Talk:General relativity/Archive 1

message: The fact that light is bent by gravity is not particular confirmation of general relativity: assuming Newtonian gravitational mechanics applies to light particles like everthing else also predicts bending.  The issue is the amount of bending and the difference between the two predictions is a testable factor of two.  But I thought something strange happened in 1919. I remember reading that there were three attempts to measure the degree of bending: one was clouded out, one was close to Newtonian predictions and one was close to General Relativity;       Eddington decide that the third was better technically than the second - and later measurements have confirmed this. --Henrygb 23:27, 19 Nov 2004 (UTC)
replies: text: To the best of my knowledge Newtonian gravitaion would only predict the bending of light if light had mass. In Einstein's model its not so much that light is bent more that the geometry of spacetime itself is curved by mass so anything travelling in a 'straight' line will follow the curve. --LiamE 15:35, 15 September 2005 (UTC)
thread_title: 1919 confirmation: bending of starlight during a solar eclipse

message: Cleon Teunissen 12:01, 15 Jan 2005 (UTC)
thread_title:  The Sagnac Effect 

message: *Tuomo Suntola, Theoretical Basis of the Dynamic Universe, 2004. . A highly promising new model of the universe.
I removed this from the article. From what I can find on the web, it is a replacement for GR that is based around the concepts of absolute space and absolute time. As far as I can tell, very few people take it seriously, as it hasn't yet been verified that it explains everything GR does, much less make new, testable predictions (in particular, one back-of-the-envelope calculation shows it getting the orbit of Mercury wrong; another says that it doesn't handle the orbital decay of binary pulsars). --Carnildo 00:59, 4 Feb 2005 (UTC)
thread_title: "Dynamic Universe"

message: I do not like this article at all, and will rewrite it when I get a chance.
1) The central principle of GR is the General Principle of Relativity, which states that the laws of physics are the same in all frames of reference.  The local uniformity of phyical constants is a consequence of the General Principle, NOT the Equivalance Principle.
2) The central tenant of GR is that spacetime is curved due to the presense of mass/energy/momentum within it as described by the Einstein Field Equations.  This should be mentioned up front, instead of being left until later.
3) GR uses several other principles which are not mentioned:  The principle of general covariance (the laws of nature are independent of the coordinate system), the principle of geodesic motion (inertial motion occurs along timelike geodesics), and local Lorentz Invariance (the rules of Special Relativity apply locally in all frames of reference).
4) The Equivalence Principle (EP) is best presented as a rule for determining whether one is in an intertial frame of reference. (The definition presented in this article is correct, but not comprehensive.) Historically, the EP is the jumping-off point for Einstein's derivation of GR.  However, it is only a part of GR and is not the best place to start a popular article on the subject in my opinion.
: please do the rewrite.  there are lotsa reviewers here that will scrutinize it, and if it's good (and accurate) your work will not go to waste.  i am solely a spectator about the subject and am looking forward to learning some of the fundamentals.  i dunno if it will help but  and  were somewhat useful to me. anything you can do to add to or elucidate that is welcome.  also, i recommend that you get yourself a wikipedia identity. r b-j 17:47, 22 Mar 2005 (UTC)
thread_title:  This article is not good. 

message: The requested ID has been obtained, and I have started drafting the changes in my sandbox.
However, I request that you do not hold your breath waiting for the rewrite.  I am going to take my time and be very deliberate about it before doing the replacement.  I may not like the current article, but I do not want to replace it with something that is almost as bad if not worse.   The time that I can devote to this is also limited, but I very much want to produce an article that gives people a sense of what GR is really about.
:ain't holding my breath.  i would suggest that you sign your comments to "Talk:" pages with four tildes " ~ ~ ~ ~ ".  then your wikipedia ID with the date and time are stamped on your comments.  are you cool with the Tex editing?  you might want to check out Wikipedia:How_to_edit_a_page or Help:Formula if you haven't yet.  good luck, and you have any encouragement from me. r b-j 17:55, 23 Mar 2005 (UTC)
replies: text:  I finally did the rewrite.  See "Rewrite ready".  (This is actually not the rewrite I started drafting back in March.  That one bogged down and was dropped, but may reappear someday as a beginner's guide to GR.  The current rewrite was started in late June as an attempt to just redo the introduction, and quickly evolved into a comprehensive rewrite.)  --EMS | Talk 21:42, 18 July 2005 (UTC)
thread_title:  More on rewrite 

