Talk:Closed timelike curve

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[edit] 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)

[edit] 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)

[edit] 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)

Wikipedia:External links#What can be done with a dead external link 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)

[edit] 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)

[edit] 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)