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Comment by pfdietz | Hacker News Reader

The situation you describe is impossible. "If you have a very large positive number that is less than zero, what happens?"

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What's the contradiction in the black hole setup?

I'm not even sure what it would mean for two black holes to be too big to collide, or where that became some kind of constraint.

I thought it was just thought that it would take too long for them to spiral into each other for it to have happened enough times in our universe

You're imagining some moment where two points merge.

These aren't points; they are (literally) opaque volumes of space, and once their outer limits broach, they have collided.

By analogy, two warships can collide, even though their centers of mass don't.

It would be difficult for two warships to collide without some of the material in one warship touching some of the material in the other one.

But there is no matter at an event horizon. That's just an imaginary line in space. It's opaque, but not solid.

If the black holes were moving fast enough, it should be possible for their event horizons to cross and then uncross, although that would immediately raise the question of what would happen to matter in the zone of overlap. Perhaps "fast enough" would exceed the speed of light?

> If the black holes were moving fast enough, it should be possible for their event horizons to cross and then uncross,

No, that is not possible.

...because?

Time dilation is infinite at the even horizon, so anything that touches it, freezes for good.

Infinities are typically indications of where a theory breaks down, not of something real.

Infinity there isn't abrupt, but steadily grows. Anything that approaches just does it slower and slower, and remains always finite, only exponential.

The event horizon is not an object that can touch something, so why is that an objection to the idea of two event horizons crossing and uncrossing?

It's time itself that stops there. No matter what you want to move, you will have a problem moving it. It might be easier to understand that Democritean idea of space as literal nothingness is a bit old, in modern science space is field, i.e. solid matter (universe), particles move in it like sound for which steel is the most transparent medium, and nothingness is the most impenetrable, because there is no foothold there, we think it's empty because we can move freely in it.

Also most black holes have matter on the event horizon, because something fell on them. Maybe it can even touch, because this matter is frozen slightly above even horizon.

Time has also stopped for photons, which does nothing to stop them from moving from place to place.

I don't see a way to read your comment that allows for the possibility that a black hole might move, which is something they do.

I'm riiiiiiight on the edge of concluding that you are a poet and the only thing you know about the words is the way they sound. Is there more to it than that?

Technically time is exponentially dilated, this introduces large relativity of synchronism: the same thing takes different time in different reference frames. You aren't completely wrong to say black holes will fly through each other, but this doesn't take the same time in all reference frames. For a distant observer it takes infinite time after their event horizons touch, and Hawking radiation evaporates everything before that.

How black holes can move is an interesting question, but as you can see, in reality there are no infinities, since stuff slows down before that. Maybe infinities could exist for an observer inside black hole, but an observer outside of black hole sees only large slow down.

I think you're playing fast-and-loose with the notion of "colliding".

First define what you mean by "colliding".

By analogy, I'm defining it as the Event Horizons intersecting, at which point nothing in side "either" black hole has any physical meaning to us ever again. For all we know, the insides could be chocolate ice cream on one half of the now-double-sized Event Horizon, and pure neutronium on the other. It's meaningless to even pretend we know anything about that volume.

Collision occurs when there is some sort of interaction between massy particles that try and fail to occupy the same space. Whatever stops you from passing through the floor counts as "collision".

An event horizon has no mass or other existence and cannot collide with anything. Within the black hole, there is mass somewhere, but generally not at the event horizon. If you're not comfortable assuming that, we can make it a definition - take this to be an example where none of the internal mass lies within the region of overlap.

What is the obstacle to the black holes separating again?

If one is within the other's event horizon, they have already collided (and are now surrounded by a common event horizon).

I have read elsewhere that all black holes are imploding, and there will be a "bounce" followed by matter emerging from the event horizons.

GR doesn't say any such thing, and anything beyond GR is speculation.

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