tag:blogger.com,1999:blog-22973357.post1933418257730337867..comments2021-08-22T06:05:44.372-04:00Comments on Sabine Hossenfelder: Backreaction: If you fall into a black holeSabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comBlogger65125tag:blogger.com,1999:blog-22973357.post-77817381583811454572018-03-09T03:05:52.079-05:002018-03-09T03:05:52.079-05:00Thanks a lot; I'll need to brush up my Penrose...Thanks a lot; I'll need to brush up my Penrose-diagram reading skills a bit to fully understand the figure...<br />PS: I really like your blog, BTWMartinBhttps://www.blogger.com/profile/15439162869843258149noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-65301491692686317902018-03-09T02:57:12.142-05:002018-03-09T02:57:12.142-05:00If the black hole entirely evaporates, the outside...If the black hole entirely evaporates, the outside observer will eventually see you crossing (just before she sees you coming out again). The causal diagram is Figure 3 of <a href="https://arxiv.org/abs/0901.3156" rel="nofollow">this paper</a>.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-55355820687835055332018-03-09T01:56:56.377-05:002018-03-09T01:56:56.377-05:00Thanks, but I'm afraid I'm still puzzled: ...Thanks, but I'm afraid I'm still puzzled: What does the external observer see? She sees me freezing at the (apparent) event horizon, never crossing it (and taking infinitely long for that), whereas I actually do cross it, but this can never be seen from the outside? If I were to send out light signals while falling, what would their worldlines look like so that they never reach the MartinBhttps://www.blogger.com/profile/15439162869843258149noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-22718447050730681232018-03-09T00:40:10.037-05:002018-03-09T00:40:10.037-05:00If the black hole evaporates, it doesn't have ...If the black hole evaporates, it doesn't have an event horizon. It has an apparent horizon. And yes, you will actually cross it. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-90776067916548320682018-03-08T13:02:52.241-05:002018-03-08T13:02:52.241-05:00Hi,
I'm very late to this party, but perhaps y...Hi,<br />I'm very late to this party, but perhaps you'll still see my comment.<br />You write:<br />"It takes you a finite time to reach the horizon of a black hole. For an outside observer however, you seem to be moving slower and slower and will never quite reach the black hole, due to the (technically infinitely large) gravitational redshift. If you take into account that black MartinBhttps://www.blogger.com/profile/15439162869843258149noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-101288381662344642016-05-02T05:38:18.404-04:002016-05-02T05:38:18.404-04:00Shantanu,
My husband read the book and tells me i...Shantanu,<br /><br />My husband read the book and tells me it's very good. Alas, I will not have time to read it any time soon, sorry. (I'll have a review of Sean Carroll's new book at some point though.)Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-86865838794936957622016-05-02T05:28:59.348-04:002016-05-02T05:28:59.348-04:00Dear Bee,
thank you for making that clear to me. T...Dear Bee,<br />thank you for making that clear to me. Turns out that reading 'Black Holes and Time Warps' (twice!) did after all leave me with at least some comprehension. And following your blog of course, which is really remarkable and certainly one of the best out there.<br />-w.Herr Wehhttps://www.blogger.com/profile/12475580610867971789noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-86744060549975171442016-05-02T03:56:26.320-04:002016-05-02T03:56:26.320-04:00Sabine, Sorry for the OT comment. But once you get...Sabine, Sorry for the OT comment. But once you get a chance would be interested in your review of Janna Levin's latest book.<br />Thinking whether to order it or not.Shantanuhttps://www.blogger.com/profile/16322812456382858228noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-76001122891939712102016-05-01T01:56:17.689-04:002016-05-01T01:56:17.689-04:00Chris,
I'm a theorist, I don't bother wit...Chris,<br /><br />I'm a theorist, I don't bother with factors of order one. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-29610889386584161172016-04-30T23:10:37.341-04:002016-04-30T23:10:37.341-04:00Hi Bee,
about the "before crossing the horiz...Hi Bee,<br /><br />about the "before crossing the horizon of a solar-mass black hole". We don't really have any of those, of course, as you appear to need at least 2.5-3.5 solar masses to make a black hole. Otherwise, you become a neutron star (or worse). Unless, of course, you are willing to wait for a long time :-) Chris Adamihttps://www.blogger.com/profile/02447043823985095127noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-839388505608686002016-04-30T23:00:42.892-04:002016-04-30T23:00:42.892-04:00No one talks much about what the infalling observe...No one talks much about what the infalling observer sees outwardly. In that infalling signals or photons would be seen or detected by the infalling observer would there be the impression that things are speeding up outside the influence of the black hole? An example might be seeing the planets of a solar system moving in their orbits. That is if an observer or probe survived a while before being Mark Thomashttps://www.blogger.com/profile/03535026336436233116noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-10565616247069106802016-04-30T01:51:41.617-04:002016-04-30T01:51:41.617-04:00Wandering Wolf,
While it is difficult to actually...Wandering Wolf,<br /><br />While it is difficult to actually compute the time in general backgrounds, the question whether the infalling time is finite or not depends only on the causal structure and hence carries over to collapse scenarios with asymptotically flat regions, provided the collapse ends at some point. (Infinitely expanded collapsing matter distributions can do all kind of weird Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-82519552086771606352016-04-30T01:46:35.111-04:002016-04-30T01:46:35.111-04:00Maurice,
