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This image from an online Italian newspaper shows photographs of one of the most powerful phenomena in the cosmos.

black hole

Nothing, not even at the speed of light $c$, can escape a black hole one it has been caught. So how is it possible mathematically that a black hole, which "swallows" the stars and gas approaching its powerful accretion disk, can then eject some of the gas into two thin jets of plasma at speeds $V_{pl}$ close to the speed of light?

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    $\begingroup$ Proposed in 1977, not yet verified. But whatever the answer is, it is not $v_{pl}>c$: en.wikipedia.org/wiki/Blandford–Znajek_process $\endgroup$
    – JEB
    Commented Apr 7, 2020 at 15:04
  • $\begingroup$ @JEB Thank you for your comment. Mine was just a hypothesis to be tempted to say that $v_{\text{pl}}>c$. I have not understood your link. $\endgroup$
    – Sebastiano
    Commented Apr 7, 2020 at 15:09
  • $\begingroup$ I think the "dash" was some non-standard dash that wiki-P uses, that this little comment box refused to interpret. – vs -. $\endgroup$
    – JEB
    Commented Apr 7, 2020 at 15:12
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    $\begingroup$ BTW: My relativistic magneto hydrodynamics class was taught by Blanford in '88, and I didn't understand it then, and haven't made progress in the meantime. $\endgroup$
    – JEB
    Commented Apr 7, 2020 at 15:21
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    $\begingroup$ How does it differ from asking how jets originate from a black hole? $\endgroup$
    – ProfRob
    Commented Apr 7, 2020 at 15:34

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The black hole is defined by its event horizon. This is the point at which the escape velocity reaches $c$. But the accretion disc forms outside the event horizon, so stuff can still escape from it. It is this outer stuff that finds its way into the jets, super-accelerated beyond escape velocity by magnetic fields being dragged round the hole.

If the escape velocity at the point of acceleration outside the black hole is $V_s$ then $c > V_{pl} > V_s$.

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