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Black holes are already pretty weird but things get even weirder when you factor in the fact that they spin!
The maths implies that inside these black holes, causality breaks down, time travel is possible and you can even travel to different universes!
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![Black holes are already pretty weird but things get even weirder when you factor in the fact that they spin!The maths implies that inside these black holes, causality breaks down, time travel is possible and you can even travel to different universes![Thread]1/24](https://pbs.twimg.com/media/EgHcYctXkAUS037.png "Black holes are already pretty weird but things get even weirder when you factor in the fact that they spin!The maths implies that inside these black holes, causality breaks down, time travel is possible and you can even travel to different universes![Thread]1/24")
The maths implies that inside these black holes, causality breaks down, time travel is possible and you can even travel to different universes!
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Before diving head first into a spinning black hole, it might be best to read through my previous thread on black holes where I explain some of the basics
https://twitter.com/PhilipLynchGW/status/1259556379985092608
But if you're ready for something a bit more advanced then let's go!!
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https://twitter.com/PhilipLynchGW/status/1259556379985092608
But if you're ready for something a bit more advanced then let's go!!
2/24
It seems like everything in space is spinning, from planets to stars to galaxies! So why not black holes?
If a star is spinning when it dies, conservation of angular momentum tells us that the resulting black hole should not just spin, but spin incredibly quickly!
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If a star is spinning when it dies, conservation of angular momentum tells us that the resulting black hole should not just spin, but spin incredibly quickly!
3/24
Since I mentioned that black holes are almost entirely empty space, it's natural to ask what is actually doing the spinning?
It turns out it's the very fabric of spacetime itself!
Think something like this (but in 4d spacetime instead of 2d)
Image: @ICRAR
4/24
It turns out it's the very fabric of spacetime itself!
Think something like this (but in 4d spacetime instead of 2d)
Image: @ICRAR
4/24
This is described mathematically by the Kerr Solution to the Einstein Field Equations.
The Spacetime has two parameters:
1) The mass of the black hole "M",
2) Angular Momentum per unit mass "a", which goes from 0 (no spin) to 1 (maximum spin)
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The Spacetime has two parameters:
1) The mass of the black hole "M",
2) Angular Momentum per unit mass "a", which goes from 0 (no spin) to 1 (maximum spin)
5/24
A non-spinning (or Schwarzschild) black hole has a very simple anatomy
We've got a point-like singularity in the middle surrounded by a spherical surface called the event horizon.
Anything that passes through the event horizon can never escape!
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We've got a point-like singularity in the middle surrounded by a spherical surface called the event horizon.
Anything that passes through the event horizon can never escape!
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However, spin makes everything a lot more complicated
(My research is teaching me this the hard way)
The best way to learn about these different regions of a black hole is to jump into them!
Image Credit: By Yukterez (Simon Tyran, Vienna)
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(My research is teaching me this the hard way)
The best way to learn about these different regions of a black hole is to jump into them!
Image Credit: By Yukterez (Simon Tyran, Vienna)
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(Note: We're going to assume that this is a supermassive black hole so that you don't get spaghettified by the extreme gravity!)
Image: @BlackHoleCam
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Image: @BlackHoleCam
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As you free fall towards the black hole, you'll also be dragged around with the rotating spacetime
Eventfully you'll get to a place, where it will be impossible to stop yourself from being dragged along without exceeding the speed of light.
This is called the Ergosphere!
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Eventfully you'll get to a place, where it will be impossible to stop yourself from being dragged along without exceeding the speed of light.
This is called the Ergosphere!
9/24
Unlike the event horizon, it's still possible to escape form here
Depending on how you do it, you can even steal energy from the black hole as you do so!
(Will cover this in a future thread)
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Depending on how you do it, you can even steal energy from the black hole as you do so!
(Will cover this in a future thread)
10/24
Next we approach the outer event horizon
Much like in the non-spinning case, when you pass through you can never escape
The mathematical reason why is quite interesting...
11/24
Much like in the non-spinning case, when you pass through you can never escape
The mathematical reason why is quite interesting...
11/24
The coordinate describing your radial distance from the black hole (r) changes sign, so it starts behaving like a time coordinate
In essence, moving towards centre of the black hole becomes as inescapable as moving forward in time!
