Thought Experiment- Galaxy Falls Into Black Hole

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amigocabal
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Thought Experiment- Galaxy Falls Into Black Hole

Post by amigocabal » 2018-12-14 01:07pm

Here is a thought experiment.

Imagine a galaxy the size and mass of the Milky Way freefalling into a black hole the size of the observable universe. Around this black hole is another galaxy orbiting at ten Schwarzchild radii.

What would the observers observe as the galaxy crosses the event horizon?

How long would it take from the time of crossing the event horizon for the galaxy to be torn apart by tidal forces?

What would observers observe as the tidal forces distort and ultimately tear apart the galaxy.

would they be able to tell that they are in fact inside a black hole?

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JI_Joe84
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Re: Thought Experiment- Galaxy Falls Into Black Hole

Post by JI_Joe84 » 2018-12-15 10:42am

Despite the gargantuan dimensions I believe the end results will be the same as the star's around Sagittarius A (which we are watching now for several years) object's falling into the Sagittarius A (from our literal outside observer position) will speed up and stretch out only to fall apart at the nuclear level and I believe even the neutrons will be torn apart by tidal forces at some point.
What the galaxy sees on the "inside" we can only guess but by that point they should have been torn apart so nothing will be seen.
There is a lot on black holes on YouTube science doc's, if you are interested in a more professional opinion.

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Re: Thought Experiment- Galaxy Falls Into Black Hole

Post by Tribble » 2018-12-15 02:31pm

Well they could survive for quite some time after crossing an event horizon that large before being eventually ripped apart, right? IIRC the bigger a black hole is the more gradual the "spaghettification" procces?
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Re: Thought Experiment- Galaxy Falls Into Black Hole

Post by Eternal_Freedom » 2018-12-15 02:49pm

True. The really dangerous tidal forces for even a "normal" supermassive black hole like Sag A* in our galaxy don't kick in until you're way past the event horizon. so this galaxy falling into a universe-massed black hole would probably survive for ages.

IIRC, a black hole with the mass of the observable universe has an event horizon radius of 13.7 billion light years, so there could literally be a whole universe within that event horizon...which would explain a few things!
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Re: Thought Experiment- Galaxy Falls Into Black Hole

Post by amigocabal » 2018-12-15 03:28pm

Eternal_Freedom wrote:
2018-12-15 02:49pm
True. The really dangerous tidal forces for even a "normal" supermassive black hole like Sag A* in our galaxy don't kick in until you're way past the event horizon. so this galaxy falling into a universe-massed black hole would probably survive for ages.

IIRC, a black hole with the mass of the observable universe has an event horizon radius of 13.7 billion light years, so there could literally be a whole universe within that event horizon...which would explain a few things!
It would be tens of billions of years before the shape if the galaxy is distorted. Tbe distortion would manifest as stars along the radial path being red shifted (as thrir distance to the galactic center increases) and the stars perpendicular to the radial path bring blueshifted.

At some point, light from.the "near" side will not even reach the "far side", forming another type of event horizon, similar to the event horizon due to an expanding universe. Sometime later, light from the "far" side will not be able to reach the "near" side until it hits the singularity. (For the purposes of this thoughtexperiment, the universe-sized black hole has negligible angular momentum, so the inner horizon would be at most microscopic in size.) This means the galaxy woulld effectively be unbound.

Star systems would still be bound, as the tidal gradient is less due to the smaller distances. In general, when the tidal force between the near and far size if an object exceeds the forces holding it together (which is gravity for galaxies and star systems), the object will be torn apart.

Planets and moons will be torn apart just seconds before atoms are torn apart.

This scenario resembles the Big Rip scenario.


I

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Re: Thought Experiment- Galaxy Falls Into Black Hole

Post by JI_Joe84 » 2018-12-19 08:41pm

Tribble wrote:
2018-12-15 02:31pm
Well they could survive for quite some time after crossing an event horizon that large before being eventually ripped apart, right? IIRC the bigger a black hole is the more gradual the "spaghettification" procces?
No every thing crossing the event horizon should all ready be pulled apart. When we look at Sagittarius A we have all ready seen stars that came to close and were stretched out before being flung out, which was intriguing in its own way as a star was frickin EGG shaped for a while as it made its closest approach and apparently maintained fusion.
Another star was pulled apart, fusion stoped, and when the gas went in towards the event horizon the pressure + energy was enough to begin fusion again.
Super massives are truly Apocalypse Engine's.

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Re: Thought Experiment- Galaxy Falls Into Black Hole

Post by amigocabal » 2018-12-20 12:01am

JI_Joe84 wrote:
2018-12-19 08:41pm
Tribble wrote:
2018-12-15 02:31pm
Well they could survive for quite some time after crossing an event horizon that large before being eventually ripped apart, right? IIRC the bigger a black hole is the more gradual the "spaghettification" procces?
No every thing crossing the event horizon should all ready be pulled apart. When we look at Sagittarius A we have all ready seen stars that came to close and were stretched out before being flung out, which was intriguing in its own way as a star was frickin EGG shaped for a while as it made its closest approach and apparently maintained fusion.
Another star was pulled apart, fusion stoped, and when the gas went in towards the event horizon the pressure + energy was enough to begin fusion again.
Super massives are truly Apocalypse Engine's.
Whether or not something is ripped apart before crossing the event horizon depends on the mass of the black hole, the tensile forces keeping the object together, the the size of the object.

Stars are huge, and are only held together by its gravity, so they are torn apart before reaching the horizon of Sag A*. By contrast, an asteroid the size of Ceres would not be torn apart until crossing the event horizon.

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Re: Thought Experiment- Galaxy Falls Into Black Hole

Post by Tribble » 2018-12-20 07:05pm

amigocabal wrote: It would be tens of billions of years before the shape if the galaxy is distorted. Tbe distortion would manifest as stars along the radial path being red shifted (as thrir distance to the galactic center increases) and the stars perpendicular to the radial path bring blueshifted.

At some point, light from.the "near" side will not even reach the "far side", forming another type of event horizon, similar to the event horizon due to an expanding universe. Sometime later, light from the "far" side will not be able to reach the "near" side until it hits the singularity. (For the purposes of this thoughtexperiment, the universe-sized black hole has negligible angular momentum, so the inner horizon would be at most microscopic in size.) This means the galaxy woulld effectively be unbound.

Star systems would still be bound, as the tidal gradient is less due to the smaller distances. In general, when the tidal force between the near and far size if an object exceeds the forces holding it together (which is gravity for galaxies and star systems), the object will be torn apart.

Planets and moons will be torn apart just seconds before atoms are torn apart.

This scenario resembles the Big Rip scenario.


I
And what if this ridiculously sized black hole was spinning and/or charged? What happens to our poor galaxy after falling into it then?
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Re: Thought Experiment- Galaxy Falls Into Black Hole

Post by Knife » 2018-12-24 12:25pm

If I understand Hawking's stuff (and I don't really) those falling into the black hole would suffer from time slowing down and eventually stopping or coming really close to stopping.
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Re: Thought Experiment- Galaxy Falls Into Black Hole

Post by Tribble » 2018-12-24 12:46pm

Knife wrote:
2018-12-24 12:25pm
If I understand Hawking's stuff (and I don't really) those falling into the black hole would suffer from time slowing down and eventually stopping or coming really close to stopping.
I thought that was the outside observer's perspective - someone falling in to a blackhole appears to slow but never quite reaches the event horizon? From the perspective of the person falling in nothing really changes (well, until s/he gets ripped apart by tidal forces)?
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