tag:blogger.com,1999:blog-22973357.post7539753990219867526..comments2021-04-12T08:59:56.641-04:00Comments on Sabine Hossenfelder: Backreaction: A brief history of black holesSabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comBlogger148125tag:blogger.com,1999:blog-22973357.post-17949591445958458142020-06-24T02:36:39.279-04:002020-06-24T02:36:39.279-04:00Well, maybe you will do a video on wormholes somed...Well, maybe you will do a video on wormholes someday and we'll discuss it again then.Pascalhttps://www.blogger.com/profile/14201150679841329835noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-8086370835981138272020-06-23T23:31:27.890-04:002020-06-23T23:31:27.890-04:00You are right about the incompleteness, sorry abou...You are right about the incompleteness, sorry about that, but this wasn't my point. I merely said just because there is a metric on a space doesn't mean there's a way to travel from any point to an other point. In any case, I don't see how that's relevant to my video which is about the history of black holes, so excuse me but good bye.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-63671621613848340452020-06-23T13:08:32.247-04:002020-06-23T13:08:32.247-04:00Sabine, I am commenting here to try and dispel com...Sabine, I am commenting here to try and dispel common misconceptions about black holes. I think this is very relevant to your video. Actually, your answer to my comment contains a related misconception: Einstein-Rosen is geodesically complete! Geometrically, this solution consists of two exterior Schwarwschild solutions glued together at the horizon.<br />In that spacetime, a geodesic going Pascalhttps://www.blogger.com/profile/14201150679841329835noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-25784120858539018212020-06-23T11:32:48.051-04:002020-06-23T11:32:48.051-04:00I don't have the time to read Wikipedia. I am ...I don't have the time to read Wikipedia. I am trying to understand why you are commenting here and it's not clear to me. Yes, if there is a metric, there are geodesics, but that doesn't mean these geodesics will connect any two points. These metrics are all geodesically incomplete, so of course they will not. Could you please be clearer what the connection of your comment is to my Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-41201754260542126552020-06-23T11:06:09.475-04:002020-06-23T11:06:09.475-04:00Also, if you want to see what I am commenting on, ...Also, if you want to see what I am commenting on, you can look up the wikipedia page on wormholes. Here is what they say on the way Einstein-Rosen relates to Kruskal-Szekeres:<br /><br />In this spacetime [Kruskal-Szekeres], it is possible to come up with coordinate systems such that if a hypersurface of constant time (a set of points that all have the same time coordinate, such that every point Pascalhttps://www.blogger.com/profile/14201150679841329835noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-54433292952016949672020-06-23T11:01:57.748-04:002020-06-23T11:01:57.748-04:00Sabine, the "non traversable" thing mean...Sabine, the "non traversable" thing means that the bridge is supposed to collapse if one tries to send matter through it. I don't know if that is right or wrong, but this not relevant here : there is a well-defined metric (you can look up in the 1935 article by Einstein and Rosen), and if there is a metric there are geodesics.Pascalhttps://www.blogger.com/profile/14201150679841329835noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-90999163750396116862020-06-23T10:53:09.730-04:002020-06-23T10:53:09.730-04:00I don't know what you are commenting on, but t...I don't know what you are commenting on, but the Einstein-Rosen bridge is a non-transversable wormhole, so I don't know what your point is with the geodesics. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-57245152268408828572020-06-23T07:26:23.242-04:002020-06-23T07:26:23.242-04:00I’ll add that there is a common misconception that...I’ll add that there is a common misconception that an Einstein-Rosen bridge somehow corresponds to the two exterior regions (I and III) in Kruskal-Szeres (KS). <br />See for instance the wikipedia page on wormholes. This is horribly wrong, and someone should fix this! One obvious reason why this is wrong is that geodesics cannot go from region I to III in KS, but only from I to II (the black holePascalhttps://www.blogger.com/profile/14201150679841329835noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-60680323761483009682020-06-22T08:56:59.450-04:002020-06-22T08:56:59.450-04:00The singularities of ordinary General Relativity c...The singularities of ordinary General Relativity can be avoided by considering the (mathematically well defined) Einstein-Yang-Mills-Dirac-Higgs System which is (heuristically) the super-classical limit of the (not mathematically well-defined) Standard Model. This system has complete solutions without singularities, solitons, and a Cyclic Universe solution. (The system has negative energy Prof. David Edwardshttps://www.blogger.com/profile/16079658994584920395noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-19348473905026632282020-06-22T06:43:32.416-04:002020-06-22T06:43:32.416-04:00Canticle wrote: "does the mathematical "...Canticle wrote: "does the mathematical "singularity" really exist?"