Menu
News
All News
Dungeons & Dragons
Level Up: Advanced 5th Edition
Pathfinder
Starfinder
Warhammer
2d20 System
Year Zero Engine
Industry News
Reviews
Dragon Reflections
Columns
Weekly Digests
Weekly News Digest
Freebies, Sales & Bundles
RPG Print News
RPG Crowdfunding News
Game Content
ENterplanetary DimENsions
Mythological Figures
Opinion
Worlds of Design
Peregrine's Next
RPG Evolution
Other Columns
From the Freelancing Frontline
Monster ENcyclopedia
WotC/TSR Alumni Look Back
4 Hours w/RSD (Ryan Dancey)
The Road to 3E (Jonathan Tweet)
Greenwood's Realms (Ed Greenwood)
Drawmij's TSR (Jim Ward)
Community
Forums & Topics
Forum List
Latest Posts
Forum list
*Dungeons & Dragons
Level Up: Advanced 5th Edition
D&D Older Editions
*TTRPGs General
*Pathfinder & Starfinder
EN Publishing
*Geek Talk & Media
Search forums
Chat/Discord
Resources
Wiki
Pages
Latest activity
Media
New media
New comments
Search media
Downloads
Latest reviews
Search resources
EN Publishing
Store
EN5ider
Adventures in ZEITGEIST
Awfully Cheerful Engine
What's OLD is NEW
Judge Dredd & The Worlds Of 2000AD
War of the Burning Sky
Level Up: Advanced 5E
Events & Releases
Upcoming Events
Private Events
Featured Events
Socials!
Twitch
YouTube
Facebook (EN Publishing)
Facebook (EN World)
Twitter
Instagram
TikTok
Podcast
Features
Top 5 RPGs Compiled Charts 2004-Present
Adventure Game Industry Market Research Summary (RPGs) V1.0
Ryan Dancey: Acquiring TSR
Q&A With Gary Gygax
D&D Rules FAQs
TSR, WotC, & Paizo: A Comparative History
D&D Pronunciation Guide
Million Dollar TTRPG Kickstarters
Tabletop RPG Podcast Hall of Fame
Eric Noah's Unofficial D&D 3rd Edition News
D&D in the Mainstream
D&D & RPG History
About Morrus
Log in
Register
What's new
Search
Search
Search titles only
By:
Forums & Topics
Forum List
Latest Posts
Forum list
*Dungeons & Dragons
Level Up: Advanced 5th Edition
D&D Older Editions
*TTRPGs General
*Pathfinder & Starfinder
EN Publishing
*Geek Talk & Media
Search forums
Chat/Discord
Menu
Log in
Register
Install the app
Install
Community
General Tabletop Discussion
*Geek Talk & Media
Is Time Travel (going backwards) Possible?
JavaScript is disabled. For a better experience, please enable JavaScript in your browser before proceeding.
You are using an out of date browser. It may not display this or other websites correctly.
You should upgrade or use an
alternative browser
.