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In essence, moving towards centre of the black hole becomes as inescapable as moving forward in time!
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For a non-spinning black hole, this is where are trip leads to it's inevitable conclusion: plunging endlessly towards the singularity
But things are more interesting for a spinning black hole because there is is a SECOND horizon inside the first!
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But things are more interesting for a spinning black hole because there is is a SECOND horizon inside the first!
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This is where things start to get really weird
If you pass through this surface, the sign on the radial coordinate flips back and you are now (somewhat) free to move around again
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If you pass through this surface, the sign on the radial coordinate flips back and you are now (somewhat) free to move around again
14/24
Inside the inner horizon there is the singularity, which is no longer a single point
Instead we have a ring along which the curvature of spacetime is infinite!
So it's sometimes called a "ringularity"
Try not to get too close, as tempting as it might be
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Instead we have a ring along which the curvature of spacetime is infinite!
So it's sometimes called a "ringularity"
Try not to get too close, as tempting as it might be
15/24
Being near a singularity like this is bad news for our concept of causality, which leads to some really crazy stuff
(Although I'm still pretty sure there aren't any bookshelves)
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(Although I'm still pretty sure there aren't any bookshelves)
16/24
We could theoretically pass through the ringularity in such a way that we form a "closed timelike curve", meaning we'd end up meeting ourselves in the past!
Protip: If you try this, make sure not to kill your own grandfather
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Protip: If you try this, make sure not to kill your own grandfather
17/24
Since we are now free to move around we could pass back out of the inner horizon
The radial coordinate would flip it's sign again and we'd be compelled to continue moving AWAY from the center and emerge outside of the outer horizon!!
We've just escaped a black hole?!
18/24
The radial coordinate would flip it's sign again and we'd be compelled to continue moving AWAY from the center and emerge outside of the outer horizon!!
We've just escaped a black hole?!
18/24
But where have we escaped to?
It turns out that we'd end up in an entirely different region spacetime than the one we've started with!
Is this a new universe? Or just a different region of our original universe?
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It turns out that we'd end up in an entirely different region spacetime than the one we've started with!
Is this a new universe? Or just a different region of our original universe?
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We'd have no way of knowing
In principle, we could then find another black hole in this new region and repeat this process over and over again
Could spinning black holes really be gateways to new universes and distant regions of spacetime?
20/24
In principle, we could then find another black hole in this new region and repeat this process over and over again
Could spinning black holes really be gateways to new universes and distant regions of spacetime?
20/24
The answer is probably not :(
Everything I've just said is entirely consistent with the mathematics of the Kerr solution
But the Kerr solution assumes that the spinning black hole is eternal and unchanging, and we know that's not the case for real black holes
21/24
Everything I've just said is entirely consistent with the mathematics of the Kerr solution
But the Kerr solution assumes that the spinning black hole is eternal and unchanging, and we know that's not the case for real black holes
21/24
Stuff is constantly falling into black holes
Even if they aren't surrounded by swirling disks of gas (which a lot of them are), they are constantly absorbing radiation from distant stars, supernovae and the cosmic microwave background
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Even if they aren't surrounded by swirling disks of gas (which a lot of them are), they are constantly absorbing radiation from distant stars, supernovae and the cosmic microwave background
22/24
This radiation would fall through the outer event horizon and accumulate on the inner horizon, causing the inner horizon to act like another singularity!
The geometry of spacetime inside the black hole would be radically different from what the Kerr solution predicts
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The geometry of spacetime inside the black hole would be radically different from what the Kerr solution predicts
23/24
But outside the outer horizon we still predict that the Kerr solution would still be a very close approximation to the spacetime of an astrophysical black hole
This prediction is something that we want to test with Extreme Mass Ratio Inspirals! 24/24 https://twitter.com/PhilipLynchGW/status/1216383143776858113
This prediction is something that we want to test with Extreme Mass Ratio Inspirals! 24/24 https://twitter.com/PhilipLynchGW/status/1216383143776858113
I hope you enjoyed this trip inside a spinning black hole!
Feel free to ask questions and let me know if there's any other threads you'd like to see! [End thread]
Feel free to ask questions and let me know if there's any other threads you'd like to see! [End thread]