I have throughout this thread only refer...Maurice,<br /><br />I have throughout this thread only referred to observers, hence not to massless particles. If you are trying to say that not all timelike geodesics reach the horizon to begin with, that is true: Luckily it is indeed possible to forever stay outside the black hole. Needless to say, I was only referring to curves that cross the horizon to begin with. <br /><br /><br />Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-36462288070075093862016-04-29T12:54:24.803-04:002016-04-29T12:54:24.803-04:00In the case of Schwarzschild black holes, the situ...In the case of Schwarzschild black holes, the situation with coordinate time and proper time is simple and straightforward; however, I am wondering what happens in other types of spacetimes, for example Vaidya-Bonnet. Is the coordinate in-fall time here still infinite ? Are there ANY spacetimes where the coordinate time is finite and well defined ? After all, I wouldn't expect to actually The Wandering Wolfhttps://www.blogger.com/profile/15148538614677929109noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-52076224464328113462016-04-29T07:06:22.973-04:002016-04-29T07:06:22.973-04:00Hi Sabine,
thanks a _lot_ for taking the time to ...Hi Sabine,<br /><br />thanks a _lot_ for taking the time to find this link for me! I get it now. Great blog - and even greater to get such answers :-)<br />Matthiashttps://www.blogger.com/profile/11262429556378215828noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-23799340505412966932016-04-29T05:09:27.138-04:002016-04-29T05:09:27.138-04:00> If the geodesic doesn't end at point X, t...> If the geodesic doesn't end at point X, the proper time to reach point X is finite. Does that make more sense?<br />At least it makes logical sense. But where is "point X"? But I now realized that your claim:<br />>If you take into account that black holes evaporate, it doesnâ€™t quite take forever, and your friends will eventually see you vanishing.<br />is certainly wrong inMauricehttps://www.blogger.com/profile/16375058316648610565noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-28306050796003970232016-04-29T05:00:59.116-04:002016-04-29T05:00:59.116-04:00Hi again Matthias,
You might find this image usef...Hi again Matthias,<br /><br /><a href="http://bustard.phys.nd.edu/Phys171/lectures/blackhole.gif" rel="nofollow">You might find this image useful</a> from <a href="http://bustard.phys.nd.edu/Phys171/lectures/blackhole.html" rel="nofollow">this website</a>. You always move within the cones. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-4943994676298539812016-04-29T04:54:18.857-04:002016-04-29T04:54:18.857-04:00Matthias,
No, the light cannot increase the dista...Matthias,<br /><br />No, the light cannot increase the distance to the singularity. All the light rays are eventually bent around and end in the singularity. It's just that locally you wouldn't be able to tell. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-39910395512910315332016-04-29T03:59:26.145-04:002016-04-29T03:59:26.145-04:00Thanks for your answer! So if in my spaceship insi...Thanks for your answer! So if in my spaceship inside the event horizon light goes in all directions, it can (locally) increase its distance from the singularity?Matthiashttps://www.blogger.com/profile/11262429556378215828noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-6472543764024386142016-04-29T01:09:57.852-04:002016-04-29T01:09:57.852-04:00Herr Weh,
Yes, that's right, your friends wil...Herr Weh,<br /><br />Yes, that's right, your friends will not only see you but everything else that ever got close to the horizon. (Depending on the direction.) And, yes, "seeing" here doesn't actually mean they might see it with their own eyes, they would need to use very sensitive instruments. In practice, you will only "see" anything approaching the horizon up to a Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-7873117498762825182016-04-28T12:04:39.395-04:002016-04-28T12:04:39.395-04:00So I've thought about this for a couple days n...So I've thought about this for a couple days now and read the comments but this is still a puzzle to me: If my "friends will eventually see [me] vanishing" only upon evaporation of the black hole, wouldn't they see all other matter that ever crossed the horizon (from its own perspective) as well? Or does "see" in that context mean "we know it's there, frozen Herr Wehhttps://www.blogger.com/profile/12475580610867971789noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-63101384340636264812016-04-28T11:30:58.023-04:002016-04-28T11:30:58.023-04:00Matthias,
The redshift, or the relative slowing d...Matthias,<br /><br />The redshift, or the relative slowing down of time, is a global effect (ie over long distances), you don't notice it locally (over short distances). You wouldn't notice anything unusual until the tidal forces become large. The easiest way to see this is that it would violate the equivalence principle, which can't happen in GR by construction. Best,<br /><br />B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-61834738017327674752016-04-28T11:24:01.076-04:002016-04-28T11:24:01.076-04:00Hon Chong,
The particles in the Hawking radiation...Hon Chong,<br /><br />The particles in the Hawking radiation *are* entangled across the horizon. This is the origin of the black hole information loss problem. As you say, this doesn't allow one to submit information though. (<a href="http://arxiv.org/abs/1507.03592" rel="nofollow">Except for one bit</a>). Best,<br /><br />B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-36560058753682706572016-04-28T11:20:47.333-04:002016-04-28T11:20:47.333-04:00Michael,
The infalling observer sees thermal part...Michael,<br /><br />The infalling observer sees thermal particles. If you fall into a black hole you can still receive signals (or images) from the outside. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-55991303004054900862016-04-28T11:16:37.007-04:002016-04-28T11:16:37.007-04:00Ivan,
Yes, one shouldn't take the exact posit...Ivan,<br /><br />Yes, one shouldn't take the exact position of the horizon too seriously.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.com