Reply to thread
Message
<blockquote data-quote="tomBitonti" data-source="post: 6043882" data-attributes="member: 13107"><p><strong>Some interesting links</strong></p><p></p><p><a href="http://arxiv.org/pdf/gr-qc/0703100.pdf" target="_blank">http://arxiv.org/pdf/gr-qc/0703100.pdf</a></p><p></p><p>28-May-2007</p><p></p><p>Stability of Closed Timelike Curves in the G¨odel Universe</p><p></p><p>Val´eria M. Rosa</p><p>Departamento de Matem´atica, Universidade Federal de Vi¸cosa, 36570-000 Vi¸cosa, M.G., Brazil</p><p></p><p>Patricio S. Letelier†</p><p>Departamento de Matem´atica Aplicada-IMECC, Universidade Estadual de Campinas, 13081-970 Campinas, S.P., Brazil</p><p></p><p>PACS numbers : 04.20.Gz, 04.20.Dw, 040.20 Jb</p><p></p><p>Stability of Closed Timelike Curves in the G¨odel Universe</p><p></p><p>From the top of page 2:</p><p></p><p>"All our experience seems to indicate that the physical laws do not</p><p>allow the appearance of CTCs. This is that, essentially, says the</p><p>Chronology Protection Conjecture (CPC) proposed by Hawking in 1992</p><p>[16]."</p><p></p><p><a href="http://physics.stackexchange.com/questions/12170/why-should-one-expect-closed-timelike-curves-to-be-impossible-in-quantum-gravity" target="_blank">http://physics.stackexchange.com/questions/12170/why-should-one-expect-closed-timelike-curves-to-be-impossible-in-quantum-gravity</a></p><p></p><p>There are a lot of other links which I haven't scanned. That second one has this:</p><p></p><p>These are some notes to complement previous answers.</p><p></p><p>Your concerns are sound: a rigorous definite way to rule out CTCs has</p><p>not been found. What we have is arguments (and quite nice looking</p><p>ones) to illustrate that every known universe with CTCs looks</p><p>unphysical.</p><p></p><p>Second, there are two nicely-written pedagogical letters written by</p><p>Kip Thorne addressing your question [1],[2]. They mainly focus on</p><p>physical aspects of the known CTC solutions, and three popular</p><p>mechanisms that could prevent CTCs: violation of the averaged null</p><p>energy conditions (the first argument cited in the post), classical</p><p>instabilities of chronology horizons, and quantum field instabilities</p><p>(following the notation of [2], section 4). Although he does not seem</p><p>to believe in CTCs personally, at the end of [2] he states that this</p><p>is still an open question:</p><p></p><p> It may turn out that on macroscopic lenghscales chronology is not</p><p> always protected, and even if chronology is protected</p><p> macroscopically, quantum gravity may well give finite amplitudes</p><p> for microscopic spacetime histories with CTCs [29].</p><p></p><p> [29] Friedman J 1992 in Proceedings of the 4th Canadian Conf. on</p><p> General Relativity and Relativistic Astrophysics eds G Kunstatter</p><p> et al (Singapore: Word Scientific) pp. 183-199.</p><p></p><p>Finally, regarding the argument against CTCs that uses logical</p><p>paradoxes, which has already appeared in the post: it is not clear to</p><p>many people whether CTCs inevitably lead to causal paradoxes. Several</p><p>studies have pointed out that causal-paradoxes of time travel could</p><p>disappear once one takes quantum mechanical effects; or maybe their</p><p>meaning could simply change [3],[4],[5],[6]. For instance, in the</p><p>framework used in the first reference the grandfather's paradox does</p><p>not violate causality. In connection with this, although it is known</p><p>that some of these models of CTCs [7],[8] lead to counter-intuitive</p><p>collapes of computational complexity classes, this is not exactly the</p><p>same as a causal paradox.</p><p></p><p>The link [1] to a Kip Thorne PDF seems to be the jackpot:</p><p></p><p><a href="http://www.its.caltech.edu/~kip/scripts/ClosedTimelikeCurves-II121.pdf" target="_blank">http://www.its.caltech.edu/~kip/scripts/ClosedTimelikeCurves-II121.pdf</a></p><p>"Closed Timelike Curves" Kip S. Thorne</p><p></p><p>One of the questions reviewed: Do the laws of physics prevent CTCs</p><p>from ever forming in classical spacetime?</p><p></p><p>From page 2:</p><p></p><p>"However, the combination of general relativity's laws and the laws of</p><p>quantum fields in curved spacetime may well provide a chronology</p><p>protection mechanism, though we might be sure of this until we</p><p>understand the laws of quantum gravity much more deeply than today."</p><p></p><p>Lots of complicated details to the PDF, which I am not qualified to</p><p>present. There is this end note:</p><p></p><p>"In summary, these studies are giving us glimpses of how CTSs</p><p>influence physics; but whether those glimpses are teaching us</p><p>something deep and important or just playing fun mental games, is far</p><p>from clear."</p><p></p><p>Note: The paper is from Feb, 1993.</p></blockquote><p></p>