message: In reference to the null result of the Michelson-Morley experiment in the section on 'Foundations' (and other results ?) it says, '...Einstein explained these results in his theory of special relativity.' I'm not convinced he 'explained' the null result, but rather he used it as an assumption in SR. Then he developed a theory of mechanics. 
As a little aside, I'm not sure we really know why the speed of light is constant in all inertial reference frames, apart from the fact that otherwise there would arise theoretical contradictions - but to explain it physically is still a mystery (I think). I suppose it's a bit like asking why (physically) does the existence of matter cause spacetime to curve. What's the exact mechanism ? Food for thought ....
: i think i have a feel for why the speed of E&M propagation should be the same for all inertial reference frames.  it really just comes from Maxwell's Eqs. and the knowledge that there is no ether medium that E&M is propagated in.  i mean, how do we tell the difference between a moving vacuum and a stationary vacuum?  if we can't, if there really is no difference between a moving vacuum and a stationary vacuum, that such a concept is really meaningless, then whether the light that you are measuring originated from a flashlight mounted on a rocket moving past you at  c/2 \  or from a stationary flashlight, how does that change the fact that a changing E field is causing a changing B field which is causing a changing E field, etc.?  that propagation of an E field and B field disturbance, which has velocity  1/ \sqrt{ \epsilon_0 \mu_0 } \ ?  how is it different?  whether you are holding the flashlight or moving past it at high velocity, Maxwell's Eqs. say the same thing regarding the nature of E&M in the vacuum.r b-j 05:28, 24 Mar 2005 (UTC)
thread_title:  [A speculative response] 

message: I've moved a few things around and included some new sections to make the article a little clearer; a lot of work still needs to be done. The 'well-known and popular metrics' have been moved to Einstein field equation. Should the maths of GR be moved to a separate article ?
The statements
"The fundamental idea in relativity is that we cannot talk of the physical quantities of velocity or acceleration without first defining a reference frame, and that a reference frame is defined by choosing particular matter as the basis for its definition. "
are incorrect. An ordinary accelerometer measures acceleration. Of course, you can say the accelerometer is stationary in some reference frame, and so, in a sense, it locally defines a reference frame, but the question of how to extend that reference meaningfully outside the world-line of the accelerometer will lead you into a morass if you are not expert, and into a lot of digressions if you are.  One asks why someone is writing a new book on relativity here, when there are so many already. There are good old books by P G Bergmann and Steve Weinberg that can be mined for descriptions, and newer books too, e.g. by Robert M. Wald. As for the second statement it was already critiqued. A reference frame can be identified by imagining a swarm of massless observers, who define the coordinate gridpoints. They need not have mass, i.e. be material; they can be hypothetical. Only requirements are that their worldlines by timelike and form a smooth family - i.e. they do not intersect.  There is a maths term for that - probably "manifold" but let someone with more maths settle that. 
Pdn 06:26, 16 Apr 2005 (UTC)
thread_title:  modifications to article 

message: I think it's about time this article had a serious facelift. The quantum mechanics article puts this one to shame. I've made a few changes: new introduction; removed a paragraph on curvature which used very loose language; replaced that with a more rigorous version. Don't want to remove large chunks, as some of the existing text has useful descriptions (and It'd be treading on other peoples' toes). Need to sift through it carefully. Moved image of spacetime curvature to start of article (grabs attention). Discussion of constancy of speed of light is already in special relativity. Mpatel
:i want to encourage those of you who understand GR to do exactly this.  usually wiki articles help me break through the ice and my understanding of a topic has been increased as a result.  sadly, i cannot say this about this article.  maybe the material is just too hard, or i am missing something.  i understand classical mechanics, a smattering of QM, SR, and a little bit of the Equivalence Principle (the man in an elevator descending at 9.8 m/s^2 is in the same instantaneous situation as other in weightless space or the man in a rocket accelerating at 9.8 m/s^2 has the same experience as one standing on the surface of the earth).  is there some way that you guys can start with the EP, and bring us to something like Einstein's Field Equation?   or to an equivalent statement like in the Baez and Bunn paper that is cited? r b-j 15:48, 20 Apr 2005 (UTC)
thread_title:  Let's sort out this article. 

message: OK, I've made a start in seriously modifying this article. I've deleted, swapped and changed loads of stuff. I propose:
(1) In Description of Theory, we talk about how curvature arises through the use of non-inertial reference frames. Then talk about the principle of general relativity and the Equivalence Principle (and maybe covariance- but that might be better in the maths section).
(2) Then discuss some of the juicy effects of GR (light bending, grav. waves, evolution of universe etc...). 
(3) Something on nonlinearity (probably in 'relationship with other physics theories' section).
(4) Something on the history of Einstein's discovery of GR. 
(5) It might be OK to mention the field equation but not state it on this page (as elegant as it is) - I think it belongs in 'Mathematical formulation of GR', as should the geodesic equation; talking of which, deriving the equations of motion from the field equations might be worth a mention.
I await comments :) Mpatel 08:38, 21 Apr 2005 (UTC)
replies: text: made some changes and then did not notice I was logged out by accident (dunno why or how). Further changes by 69.244.72.110
this morning were mine. 
Pdn 12:31, 21 Apr 2005 (UTC)
thread_title:  Sorting out article. 

message: Included a subsection on topology of spacetime; might link in nicely with a brief discussion of black holes, wormholes etc. 
Is the section on foundations needed ? I think it's a little lengthy and possibly completely unnecessary - should it be moved somewhere else ? Mpatel 11:33, 22 Apr 2005 (UTC)
replies: text: Agreed. I just made a start in improving the article. The more modifications that are made to this article by more people, the better. Mpatel 09:23, 23 Apr 2005 (UTC)
thread_title:  More modific's