tag:blogger.com,1999:blog-22973357.post2236597761458717023..comments2023-09-27T07:44:19.769-04:00Comments on Sabine Hossenfelder: Backreaction: The Equivalence PrincipleSabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comBlogger104125tag:blogger.com,1999:blog-22973357.post-80089189655621265392008-09-08T21:09:00.000-04:002008-09-08T21:09:00.000-04:00A closed timelike loop allows you to go back to a ...A closed timelike loop allows you to go back to a point in space <I>and time</I>. That means it is a curve of non-zero length on which you'd have your own eigentime, but you could re-visit a time in the past. It causes the usual time-travel conundrums and is generally considered to be something you better avoid in your theory. Best,<BR/><BR/>B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-90179317839465732132008-09-08T21:05:00.000-04:002008-09-08T21:05:00.000-04:00Hi Bee,You said: unless your spacetime allows clos...Hi Bee,<BR/>You said: <I>unless your spacetime allows closed timelike loops you will have to accelerate one of the twins to get them to meet again which breaks the symmetry</I><BR/><BR/>What do closed timelike loops do? I am trying to figure out the Magellan variation of the paradox, the one with cylindrical universe. Can you please help? <BR/>Thanks!<BR/>AditiStargazerhttps://www.blogger.com/profile/16482382083784806186noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-59280710888792683052008-08-28T20:48:00.000-04:002008-08-28T20:48:00.000-04:00http://www.blogodoom.com/images/girls_with_math_on...http://www.blogodoom.com/images/girls_with_math_on_backs.jpg<BR/><BR/>My choice of general relativity continues to be correct.eric gissehttps://www.blogger.com/profile/10598878490537720448noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-84613760524950455762008-08-22T21:30:00.000-04:002008-08-22T21:30:00.000-04:00HiBee,very nice explanation.Btw since you write ex...Hi<BR/>Bee,<BR/>very nice explanation.<BR/>Btw since you write excellent articles you should<BR/>blog on the speed of gravity controversy <BR/><A HREF="http://physics.wustl.edu/cmw/SpeedofGravity.html" REL="nofollow">in 2003 </A> I'd like to know your take on it.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-22973357.post-42318759043217851742008-08-22T12:35:00.001-04:002008-08-22T12:35:00.001-04:00No time to read all these comments, but an excelle...No time to read all these comments, but an excellent place to learn about the twin paradox is in Schutz's <A HREF="http://www.amazon.com/First-Course-General-Relativity/dp/0521277035/ref=pd_bbs_sr_2?ie=UTF8&s=books&qid=1219422652&sr=8-2" REL="nofollow">book</A>.<BR/><BR/>Also, take a look at the nice front cover (and explanation inside) of Koks' <A HREF="http://www.amazon.com/Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-22973357.post-69916951243477377922008-08-22T12:35:00.000-04:002008-08-22T12:35:00.000-04:00No time to read all these comments, but an excelle...No time to read all these comments, but an excellent place to learn about the twin paradox is in Schutz's <A HREF="http://www.amazon.com/First-Course-General-Relativity/dp/0521277035/ref=pd_bbs_sr_2?ie=UTF8&s=books&qid=1219422652&sr=8-2" REL="nofollow">book</A>.<BR/><BR/>Also, take a look at the nice front cover (and explanation inside) of Koks' <A HREF="http://www.amazon.com/Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-22973357.post-10977954613683219062008-08-19T11:05:00.000-04:002008-08-19T11:05:00.000-04:00Reductionism paved the way for Emergence. Emergenc...Reductionism paved the way for Emergence. Emergence from reductionism, is creatively free.<BR/><BR/>A proposition then:These two avenues exist in the idea of "The Equivalence Principle?" At the "same time."PlatoHagelhttps://www.blogger.com/profile/00849253658526056393noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-25543522980036966602008-08-18T14:41:00.000-04:002008-08-18T14:41:00.000-04:00Dr Who.Please write down a prediction of General R...Dr Who.<BR/><BR/>Please write down a prediction of General Relativity that cannot be derived from the Spin 2 propagating in a flat background formalism.<BR/><BR/>You cannot of course, because the two are equivalent, as they must be. In fact, entire textbooks written by Nobel Laureates treat the subject.<BR/><BR/>Which language you choose to express physics in, is inconsequential. Sometimes Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-22973357.post-69513297112443010642008-08-18T13:44:00.000-04:002008-08-18T13:44:00.000-04:00“Orin, all of us who take Quantum Mechanics to be ...