Talk:General relativity/Archive 6

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Paper source?

My Google skills fail me. Does anyone have a link to the 1915 paper in English (e.g., a PDF)? I just heard an interview on episode 13 of the Scientific American podcast in which Frank Wilczek talks of reading the paper and liking Einstein's writing style (around minute 6). —Ben FrantzDale 14:53, 8 May 2006 (UTC)

Intro [Isn't GTR defined as a theory of accelerated motion and gravitation?]

For more clarity, the subject matter of this discussion is about the following phrase (emphasis added):

General relativity (GR) as published by Albert Einstein in 1915 was an extension of the special relativity for accelerated frames and included a theory of gravitation.

According to CH that is summarized by (emphasis added): Is GTR a theory of gravitation or a theory of accelerated motion? Our difference in understanding of the subject matter almost certainly played a major role in the disagreement below. Harald88 21:10, 22 May 2006 (UTC)

User 87.217.88.42 made a very correct improvement to the intro, which was disliked by Hillman and Mpatel; I now combined the versions.

Perhaps some people here are unaware of (or simply forgot!) the meaning of "SRT" and "GRT", as well what gravitation has to do with it. However, Einstein's introduction in his 1916 paper that is linked from the article is very clear about it. Note also that the use of GRT for accelerated frames is discussed in twin paradox and that it probably will be mentioned in special relativity (see the recent change, rv and question about "accelerated systems" in History). Wikipedia encourages articles that disagree with other more accurate articles to harmonize the information. Harald88 23:22, 14 May 2006 (UTC)

Harmony is nice, but it helps if the information is correct first and foremost. I do not disagree that GR started as an extension of SR to cover accelerated frames of reference. However, once Einstein was done, he had a full-fledged theory of gravitation. You seem to have no appreciation for what GR is, just some knowledge about its initial inspiration. --EMS | Talk 06:00, 15 May 2006 (UTC)

You seem to have a different understanding of English than I have: The intro as you reverted to presents the facts wrongly, while the merge of your version with that of 87.217.88.42 presents the facts as you also state here above. An alternative is to not refer to Einstein in the intro; but misrepresentation isn't an acceptable solution. Harald88 19:11, 15 May 2006 (UTC)

You are the one doing the misrepresentation IMO. GR is fundamentally a theory of gravitation. It incorporates the special theory as a special case, but is much, much more than a simple extension of SR. --EMS | Talk 20:37, 15 May 2006 (UTC)

P.S. Have you read the whole article? I find it hard to believe that you would be arguing about this point if you had. --EMS | Talk 20:43, 15 May 2006 (UTC)

Where does the article state that it's "a simple extension of SR"?? You read words things that I can't find. Anyway, the Wikipedia method is to refine each other's contributions, so please remove the word "simple" from wherever it may be. And yes I read the whole article some time ago; but I find it hard to believe that you would argue about what Einstein wrote if you had read it. By his definition (and he was the one who coined the terms "SRT" and "GRT"), GRT led to a theory on Gravitation (Einstein 1916). Thus, as I stated above: take it or leave it, but please don't misrepresent publications. Harald88 22:02, 16 May 2006 (UTC)

When we write about a topic on Wikipedia, we write about our current knowledge, conception and formulation of it. Einstein did introduce GR as an extension of SR, but he wanted (and realised that an extension of GR would give) a theory of gravitation (via the equivalence principle). Like EMS suggested, you have to read the whole article to get a snapshot of what we now believe GR to be. The history and other details are explored in other articles mentioned in this one. The fact that Einstein initially wanted an extension of SR is an important fact, but nowhere near as important as actually defining what GR is, which is what we ought to be doing when defining GR at the start of the article. It's a bit like saying that electromagnetism should be defined as an extension of the separate theories of electrostatics and magnetism, but electromagnetism is much, much more than that ! MP (talk) 22:10, 15 May 2006 (UTC)

