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These were considered before General Relativity, using Newton's Law. A complete gravitational collapse was possible, although problematic. They didn't call it a " Black Hole", but that statement in an early paragraph is misleading, and implies that gravitational collapse had never occurred to anyone before GR. — Preceding unsigned comment added by 184.147.125.176 (talk) 06:15, 3 December 2013 (UTC) 77Mike77 (talk) 12:17, 3 December 2013 (UTC)
I couldn't find what I was looking for, but found this http://www.amnh.org/education/resources/rfl/web/essaybooks/cosmic/cs_michell.html , which has the idea of a "black hole" in all but name. 1783, hypothetical star so massive that the escape velocity exceeds the speed of light, so it is, effectively, black.77Mike77 (talk) 12:17, 3 December 2013 (UTC)
Did I suggest that? I thought I was suggesting that BHs are possible with Newtonian gravity. Once the field is stronger than the material's ability to resist, the object has to collapse. The only problem is that of the infinities at the centre point. In GR, time slows to a stop at the event horizon, when viewed from the universe outside, so the singularity at the centre only exists for someone who falls through; for those outside, the BH is hollow, and the central singularity only exists in a hypothetical infinite distance into the future. I'll think about how to make it clearer with minimal change, and get back.77Mike77 (talk) 12:17, 3 December 2013 (UTC)
I'm thinking of replacing these two sentences, "Einstein's theory has important astrophysical implications. For example, it implies the existence of black holes—regions of space in which space and time are distorted in such a way that nothing, not even light, can escape—as an end-state for massive stars."...with this: "Einstein's theory has important astrophysical applications. For example, the mystery of what happens when a star undergoes catastrophic gravitational collapse was explained, leading to the modern concept of black holes—regions of space in which space and time suffer such extreme gravitational distortion that nothing, not even light, can escape—as an end-state for massive stars." Does anybody object to this, or have any suggestions to make it better?77Mike77 (talk) 23:30, 4 December 2013 (UTC)
The only "flowery" word I can see is "mystery". Let me know if you can think of a non-flowery synonym. I was trying to keep the flow of the article intact, and not turn it into another edit disaster where a huge paragraph of dull prose replaces a sentence. It's just plain wrong the way it is. If this is like most other articles, someone will revert it back to its current incorrect version once people have spent a few hours straightening it out, so I'll probably just leave it wrong if there is no simple synonym for "mystery".77Mike77 (talk) 22:18, 5 December 2013 (UTC)
How about this? "Einstein's theory has important astrophysical applications. For example, the problems involved in trying to find a Newtonian description of what happens when a star undergoes catastrophic gravitational collapse were resolved, leading to the modern concept of black holes—regions of space in which space and time exhibit such extreme gravitational distortion that nothing, not even light, can escape—as an end-state for massive stars."77Mike77 (talk) 02:22, 6 December 2013 (UTC)
I give up then. The original sentence is still wrong, because it gives the false impression that nobody had previously thought of a situation where light could not escape because of gravity, when this clearly had been thought of (Mitchell, and those who commented on Mitchell), and so I guess that the article will simply remain with this serious flaw, since any attempt to correct that error will be rejected. Despite the subtle inaccuracies in my statement, it is still an improvement on the existing mistake. Unfortunately, the mistakes in the article itself do not come under as much scrutiny as attempts to rectify those mistakes. Oh well, another defective Wikipedia article remains defective...what else is new?77Mike77 (talk) 15:27, 6 December 2013 (UTC)
