# 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:** 20\ **Page:** 1
**Author:** ![Nansbread1](https://avatars.discourse-cdn.com/v4/letter/n/6a8cbe/32.png) [@Nansbread1](https://boards.straightdope.com/u/Nansbread1)\ **Post date:** [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/840453/1 "2019-09-18T19:50:56Z")
Using current instrumentation would it take 25000 years for us to realise that our nearest galaxy has disappeared. ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [September 18, 2019, 7:55pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/2 "2019-09-18T19:55:43Z")
Black holes are not vacuums, if the sun was replaced by a black hole with the same mass we wouldn’t find out for 8 minutes and 20 seconds, if the Canis Major Dwarf Galaxy was replaced by a singular black hole it would take 25,000 to notice as you mentioned. The _speed of light_ is really the _speed of causality_. No instrumentation we will ever have will change that. But until you are very very close orbiting a black hole and a similar star or galaxy or … is the same. ---
**Author:** ![Ashtura](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ashtura/32/2963_2.png) [@Ashtura](https://boards.straightdope.com/u/Ashtura)\ **Post date:** [September 18, 2019, 7:55pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/3 "2019-09-18T19:55:49Z")
Well, nothing can go faster than light, so, yeah, it would take at minimum 25K years. ---
**Author:** ![Nansbread1](https://avatars.discourse-cdn.com/v4/letter/n/6a8cbe/32.png) [@Nansbread1](https://boards.straightdope.com/u/Nansbread1)\ **Post date:** [September 18, 2019, 8:06pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/4 "2019-09-18T20:06:34Z")
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. ---
**Author:** ![Slash1972](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/slash1972/32/6461_2.png) [@Slash1972](https://boards.straightdope.com/u/Slash1972)\ **Post date:** [September 18, 2019, 8:10pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/5 "2019-09-18T20:10:50Z")
> [@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. What the hell? My day was going so well too! :(☹ ---
**Author:** ![D18](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@D18](https://boards.straightdope.com/u/D18)\ **Post date:** [September 18, 2019, 8:18pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/6 "2019-09-18T20:18:02Z")
> [@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. 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. ---
**Author:** ![Nansbread1](https://avatars.discourse-cdn.com/v4/letter/n/6a8cbe/32.png) [@Nansbread1](https://boards.straightdope.com/u/Nansbread1)\ **Post date:** [September 18, 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/840453/7 "2019-09-18T21:05:28Z")
So by my understanding if there was a destruction of the milky way galaxy then the destruction will be seen by us on Earth as such As a tiny Mars like speck in the sky. Then within 3 minutes it will be upon us. Using the Mars example as thats how long it takes for light to reach us from Mars. 3 minutes. Or will it cover the whole sky in an instant and we will not see the speck which grows bigger in the sky. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [September 18, 2019, 9:39pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/8 "2019-09-18T21:39:12Z")
It’s impossible for a black hole to swallow a whole galaxy “today.” A typical galaxy is over 100,000 light-years in diameter. Even if a hyper-massive (?) black hole suddenly appeared in the middle of a galaxy, so massive that all the stars within that galaxy immediately starts falling towards it **at the speed of light** (which is completely absurd), it would take 50,000 years for it all to get swallowed. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [September 18, 2019, 9:47pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/9 "2019-09-18T21:47:33Z")
> [@Nansbread1](#): > > So by my understanding if there was a destruction of the milky way galaxy then the destruction will be seen by us on Earth as such > As a tiny Mars like speck in the sky. > > Then within 3 minutes it will be upon us. > > Using the Mars example as thats how long it takes for light to reach us from Mars. 3 minutes. > > Or will it cover the whole sky in an instant and we will not see the speck which grows bigger in the sky. The explody part of a supernova does not last that long. But, afterwards, it does take a couple of weeks to reach maximum brightness. Also, it is individual stars that collapse and explode, not the entire galaxy. Galaxies like the Milky Way do contain supermassive black holes which, under certain conditions, can accrete matter and emit stupid amounts of energy in the form of active galactic nuclei, but, again, the black hole does not destroy the galaxy. Now, if you watch for a while you will see M31 get bigger, and bigger, and bigger… as it plows into the Milky Way; that will take a little longer than 3 minutes, though, and also it is perhaps not right to say the Milky Way will be destroyed, as much as merge into something new. ---
**Author:** ![FastDan1](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fastdan1/32/427_2.png) [@FastDan1](https://boards.straightdope.com/u/FastDan1)\ **Post date:** [September 18, 2019, 9:51pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/10 "2019-09-18T21:51:13Z")
> [@Nansbread1](#): > > Using the Mars example as thats how long it takes for light to reach us from Mars. 3 minutes. Nitpick - That’s when Mars is at it’s closest point to Earth. It varies from approx 54.6 million km to 401 million km. So, closer to 21 minutes when it’s at its furthest point. ---
