It is indeed! And, in case you don't know it, a similar series [The Periodic Table of Videos](http://www.periodicvideos.com/)^ohthehair! is just as great, only it deals with chemistry, not physics.

This is an awesome video! Thanks for that, it explains a lot. The rest of the series seems pretty great as well!

>However, my thinking is that if another black hole came by, would it be possible for it to "suck the other black hole in"? And if so, would that qualify as matter exiting the first?

Yes and no.  The black holes could merge.  Stuff inside one event horizon would never get out -- the event horizons may combine; but matter never escapes to the universe at large.

>because the geometry of spacetime is curved to the point where all trajectories that are either parallel to or directed away from the center lie in the past.

This seems like a strange way of phrasing this.  Is this different than saying that you would have to go faster than light to get out of the gravity well?


All that would happen would be that the two black holes would merge into one bigger one. your confusing the difference in matter being able to escape and the space curvature being changed.

visualising it as a curved surface helps you see it better as the two fall into one big hole.

The matter can interact as it falls into the hole.It just cannot escape. Before spaghettification you would still be able to see 'out' of the hole but nothing would ever 'reach' the singulairty.

Very weird stuff going on there.

> Wait, so there is no matter (mass) between the event horizon and the singularity?

Not for very long. It's impossible for any matter between the event horizon and the singularity to either increase or maintain its radial distance from the center, because the geometry of spacetime is curved to the point where all trajectories that are either parallel to or directed away from the center lie in the past.

> I know that gravity acts as a point source, but I'm interested in what would happen to this matter (if indeed it exists) in between the singularity and the event horizon.

Not only does no one know, no one can *ever* know. It's possible that there exists some quantum-scale interaction that prevents matter and energy from collapsing to a point of zero volume. But once anything crosses the event horizon, it ceases to matter, in the most literal sense possible.

i think we just (currently) have no way of knowing what's between the event horizon and the point mass.

Wait, so there is no matter (mass) between the event horizon and the singularity? Or are you saying that the mass in between is negligible in comparison to the singularity's mass and thus can be ignored?

I know that gravity acts as a point source, but I'm interested in what would happen to this matter (if indeed it exists) in between the singularity and the event horizon.

How will it be spinning if it's a point object?

It's actually impossible to have closed orbits inside the event horizon, but yes, they will merge and eventually settle down into one spinning black hole.

Why would it spiral in, and not simply keep orbiting forever?

They wouldn't be able to form closed orbits within either event horizon, just one spiralling towards the other. Whichever one is less massive will spiral into the other, merging at the singularity.

If two black holes are orbiting one another within their event horizons, can they be described as a single black hole with a certain angular momentum?

> What happens to the spaceship that happens to be simultaneously within the event horizon of both black holes?

It wouldn't be there for very long. Once inside the event horizon of a black hole, it is impossible to maintain a positive radial distance from the center for infinite time. How long you can last is a function of a lot of things, not least of which is the mass of the black hole and your own physical size. But you cannot last for long.

So then it isn't necessary for the black holes to merge, like if they're moving rather fast to begin with? What happens to the spaceship that  happens to be simultaneously within the event horizon of both black holes?

One thing needs to be added here. If there is no stuff in a disk around the black hole, nothing will happen. All the mass of the black hole is concentrated in the singularity, the event horizon only matters for stuff. They should behave as point masses moving in a geometry described by general relativity.

They would merge together as one black hole or just orbit each other.

Black holes merging are thought to be one of the most energetic events in the universe, so much so that it's one of the things they're looking for at [this gravitational wave observatory](http://en.wikipedia.org/wiki/LIGO).

So you mean like a Lagrangian point between two black holes which would happen to be within their event horizons? It makes sense from my perspective, but I'm also quite certain a physicist would say that isn't possible...

> So let's say we have a ship just inside the event horizon, pushing it's stardrive to the max so that it's just barely stationary.

We can't. This is not possible. Inside the event horizon of a black hole, remaining stationary is exactly equivalent to going backwards in time. It cannot be done.

So let's say we have a ship just inside the event horizon, pushing it's stardrive to the max so that it's just barely stationary.  Now another black hole comes shooting by, and for a brief instant, the Lagrange points between the black holes is exactly where the spaceship is.  Can it escape at that one instant?  Or if not totally escape, can it head over to the other black hol  in that instant and be trapped inside that one instead?

You seem to be thinking of the event horizon of a black hole as a thing. It isn't. It's just a boundary delineating two regions of spacetime where different things are possible. Outside a gravitational event horizon, it's possible — at least on paper! — for an object undergoing acceleration to maintain a constant radial distance from the barycentre. At or within the event horizon, this is no longer possible. (The definition of the event horizon is the manifold in spacetime where only light itself can maintain a constant radial distance.)

