Since it cannot radiate light and therefore neither heat, if any reactions which generate heat were to occur inside a Black Hole, that would seem to imply an infinitely high ever increasing temperature, or perhaps none whatsoever, that the intense radiation from the accretion discs and plasma jets is a necessary transformation of mass into a form which can cross the event horizon...
I have never seen any reference to the temperature of a singularity, or if such a thing could have one, is there any theory of what it might be?

  • $\begingroup$ Black holes do radiate, see Hawking radiation. And the more matter they absorb, the colder they get. $\endgroup$ – AccidentalBismuthTransform Apr 30 '19 at 5:39

One can arrange a heat-producing reaction to take place inside the event horizon of a black hole. For example, I can drop two cold blocks of matter on trajectories so that they collide inside the horizon, producing heat. There is nothing special about the spacetime inside the horizon in this respect except that the heat from the collision will not be seen by outside observers due to the horizon. What is unusual about this region is that in a short time (as experienced by the objects) they - and the heat emissions - will meet the singularity and at this point we have no theory describing what happens.

Since the topology of the region is such that the singularity is more like a point in time than a place in space there is also no lingering heat in the interior space nor any sense of temperature of the singularity.

Event horizons do not care if things crossing them are energy or matter. The reasons for the accretion disks and jets are different: non-black hole object like stars being formed and neutron stars also have disks and jets. Basically disks happen because matter is interacting and slowly shedding angular momentum and potential energy through turbulent interactions, and jets happen because the resulting plasma produces strong magnetic fields and blocks radiation in the equatorial direction.

  • $\begingroup$ I generally agree with your answer. Only one point IMO requires a better explanation. Event horizons do not care if things crossing them are energy or matter. How do you differentiate between matter and energy? $\endgroup$ – Elio Fabri Apr 30 '19 at 9:31
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    $\begingroup$ @ElioFabri - In GR there is no real difference, since they are all handled by the stress-energy tensor $T_{ij}$ anyway. Maybe I should try to add something about how both timelike and null geodesics pass through the horizon, but I fear it might get rather technical. $\endgroup$ – Anders Sandberg Apr 30 '19 at 11:03
  • $\begingroup$ From your reply I think that by "matter" you meant massive particles, by "energy" massless ones. Don't you? It's a terminology not uncommon among physicists that I don't particularly like because I find it confusing. I read "energy" as a physical quantity, measurable in joules or other unit. "Matter" sounds to me as a more philosophical term to be used with much care in physics, especially when talking to unskilled people. $\endgroup$ – Elio Fabri May 1 '19 at 7:47

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