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Old 09-18-2019, 02:50 PM
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If our nearest Galaxy was swallowed up by a blackhole today, when would we find out.


Using current instrumentation would it take 25000 years for us to realise that our nearest galaxy has disappeared.
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Old 09-18-2019, 02:55 PM
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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.

Last edited by rat avatar; 09-18-2019 at 02:56 PM.
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Old 09-18-2019, 02:55 PM
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Well, nothing can go faster than light, so, yeah, it would take at minimum 25K years.
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Old 09-18-2019, 03:06 PM
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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.
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Old 09-18-2019, 03:10 PM
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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!

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Old 09-18-2019, 03:18 PM
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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.
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Old 09-18-2019, 04:05 PM
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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.
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Old 09-18-2019, 04:39 PM
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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.

Last edited by scr4; 09-18-2019 at 04:39 PM.
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Old 09-18-2019, 04:47 PM
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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.
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Old 09-18-2019, 04:51 PM
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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.
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Old 09-18-2019, 04:57 PM
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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.
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Old 09-18-2019, 05:58 PM
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"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.

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Old 09-19-2019, 02:17 AM
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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
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Old 09-19-2019, 02:46 AM
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"The bullet's already been fired."

From a movie nobody saw.
I did it thirty-five minutes ago.
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Old 09-19-2019, 03:30 AM
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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.
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Old 09-19-2019, 04:38 AM
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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, wed have seen it in the process of getting swallowed up for as long as weve had the means to observe said galaxy?
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Old 09-19-2019, 08:35 AM
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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.
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Old 09-19-2019, 09:25 AM
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I'm hoping earth gets flung off into the void as a rouge planet.
I've always looked good in red.
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Old 09-19-2019, 09:26 AM
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Old 09-19-2019, 10:02 AM
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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."

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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)?
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Old 09-19-2019, 10:49 AM
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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.
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Old 09-19-2019, 12:31 PM
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Old 09-19-2019, 02:12 PM
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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.

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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.
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Old 09-19-2019, 03:15 PM
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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.
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Old 09-19-2019, 04:05 PM
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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?
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Old 09-19-2019, 07:46 PM
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Old 09-20-2019, 08:54 AM
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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. 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.
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Old 09-20-2019, 09:43 AM
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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.
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Old 09-20-2019, 02:38 PM
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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.)
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Old 09-20-2019, 10:57 PM
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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.
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Old 09-21-2019, 06:59 AM
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Scenarios like these are why I carry insurance against catastrophic loss.
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Old 09-21-2019, 07:23 AM
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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)
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Old 09-21-2019, 07:33 AM
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Scenarios like these are why I carry insurance against catastrophic loss.
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