I was just posting an edit to my comment, but got a posting conflict. I wanted to say that you had a valid point re Mitchell's "Dark Star" not collapsing to a point, and that you are right that the light not escaping is a distinct concept from gravitational collapse. I had somehow got the idea from something I read, that the idea (that a gravitational field could be stronger than the material's ability to resist gravitational collapse) was first thought of in Newtonian terms, where the star would collapse to a point with infinite density and infinite gravitational field strength, and that those infinities were part of the problem. I think what I read must have been hypothetical, i.e. the author was posing a Newtonian "solution" to contrast it with the Relativistic one. I have a B.Sc. with a physics major, and am an "avid amateur" in following physics, so I understand Special Relativity very well, but never took post-grad courses in General Relativity, just a little tensor analysis in final year, and so I am certainly no "expert" on any formal treatment of GR. There is an important role to play in translating the abstract into terms understandable to the average reader, which I think is part of what wikipedia is supposed to do, even thought that goal is too often lost here. Anyway, I'll have another go at it and post it soon. Thanks for your "colourful" comment.77Mike77 (talk) 20:04, 6 December 2013 (UTC)
Okay, this is a new attempt. "For example, the topic of extremely strong gravitational fields, and their effect on light, was clarified. In 1783, John Mitchell hypothesized that light might interact with gravity, and suggested the possible existence of "Dark Stars", which were massive stars with gravitational fields so strong that light could not escape. His analysis was based on the Newtonian view that light consisted of "pellets" traveling at a certain speed, and that if the gravitational field of the star were strong enough, the "escape velocity" would exceed the speed of light, so that the light could not escape. General Relativity generated a completely different analysis of this problem, which took into account the wave nature of light, and the possibility of a catastrophic gravitational collapse in which the massive star would collapse down to a point, leading to the modern concept of black holes—regions of space in which space and time exhibit such extreme gravitational distortion that nothing, not even light, can escape—as an end-state for massive stars." I've tried to keep the "flow" of the original statement, even though I've lengthened this part. There is a wikipedia entry for "Dark Star", which I can link to that phrase. I used the word "point" instead of "singularity", because the lay reader won't know what a singularity is (I could link the word "point" to the article https://en.wikipedia.org/wiki/Gravitational_singularity). What do you think? — Preceding unsigned comment added by 77Mike77 (talk • contribs) 20:32, 6 December 2013 (UTC)
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Hello, I've just read in section "Quantum Gravity" the following:
"Another approach starts with the canonical quantization procedures of quantum theory. Using the initial-value-formulation of general relativity (cf. evolution equations above), the result is the Wheeler–DeWitt equation (an analogue of the Schrödinger equation) which, regrettably, turns out to be ill-defined.[177] " The source given is the only one: Kuchař, Karel (1973), "Canonical Quantization of Gravity", in Israel, Werner, Relativity, Astrophysics and Cosmology, D. Reidel, pp. 237–288, ISBN 90-277-0369-8
Wheeler-DeWitt equation is pretty much out there influencing physics research, and it is extremely strange that here it is called "ill-defined" and only one reference dated 1973 is given.
There has been recent experimental research confirming the absence of time for an external observer of a universe: http://arxiv.org/pdf/1310.4691v1.pdf
It's highly ridiculous for a Wikipedia article to have qualitative statements about theories unless they have completely been dismissed by the entire scientific community through numerous failed attempts to verify them. — Preceding unsigned comment added by Whitely3000 (talk • contribs) 17:35, 24 February 2014 (UTC)
I think there should be a (sub)section on the expected domain of validity of GR in the article. When reading about GR (whether popular or real thing), one is always fed the message that "most physicists expect..." and hence quantization is necessary. (The breakdown is just around the corner...) From the little I know myself, the circumstances are pretty extreme when one can surely expect GR to break down (giving incorrect predictions). Note: This is not something that can be disposed of by saying GR has already broken down because black holes exist and singularities don't. I think it would be interesting for the reader to get "pretty extreme" quantified in terms of, say, distance from the singularity of a really nasty black hole, or anything else suitable.
