# If our nearest Galaxy was swallowed up by a blackhole today, when would we find out. **URL:** \ **Category:** Factual Questions\ **Created:** [September 18, 2019, 7:50pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453 "2019-09-18T19:50:56Z")\ **Posts on this page:** 13\ **Page:** 2
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [September 19, 2019, 3:49pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/21 "2019-09-19T15:49:12Z")
> [@CookingWithGas](#): > > Wait. Wouldn’t we have been torn to smithereens by tidal forces long before we would reach the central singularity (at which point we would run into Matthew McConaughey floating around behind a bookcase)? The strength of the tidal forces depend on the size and mass of the black hole. A relatively small hole will have a huge disparity over even so small a length as six feet. A gigantic black hole the size of the universe might have a discrepancy between your head and foot so tiny that you’d never know the difference until it’s too late. ---
**Author:** ![Ike\_Witt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ike_witt/32/283_2.png) [@Ike\_Witt](https://boards.straightdope.com/u/Ike_Witt)\ **Post date:** [September 19, 2019, 5:31pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/22 "2019-09-19T17:31:11Z")
> [@Telemark](#): > > I’ve always looked good in red. > [@Ponderoid](#): > > Not enough makeup in the world to cover up that mistake. God damn it :smack: ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [September 19, 2019, 7:12pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/23 "2019-09-19T19:12:03Z")
> [@Nansbread1](#): > > So it is entirely possible that beyond our solar system, most of the rest of the Milky Way and all other billions of Galaxies in the Universe have all been destroyed, as I write this sentence, and have been destroyed for the entire history of man, and we don’t know about it and wont find out for another 25 millenia minimum. > > How do scientists and physicists grapple with this concept. They ignore it. > [@D18](#): > > IANAP, but my understanding is they revel in this concept, because it means looking at objects far away is the same as looking at objects back in time. “Revel” is not the word I would use, but whatever. At any rate, this is mainly useful for things at cosmological distances, i.e. more than about half the age of the universe. We don’t need the effect to study, for example, the lives of stars. Stars are continually being made, so all they have to do is study a variety of stars and figure out which are younger and which older and how they change over their lifetimes. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 19, 2019, 8:15pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/24 "2019-09-19T20:15:22Z")
Event horizons _do_ expand at c, if the mass is present. In an “ordinary” black hole formation, you’ve got a star collapsing until, at some point, a black hole starts forming in the middle, expanding outwards from 0 radius to the radius corresponding to the whole mass, at a rate of c. As for forming without anyone knowing, imagine a spherical shell, a very great distance away, consisting of a bunch of ludicrously powerful lasers. All of the lasers aim straight inwards at us, and all fire a brief but powerful pulse simultaneously. If the pulses are powerful enough, and precisely-enough aimed, then they will form a black hole when they reach some distance from us. At that point, an event horizon will expand outwards from the point where they will eventually converge, and will cross over various observers who will have no indication whatsoever that anything unusual has happened, until the lasers actually reach them. And for the poor souls right at the center, that moment is exactly the same as the moment when they reach the central singularity. Key to this is that an event horizon is not a local phenomenon. If you can send something (even just a photon) from your location to a point an infinite distance away, then you’re not inside an event horizon. If you’ve already sent something, and it’s now a long (but still finite) distance away, then you don’t know yet if, at the moment you sent it, you were inside a horizon, because it isn’t yet infinitely far off. And of course, you know even less about the times after when you sent it. It takes an infinite amount of time to confirm that any given point is not inside a horizon. ---
**Author:** ![ASL\_v2.0](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/asl_v2.0/32/3525_2.png) [@ASL\_v2.0](https://boards.straightdope.com/u/ASL_v2.0)\ **Post date:** [September 19, 2019, 9:05pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/25 "2019-09-19T21:05:26Z")
> [@Chronos](#): > > Event horizons _do_ expand at c, if the mass is present. In an “ordinary” black hole formation, you’ve got a star collapsing until, at some point, a black hole starts forming in the middle, expanding outwards from 0 radius to the radius corresponding to the whole mass, at a rate of c. Can you explain how this could be true in all but the most mundane sense that the effect of a change in mass will be transmitted at c? Setting aside the very brief period of time during which a star collapses and a black hole is formed, is there ever a time when the event horizon could grow, relative to the singularity, from a radius r to a radius 2r in a time t where r/t=\>c? ---
**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\ **Post date:** [September 20, 2019, 12:46am UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/26 "2019-09-20T00:46:01Z")
> [@Ponderoid](#): > > Not enough makeup in the world to cover up that mistake. I thought **Ike** meant that Earth scored a single point. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [September 20, 2019, 1:54pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/27 "2019-09-20T13:54:26Z")
I’d say based on our current knowledge, it’s probably impossible for a black hole to be literally the size of a galaxy. The radius of the event horizon is directly proportional to its mass. If the mass of an entire galaxy is turned into a black hole, its radius will still only be [0.25 light-year](https://en.wikipedia.org/wiki/Schwarzschild_radius#Parameters). For it to be the size of a galaxy, it would have to swallow 200,000 galaxies. But because black holes start out small, it’s actually very difficult for stuff to fall into it. Even when something falls _towards_ it, if it doesn’t directly hit the event horizon, it will just skim past and be flung far away, or merge with the accretion disk. ---
