<p>An accretion disc is stuff orbiting around the black hole as it is falling into it.  Or it would be possible that the particles are in some sort of orbit around the black hole.  This disc would be outside of the event horizon so it isn't really "in" the black hole.</p>

<p>The jets are formed in a similar fashion.  Particles spinning into the black hole generate radiation which gets appears to be "emitted" by the black hole.</p>

<p>Things only appear to be coming out of the black hole but they are really happening outside of the event horizon.  The disc and jets are formed by the interaction of particles as they are moving around the black hole and entering the event horizon.</p>

CC BY-SA 3.0
2013-10-30T17:55:49.613Z
2013-10-30T17:55:49.613Z
Schleis
<p>When it comes to accretion disks, nothing is coming <em>out</em> of the black hole. That's just orbiting matter, though it is swirled around a bit by frame dragging. Even at high gravity, the ability to orbit around a massive body still exists. The gravitational force is already being "used up" to cause the orbiting (it accounts for the centripetal force), so there is no need for the gas to fall in.</p>

<p>As for jets, as far as I can tell there is no single explanation (I am not sure of this). One candidate explanation is the <a href="http://en.wikipedia.org/wiki/Blandford%E2%80%93Znajek_process" rel="nofollow noreferrer">Blandford-Znajek process</a><sup>1</sup></p>

<p>The following image is from <a href="http://rads.stackoverflow.com/amzn/click/0393312763" rel="nofollow noreferrer"><em>Black Holes and Time Warps: Einstein's Outrageous Legacy</em>, by Kip S. Thorne</a>:</p>

<p><img src="https://i.sstatic.net/jwVJ8.png" alt="enter image description here"></p>

<p>Basically, most black holes rotate, and sometimes the intense rotation can cause forces that overcome gravity, even by a few orders of magnitude.</p>

<p>When a black hole spins, magnetic field lines anchored to it<sup>2</sup> spin along with it. Plasma (from the accretion disk) is then flung out along these lines, similar to what happens when you put a marble in a conical cup and rotate it. This is depicted in the first image.</p>

<p>In the second image, current passes through the field lines (I do not understand this one as well as the first, however <a href="https://physics.stackexchange.com/questions/20871/blandford-znajek-process-why-how-does-the-current-flow-along-the-magnetic-field">this post has a reasonable explanation</a>), accelerates plasma with a mechanism similar to an <a href="http://en.wikipedia.org/wiki/Railgun" rel="nofollow noreferrer">electromagnetic railgun</a>. This is another way of creating jets.</p>

<p>Note that the energy here comes from the rotational energy of the BH, <em>not</em> the mass-energy of the "contents" of the BH (which is lost to the universe unless we consider Hawking radiation)</p>

<p>(I shall have a closer look at the paper when I have time and update the answer accordingly. Comments appreciated)</p>

<p><sup>1. <a href="http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?db_key=AST&amp;bibcode=1977MNRAS.179..433B&amp;letter=.&amp;classic=YES&amp;defaultprint=YES&amp;whole_paper=YES&amp;page=433&amp;epage=433&amp;send=Send%20PDF&amp;filetype=.pdf" rel="nofollow noreferrer">Blandford, R. D., &amp; Znajek, R. L. (1977). Electromagnetic extraction of energy from Kerr black holes. <em>Monthly Notices of the Royal Astronomical Society, 179, 433-456.</em></a></sup></p>

<p><sup>2. While the no-hair theorem forbids a naked black hole from posessing magnetic field lines, one with an accretion disk may have them as the field lines cannot "escape" through the disk.</sup></p>

CC BY-SA 4.0
2019-12-02T14:42:50.687Z
It is maybe worth mentioning that jets occur in different objects and on different scales: protostars, neutron stars, black holes, active galactic nuclei, etc. Thus it seems to be a universal phenomenon.
Hartmut Braun
CC BY-SA 3.0
2015-05-31T03:37:54.923Z
Hawking radiation originates outside the Schwarzschild radius, so nothing is leaving the BH; it is being annihilated by antiparticles.
LDC3
CC BY-SA 3.0
2013-10-29T08:23:29.660Z
2013-11-15T08:49:20.690Z
2017-04-13T12:40:15.377Z
Manishearth
<p>If supermassive black holes possess the gravity required to prevent the escape of photons and other mass particles, why do jets and accretion disks form? </p>

<p>They seem to indicate that something is coming <em>out</em> of the black hole, while mathematics and physics seem to tell us that this is impossible.</p>

<p>It would seem that supermassive black holes only exert gravity towards themselves. </p>

CC BY-SA 3.0
2013-10-29T08:04:40.133Z
I have very little understanding of black holes, but maybe the accretion disk is another effect produced by the conservation of angular momentum? Just like Protoplanetary disks, galaxies and so on.
Arne
<black-hole><supermassive-black-hole><astronomical-jet>
Why do black holes have jets and accretion disks?
Naftuli Kay
# Why do black holes have jets and accretion disks?