Also, provided GR is correct (within its expected domain of validity), can we ever expect to experimentally verify a quantum theory of gravity? Sure, such a theory might predict other things that can be detected, but I personally doubt that we will be able to produce graviton showers (instead of gravitational waves or whatever) at CERN. I understand I'm being vague here, but some skilled and knowledgeable editor might have an idea about good stuff to spice up the article with. YohanN7 (talk) 02:58, 4 July 2014 (UTC)
Objects do not follow the geodesic because the gravitational field is in non-inertial motion. If there are two objects under their mutual gravitational field, both will be accelerated and follow a non-inertial motion. If one is a photon, the other will still follow non-inertial motion. Saying the object follow the geodesic is an approximation but not their true path. Shawn H Corey (talk) 14:05, 21 August 2014 (UTC)
I feel it may be beneficial to include some discussion of the outstanding challenges encountered by the theory, and preferably in its own section. Certainly, the notion of singularities is one where most physicists are uncomfortable to the point where I really can't understand why it seems to be so universally espoused. Then of course there are the broader cosmological observations of dark matter and dark energy, which are almost beyond direct measurement despite comprising 95% of the universe. On top of that, we have inflation, and the elephant in the room - the fact the big bang could occur at all given that it should have by all rights gravitationally collapsed on itself.
As Lord Kelvin would no doubt attest, we still have a few clouds on the horizon. I fear I do not share his optimism, however. 150.203.179.56 (talk) 03:49, 8 September 2014 (UTC)
Not being a physicist, I would like to throw another note on style. We have two articles, one for the general theory of space (general relativity theory, this article), the other for an introduction into it. However, the physical section in this article starts by a phrase: "General relativity can be understood by examining..." I think that this (technical) article should, like all other articles on other subjects, state what the subject of the article is, while the introductory article should contain the opinionated discussion of how the subject can be best understood; we need to separate the concerns and get two essentially different articles. I do not negate that this section may be potentially applicable into the article, but I think that in any case it should go after an essential presentation of the subject (what does it consist of, into where are applied its parts in the process of cognition, how is it used in real-life cognition, what most discussed&weighted properties does it have), if it should go at all in this article, and not say, in the historic article or in the introductory article, or even in a wikibook. - Evgeniy E. (talk) 13:19, 1 January 2015 (UTC)
«In 1917, Einstein applied his theory to the universe as a whole, initiating the field of relativistic cosmology. In line with contemporary thinking, he assumed a static universe, adding a new parameter to his original field equations—the cosmological constant—to reproduce that "observation".» Irony is out of place here; we know very well that that was not an observation, so let us not "affect ignorance" for no purpose. I think we need a different word. - Evgeniy E. (talk) 12:30, 1 January 2015 (UTC)
This added image]:
I think the license description and caption needs modification if the image is to be retained. —Quondum 16:39, 16 February 2015 (UTC)
The formulation of GR in terms of a metric cannot be fundamentally correct, because it does not allow coupling to fermions, and fermions exist. We need the tetradic formulation to couple to fermions, see spin connection.
Need a section on the tetradic formulation and links to spin connection and Tetradic Palatini action? Ibayn T 15:01, 31 December 2014 (UTC)
On point 2...In "Quantum field theory" by Michio Kaku:
"However, the coupling of gravity to spinor fields leads to an immediate difficulty: There are no finite dimensional spinorial representations of the group GL(4). This prevents a naive incorporation of spinors into general relativity. There is, fortunately, a trick that we may use to circumvent this problem. Although spinor representations do not exist for general covariance, there are, of course, spinorial representations of the Lorentz group. We utilize this fact and construct a flat tangent space at every point in the space."
The need for tetrads to couple fermions to gravity is quoted in many other places.
I would say that point 3 is not how fermions are "usually understood and described in most reliable sources".