<br />It certainly does not exist in Einstein-Rosen bridges. This is the alternative to Kruskal-Szekeres mentioned in my previous <br />message. When a star collapses, the excess matter could be ejected through an Einstein-Rosen bridge instead of being crushed in a singularity. This is basically the mechanism Pascalhttps://www.blogger.com/profile/14201150679841329835noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-21573991805763864662020-06-22T05:06:33.806-04:002020-06-22T05:06:33.806-04:00Sabine, From my A&A view I've long struggl...Sabine, From my A&A view I've long struggled with the increasing gap between theory and observation. Study of AGN & quasar jets since Martin Rees etc in the '70s shws accretion, 'counter wound' toroidal acceleration, and the opposing helical outflows not only ejecting all accreted matter (most re-ionised) but actually resulting in MORE mass than in the accreted galaxy discPeter Jacksonhttps://www.blogger.com/profile/13905359947213961259noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-61723442276168564252020-06-08T10:45:30.289-04:002020-06-08T10:45:30.289-04:00If anyone is still following this thread, I have s...If anyone is still following this thread, I have some news about the Krsukal-Szekeres extension and its uniqueness property (or lack thereof). I discussed this issue with colleagues who are actual experts of the maths of general relativity. As it turns out, the uniqueness property mentioned on wikipedia holds only for metrics with signature (-++++) everywhere. In particular, the metric is not Pascalhttps://www.blogger.com/profile/14201150679841329835noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-50690888400074555792020-05-22T11:16:21.718-04:002020-05-22T11:16:21.718-04:00Thank you for all the references! In the meantime,...Thank you for all the references! In the meantime, the author of the chapter cited on wikipedia pointed me to his arxiv paper https://arxiv.org/pdf/0807.2309.pdf (see Theorem 4.5). Looks like very serious, non-cranky math so I am sure that Sabine won't mind the link.Pascalhttps://www.blogger.com/profile/14201150679841329835noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-43739475750201955032020-05-22T07:39:37.360-04:002020-05-22T07:39:37.360-04:00Pascal, the 1960 journal article by Kruskal is inf...Pascal, the 1960 journal article by Kruskal is informative: Maximal Extension of Schwarzschild Metric (Physical Review, Volume 119, pp. 1743-1745). Another useful reference is "Maximal Analytic Extension of the Kerr Metric," (Journal of Mathematical Physics, Volume 8, Number 2, pp. 265-281). These articles are reprinted in a resource booklet: Black Holes Selected Reprints (1982, Gary Alanhttps://www.blogger.com/profile/15299444226289034923noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-12611512525968043192020-05-20T00:23:06.211-04:002020-05-20T00:23:06.211-04:00Pascal, this is actually not a bad question. The a...Pascal, this is actually not a bad question. The answer is that the mass of the central spherical object as seen from the outside includes anything that fall on it, even before it is done falling. With black holes, due to the gravitational time dilation, the infalling matter close to the horizon would be "seen" as smeared all over the horizon, just as it fades into the infrared, Sergeihttps://www.blogger.com/profile/15415152744605672336noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-56002989452570626342020-05-19T06:32:31.797-04:002020-05-19T06:32:31.797-04:00Lawrence Crowell: "A trillion stars has a col...Lawrence Crowell: "A trillion stars has a collective Schwarzschild radius of around 3×10^{12}km."