I have no issues with what GRT nowadays is regarded to be; if it's not as stated in the intro, then you have an issue with your own formulation of it! The way it is formulated now, it disagrees with othr sources, starting with Einstein 1916; moreover, quite some teachers still regard it as a theory of accelerated frames as well as gravitation. Harald88 22:02, 16 May 2006 (UTC)

Harald is apparently referring to this edit by the Jazz Telecom anon in Madrid. Harald, we are not unaware of this issue; in fact we were just talking about a closely related issue (see Where to move Ian Benyon's additions in most recent archive)! As I mentioned then, I was planning my own version before Ian provided a very different article than the one I had in mind, which would have addressed this specific point. In any case, the real issue is how to best serve our readers. There is a lot of say about general relativity, so in the main article we must be very judicious in what we choose to say in the article, rather than in articles on specialized aspects. I suggest you go argue with Ian Beynon (talk · contribs) in Talk:Hole argument (I trust you know the connection between Einstein's "hole problem" and gtr as something-other-than-a-mere-theory-of-gravitation. ---CH 04:22, 16 May 2006 (UTC)

I was indeed referring to that reformulation of the intro which accurately presented Einstein's introduction to the subject and which is very easy to understand (certainly for someone who is supposed to understand something of what follows!); as a Wikipedia reader I want to be informed in an as simple but still accurate way. Harald88 21:56, 16 May 2006 (UTC)

I don't know in how many different ways we can explain this to you Harald, but you fail to realise that the basic premise of GR is that the phenomena we call gravitation is a curved spacetime caused by matter/energy. The defining feature of GR is not that it's an extension of SR. GR is principally a theory of gravitation and it is this point that needs to be emphasised in the intro., not the fact that it's an extension of SR. Amongst other things, the extension provides a nice way to compare the 2 theories and this is explained later in the article. It's all very well trying to write an article which is easily understood, but we must emphasise the right points in the right places in the article. MP (talk) 22:29, 16 May 2006 (UTC)

Harald88 - You don't care to listen, do you? Your pet reformulation is not accurate! Item: In 1907 and again in 1911, Einstin used special relativity alone to demonstrate the existance of gravitational time dilation, gravitational redshifing, and even the bending of light by the Sun. So special relativity itself applies in accelerated frames of reference. However, Einstien needed GR to get the correct value for the bending of light, as well as to explain how mass causes objects to be accelerated towards other massive objects. Spacetime curvature is something totally alien to special relativity, for that reason general relativity is not accurately described as an extension of special relativity.

As for making it understandable, I am willing to work on it and would appreciate suggestions, but the underlying concepts are very advanced even when simplified as much as possible. I sympathize about wanting the explanation to be simple, but in choosing between "simple" and "accurate" I will always choose the accurate wording over a simpler but misleading one. --EMS | Talk 22:39, 16 May 2006 (UTC)

EMS and Mpatel, SRT is an extension of classical mechanics. Indeed I don't understand what problems you have with the meaning of such a standard expression that apparently doesn't mean to you what it means to me and others. Sorry, but it's no problem to replace that word by "generalized" or what ever word it was that for example Einstein chose.

Apart of that, EMS you are mistaken to claim that the demonstration of gravitational time dilation was possible with SRT alone: instead he used the equivalence principle - the foundation of GRT.

It's good that EMS and I are likeminded in principle; it's a pity that on some points he and I mutually disagree about each other's claimed "accuracy". Harald88 23:29, 16 May 2006 (UTC)

"A little knowledge is dangerous" - The equivalence principle dictated which frames of reference are the inertial ones. It in no way influences the apperance of the effects mentioned above in accelerated frames of reference. Furthermore, SR and the equivalence principle are totally inadequate for the formulation of general relativity. At best, they lead you to the contradiction that forced Einstein to devise GR. --EMS | Talk 23:57, 16 May 2006 (UTC)

Apparently we agree that SRT doesn't predict gravitational time dilation, and it wasn't me who suggested that SRT+the equivalence principle would be sufficient for full GRT. Harald88 19:36, 17 May 2006 (UTC)

No we don't agree on that.

