On 20 October 2014 03:54, John Clark <johnkcl...@gmail.com> wrote: > On Sat, Oct 18, 2014 at 6:40 PM, LizR <lizj...@gmail.com> wrote: > > >>> Gödel shows that there are solution of Einstein's equation of >>>> gravitation with closed timelike curves, making them consistent. >>>> >>> >>> >> But only if you assume that the Universe is rotating, and >>> experimental evidence proves that it is not. And only if you assume that >>> Einstein's General Theory of Relativity is 100% correct, and we know it >>> can't be, it's the best theory of gravity we have but it can't be the final >>> word because it doesn't take Quantum Mechanics into account. >>> >> >>> >> > That isn't quite true. Several GR based time machines have been >> proposed - Tipler cylinders and wormholes, at least, as well as a Godel >> universe. >> > > Tipler found a solution in General Relativity that shows that an > infinitely long, extremely dense cylinder made of Neutronium (the stuff of > Neutron Stars) and spinning at almost the speed of light would be able to > tip a light cone enough to act as a time machine and allow you to visit the > past, but there are two very important problems. > > 1) Nobody knows if a cylinder of finite length would also work, the math > is too difficult to figure out. > > 2) The very rapid rotation would cause the cylinder to fly apart. This is > much more than just an Engineering difficulty, no known force in Physics > would be strong enough to hold the cylinder together, not even the strong > nuclear force. So for this to work new Physics would need to be found, > which is just another way of saying that as far as we know now it's > impossible. >
Hence my use of "quite" in the above quote. Time machines strike me as extremely unlikely, but we can't say they are impossible on the basis that GR has to be 100% true, or that the universe has to be rotating, which was the original point I was replying to. -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to everything-list+unsubscr...@googlegroups.com. To post to this group, send email to everything-list@googlegroups.com. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