<br /><br />And the whole cosmos? What might be its Schwarzschild radius? May we live in a black hole? I would suggest that the average distance between particles in the whole cosmos for forming a Schwarzschild radius would be greater than in your example? Right? Or let's take just the weristdashttps://www.blogger.com/profile/04693023273675933748noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-1996047486713465832020-05-18T14:34:44.101-04:002020-05-18T14:34:44.101-04:00I have a question about the Kruskal-Szekeres (KS) ...I have a question about the Kruskal-Szekeres (KS) coordinates. Any other solution seems to be rejected outright by the experts because the KS coordinates are *the* maximal analytic continuation of the Schwarzchild solution. So there can’t be another sensible solution than KS, and that’s the end of the story. Here there seems to be a uniqueness theorem lurking in the background. Does anyone on Pascalhttps://www.blogger.com/profile/14201150679841329835noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-47102760676903301092020-05-18T09:50:48.532-04:002020-05-18T09:50:48.532-04:00Werisidas: You should be able to make an estimate ...Werisidas: You should be able to make an estimate on your own. It is easy, but the answer would be about 7×10^6km, or about 10 times the diameter of a star. Also this is all Euclidean, for the curvature of spacetime is not huge.Lawrence Crowellhttps://www.blogger.com/profile/12090839464038445335noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-73557373062134967442020-05-18T06:56:12.790-04:002020-05-18T06:56:12.790-04:00to Lawrence Crowell7:53 AM, May 13, 2020
in your ...to Lawrence Crowell7:53 AM, May 13, 2020<br /><br />in your example, what is the average distance between the ideal middle points of a pair of adjacent particles which are possible under such conditions? And what kind of forces works between them dominantly? Or is the answer for such a question beyond known physics?weristdashttps://www.blogger.com/profile/04693023273675933748noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-33077334134761457812020-05-17T14:54:15.338-04:002020-05-17T14:54:15.338-04:00Sergei, you're saying that the effect of the t...Sergei, you're saying that the effect of the traveller's mass inside the horizon cannot be felt outside. But the sad fate of the traveller is that he will end up in the central singularity, thereby increasing the mass of the black hole. I suppose that the Schwarzchild solution should then be replaced by a new one, taking into account the increase in mass of the black hole. Won't this Pascalhttps://www.blogger.com/profile/14201150679841329835noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-68265278413291135992020-05-17T04:43:56.173-04:002020-05-17T04:43:56.173-04:00I am aware that the standard way of approaching th...I am aware that the standard way of approaching this issue is via the Kruskal-Szekeres coordinates, but it's not the only one.Pascalhttps://www.blogger.com/profile/14201150679841329835noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-12983141107746688592020-05-17T01:23:30.851-04:002020-05-17T01:23:30.851-04:00Pascal,
Look, it's called "geodesically ...Pascal,<br /><br />Look, it's called "geodesically incomplete" and it's a math thing and you can look it up and there's a proof and that's that. I don't care at all how you want to glue together different sides of a static metric, because the time-independent solution is not realistic anyway. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-64661349032250301522020-05-16T20:48:11.423-04:002020-05-16T20:48:11.423-04:00When someone sees a drop of water from rain, did t...When someone sees a drop of water from rain, did the drop always exist?Michael John Sarnowskihttps://www.blogger.com/profile/00528454593064091302noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-63183992725220773752020-05-16T18:43:37.051-04:002020-05-16T18:43:37.051-04:00> If that’s the case, can you explain why the e...> If that’s the case, can you explain why the effect of the mass of the collapsed star, which is also trapped inside the horizon, *can* be felt outside ?<br /><br />Yes, yes, I can. It's basic general relativity. But the important part is that no changes inside the horizon can propagate outside. You can conclude as much already after noticing that there is no causal connection from inside Sergeihttps://www.blogger.com/profile/15415152744605672336noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-83480706075406013792020-05-16T16:05:02.966-04:002020-05-16T16:05:02.966-04:00Sabine wrote: “If you don't like the interior...Sabine wrote: “If you don't like the interior solution, you have to cope with geodesics that just end at finite proper time which makes absolutely no sense.”<br />This is not quite true. See my previous comment on the change of variables which makes the interior (including the singularity) disappear. The situation is sort of like in my other comment about the hyperbola x^2-y2=1. Imagine a Pascalhttps://www.blogger.com/profile/14201150679841329835noreply@blogger.com