• SR alone is sufficient to infer the existance of gravitational time dilation.
• The equivalence principle determines the contexts in which the SR prediction applies.
• General relativity uses spacetime curvature to explain how the equivalence principle works on all sides of a massive body (which SR cannot explain).

If you knew GR, I would not have to explain that to you. --EMS | Talk 21:44, 17 May 2006 (UTC)

SRT ignores gravitational time dilation and it surprises me that some people don't know that effects of gravitational fields are not part of it. Einstein explained that rather well IMO.Harald88 23:01, 17 May 2006 (UTC)

P.S. SR may build on classical mechanics but it is not an "extension" of it. The second postulate (as extended with the aid of the first postulate) is fundamentally incompatible with classical mechanics. A similar break exists between SR and GR. --EMS | Talk 00:00, 17 May 2006 (UTC)

Their predictions merge at low speeds and near-constant gravitation - which is why SRT and even classical mechanics are still in use. As I already emphasized, I welcome it if you come up with a better phrasing. As it stands, the intro poorly reflects the reference that it points to, contrary to what it suggests to a fresh reader. Harald88 19:36, 17 May 2006 (UTC)

The introduction will not be changed by any of the three of us to reflect your inappropriate viewpoint. Nor will we permit such a change to occur. Period. --EMS | Talk 21:44, 17 May 2006 (UTC)

BTW - Have you read the introduction fully? After all, special relativity is mentioned in it as being "unified" with other insights in the process of creating GR. What you want to do it to emphasize the SR part of that relationship, but that makes the intro misleading. --EMS | Talk 21:53, 17 May 2006 (UTC)

The issue won't go away; now you are in the majority, but in the long run such misleading introductions have little chance of staying. Apparently you have not even the slightest idea of what Einstein wanted to achieve with GRT: it has little to do with either SRT or gravitation.

In fact, the article hardly discusses the subject of Einstein's general relativity of all motion, despite the fact that according to Einstein, GRT is based on "an extension of the postulate of relativity" "to systems of reference in all kinds of motion" - Einstein 1916. Thus, I now found back the source of user 87.217.88.42's word choice "extension": it comes directly from the official English translation of Einstein's introduction to GRT! Harald88 23:01, 17 May 2006 (UTC)

[removed offensive remark - sorry for loosing my patience! - at request of EMS. Harald88 06:29, 4 July 2006 (UTC)]

I have reverted changes by an anon which appear to be pushing Harald's non-maintream POV. ---CH 21:09, 20 May 2006 (UTC)

No. It's not Harald88, but may be the anon who set Harald88 off (although I doubt that). This editor is not keying on the things that Harald claims are important to him. --EMS | Talk 22:29, 20 May 2006 (UTC)

To avoid possible confusion: the anon I reverted is the online-age anon, who has used IPs including 205.173.93.39 and 205.173.93.41 and who may be geolocated in the Northeastern US, possibly near Bronx, NY. The most recent anon edits are from the IP 67.168.57.6, aka the Bellevue area comcast.net anon, geolocated near Bellevue, WA. As always, geolocations correspond to the machines with the stated IP address. And Harald, fear not, we at WikiProject GTR intend to find the proper place to discuss the issue which concerns you, and we will link between this article to that discussion. However, this article is a survey of a huge topic which is the subject of many long textbooks, monographs, and research papers, so neccessarily we must choose carefully what topics to include at all, and of the included topics, which to stress, and in what order to mention them. Clearly, the community of knowledegable editors wishes to allow us to proceed with this plan.---CH 21:20, 21 May 2006 (UTC)

Hillman our profound disagreement about what is a correct way to introduce subject matter doesn't make me doubt your good intentions, nor your knowledge of the current view on modern GRT; and I appreciate your clarification. But there is a little glitch: Wikipedia can't find Wikipedia:WikiProject GTR. Harald88 21:22, 22 May 2006 (UTC)