[QUOTE="tomBitonti, post: 6043882, member: 13107"] [b]Some interesting links[/b] [url]http://arxiv.org/pdf/gr-qc/0703100.pdf[/url] 28-May-2007 Stability of Closed Timelike Curves in the G¨odel Universe Val´eria M. Rosa Departamento de Matem´atica, Universidade Federal de Vi¸cosa, 36570-000 Vi¸cosa, M.G., Brazil Patricio S. Letelier† Departamento de Matem´atica Aplicada-IMECC, Universidade Estadual de Campinas, 13081-970 Campinas, S.P., Brazil PACS numbers : 04.20.Gz, 04.20.Dw, 040.20 Jb Stability of Closed Timelike Curves in the G¨odel Universe From the top of page 2: "All our experience seems to indicate that the physical laws do not allow the appearance of CTCs. This is that, essentially, says the Chronology Protection Conjecture (CPC) proposed by Hawking in 1992 [16]." [url]http://physics.stackexchange.com/questions/12170/why-should-one-expect-closed-timelike-curves-to-be-impossible-in-quantum-gravity[/url] There are a lot of other links which I haven't scanned. That second one has this: These are some notes to complement previous answers. Your concerns are sound: a rigorous definite way to rule out CTCs has not been found. What we have is arguments (and quite nice looking ones) to illustrate that every known universe with CTCs looks unphysical. Second, there are two nicely-written pedagogical letters written by Kip Thorne addressing your question [1],[2]. They mainly focus on physical aspects of the known CTC solutions, and three popular mechanisms that could prevent CTCs: violation of the averaged null energy conditions (the first argument cited in the post), classical instabilities of chronology horizons, and quantum field instabilities (following the notation of [2], section 4). Although he does not seem to believe in CTCs personally, at the end of [2] he states that this is still an open question: It may turn out that on macroscopic lenghscales chronology is not always protected, and even if chronology is protected macroscopically, quantum gravity may well give finite amplitudes for microscopic spacetime histories with CTCs [29]. [29] Friedman J 1992 in Proceedings of the 4th Canadian Conf. on General Relativity and Relativistic Astrophysics eds G Kunstatter et al (Singapore: Word Scientific) pp. 183-199. Finally, regarding the argument against CTCs that uses logical paradoxes, which has already appeared in the post: it is not clear to many people whether CTCs inevitably lead to causal paradoxes. Several studies have pointed out that causal-paradoxes of time travel could disappear once one takes quantum mechanical effects; or maybe their meaning could simply change [3],[4],[5],[6]. For instance, in the framework used in the first reference the grandfather's paradox does not violate causality. In connection with this, although it is known that some of these models of CTCs [7],[8] lead to counter-intuitive collapes of computational complexity classes, this is not exactly the same as a causal paradox. The link [1] to a Kip Thorne PDF seems to be the jackpot: [url]http://www.its.caltech.edu/~kip/scripts/ClosedTimelikeCurves-II121.pdf[/url] "Closed Timelike Curves" Kip S. Thorne One of the questions reviewed: Do the laws of physics prevent CTCs from ever forming in classical spacetime? From page 2: "However, the combination of general relativity's laws and the laws of quantum fields in curved spacetime may well provide a chronology protection mechanism, though we might be sure of this until we understand the laws of quantum gravity much more deeply than today." Lots of complicated details to the PDF, which I am not qualified to present. There is this end note: "In summary, these studies are giving us glimpses of how CTSs influence physics; but whether those glimpses are teaching us something deep and important or just playing fun mental games, is far from clear." Note: The paper is from Feb, 1993. [/QUOTE]
Insert quotes…
Verification
Post reply
Community
General Tabletop Discussion
*Geek Talk & Media
Is Time Travel (going backwards) Possible?
Top