Also, the Schwarzchild radius really doesn't have anything to do with anything in this context. That's just a number that defines a sphere that, if all of the mass of a given body were confined within that sphere, that sphere would collapse into a black hole.

How do you define a sphere when space is asymmetrically warped?  There is not always a reasonable measure of distance to the singularity.  (Note that the Schwarzschild coördinates aren't even that great for a static black hole.  Their only real advantage is the smooth connection to spherical coördinates far away, but constant r is undergoing acceleration.)

I still think it is an interesting question, even if the terminology is a bit imprecise.

Let's substitute "event horizon" with "Schwarzchild radius" and ask again:

Can a black hole's event horizon be deformed by an external gravity source that it would not fully emcompass the sphere formed by its Schwarzchild radius?  Could there exist a dead zone between two black holes such that, while within the Schwarzchild radius of both, is within neither's event horizon? 

My guess is no, but I can't prove it off the top of my head.

This is a case where you should "step-back, and unask the question, because it is based on false premises".  The horizon of a black hole does not exist in the same way anything else does.  Locally, an observer going through the horizon does not, *can* not, notice anything different than anywhere else.  The horizon is something we define about the system in our analysis of it.   It's the boundary past which things can't escape.  To even get a rough idea of this, we need to know the entire future evolution of the system, including anything like other black-holes swinging by.

Maintaining this definition of the horizon during interactions means that when a black hole comes near another, the horizon will change, but any matter or light, any *trajectories* behind the horizon will remain behind the horizon -- if it didn't remain behind the horizon, then our definition for the horizon earlier was flawed.  Stuff that fell through the horizon is not waiting right behind it waiting to get out --

I asked a similar question before, but didn't really get a satisfactory answer, so I'll ask again.

Consider this situation where two black holes come near each other. I apologize for thinking in terms of classical gravity here, but there should be a region directly between them where their gravity largely "cancels out", which would suggest to me that this region would be outside their event horizon. So I would imagine their horizons warping a little before getting close enough to merge.

I would also imagine that a region *just within* the horizon of one of the black holes could be made to be outside the horizon due to this warping, and thus any matter that was captured but still hadn't fallen far could be "freed".

Can someone in the know please explain what is wrong with this?

Nothing exits anything. They just collide and merge.

http://www.youtube.com/watch?v=4m-ZVsLf070

i'm not a scientist... i'm not even batman.  but it seems to me like they'd just combine, kinda like what happens if a rock hits a planet - the mass never left the rock, but was added to the mass of the planet.

What happens when the two singularities are not within the event horizon of the other, they are accelerating away from each other, and the event horizons overlap?  What happens to the stuff overlapped by both event horizons?  Which way does it fall?  Is it possible to escape if you are exactly between both horizons?

This is pretty much my area of research. Like others have said, the black holes merge, and after a ringdown period, the new object is a black hole with roughly the mass of the sum of the masses of the two black holes.

The part where you said matter is emitted from the black hole.

This does not occur, and in fact is physically impossible.

I have a ridiculous amount of respect for Lenny Susskind. I mean, you just have no idea, seriously. But I would caution one to not let his interpretations of quantum information theory (which may or may not end up being valid) distract one from the hard facts of general relativity, that's all. Quantum information theory is an interesting and promising area of inquiry, but it must necessarily conform to the predictions of general relativity.

RobotRollCall The way I understood Hawking Radiation was layed out to me in a book titled "[The Black Hole War](http://www.amazon.com/Black-Hole-War-Stephen-Mechanics/dp/0316016411/ref=sr_1_1?ie=UTF8&qid=1295048365&sr=8-1)" by Leonard Susskind.  
I admit the book is an easy to read simplified view of blackholes so a layman can easily understand some concepts but i thought this is how it worked.  
How is it a misleading characterization?  
Can you please explain this?

That's actually quite a misleading characterization of Hawking radiation, unfortunately.

> My understanding of black holes is that once matter passes the even horizon, it can never make it back out.   

Matter (entropy) is re-arranged within the black hole and is emitted in what's called [Hawking Radiation](http://en.wikipedia.org/wiki/Hawking_radiation). It then slowly loses all of it's heat and entropy and eventually evaporates. 

Is it possible that black holes do not exist because as a star collapses time slows down asymptotically so that a true event horizon is never quite formed?

What if two black holes are accelerating away from each other, but their event horizons currently overlap?  Would the matter perfectly in between the ven diagram part of both horizons be able to escape?

Tell your teacher that you're allowed to go to 12th grade.

How many black holes does it take to make a big bang?

My understanding of black holes is that once matter passes the even horizon, it can never make it back out. However, my thinking is that if another black hole came by, would it be possible for it to "suck the other black hole in"? And if so, would that qualify as matter exiting the first?

What would happen if the event horizons of two black holes touched?