“Orin, all of us who take Quantum Mechanics to be a complete theory condescend to Einstein.”<BR/><BR/>I don’t know what you have in mind, but if you claim “non-relativistic” QM at that time (without Principle of Local Gauge Invariance) was complete and that QT is complete today you are hopelessly naïve.<BR/><BR/>“That is at the essence of science and mathematics.”<BR/>I guess you condescend to Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-22973357.post-21599235656026037102008-08-18T13:41:00.000-04:002008-08-18T13:41:00.000-04:00Proper time to quote John Archibald Wheeler. Well,...Proper time to quote John Archibald Wheeler. Well, somebody's got to.<BR/><BR/>"Matter tells spacetime how to curve; spacetime tells matter how to move."<BR/><BR/>As to Poe and Dodgson, I'll add that Jonathan Swift wrote that Mars had two small moons, one so close that it rises in the West and sets in the East, many years befor Asaph Hall discovered Phobos and Deimos.<BR/><BR/>Finally, I'm biasedAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-22973357.post-7560084377248904702008-08-18T12:57:00.000-04:002008-08-18T12:57:00.000-04:00Arun: "Sigh, I'll try one last time."I don't know ...Arun: "Sigh, I'll try one last time."<BR/><BR/>I don't know Arun, this seemed pretty intentional to me, to say the least.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-22973357.post-14086139559729720012008-08-18T12:45:00.000-04:002008-08-18T12:45:00.000-04:00There was no intent to be condescending and I only...There was no intent to be condescending and I only apologize for where I added to the confusion.<BR/><BR/>-ArunArunhttps://www.blogger.com/profile/03451666670728177970noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-91572900938803667192008-08-18T12:39:00.000-04:002008-08-18T12:39:00.000-04:00Orin: "This interpretation goes hand in hand with ...Orin: "This interpretation goes hand in hand with Einstein's original 'Machian' ideas and motivation for SR, and one of the reasons I have always found SR beautiful. In other words, reality is defined by the motion of objects relative to each other, not relative to some 'objective' coordinate system."<BR/><BR/><BR/>Orin, this is GR you are talking about not SR. In GR this is formulated with the Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-22973357.post-16117727157315301462008-08-18T12:31:00.000-04:002008-08-18T12:31:00.000-04:00OK, better worded: Why is the field from an extend...OK, better worded: Why is the field from an extended planar mass curved (if it is, and in space-time in the net since of course it is still a parallel field) rather than the flat field of the Rindler elevator? txNeil Bateshttps://www.blogger.com/profile/04564859009749481136noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-69803102155258800132008-08-18T12:27:00.000-04:002008-08-18T12:27:00.000-04:00OK thanks Bee. For anyone who wants to try, this i...OK thanks Bee. For anyone who wants to try, this is perhaps the more cogent question: <I>why</I> is the allegedly curved g-field (if G. Egan was right ...) of an extended planar mass not like the elevator flat field?<BR/><BR/>PS: thanks andrew for excerpt from Lewis Caroll about the EP. Perhaps even more "amazing", Edgar Allen Poe first came up with the modern-like explanation of how to avoid theNeil Bateshttps://www.blogger.com/profile/04564859009749481136noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-71624180048176411412008-08-18T12:18:00.000-04:002008-08-18T12:18:00.000-04:00Hi Orin,Well, I am genuinely sorry if I misunderst...Hi Orin,<BR/><BR/>Well, I am genuinely sorry if I misunderstood you. It seemed to me you were trying to say there's something funny with the explanation we've been trying to give you. If not, then I think the issue is settled anyway. Thanks for your interesting comments. Best,<BR/><BR/>B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-15411077865734453442008-08-18T12:07:00.000-04:002008-08-18T12:07:00.000-04:00Hi Bee,The connection of both lines in that point ...Hi Bee,<BR/><BR/><I>The connection of both lines in that point you dislike does matter. As I said already above though I don't know why you're hanging yourself up on this unphysical limit with infinite acceleration in one point - just because the limit looks unintuitive? There is nothing 'hand-wavy' here, there are no 'subleties' and there is no paradox.