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I understand it is possible to do it in a black hole, and its also possible to reach the event horizon of a black hole and increase its size.

But in a white hole, no wave (electromagnetic or gravitational) can reach the white hole's event horizon. So, if you apply the penrose process to a white hole, would you be affecting the speed of rotation near its event horizon?

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  • $\begingroup$ Anything is possible with white holes $\endgroup$ Jul 30 '19 at 18:35
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A white hole is the time-reverse of a black hole. So, a "white hole penrose process" necessarily is the time-reverse of the ordinary penrose process, which would be a particle absorbing a wave or particle coming from the event horizon, leaving the white hole "spun up".

If this sounds weird, it's because it is weird, in the same way that the advanced solutions of Maxwell's equations are weird.

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  • $\begingroup$ Could you explain further this case? $\endgroup$
    – Manuel
    Jul 31 '19 at 17:52
  • $\begingroup$ @Manuel: what's to explain? you literally just take the "forward" black hole solution, time-reverse the paths, pretend that it's your new "forward" time direction, and you have a valid path in a white hole spacetime. That's in fact what a white hole is. So, the white hole emits a particle that has negative mass relative to infinity, it picks up net spin, and the negative mass accelerates the nearby particle, exactly the opposite of the black hole penrose process. $\endgroup$ Jul 31 '19 at 18:47
  • $\begingroup$ @safesphere: the time reverse of a closed orbit is a closed orbit. A white hole horizon is absolutely repulsive, though. $\endgroup$ Aug 1 '19 at 13:40
  • $\begingroup$ @safesphere: you cannot radially fall into a white hole. You radially fall out of it. The white hole singularity lives in the past of all timelike and null orbits that intersect it, just as the black hole singularity lives in the future of all timelike and null orbits. A white hole is the time reverse of a black hole, full stop. That's what it is. it's an artifact of the fact that einstein's equations have time reversal symmetry, but an object falling into a black hole does not have a reverse process unless you "reverse the horizon" somehow. $\endgroup$ Aug 1 '19 at 17:17
  • $\begingroup$ @safesphere: look at the Kruskal diagram of the spacetime with both holes. There's no ambiguity or puzzle here. $\endgroup$ Aug 1 '19 at 17:51

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