That's because it is perpetually on the verge of "going public" :-/ See User:Hillman/Wikiproject_GTR_draft. ---CH 01:41, 24 May 2006 (UTC)

quantum mechanics is more than just quantum gravity

The section on quantum mechanics and its interface with general relativity has a glaring omission: quantum field theory in curved spacetime. We need a paragraph with a link to a main article. Now who's going to write it? :-) -lethe ^{talk +} 02:08, 24 May 2006 (UTC)

Please clarify the caption

The caption to a very important graphic in this article states:

Two-dimensional visualization of space-time distortion. The presence of matter changes the geometry of spacetime, this (curved) geometry being interpreted as gravity. Note that the white lines do not represent the curvature of space, but instead represent coordinate system imposed on the curved spacetime which would be rectilinear in a flat spacetime

I simply don't understand this at all. This paragraph starts by saying that matter indeed changes the geometry of spacetime, but then seems to contradict itself. This needs to be rewritten much more carefully. And if a careful explanation of this figure takes more than one paragraph, then please move the explanation to the body of the article. RK 00:26, 24 June 2006 (UTC)

The figure (Earth on a rubber sheet with a grid on it) is actually a very misleading one, and should probably be replaced. It's used to explain gravity to children, but rather than representing relativistic space curvature, it's actually just showing potential well depth for Newtonian gravity. The caption looks like someone's attempt at describing this caveat, while adding unrelated information about coordinate systems. I think the image should probably be moved to gravitation, and given a more appropriate description.

The best figure I've seen used to explain spacetime curvature is one that plots the paths of two objects falling through a hole bored in a (nonrotating) planet (plotting position vs. time, one axis for each). The paths diverge, then become parallel, then converge again. It's pointed out that lines that are parallel at one point converge like this on positively curved surfaces. If nobody else makes one, I can try hacking together a figure, but anything I make will be of pretty poor quality (my drawing program is "xfig"). You can do a similar demonstration for negatively curved spacetime by making a similar plot of objects' paths diverging when falling towards the planet (while still above its surface). --Christopher Thomas 06:27, 24 June 2006 (UTC)

The (in)famous rubber sheet picture should be eliminated. If you can try to do a better picture in principle, we'll surely can find someone for nicer version (I don't volunteer, but for very simply drawings. My drawing program is notepad, to directly produc SVG, or sometimes to hack a Python/matplotlib program to do it). --Pjacobi 19:10, 24 June 2006 (UTC)

All of your points are well taken. Nonetheless, I don't think that it is a bad idea to have some kind of graphical analogy to show how gravity works in General Relativity. The (in)famous diagram is actually used by physicists studying General Relativity when explaining the idea to students and laypeople (e.g. NOVA's "The Elegent Universe"). I don't see a problem using this diagram as long as it is made clear in the caption that this only shows an analogy. In the text of the article we can elaborate more about in what ways this analogy is useful, and in which ways it is not. All analogies are misleading if taken literally, yet illuminating if understood in context. I think I can write some useful text clarifying the issue. I'll give it a go, and I'd appreciate if you would tell me what you think. RK

I think that having a very misleading figure is worse than having no figure at all. I'll see what I can do about drawing placeholders for the more accurate ones. --Christopher Thomas 18:26, 2 July 2006 (UTC)

Placeholder replacement figures

I've finished making placeholder versions of figures that more accurately show how spacetime curvature works. If these are deemed useful, they can either be put in as-is or replaced with prettier versions (my drawing and animation tools consist of "xfig" and "convert", respectively). Omit "temp" for the filenames when creating the nicer versions. Sample use and captions are below. Comments are welcome. The main problem I see is that putting these in the article as-is would take up large amounts of space. --Christopher Thomas 01:28, 3 July 2006 (UTC)

Objects moving through a region of positive spacetime curvature (a borehole through the Earth). The world lines in the spacetime diagram are parallel at points B and D, but intersect at points A and C. This intersection of parallel lines indicates that spacetime is positively curved.

Objects moving through a region of negative spacetime curvature (falling towards the Earth). The leftmost animation shows the object cluster falling, while the middle animation shows the components of the cluster moving in response to tidal forces. The world lines are plotted with position measured with respect to the blue test object (its position is treated as constant), to show tidal effects. There are two world lines that pass through point X on the spacetime diagram, and none of these intersect the worldline of the blue test object. This drawing of multiple lines that don't intersect the blue world line indicates that spacetime is negatively curved.