Ibayn T 15:01, 1 January 20145 (UTC) — Preceding unsigned comment added by 86.152.185.4 (talk)
Not sure what
"To call it the "tetrad formulation" is also a misnomer, and it seems to relate to the relaxing of an assumption and has nothing to do with tetrads: in particular, it relates to the coupling between the metric and the affine connection, if the article Tetradic Palatini action is to be taken literally."
means. Possibly you have missed the point of what a first order formulation of the action principle is about? Perhaps I need to explain it better in the article? The general idea of first order formulations is introduced, for example, in Ray D'Inverno there in the context where you take the metric and affine connection as independent variables both to be varied over. There it is explained that this first order formulation is a more elegant way of proceeding...
With the Tetradic Palatini action we take the tetrad and the spin-connection as independent variables. This independent spin-connection defines a "new" curvature tensor yep. Variation of the action due to variation of the teterad field only directly gives Einstein's equations all be it involving this "new" curvature tensor (and the tetrad). Variation with respect to the spin-connection implies the "compatibility condition" which uniquely gives the spin connection in terms of the tetrad - it also allows this "new" curvature tensor to be identified with usual curvature tensor written in terms of tetrads (so end result is all written in terms of tetrads!)...What this all tells you is that if you had written the action solely in terms of tetrads in the first place (and never introduced the spin-connection as an independent variable - given it in terms of these tetrads. This is the second order formulism and probably what you would call the tetradic action), varying the action with respect to tetrads you would have got out Einstein's equations written in terms of tetrads - the first order formulation is just a more elegant way of arriving at this result.
Ibayn T 8 January 2015.
Good evening. I hereby inform you that this this article - a featured article - possess many sections and information on any source (see the link to my edition). Someone who understands the subject, or that is "responsible" for its quality, or know how to do it could add sources? I really appreciate it. --Zoldyick (talk) 07:22, 30 June 2015 (UTC)
Where is the source for all that text? I am exercising my right player and checker. I want to check the source. Where / what is the source? --Zoldyick (talk) 08:18, 30 June 2015 (UTC)
So, I added a primary source on the closed timelike curve (CTS). It remains to find a second source, and I think this, Closed Timelike Curves by Kip Thorne should suffice. I don't have time a t m to fill out all details about the publication (presumably conference proceedings), but CiteSeer says it is cited, hence published. (The link is to a preprint.)
Interestingly, according to Thorne, Gödel wasn't first to find a CTC. YohanN7 (talk) 12:18, 26 November 2015 (UTC)
In the summery of the image featuring a simulated black hole, it says a "distance of 600 kilometers" - should this really be light years? It seems like 600 kilometers is really close in astronomical units
Tossing in "thus opens new beginning of gravitational wave astronomy" atas the end of an otherwise coherent sentence was an insignificant improvement and part of an overall damaging edit. Mansi Kasliwal (who is one of the authors of a paper we cite elsewhere an earlier paper we cite elsewhere) is an astro PhD & visiting assoc. prof. at the institution and may even be the best qualified source we can quickly quote, b. But the butchery into word-salad of whatever she said, and the lack of info about how close to the work she is, make that edit a very definite minus in the form that i found and removed.
-- Jerzy•t 10:15 & 10:45, 16 February 2016 (UTC)
Re quote: the results are the vacuum Einstein equations, R_{\mu\nu}=0.\ This implies that away from sources (matter and energy), the Ricci curvature is zero. Clearly, spacetime is curved near but outside the sun. Further explanation is needed. — Preceding unsigned comment added by Ian R Bryce (talk • contribs) 07:11, 25 March 2016 (UTC)
People have suggested me to create an animation of a 3D cubical grid getting distorted by a spherical mass. Along the points where the grid crosses, tiny clocks would be added with hands spinning at different rates, with slower clocks near the mass. The idea is to have it as a looped GIF or a small video.
This would be created in order to better convey the distortion of 3D space and time, and to avoid the rubber sheet model.
I'm not really formally well-versed in GR, and at this point what I have is merely qualitative. I think it is fine since the exact details won't work well in a diagram anyway. However, I'm wondering if there's a specific way to distort the grid that would be more accurate. For instance, should the distortion to the grid be conformal? Can we convey some useful information with colors on the grid?
