Timeline for Can black holes form in a finite amount of time?
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30 events
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| May 23, 2022 at 1:52 | comment | added | sf_jeff | "Why do most people (physicists?) associate events happening with "seeing" them happen: "If we cannot see it, it didn't happen or happens in the future". ... Am I missing something?" Yes. You are missing limiting behavior. So, when mass is falling into the black hole you can see spectrum shifting. This is also associated with time dilation. This means that if some mass has spent 10,000 years falling in, it spent a much larger amount of time in the observer's clock. Nothing to do with horizon effects! | |
| Sep 1, 2021 at 16:04 | comment | added | Damir Tenishev | If time at the Event Horizon for the external observers "slows down" to zero (distance between events became huge), how one should see the interaction of such "frozen in time" object with the neighbors? Let's say, the Black Hole formed somewhere, but it moves through space and between other stars. If it "hold" at the "moment of emergency", how will it interact with other stars for the observer if it is frozen at creation place and at the same time already near another star? | |
| Sep 1, 2021 at 16:02 | comment | added | Damir Tenishev | Well, let's forget about what is in the Black Hole "now" and "seeing". Let's focus only on one important (for observer) question: is the Black Hole "observable" by the outside observer? Am I got this correctly that an external observer will see the Black Hole "always forming" and never gets formed? So, from observational point of view, there couldn't be (observed even by side effects) any Black Holes? | |
| Dec 9, 2020 at 18:02 | comment | added | Edouard | Dust collapse differs from the stellar collapse discussed by the OP, as it's on a much larger scale: I believe that only one instance of it (in Sagitarrius A) has been observed, and it's been discussed on the Astronomy Stack Exchange. | |
| Oct 16, 2019 at 11:15 | review | Suggested edits | |||
| Oct 16, 2019 at 14:29 | |||||
| S Oct 16, 2019 at 11:00 | comment | added | Jean-Paul Dozier | (cont.) Another example, if the distant observer or universe can only see Hawking radiation once the BH and its horizon really disappeared from the universe, it would be necessary to be able to express this disappearance according to a universal time, which does not exist not in GR. Ditto for the formation of the horizon. The fact that there is no universal time in GR to describe the history of the universe and history in the universe is not the solution but the problem behind all these questions. | |
| S Oct 16, 2019 at 11:00 | comment | added | Jean-Paul Dozier | @Ben Crowell, @ user1062760 It is an argument that (among others) shows a problem of coherence in GR. For example, it can not be said simultaneously that the horizon is an appearance due to the difference of the reference frames (and in fact, as on the sea, it retreats when one approaches it), but that it is also an objective reality which produces physical effects (locally, it can only be traversed in one direction, it produces Hawking radiation, etc.). (cont.) | |
| Jun 4, 2018 at 12:43 | vote | accept | Itai Bar-Natan | ||
| Mar 13, 2018 at 23:43 | comment | added | user1062760 | Seriously? Like really? You're considering the fact that one cannot see the black hole form with naked eye as an argument? | |
| Feb 5, 2018 at 16:06 | comment | added | user4552 | @Jus12: I agree with you that equating "seeing" with "being" would be a mistake here. I think the OP implicitly seems to have made that mistake. I don't think physicists generally make that mistake, although you may see it in popularizations or in writing by people who lack specialized expertise. | |
| Dec 4, 2017 at 9:53 | comment | added | Jus12 | Why do most people (physicists?) associate events happening with "seeing" them happen: "If we cannot see it, it didn't happen or happens in the future". Just because light from beyond the visible universe cannot reach us doesn't mean it lies in our future. Am I missing something? | |
| Jul 30, 2017 at 17:47 | comment | added | magma | Ben, what software did you use to draw the diagrams? | |
| Sep 17, 2015 at 17:53 | comment | added | Timaeus | @Anixx Including more than the outside is actually a bit silly for observers that stay outside since the inside can't affect things outside. | |
| Sep 17, 2015 at 17:50 | comment | added | Timaeus | @Anixx The region where E1 and E2 are and the whole region outside the event horizon is a fine region where every event has its own past included. Its a thing you can do science with. And by complementarily is a perfectly fine region. Sure it has timelike curves that stop after a finite proper time. But so does the bigger region that includes the inside. It isn't better that the curvature blows up for curves that go inside, all that matters is the curve stops after a finite proper time. The region outside has every event include its past so you can do science with it. | |
| Sep 17, 2015 at 16:47 | comment | added | Anixx | @Timaeus this diagram shows all these regions. From r=0 to the horizon and above. And time frame from before BH forms to its disappearance. | |
| Sep 17, 2015 at 15:50 | comment | added | Timaeus | @Anixx The diagram shows casually connected regions, it does not contain a drawing of the region from whence Hawking Radiation emerges (except that last burst). The region from where Hawking radiation emerges is always below the event horizon. Which by black hole complementarity might as well be the point r=0 (for the outside observers) though you have to fix a coordinate system before you can really say something like that. | |
| Sep 17, 2015 at 7:01 | comment | added | Anixx | @Timaeus light travels along the 45 degree line so according this diagram all Hawking radiation travels along the horizon tll the final burst. | |
| Sep 17, 2015 at 1:55 | comment | added | Timaeus | @Anixx No. It is a conformal diagram, that corner at the right end of the dotted line is simply the last burst of Hawking radiation. The black hole got smaller ever since the horizon formed (or since its temperature dropped below background) just like that infalling matter was always falling inwards even though the picture makes it look like it is going out. The diagram makes causal structure easy to see, other structure not so easy. So regardless of how big the event horizon gets or shrinks we draw it as a 45 degree line to indicate that the inside stays in, it doesn't mean the size is fixed. | |
| Sep 10, 2015 at 13:57 | comment | added | Anixx | According to the last picture E2 sees Hawking radiation all at one moment | |
| Mar 22, 2015 at 11:57 | comment | added | Emil | One question (as I'm still struggling a bit with Penrose diagrams): In figure 2, would there also always be a sensible "now slice" for which nothing has crossed the event horizon yet? | |
| Mar 22, 2015 at 11:17 | comment | added | Emil | What a great answer! | |
| Nov 17, 2014 at 0:08 | history | rollback | user4552 |
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| Nov 14, 2014 at 22:57 | history | answered | user4552 | CC BY-SA 3.0 |