Talk:Closed timelike curve

Not all worldlines are timelike
A worldline, by definition, is a path (curve) in spacetime. Worldlines may be timelike, null or spacelike. Material particles have timelike worldlines and, for example, photons have null worldlines. ---Mpatel 16:36, 20 Jun 2005 (UTC)


 * And tachyons would have spacelike worldlines, if they existed, which they probably do not.---CH (talk) 10:47, 2 September 2005 (UTC)

Cauchy Horizons
"A CTC therefore results in a Cauchy horizon"? I don't think that makes sense. Take a quotient of Minkowski spacetime in which you identify t = 0 and t = 1. That breaks the global Lorentz symmetry, but this is trivially a vacuum solution. (And isn't this called a Tipler cylinder? No-one who mentioned that term really explained what it means--- bad, bad!)  Anyway, you now have CTCs, but where is the Cauchy horizon?!---CH  (talk) 10:47, 2 September 2005 (UTC)

My understanding of a Cauchy horizon is that it simply marks a region of spacetime which remains unpredicable based upon knowledge of past spacetime. If I were to invoke a form of the grandfather paradox to explain it, I'd say that a child could be born in a CTC with himself as his own father, and then proceed to perform hijinks throughout the world (including fathering himself). Since it would be impossible to predict the child's birth and hijinks using only knowledge from before the CTC, all predictions made for the child's lightcone would be suspect - potentially affected by an unknowable agent. Thus, the child's lightcone defines a Cauchy horizon. As Carroll puts it, none of the points containing CTCs are in the domain of dependance for some previous surface, since the CTC itself does not pass through that surface. He also claims singularities result in Cauchy horizons. Perhaps this all could be explained better in the article. --Hyandat 15:09, 2 September 2005 (UTC)

Issue with external link "A Primer on Time Travel"
Currently the external link labled "A Primer on Time Travel" (which is coded to link to http://www.readmag.com/Columns/timetravel.htm) is being redirected to the URL http://www.readjunk.com/, where the desired information is missing. Google contains a cached version of the original article from 17 Oct, 2006 at this URL. In cases where relevant information from old external links is no longer available on the original site, is it acceptable to redirect the link to Google cache, or should the link just be removed entirely? --Seph Vellius 02:32, 23 October 2006 (UTC)


 * External links suggests using the Internet Archive Wayback Machine; indeed User:Gwern has already done so. I think the Google cache will disappear after some time. -- Jitse Niesen (talk) 04:18, 23 October 2006 (UTC)

More gogglygook
I can't help but shake my head when I read most physics articles on the wiki. They are invariably filled with technobabble and rarely include a plain-english explaination of what they're talking about. This one is better than most, but nevertheless:

1) never explains what a lightcone is, or why we care 2) never explains how one tilts a lightcone 3) never explains the "timelike" term in english (ie, "time travel") 4) mentions the Tippler cylinder only in passing etc.

The problem is my familiarity with GR is passing at best, so while I think I can clean this up, I will certainly introduce errors in the process. Please fix in behind me.

Maury 16:20, 14 March 2007 (UTC)

Error?
The article has this line:


 * If the object were in free fall it would travel up the t axis,

Wouldn't it also travel along the x-axis, drawing a straight line on the graph?


 * if it accelerates it moves across the x axis as well.

If it accelerates, it moves with a changing velocity across the x axis (as opposed to a constant one above)

David 17:09, 15 August 2007 (UTC)

No, there is not any error and the article says correctly the concept. In free fall, the object in it's own reference frame only feels the passage of time on itself and it wouldn't interpret the free fall as moving in new space locations. As a matter of fact, the object in free fall doesn't have any sense of force imposed on it. However, in the reference frame of an external observer, the object is both under time passage and accelerating to find a new space location among the possible future ones. — Preceding unsigned comment added by Alirezanadi (talk • contribs) 19:37, 26 December 2014 (UTC)

Alireza 23:09, 26 December 2014

Technical, and needs more references
I tried to read through this article multiple times, but was unable to understand more than a handful of sentences. I have the vaguest idea of what closed-time curves are supposed to do, but no real idea of why, how, or if I can reasonably be expected to believe that they might exist. I also noted a lack of citations for this article. If someone could find a few that explained this concept in layman's terms, I feel that that would greatly improve this article. --Sennsationalist (talk) 11:37, 9 November 2014 (UTC)

The article says correctly the main points about the concept of closed timelike curves. You are to imagine the discrepancy between flat and curved spacetime to finally understand these curved worldlines. The curvedness of spacetime is the cause for curving the expected straightened worldlines for any event near the massive objects like stars. If the curvedness of spacetime near the star be severe enough, it causes the worldline to be curved towards the star so that the end of the worldline reaches it's starting point and a closed timelike curve comes into existence--Alireza Nadi(talk) 23:17, 26 December 2014

Van Stockum, Gödel and the discovery of the Closed timelike curve
In the introduction of the article this is said about Gödel's solution to the equations of GR: 'In mathematical physics, a closed timelike curve (CTC) is a world line in a Lorentzian manifold, of a material particle in spacetime that is "closed", returning to its starting point. This possibility was discovered by Kurt Gödel in 1949, who discovered a solution to the equations of general relativity (GR) allowing CTCs known as the Gödel metric; and since then other GR solutions containing CTCs have been found, such as the Tipler cylinder and traversable wormholes.'

The fact that the solution was discovered by Gödel does not mean that Gödel was the first to theorize the possibility of a Closed timeline curve (which the introduction suggests). Willem Jacob van Stockum in his 1937 article: ''Stockum, W. J. van (1937). "The gravitational field of a distribution of particles rotating around an axis of symmetry.". Proc. Roy. Soc. Edinburgh. 57'' already theorized the possibility of a Closed timeline curve. I'm not sure if he was the first one, but the supplied source (Hawkins) is not coming from an scientific work (nor from someone who specialized in the history of science) and nor does it actually states that Gödel 'discovered this possibility'. Rather, Hawkins wrote that Gödel was the first to discover a solution to the equations of GR. C.Gesualdo (talk) 19:54, 3 January 2017 (UTC)