**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 18, 2019, 9:57pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/11 "2019-09-18T21:57:56Z")
Earth is part of the Milky Way Galaxy. If the entire Galaxy is swallowed, then we are, too. That said, it is actually possible that we have been swallowed by a black hole, and just won’t realize it until we reach the central singularity and just abruptly end. It would take a cosmic conspiracy to set up the conditions for this, but we can’t actually rule it out. ---
**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 18, 2019, 10:58pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/12 "2019-09-18T22:58:40Z")
“The bullet’s already been fired.” — From a movie nobody saw. But that line’s supposed to help us make peace with the inevitability of death. Or something. ETA: But I just want to be clear on this. Black holes do not work instantaneously, or even at the speed of light. We would see the other galaxy (or the rest of our galaxy) getting swallowed up by the black hole long before the black hole reached us. There is, for example, a black hole at the center of our galaxy. We know this because we see how other objects at the center of our galaxy interact with it. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [September 19, 2019, 7:17am UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/13 "2019-09-19T07:17:16Z")
> [@Nansbread1](#): > > So by my understanding if there was a destruction of the milky way galaxy then the destruction will be seen by us on Earth as such > As a tiny Mars like speck in the sky. > > Then within 3 minutes it will be upon us. > > Using the Mars example as thats how long it takes for light to reach us from Mars. 3 minutes. > > Or will it cover the whole sky in an instant and we will not see the speck which grows bigger in the sky. I am confused by this; the apparent size of Mars and the distance to Mars have no specific relationship to the apparent size of something else, somewhere else ---
**Author:** ![Smapti](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/smapti/32/17938_2.png) [@Smapti](https://boards.straightdope.com/u/Smapti)\ **Post date:** [September 19, 2019, 7:46am UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/14 "2019-09-19T07:46:56Z")
> [@ASL\_v2.0](#): > > “The bullet’s already been fired.” > > — From a movie nobody saw. I did it thirty-five minutes ago. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [September 19, 2019, 8:30am UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/15 "2019-09-19T08:30:00Z")
> [@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. Or it happened 24999 years and 364 days ago and we find out about it tomorrow. ---
**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, 9:38am UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/16 "2019-09-19T09:38:37Z")
> [@Richard\_Pearse](#): > > Or it happened 24999 years and 364 days ago and we find out about it tomorrow. This thread is fun and all, but, kidding aside, do we all understand that while it is true light cannot escape the event horizon of a black hole because the escape velocity is greater than the speed of light, the event horizon itself does not expand at light speed, much less instantaneously, meaning that it would take a while for the nearest galaxy to be swallowed up by a black hole, meaning that if it were going to be swallowed up tomorrow, we’d have seen it in the process of getting swallowed up for as long as we’ve had the means to observe said galaxy? ---
**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, 1:35pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/17 "2019-09-19T13:35:11Z")
> [@DPRK](#): > > and also it is perhaps not right to say the Milky Way will be destroyed, as much as merge into something new. I’m hoping earth gets flung off into the void as a rouge planet. ---
**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\ **Post date:** [September 19, 2019, 2:25pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/18 "2019-09-19T14:25:09Z")
> [@Ike\_Witt](#): > > I’m hoping earth gets flung off into the void as a rouge planet. I’ve always looked good in red. ---
**Author:** ![Ponderoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ponderoid/32/19221_2.png) [@Ponderoid](https://boards.straightdope.com/u/Ponderoid)\ **Post date:** [September 19, 2019, 2:26pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/19 "2019-09-19T14:26:08Z")
Not enough makeup in the world to cover up that mistake. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [September 19, 2019, 3:02pm UTC](https://boards.straightdope.com/t/if-our-nearest-galaxy-was-swallowed-up-by-a-blackhole-today-when-would-we-find-out/840453/20 "2019-09-19T15:02:43Z")
I think the OP is more interested in the concept of how long it takes information to travel rather than the specifics of black holes. We have had other threads like, “If the sun suddenly disappeared, when would we find out?” I think the OP is using the black hole as a kind of _deus ex machina_. The upshot of it is, “Wow, it’s mind-bending that something could happen and it could take 25,000 years for us to know it.” > [@Chronos](#): > > That said, it is actually possible that we have been swallowed by a black hole, and just won’t realize it until we reach the central singularity and just abruptly end. It would take a cosmic conspiracy to set up the conditions for this, but we can’t actually rule it out. 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)? [Next 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=2)