At the moment, I'm not using an infinite grid as that looks really messy. I'm restricting myself to a 3x3x3 grid, as it conveys the idea just fine without clutter.
I'd love if someone could pitch in to help. Cheers! — LucasVB | Talk 05:26, 10 April 2016 (UTC)
Does anyone else think we should examine the sea of blue in this article and reduce the amount of overlinking? Tayste (edits) 03:31, 26 July 2017 (UTC)
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Several editors disagree on the importance of the statement that general relativity is a beautiful theory. Is this an important point to make, and if so, where in the article should this statement go? A statement to this effect has been in the article (with some discussion on its precise wording) since this version from May 22, 2017. (See discussion and consensus above.) Entire books have been written on the importance of beauty in physics. One recent collection of papers flatly states in its title that general relativity is the most beautiful of theories.[1] Do we remove this statement from the article because a few editors do not feel awed by general relativity and don't agree with previous consensus?
General relativity is universally acknowledged as a theory of extraordinary beauty, such that it has often been described as the most beautiful of all existing physical theories.[2]
Prokaryotic Caspase Homolog (talk) 06:16, 7 May 2018 (UTC)
References
Beauty is in the eye of the beholder. This article is about a beautiful(imho) physical theory. In itself it has no intention to be beautiful, it is -maybe universally- perceived as such, but as theory it is there to give a model of physical reality. This is the core topic. Of course, an article about a theory of this deepness and importance may also deal with its acceptance, but its degree of being worshiped, as is sourced by abundant eulogies, or combated by more rare theorists, considered partly as cranks, does not belong in the leade, rather to History, or -considering the fundamental importance- in a new section "Acceptance/Valuation". I do not want the part of WP, considered as floridly formulated, to grow, and I would not bold the consensus from the above thread, but rather scare it. Purgy (talk) 07:18, 7 May 2018 (UTC)
I think the remark should stay, as above we are saying that people find it beautiful, not that it inherently is so. Absolutelypuremilk (talk) 12:58, 10 May 2018 (UTC)
The article says, "Soon after publishing the special theory of relativity in 1905, Einstein started thinking about how to incorporate gravity into his new relativistic framework." Is this really true? My understanding is that Einstein wanted to include acceleration in relativity, to have all reference frames equivalent. He even says so in his book _Relativity: The special and general theory_. It was only when he realized that acceleration and a gravitational field are physically the same thing that gravity came into consideration. Betaneptune (talk) 16:37, 8 November 2018 (UTC)
In the rocket figure it says, "According to general relativity, objects in a gravitational field behave similarly to objects within an accelerating enclosure. For example, an observer will see a ball fall the same way in a rocket (left) as it does on Earth (right), provided that the acceleration of the rocket is equal to 9.8 m/s2 (the acceleration due to gravity at the surface of the Earth)."
This is a _postulate_ of general relativity, not a result thereof, and was known by Galileo, although in a different form. Both imply the equivalence of inertial and gravitational mass.
Also, it is better to use a rope pulling a hook on top of the elevator car than a rocket engine. This is so the rider can look at the hook and rope and still have no way to conclude whether the elevator car is hanging or accelerating. From Einstein's book _Relativity: the special and general theory_: ". . . he discovers the hook in the middle of the lid of the chest and the rope which is attached to it, and he consequently comes to the conclusion that the chest [or our elevator car] is suspended at rest in the gravitation field." This is rather hard to do with a rocket instead of a rope and hook! Also, the elevator car would lose mass, requiring a changing force, and hence thrust, and would run out of fuel at some point. The rope and hook is a much better way to demonstrate the equivalence of acceleration and gravitation.
