from you. You don't own wikipedia and neither do i. As long as we'll construct a QFT version of gravitational interactions, general relativity is disqualified as a physical model of nature and gravitation becomes just like the other forces.

Peace -- 81.218.230.86 20:42, 13 Jun 2005 (UTC)

uh, oh. Some misunderstanding about scientific theories and models again. Even Newtonion gravitation is still a valid model and theory of gravition, as it proofs useful for description and calculation of a wide range of phenomena. Of course by now we know, where its usefullness breaks down and GR delivers better predictions. There is a widespread belief, that GR will break down at "Planck scale", but neither are details clear nor will this change anything on its range of usefull applications. --Pjacobi

Hallo Pjacobi,

AFAIK general relativity breaks down when space-time becomes infinitely curved. Unfornately we will never reach Planck scale but we should be able to construct a QFT version of gravitational interactions. -- 81.218.230.86 22:38, 13 Jun 2005 (UTC)

Pjacobi & others - All that answering this person is doing is giving him a soapbox to stand on.
He has admited that he cares not for GR, and has an almost religous belief that a QFT will solve all of the ills of GR.
And this comes from the same idiot who thinks that gravity is space-time curvature. You fascists cannot let go of old ideas. Btw i reverted your edits about the time and physical space article. -- 81.218.230.86 16:29, 14 Jun 2005 (UTC)
Edit of time re-reverted, but your link to dimension was kept. (That at least was a good idea/edit.) --EMS | Talk 19:07, 14 Jun 2005 (UTC)
Never mind that Wikipedia is not the place for original research or personal speculative opinions. I suggest that we leave him be, except to revert his edits to articles as needed and if deserved. --EMS | Talk 02:25, 14 Jun 2005 (UTC)