World lines drawn in a) positively curved spacetime and b) negatively curved spacetime. In positively curved spacetime, for an initial world line with an event (point) that isn't on the world line, any new world line that passes through the event will intersect the original world line ("parallel" world lines aren't possible). In negatively curved spacetime, for an initial world line with an event (point) that isn't on the world line, more than one distinct world line can be drawn through the event, without any of these new world lines intersecting the original world line. In flat spacetime, exactly one "parallel" world line can be drawn.

Students beware

I tried to create a WikiProject called WikiProject GTR, but this failed.

Due to the very frequent vandalism of this article, I made few if any substantial edits to it, and have mostly not even tried to monitor it, but from time to time I did point out specific problems in this talk page. However, 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. While as of June 2006 there are still some Wikipedians knowledgeable about gtr, given what I view as the failure of WP as a source reliable information (see recent comments by our founder, Jimbo Wales), and given the lamentable past history of this article, which concerns a topic which attracts many cranks, I have reason to believe that at least some future versions of this article are likely to contain slanted information, misinformation, or disinformation. Beware also of external links to other websites, some of which may be cranky. See Relativity on the World Wide Web for a website which is very out of date, but at least provides mostly accurate information. (This is now displayed by someone else, but the original version was created by myself.)

Good luck to all students in your search for information, regardless!---CH 23:47, 30 June 2006 (UTC)

Citation for the dismissal of Weyl's gauge formulation

Hi, does anyone have a citation for Einstein's dismissal of Einstein's gauge formulation of GR? I've heard this before, and I know it's part of the folklore of the subject, but I'm not certain if I have ever seen it in print. Thanks for indulging me, Silly rabbit 13:06, 1 July 2006 (UTC)

Star Trek

I saw an episode of Star Trek: TNG a few days ago, in which an alien woman blew up her spaceship to open a rift in space-time to demonstrate how warp drive is literally tearing the universe apart. The Enterprise had to enter the fringes of the rift in order to rescue another Federation ship, but entering the rift while using warp drive would simply exacerbate the problem. They planned to build up momentum at warp 9, then deactivate the warp core and "coast" into the rift. However, they only had a short amount of time to reach the ship before the gravitational forces pulled it out of their range. The other ship (the one that needed to be rescued) activated its warp core inside the rift and caused it to expand. The ending of the episode aside (in which the Enterprise "surfs" out of the rift), isn't this episode somehow violating relativity? When using warp drive, the ship is actually encased in a space-time "bubble" of sorts while the ship pulls on the space around it to move at high speeds; thus, the ship is not actually moving in the strictest sense. However, higher warp speeds (such as the Enterprise-D's maximum of 9.3 or something) are extremely fast, and without the warp field around the ship relativity would apply again. So to my view, Star Trek violated relativity: once the Enterprise deactivated her warp core, relativity would have applied again and it's highly unlikely that the Enterprise would have reached the second ship in time. Am I getting something horribly wrong here, or did the episode's creators kind of mess up? (At any rate, could someone tell me which episode that is, because it was really cool!) --71.219.52.183 13:29, 9 July 2006 (UTC)

The Enterprise D has a maximum speed of warp 9.9, and a maximum _sustainable_ speed of somewhere between 9.2 and 9.6 (don't recall offhand). The episode in question was Force of Nature (TNG episode). Long story short, trying to apply real-world physics to Star Trek episodes only results in a headache (the writers weren't scientists). --Christopher Thomas 01:55, 12 July 2006 (UTC)

Fundamental principles

The first two points in the set of fundamental principles seem to be the same:

General relativity is based on a set of fundamental principles which guided its development. These are:

• The general principle of relativity: The laws of physics must be the same for all observers (accelerated or not).
• The principle of general covariance: The laws of physics must take the same form in all coordinate systems.