Additionally, they don't behave similarly; they behave identically, which is essential to the point. Betaneptune (talk) 16:58, 8 November 2018 (UTC)
"for an observer in a small enclosed room, it is impossible to decide, by mapping the trajectory of bodies such as a dropped ball, whether the room is at rest in a gravitational field, or in free space aboard a rocket that is accelerating at a rate equal to that of the gravitational field"
I find this misleading because it seems to imply a constant acceleration for the rocket, but the gravitational field varies with 1/r2. So for this to be true mustn't the acceleration of the rocket vary according to the height of the ball above the floor? This seems impractical. (There are devices on earth that are capable of measuring the variation in the gravitational force over a distance as small as 1 metre, so this effect should be detectable.) Boardhead (talk) 14:00, 12 April 2019 (UTC)
Finding the above sentence a bit awkward, I changed "probably" to "among", but my edit was reverted. While the cited source indeed says "probably", the encyclopedia does not need to parrot the sources' wording. To view something as "probably the most beautiful" is to consider it "among the most beautiful". The latter of the two isn't as specific, but the difference is minor, and thus the former isn't necessary when it makes the wording this way. AndrewOne (talk) 08:03, 8 August 2017 (UTC)
While that book was written long ago, it has been revised and brought up to date throughout the decades. That passage was kept. L&L did not splatter around assessments of theories (or their authors), but they made one exception, that for Einstein and his theory (throughout the whole L&L series). General relativity does have a status that no other theory has. The status is not so much about absolute fundamental correctness, but it is about its enormous predictive power and beauty. Like I said above, it is easy to find references in modern books by both mathematicians and physicists supporting this general observation. Its status didn't exactly diminish after the first observation of gravitational waves, and string theorists set out to prove GR (on all reasonable scales) right, not wrong. YohanN7 (talk) 10:56, 8 September 2017 (UTC)
An subjective statement like "General relativity is considered probably the most beautiful of all existing physical theories" is almost impossible to properly source. No matter how well regarded the cited source, the authors clearly are expressing their opinion. As such the cited quote can only be used to verify the authors' opinion on the matter. To verify the statement including "is considered", you would need to cite a reliable source that establishes that this opinion is widely held (through sociological research or something). Lacking such a source a formulation involving "has been described as" comes much closer to be verifiable. (Don't get me wrong, personally I definitely share the opinion that GR is one of the most elegant theories out there, but I also recognize that that is my opinion.)TR 14:46, 19 October 2017 (UTC)
The following Wikimedia Commons file used on this page has been nominated for deletion:
Participate in the deletion discussion at the nomination page. —Community Tech bot (talk) 07:36, 13 July 2019 (UTC)
To a third party observer like me who watches the page, this seems ridiculous (both are true). I suggest to form a consensus here for one or the other instead of constantly switching from one to another... Thanks, —PaleoNeonate – 12:25, 23 July 2019 (UTC)
Well, or for an alternative that conveys more clearly the intended meaning, of course. —PaleoNeonate – 12:25, 23 July 2019 (UTC)
@DVdm: The original short description contradicts the article. It reads, "Theory by Albert Einstein, covering gravitation in curved spacetime." In reality, general relativity tells us that gravitation manifests itself as the curvature of spacetime. Again, that's not what the article says. My alternative is correct and does not contradict the article. Nerd271 (talk) 15:42, 22 June 2019 (UTC)
The following Wikimedia Commons file used on this page or its Wikidata item has been nominated for deletion:
Participate in the deletion discussion at the nomination page. —Community Tech bot (talk) 23:37, 9 February 2020 (UTC)
Shall we create a separate page for the notable books on general relativity at various levels? This article is already long in its current state. We do have pages for lists of textbooks in classical mechanics and quantum mechanics, of thermodynamics and statistical mechanics, and in electromagnetism. Nerd271 (talk) 00:37, 14 June 2020 (UTC)
Einstein recognized the 4 dimensions of spacetime: 3D regular space + 1D time. 2601:589:4800:9090:489:97E6:2CC8:D11E (talk) 13:01, 21 August 2020 (UTC)