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Comment by fguerraz | Hacker News Reader

With the current theories, I don't think it matters (pun intended) if a matter and anti-matter black holes merge.

E=mc2 (+change)

In normal space that would have liberated a huge amount of energy, but in sufficiently curved space, the energy stays inside the back hole, mass is conserved, nothing to see (literally).

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I think the question is, will the spacetime stay sufficiently curved if it's now curved by pure energy whizzing around at light-speed or will it now, without the inertia of the matter, fall into some weird growing resonance and ultimately shatter and relax.

It's not exactly whizzing around at light speed in the way that you imagine if it's beneath an event horizon. The entire point of an event horizon is that everything beyond it physically cannot move outward. The horizon isn't some kind of physical barrier, it is just the farthest distance at which the black hole's gravity becomes inescapable, that doesn't stop applying just because you've moved past the horizon, and in fact it only becomes more true.

If you imagine any sphere beneath the event horizon which is centered on the singularity, that sphere is just as inescapable as the farthest such sphere which we call the event horizon.

Additionally, the event horizon is inescapable to everything. Even gravitation is transmitted at the speed of light, and cannot outrun the cascade of inwardly falling spacetime beneath the event horizon. So no possible change in the distribution of mass within a black hole could have any effect on the configuration of spacetime at the event horizon, or even on any spacetime further from the singularity than itself.

The point of event horizon is that everything at it expeiriences infinite gravitational time dilation in relation to everything even barely outside it.

So talking about things inside event horizon is about as physical as talking about things traveling at superluminal speeds. You can consider such object, it just takes more than infinite acceleration time to get there.

I don't really understand why people think they can get rid of the discontinuity at event horizon by changing coordinate system. It's as if we were considering function 1/x in new coordinate system xi where xi is defined as 1/x and suddenly we have a nice continuus function without anything special happening around 0. It's a mathematical sleigh of hand.

Sure, from the point of view of infalling matter nothing special happens at event horizon but why would we, outside people, should care about this perspective since the universe will evaporate before this infalling matter crosses event horizon?

So when black hole becomes made of mostly photons, they are all still outside of the event horizon, because infinite time has not yet passed outside.

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