ps3 Posted November 28, 2008 Share Posted November 28, 2008 are they physicaly real? what is thought to prevent them? Link to comment Share on other sites More sharing options...
ajb Posted November 28, 2008 Share Posted November 28, 2008 (edited) Good question... This is really a topic of current research. Honestly, I don't think it is fully understood if CTC's are physical or not. What I can say is that calculations in semi-classical gravity suggest they may not be physical. To be a little more specific, the expectation value of the energy momentum tensor for a field diverges as one approaches a CTC. However, I do not think that this has been formulated as a "theorem", but only shown to be true by example. Also, one really needs to have a full quantum theory of gravity in order to have the final say on this. Edited November 28, 2008 by ajb multiple post merged Link to comment Share on other sites More sharing options...
ps3 Posted November 29, 2008 Author Share Posted November 29, 2008 ive read or been told,im not sure,that QG will probly prevent ctc from forming. Link to comment Share on other sites More sharing options...
ajb Posted November 29, 2008 Share Posted November 29, 2008 ive read or been told,im not sure,that QG will probly prevent ctc from forming. Maybe as semiclassical gravity suggests they may not be physical. However, without a full quantum theory of gravity to hand it is not really possible to say either way without some speculation. Link to comment Share on other sites More sharing options...
ps3 Posted December 1, 2008 Author Share Posted December 1, 2008 would something be wrong if ctc's did not exist? causality would be violated if they existed? what would be proof that ctc's exist? Link to comment Share on other sites More sharing options...
ajb Posted December 2, 2008 Share Posted December 2, 2008 would something be wrong if ctc's did not exist? causality would be violated if they existed? what would be proof that ctc's exist? In the context of general relativity it is not clear if space-times with CTC's are unphysical. Either way, they are interesting space-times and need to be studied to get a deeper understanding of general relativity. They would appear to violate causality, but it is possible to have consistent dynamics. Maybe our ideas about causality need modifying? CTCs exist in solutions to Einstein's Field equations. That is they mathematically exist. Now, your question is really do they "physically exist?". Which is really the same as are the solutions to the field equations that have CTCs physical. I think this is still an open question to a large extent. Hawking (and others) have shown that in semiclassical gravity the expectation value of the energy-momentum tensor diverges as you approach a CTC. This is suggests that CTCs are unphysical. However, the only "real proof" so far is that we have not found one nor have we found ant time travellers. To recap, I do not think it is very clear if CTCs are physical or not. Classically, general relativty permits them. Semiclassically, it looks more doubtful. As for a full quantum theory of gravity who knows? Link to comment Share on other sites More sharing options...
ps3 Posted December 2, 2008 Author Share Posted December 2, 2008 (edited) so just becuase relativity allows something doesnt mean it can work in the universe? so relativty could allow it but not the universe? so the solutions would have to be wrong for there to be no ctc's? or can the solution be right without any ctc's? Edited December 3, 2008 by ps3 Link to comment Share on other sites More sharing options...
ajb Posted December 3, 2008 Share Posted December 3, 2008 The Einstein field equations by them self are not "strong enough" to decide if a classical space-time is physical or not. Generically, you will need further conditions on the matter content such as energy conditions. Deciding exactly what these conditions are is still non-trivial and people work on this today, particularly in semiclassical gravity. Not all solutions to the field equations have CTCs. You could argue that CTCs are unphysical and hence any space-time with them should be discarded. However, this is not very scientific in my view, nor that of many other people. (This is independent of if you think time travel is possible or not). Link to comment Share on other sites More sharing options...