(Asked by: username_0 on 2013-10-29)

If supermassive black holes possess the gravity required to prevent the escape of photons and other mass particles, why do jets and accretion disks form?

They seem to indicate that something is coming *out* of the black hole, while mathematics and physics seem to tell us that this is impossible.

It would seem that supermassive black holes only exert gravity towards themselves.

**Tags:** black-hole, supermassive-black-hole, astronomical-jet
> **Comments:**
> **username_1**: I have very little understanding of black holes, but maybe the accretion disk is another effect produced by the conservation of angular momentum? Just like Protoplanetary disks, galaxies and so on.  

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## 2 answers

### Answer by username_3 on 2013-10-29. Score: 14 | Accepted answer

When it comes to accretion disks, nothing is coming *out* of the black hole. That's just orbiting matter, though it is swirled around a bit by frame dragging. Even at high gravity, the ability to orbit around a massive body still exists. The gravitational force is already being "used up" to cause the orbiting (it accounts for the centripetal force), so there is no need for the gas to fall in.

As for jets, as far as I can tell there is no single explanation (I am not sure of this). One candidate explanation is the [Blandford-Znajek process](http://en.wikipedia.org/wiki/Blandford%E2%80%93Znajek_process)1

The following image is from [*Black Holes and Time Warps: Einstein's Outrageous Legacy*, by Kip S. Thorne](http://rads.stackoverflow.com/amzn/click/0393312763):

![enter image description here](https://i.sstatic.net/jwVJ8.png)

Basically, most black holes rotate, and sometimes the intense rotation can cause forces that overcome gravity, even by a few orders of magnitude.

When a black hole spins, magnetic field lines anchored to it2 spin along with it. Plasma (from the accretion disk) is then flung out along these lines, similar to what happens when you put a marble in a conical cup and rotate it. This is depicted in the first image.

In the second image, current passes through the field lines (I do not understand this one as well as the first, however [this post has a reasonable explanation](https://physics.stackexchange.com/questions/20871/blandford-znajek-process-why-how-does-the-current-flow-along-the-magnetic-field)), accelerates plasma with a mechanism similar to an [electromagnetic railgun](http://en.wikipedia.org/wiki/Railgun). This is another way of creating jets.

Note that the energy here comes from the rotational energy of the BH, *not* the mass-energy of the "contents" of the BH (which is lost to the universe unless we consider Hawking radiation)

(I shall have a closer look at the paper when I have time and update the answer accordingly. Comments appreciated)

1. [Blandford, R. D., & Znajek, R. L. (1977). Electromagnetic extraction of energy from Kerr black holes. *Monthly Notices of the Royal Astronomical Society, 179, 433-456.*](http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?db_key=AST&bibcode=1977MNRAS.179..433B&letter=.&classic=YES&defaultprint=YES&whole_paper=YES&page=433&epage=433&send=Send%20PDF&filetype=.pdf)

2. While the no-hair theorem forbids a naked black hole from posessing magnetic field lines, one with an accretion disk may have them as the field lines cannot "escape" through the disk.

> **Comments:**
> **username_5**: Hawking radiation originates outside the Schwarzschild radius, so nothing is leaving the BH; it is being annihilated by antiparticles.  
> **username_4**: It is maybe worth mentioning that jets occur in different objects and on different scales: protostars, neutron stars, black holes, active galactic nuclei, etc. Thus it seems to be a universal phenomenon.  

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### Answer by username_2 on 2013-10-30. Score: 3

An accretion disc is stuff orbiting around the black hole as it is falling into it. Or it would be possible that the particles are in some sort of orbit around the black hole. This disc would be outside of the event horizon so it isn't really "in" the black hole.

The jets are formed in a similar fashion. Particles spinning into the black hole generate radiation which gets appears to be "emitted" by the black hole.

Things only appear to be coming out of the black hole but they are really happening outside of the event horizon. The disc and jets are formed by the interaction of particles as they are moving around the black hole and entering the event horizon.


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