Talk:Hartle–Hawking state

message: This page could really do with a plain English summary - I came to it looking for elucidation of the term "Hartle-Hawking boundary condition" from the Big Bang article, but that's not what this article offers.Rayray 10:32, 5 April 2006 (UTC)
replies: text: Seconded. --Christopher Thomas 00:51, 25 June 2006 (UTC)
replies: text: Absolutely. Just came to the discussion page to say exactly the same thing. 193.129.65.37 11:03, 29 December 2006 (UTC)
replies: text:  Fourthed - I'm afraid this page is utter incomprehensible gibberish. Will see if I can contact original author for a clarification. DewiMorgan 03:30, 30 May 2007 (UTC)
replies: text:  Motioned carried. Anyone up for it?  —Preceding unsigned comment added by Atrosaro (talk • contribs) 01:54, 5 September 2007 (UTC)
thread_title: Plain English summary needed

message: Dear friends, the Hartle-Hawking state is a rather complicated technical construct. It is the wavefunction of the Universe - a notion meant to figure out how the Universe started - that is calculated from Feynman's path integral in a specific way, using particular boundary conditions. I doubt there is a correct but much more penetrable explanation. If popular articles mention it, I don't believe that their authors understand what they are writing unless they are quantum gravity experts.  But good luck, Lubos  --Lumidek 21:58, 4 June 2007 (UTC)
replies: text: Thanks! I smooshed some of your words in there and hopefully didn't munge the meaning too badly. But feel free to smoosh it back, or smoosh some more! Be bold! Basically, it needs to change from an incoherent mishmash of linked terms to an explanation of what it is, what it does, what it means, and what it's used for, by whom, and why that's important. DewiMorgan 20:17, 4 July 2007 (UTC)

text: I understood the point of their theory (from A Brief History Of Time) as that there were nothing before the inflation, but that the time started with inflation. There was never a Planck time. This article is in discord with my impression. Rursus dixit. (mbork3!) 16:44, 14 July 2012 (UTC)
replies: text: There is still something wrong here. "According to the theory time diverged from three state dimension - as we know the time now - after the Universe was at the age of the Planck time." Besides that the grammar of this sentence could be improved, what does "the age of the Planck time" (indicating a time span) even mean, when time is said to emerge after that, which again is a paradox: "after"!? The same problem exists with the word "pre-Big Bang" earlier in the article -- that seems meaningless and confusing. Koornti (talk) 23:52, 7 December 2012 (UTC)
thread_title:  Complicated 

message: It seems that the first paragraph "infinitely finite" is an oxymoron. Pass a Method talk 23:19, 2 March 2013 (UTC)
thread_title:  Finite 

message: The article mentions that the model requires the universe to have initially been a singularity in space and time, but every other source I've found that discusses this model says that it replaces the singularity model because that model didn't handle quantum mechanics properly. Example: Stephen Hawking's Universe: NO-BOUNDARY UNIVERSE  — Preceding unsigned comment added by 161.84.227.12 (talk) 18:13, 5 February 2014 (UTC)
thread_title:  Singularity 

message: When one is an observer walking towards the horizon (ie: the apparent edge of the Earth), the curvature of the Earth, and the distance travelled towards the horizon ensures that new parts of the Earth come into view so that we never actually see 'the end of the Earth'.  It is the fact that the geodesics on Earth are curves (great circles, in fact) which causes us not only to be bound to the Earth BUT also for our field of view to become a different 'patch' of the Earth (the field of view of an observer on Earth can be likened to a disk - though it's actually a curved base of a spherical cone, and this 'tracks' the observer when they are walking along the Earth on their spherical geodesic).  NOW, extending this analogy to the time close to the Big Bang, I imagine that (i) We are going to have to make use of real variables due to the fact that we will have to 'zoom' closer and closer into the finer and finer structure of space-time to a very high level (ie: potentially so that we can see the Planck scale of the universe), (ii) The geodesics will be determined by General Relativity AND Quantum Mechanics, (iii) our 'field of view' will be replaced with a suitable higher dimensional analogue (I'm not going to put my head out by saying 'Hubble Sphere', but it would likely be some naturally related concept), (iv) Our 'field of view' will be limited by the curvature of spacetime as in the sphere analogy, (v) The whole point of the sphere analogy was that what appears to be the beginning of time (represented by the horizon of the Earth), in actuality does not exist but is an optical illusion.  This points to the possibility that (a) There is no beginning in time (as it's an optical illusion) or (b) The beginning of time exists but that the beginning of time is somehow 'enfolded' into spatial dimensions as to never be 'visible' or (c) Some other, likely weird, possibility....  
I can imagine that the spherical geometry example is amenable to mathematical fleshing out (spherical geometry is not as hard as General Relativity, eg: the Ricci curvature of a Sphere). 
Also, the article states "According to the theory time diverged from three state dimension—as we know the time now[clarification needed]—after the universe was at the age of the Planck time."  I think what is meant is that "time diverged from the three spatial dimensions - as we know time now - after ...".  This would seem to jibe with ideas in String theory where dimensions are 'compacted' or somehow 'enfolded' together at a microscopic level of detail.  
ASavantDude (talk) 02:26, 9 August 2016 (UTC)
thread_title: Mathematical Details or an Analogy Fleshed out

message: For those who are acquainted with Hugh Everett's "many worlds" theory (actually, it was physicist Bryce DeWitt who coined that term), hearing that Stephen Hawking had a wave function of the universe theory as well begs the question, are there differences between the two theories? 2600:8801:BE31:D300:BCC4:6E49:B609:F667 (talk) 18:23, 23 August 2022 (UTC)
thread_title:  How does the Hawking-Hartle "wave function" of the universe differ from the "universal wave function" of Hugh Everett?  

message: In the "technical explanation" section, the article reads: "The Hartle–Hawking state is the wave function of the Universe — a notion meant to figure out how the Universe started — that is calculated from Feynman's path integral." I question whether the so-called "Hartle-Hawking" state and "the wave function of the universe" are the same thing. From a boundaryless pre-big bang state a "wave function" emerged, similar to the wave function of a superposed quantum system. In his later writings, Hawking described the primordial wave function as a superposition of possible, or probable, histories of the universe. .. James. 2600:8801:BE01:7C00:C54D:2908:3406:2610 (talk) 14:30, 19 March 2023 (UTC)
thread_title:  Is the article conflating two things? 

message: A memorial for Hartle:
* https://news.ucsb.edu/2024/021364/physicists-gather-remember-father-quantum-cosmology
says
* "In the 1970s Hartle shifted his focus toward black holes, collaborating with astrophysicist Stephen Hawking to develop the Hartle–Hawking state. This formula describes the quantum state of a black hole in thermal equilibrium, with the matter falling into it counterbalanced by the Hawking radiation it emits."
Johnjbarton (talk) 22:00, 30 March 2024 (UTC)
thread_title:  This article is way off base