AlphaNumeric Posted December 4, 2008 Share Posted December 4, 2008 (edited) Just to let everyone know, ps3 is known as Wolv1 on SciForums and Deuce2 on PhysOrg. Myself and the posts known as Rpenner and Prometheuse (on PhysOrg) and BenTheMan (a mod on Sciforums and a poster on PhysOrg) have all answered the thread starters questions multiple times. He just asks the same questions again and again and again. He was banned from SciForums for repeatedly ignoring the answers we provided and spamming several people's PM boxs with the same questions. The aforementioned people are either physics graduates or current postgraduates, so it's not like he's asking questions to people who don't know. He will continue to ask these questions and I'm willing to be he's already PM'd a few of you. Don't bother replying, he'll just ask the same question rephrased again and again. We can't work out why he's got such an obsession with it. Edited December 4, 2008 by AlphaNumeric Link to comment Share on other sites More sharing options...
ps3 Posted December 4, 2008 Author Share Posted December 4, 2008 Just to let everyone know, ps3 is known as Wolv1 on SciForums and Deuce2 on PhysOrg. Myself and the posts known as Rpenner and Prometheuse (on PhysOrg) and BenTheMan (a mod on Sciforums and a poster on PhysOrg) have all answered the thread starters questions multiple times. He just asks the same questions again and again and again. He was banned from SciForums for repeatedly ignoring the answers we provided and spamming several people's PM boxs with the same questions. The aforementioned people are either physics graduates or current postgraduates, so it's not like he's asking questions to people who don't know. He will continue to ask these questions and I'm willing to be he's already PM'd a few of you. Don't bother replying, he'll just ask the same question rephrased again and again. We can't work out why he's got such an obsession with it. ya becuase you guys don't give simple answer's like these guys on here. Link to comment Share on other sites More sharing options...
AlphaNumeric Posted December 4, 2008 Share Posted December 4, 2008 We gave you the answers on both forums. You asked hundreds of times the same question. Initially you get answers which were indepth, because a simple yes or no isn't enough, but you continued asking again and again and eventually, after the dozens of threads you started, we got tired of your display of ignorance and unwillingness to accept the answers and you got banned. And they haven't given straight forward answers here, they have said "Depends" and "We're not entirely sure". Why are you obsessed so much with ctcs? Link to comment Share on other sites More sharing options...
ps3 Posted December 4, 2008 Author Share Posted December 4, 2008 im doing a school project on them which has to be done soon. Link to comment Share on other sites More sharing options...
AlphaNumeric Posted December 4, 2008 Share Posted December 4, 2008 But given you don't want any kind of indepth answer and all you want is a yes or a no, your project is going to be pretty crap. You don't enter into a discussion, you just ask the same question again and again. Also, no school project is going to involve giving you a highly complex, very mathematical, topic like closed time-like curves. Guest gave you a formal definition of closed time-like curves on SciForums and you don't understand it. All your posts just ask the same thing. You are not making any attempt to do the research yourself and if you did, it would be completely over your head. You don't understand that closed time-like curves are not related to antimatter (antimatter doesn't require curved space-time) and you will no doubt ask another inane question on that. Basically, if this is your method of doing school projects I imagine you're pretty crap at science. Link to comment Share on other sites More sharing options...
ps3 Posted December 4, 2008 Author Share Posted December 4, 2008 well,my project is more on time travel.have you heard of yakir aharonov? Link to comment Share on other sites More sharing options...
swansont Posted December 5, 2008 Share Posted December 5, 2008 Just to let everyone know, ps3 is known as Wolv1 on SciForums and Deuce2 on PhysOrg. Myself and the posts known as Rpenner and Prometheuse (on PhysOrg) and BenTheMan (a mod on Sciforums and a poster on PhysOrg) have all answered the thread starters questions multiple times. He just asks the same questions again and again and again. He was banned from SciForums for repeatedly ignoring the answers we provided and spamming several people's PM boxs with the same questions. The aforementioned people are either physics graduates or current postgraduates, so it's not like he's asking questions to people who don't know. He will continue to ask these questions and I'm willing to be he's already PM'd a few of you. Don't bother replying, he'll just ask the same question rephrased again and again. We can't work out why he's got such an obsession with it. I realize this is motivated by a desire to be helpful, and your desire to improve the quality of the boards is appreciated. However, it should be noted that the staff isn't going to act based on behavior exhibited elsewhere. As along as a poster abides by the rules here, they are welcome to post questions and stimulate discussion. If they are breaking the rules, though, all bets are off. Link to comment Share on other sites More sharing options...
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