a suspicion that is confirmed by general principle of relativity and principle of general covariance both taking you to the same page, general covariance. I suggest the first bullet point be eliminated or merged with the second. --Michael C. Price ^talk 23:09, 11 July 2006 (UTC)

Hello MichaelC. I understand why you may think that the 2 principles are the same, but technically, they are not. The first one refers to the laws of physics, whereas the second one refers to how we write the equations that we believe describe physical phenomena. Clearly, these are different. The redirect must be fixed. I never quite got round to doing it, but will do so sometime soon. Thanks. MP (talk) 20:40, 12 July 2006 (UTC)

OK, I see the distinction, although I'm not sure if they need seperate points: I'll suspend judgement until general principle of relativity is updated.  :-) BTW I updated another link that got accidentally(?) rolled back with your revert. --Michael C. Price ^talk 21:07, 12 July 2006 (UTC)

I'll try and create the general principle of relativity page sometime soon. Sorry about the accidental revert which killed the link - I didn't realise that there was a page called Local Lorentz covariance - we need such a page. Thanks for creating it. MP (talk) 21:14, 12 July 2006 (UTC)

Finally got round to creating general principle of relativity - now it just needs some proper content. MP (talk) 18:10, 28 August 2006 (UTC)

Common Index Convention

From all texts that I've read (Misner, Wheeler, Thorne, Weinberg, Hartle, etc.) it is common convention to use Greek indices as opposed to Latin. The Latin indices imply that they range from 1 to 2, while the Greek indices imply a range of 0 to 3. In 4 dimensional spacetime (which general relativity was conceived in), the range 0 to 3 is need (0 being the time dimension). As such, I propose to change all the Latin indices to Greek. Also, there seem to be some misplaced indices in some of the equations. However, before making changes, I'm going to wait till I have a change to take a look through books to double check. --Archmagusrm 19:00, 12 July 2006 (UTC)

Greek = space-time indices 0-3

Latin (a,b,c..) = Vierbein indices

Latin (i,j,k,..) = spatial 1-3 (not 1-2 as above, a typo I presume)

--Michael C. Price ^talk 19:14, 12 July 2006 (UTC)

Thank you for the enlightenment. I must admit, I wasn't aware of Vierbein indices until now. And yes, the 1-2 was a typo, sorry for the confusion. Could you please explain why the article uses the Vierbein indices as opposed to the space-time indices? --Archmagusrm 02:56, 13 July 2006 (UTC)

I would think, no reason at all except that were easier to type. Greek indices would certainly look more standard (and important for clarity where Verbein indices exist as well, such as in supergravity). Probably other articles need standardising as well. Please do make the changes! PS I've updated the EFEs. Probably the other sections need standardising as well. --Michael C. Price ^talk 09:42, 13 July 2006 (UTC)

You are giving yourself a bunch of work all over the place with this, but it you feel strongly about it, go ahead. Just be aware that the use of a consistent convention, especially within a given article, is more important than what convention is used. For the most part, editors have chosen to use the classical tensor notation and not been concerned about issues of notation. --EMS | Talk 15:25, 13 July 2006 (UTC)

It may also help to look at Wikiproject GTR, which is still not officially a project, but does attempt to organise and standardise notations etc. in relativity articles. MP (talk) 19:09, 13 July 2006 (UTC)

Cause of curvature

What causes mass to curve space-time? Also, what are the geodisics that entities follow through space-time called? --scienceman 16:49, 13 July 2006 (UTC)

In response to your questions:

• What causes mass to curve space-time ? - General relativity has no answer to this; it's just an assumption of the theory. This is one of the unsolved problems of physics - if we could answer it, then it would provide us with much valuable information for a theory of quantum gravity.

• What are the geodesics that entities follow through space-time called? - I'm not sure I understand this question (does the question make any sense?). The paths that free particles and light beams follow in spacetime are called geodesics. Worldlines are trajectories (paths) of particles and light beams whether they are or are not being acted upon by external forces. So, all geodesics are worldlines, but not all worldlines are geodesics. MP (talk) 19:15, 13 July 2006 (UTC)