**Author:** ![RickJay](https://avatars.discourse-cdn.com/v4/letter/r/bb73d2/32.png) [@RickJay](https://boards.straightdope.com/u/RickJay)\ **Post date:** [September 20, 2019, 2:43pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/28 "2019-09-20T14:43:36Z")
> [@Nansbread1](#): > > So it is entirely possible that beyond our solar system, most of the rest of the Milky Way and all other billions of Galaxies in the Universe have all been destroyed, as I write this sentence, and have been destroyed for the entire history of man, and we don’t know about it and wont find out for another 25 millenia minimum. > > How do scientists and physicists grapple with this concept. But they haven’t. You can look up and see them. The idea that “maybe they were destroyed yesterday” is irrelevant. The speed of information is the speed of light. We are here on Earth, not in the Andromeda Galaxy, and the reality of Andromeda’s existence is always subject to the speed of light. If Andromeda existed at a point in the past that, because of the speed of light, makes it appear to exist to us, then it exists. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [September 20, 2019, 7:38pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/29 "2019-09-20T19:38:32Z")
I thought one of Hawking’s last theories was that black holes “leak”. Thus there are no quantum (mini) black holes left over from the big bang because they would be small enough to leak enough matter to be not-black not-holes a long time ago. Of course a super-nova sized black hole will not leak enough mass to make a difference… But as mentioned, a black hole swallowing a galaxy (a questionable feat) would be obvious, as parts of the galaxy would be disappearing very slowly. Even if we grant that in the denser areas stars are only a few light-weeks apart, we would see and advancing tide of stars snuffing out (IIRC flashing into nothing as they pass the event horizon). Even giving the black hole the unlikely ability to expand at c (it won’t) you’d see a progressive snuffing out of visible chunks of the remote galaxy over decades and centuries. Let’s pick on the Lesser Magellanic Cloud - 7,000ly across, 200,000 ly from us. It appears about 4.2 degrees across. So absolute top end case, it’s going to disappear over 7,000 years. And then, it will be 200,000 years before that world-killing whatever reaches us. But evidence of the world-killer, an expanding whatever, as it approaches the LMC would be evident too - if it is a black hole, we’d see flashes of this, that and whatever dropping in, plus gravity lensing effects around the perimeter. So if the LMC is in the throes of destruction, not to be seen for 200 millennia, we’d still be able to see evidence of the black hole sneaking up on it. But then, as already pointed out, the amount of mass needed to create a black hole like that is simply too much - I think if the universe had that much mass it would be collapsing anyway. Larry Niven in his _…of Worlds_ series, suggests the problem is not that - it’s that the central black hole in our galaxy is as it expands emitting a blast of lethal radiation and this is slowly sterilizing the galaxy as it spreads out. We may have about 25,000 years to go. (Much like how a supernova blast will sterilize worlds for light-years around.) ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [September 21, 2019, 3:57am UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/30 "2019-09-21T03:57:30Z")
> [@ASL\_v2.0](#): > > Can you explain how this could be true in all but the most mundane sense that the effect of a change in mass will be transmitted at c? Setting aside the very brief period of time during which a star collapses and a black hole is formed, is there ever a time when the event horizon could grow, relative to the singularity, from a radius r to a radius 2r in a time t where r/t=\>c? Locally the event horizon of BH always expands outwards at c, this is a result of the technical definition of a black hole event horizon. On the other hand when the black hole has ‘settled’ (ignoring any further accretion and Hawking radiation) the event horizon has a fixed radius in static/stationary coordinates, so it seems fair to ask how quickly its radius goes from zero to its settled radius. Unfortunately static/stationary coordinates can’t answer how quickly the event horizon forms as static/stationary coordinates map the event horizon to the infinite (null) future and technically during BH formation there aren’t any static/staionary coordinates. You could answer ty and measure how quickly the event horizon forms by using in-falling observers and I think the most sensible answer you would get, using this technique, is that the horizon intially expands out at c, but the expansion smoothly decelerates to effectively zero in a fairly short space of time. ---
**Author:** ![kayaker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kayaker/32/441_2.png) [@kayaker](https://boards.straightdope.com/u/kayaker)\ **Post date:** [September 21, 2019, 11:59am UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/31 "2019-09-21T11:59:41Z")
Scenarios like these are why I carry insurance against catastrophic loss. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [September 21, 2019, 12:23pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/32 "2019-09-21T12:23:18Z")
I think the main point is that unless the giant black hole is moving and doing a pac-man on the galaxy - a stationary black hole having swallowed a star will not result in the event horizon expanding to reach the next star; not even close. It may draw the nearest stars toward it, but that would be a very very slow process given the distances, and there’s a good chance that random stellar motions mean the next star would simply zip past in a hyperbolic orbit rather than getting swallowed. (Much as interstellar comets do with our sun) ---
**Author:** ![running\_coach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/running_coach/32/15836_2.png) [@running\_coach](https://boards.straightdope.com/u/running_coach)\ **Post date:** [September 21, 2019, 12:33pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/33 "2019-09-21T12:33:32Z")
> [@kayaker](#): > > Scenarios like these are why I carry insurance against catastrophic loss. [Lehrer] Lloyds of London will be _loaded_ when they go![/Lehrer] [Previous page](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453.md?page=1)