</I><BR/><BR/>Wow. No. This is exactly whatOrinhttps://www.blogger.com/profile/13399554077097548978noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-12268523392999632012008-08-18T11:51:00.000-04:002008-08-18T11:51:00.000-04:00hmm, though maybe it should have been 'proper time...hmm, though maybe it should have been 'proper time of the curve' and not 'on the curve'? Sorry if that was confusingly formulated.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-13687313444801645542008-08-18T11:48:00.000-04:002008-08-18T11:48:00.000-04:00Hi Orin,What Arun and Dr. Who patiently told you i...Hi Orin,<BR/><BR/>What Arun and Dr. Who patiently told you is correct. You apparently also did not quite understand what I initially said. I said the proper time on the curve is continuous in the limit when you squeeze all the acceleration into one point, not about the limit of the point itself. This limiting situation is not identical to having one twin moving away from Earth and somebody else Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-73954189515335539162008-08-18T11:34:00.000-04:002008-08-18T11:34:00.000-04:00Anonymous,I believe this is responding to your poi...Anonymous,<BR/><BR/><I>I believe this is responding to your point - just as in the Euclidean plane, the lack of a preferred coordinate system doesn't mean we can't draw a triangle.</I><BR/><BR/>Ah, yes. I now realize I misinterpreted Arun's statement. What I am saying: <BR/><BR/>My interpretation of SR was that 'lack of a prefered coordinate system' went beyond what that of the Euclidean plane, Orinhttps://www.blogger.com/profile/13399554077097548978noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-84979338615456721362008-08-18T11:31:00.000-04:002008-08-18T11:31:00.000-04:00Hi Cecil,For one, the way I have stated the EP it ...Hi Cecil,<BR/><BR/>For one, the way I have stated the EP it doesn't speak about inertial and gravitational masses. As to your question about the definition of gravitational mass, for that you'd need the field equations. To begin with please note it is generally not only a mass that gravitates but the stress-energy tensor, it is just for testparticles in the appropriate limit that one has this Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-10422174560012667942008-08-18T11:24:00.000-04:002008-08-18T11:24:00.000-04:00Hi Neil,Well, I think you've answered your own que...Hi Neil,<BR/><BR/>Well, I think you've answered your own question. A spacetime either has a curvature or it hasn't. I don't know how you want to 'model' a spacetime with curvature by one without. As to the differences, I don't know, I'd compute the geodesics and see what they do. Best,<BR/><BR/>B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-75953646643317834422008-08-18T11:17:00.000-04:002008-08-18T11:17:00.000-04:00Bee, your outline of the Equivalence Principle fit...Bee, your outline of the Equivalence Principle fits in with my understanding (based on middle-brow explanations) that a "uniform" and parallel gravity field is just like the environment in an elevator with constant (proper) acceleration. Even then I came to appreciate the need to adjust for "Rindler coordinates" arising from the Lorentz contraction of the accelerating Neil Bateshttps://www.blogger.com/profile/04564859009749481136noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-53051728510856076932008-08-18T09:56:00.000-04:002008-08-18T09:56:00.000-04:00Descendants?Reflections on RelativityIn any case, ...Descendants?<BR/><BR/><A HREF="http://www.mathpages.com/rr/s8-06/8-06.htm" REL="nofollow" TITLE="On Gauss's Mountains">Reflections on Relativity</A><BR/><BR/><I>In any case, it seems reasonable to agree with Buhler, who concludes in his biography of Gauss that "the oft-told story according to which Gauss wanted to decide the question [of whether space is perfectly Euclidean] by measuring a PlatoHagelhttps://www.blogger.com/profile/00849253658526056393noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-46563098383767778772008-08-18T09:54:00.000-04:002008-08-18T09:54:00.000-04:00Cecil Kirksey: Even though my involvement in that ...Cecil Kirksey: Even though my involvement in that question about EP operating on different 'types' of matter was about 15 yrs ago (I've not kept up with the latest research), I don't think the answer is clear yet. See my <A HREF="http://cosmicvariance.com/2006/05/18/dark-energy-task-force-report/#comment-23337" REL="nofollow">old comment</A> on cosmicvariance and a more recent <A HREF="http://amaragrapshttps://www.blogger.com/profile/15769062084934190681noreply@blogger.com