When Black Holes Collide

EricTheGreen writes "CNN.com reports on a pair of black holes in a mating dance that can only end badly for both of them. Fortunately they've still got several million years for the emotional rush to wear off and realize what a terrible mistake they're both making..."

Comments

yup...

[3.14159265]

gravity sucks...

LISA

[alta]

Neat, a new telescope thing called LISA will be able to detect the merger. If they can keep the power on for a few million years.

Sooner than you think

[Skevin]

> Fortunately they've still got several million years

Umm, how many light years away is this? Sure, it might take million years for the *light* from the spectacle of them merging to reach us, but if they're millions of light years away (center of the galaxy?), they may have already merged.

I've always speculated as whether gravity travels like light. Would "gravity waves" from the merge be felt here on earth the instant it happened, or would it take the same amount of time as light/electromagnetic radiation to reach us?

Re:Sooner than you think

[BlewScreen]

I've always speculated as whether gravity travels like light. Would "gravity waves" from the merge be felt here on earth the instant it happened, or would it take the same amount of time as light/electromagnetic radiation to reach us?

If you take a look at this book, you'll find that there is a way to measure the "speed of gravity" (according to the author) and that it is indeed faster than the (current) speed of light.

I'm not going to agree or disagree with what he puts forth, but if you're interested in questions such as the one you propose above, you'll probably find the book interesting. The supposition is that the speed of light and the speed of gravity were, at the time of the big bang, equal, and that the speed of light has gradually slowed over time.

I think the answer the author would give to your question is that the "gravity waves" you mention would arrive before the light would, but it would not be instant.

-bs

Re:Sooner than you think

[Luyseyal]

Scientists measured the speed of gravity a few years ago... short answer: it travels at the speed of light as predicted by Einstein's equations.

Cheers,
-l

Re:Sooner than you think

[ComaVN]

Even if (just a hypothesis) gravity waves reaches here at the instant it happened, it means that it is not detectable, since if we can detect it, it means information travelled at more than the speed of light.

Alternatively, it could mean that no information can travel at more than the speed of light, except in the form of gravity waves.

I mean, shouldn't "No Information can travel at the speed more than that of light" really be "There's no known mechanism by which information can travel at a speed more than that of light"?

Re:Sooner than you think

[Chowderbags]

Wha? The word "gravity" by itself isn't a force. It's a concept. The "force of gravity" is the force felt by two objects pulling on eachother, which you could calculate using (G(m1)(m2))/(r^2). Since we can calculate the mass of the black holes based on the speed of the dust orbiting a particular distance from the center, we could find the real force that the black holes exert on the earth (which, yes, would be small, since the objects are so far away). The problem, though, is that since the black holes are merging, it's going into crazy Einsteinian physics. Two rips in spacetime are coming together, with unknown consequences, one of which could be noticible (to instruments, probably not to the man on the street) gravitational effects which could show the true nature of gravity.

Re:Sooner than you think

[Feanturi]

Gravity used to go the same speed as light, but then it pissed off Chuck Norris and has to go faster now to get away.

What...Goatse Guy met Tubgirl?

[R2.0]

Come on - tell me no one else thought of that?

Re:What...Goatse Guy met Tubgirl?

[Cheapy]

No one thought of that.

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Stop!

[Hikaru79]

Fortunately they've still got several million years for the emotional rush to wear off and realize what a terrible mistake they're both making...

Black holes hate it when you anthropomorphise them!

something I always wondered

[F�an�ro]

Something I always wondered:
When two black holes are close together, then something that has exactly the same distance to each of them should not fall into either one.

What happens when they are so close that their event horizons overlap?

Shouldn't there always be some flat zone between them that is not part of either event horizon?

So how can they merge?

Re:something I always wondered

[Scott Lockwood]

I'm just speculating here, but I'm betting you have a RAZOR thin 'line' between the two. You're still squished into spaghetti at that line, and once they 'touch' event horizons, it's only a (short?) matter of time before their center's merge and form one, larger black hole, whith one, larger event horizon. It's not like there is any force that will cause them to repell one another, so they likely arc together in an ever tightening vortex until they merge. The closer they get, the faster they go, as well.

Re:something I always wondered

[Bob3141592]

Something I always wondered: When two black holes are close together, then something that has exactly the same distance to each of them should not fall into either one. What happens when they are so close that their event horizons overlap? Shouldn't there always be some flat zone between them that is not part of either event horizon? So how can they merge?

There's a difference between the strength of a gravitational field and a gravitational gradient. It's like at the center of the Earth. The gravitational gradient there (relative to the Earth's field) is zero, but the force of all that overhanging rock is pretty high. You wouldn't float there comfortably with no force acting on you. You'd be squished.

And that's in a conventional, Nwtonian view of gravity, which is where most people are comfortable thinking about these things. In the relativistic world things get a bit more complicated. The gravitational field itself has energy, and energy at sufficiently high densities has an appreciable mass equivalence and so itself gravitates. At high enough values, like at the event horizon of a black hole, this kind of positive resonance causes the equations describing the system to diverge and the solutions go to infinity, and this divergence is called a singularity.

The event horizon isn't a physical thing, it's the point where the divergence is assured. You can't really think of a black hole as a single hard little ball agt the center of a black hole surrounded by black empty space up to the event horizon, though I believe that's now most people think of it. All spatial and temporal points within the event horizon are indistinguishable - but it's be somewhat misleading to say that they're all the same point either, because the equations that describe those points can't be solved rationally since they contain infinities and it's like asking how infinity +1 is different from infinity + 2.

If you were able to maneuver in space such that you were always equidistant from two black holes of identical mass, you would float around comfortably as long as the bh's were sufficiently far from you. As they approached, you'd feel significant tidal stretching. As the bh's got closer, you would be stretched further, and smaller regions even closer to that exact midpoint would feel increased stretching. At the point where they merged, even the infinitestimal point at the exact center would be stretched to infinity (that one zero volume point could not resist the force that was stretching it out to fill the volume of the whole universe). Of course, this is a somewhat poetic way to describe events that cannot really be described because the physical equations contain infinities and have no meaningful interpretations.

At times like that, poetry is all you can do. It's hard to resist making analogies with this scenario and the creation of the universe, but such analogies, like any other analogy what talk about on or inside the event horizon of a black hole, are meaningless here. But it's still fun.

Re:something I always wondered

[vertinox]

It would be an instable position, but a small spaceship might be able to maintain that position?

I'd have to give a big resounding no. IANAAP (I Am Not An Astrophysist), but it would be generally assumed that since we are 3 dimensional, some of our atoms would fall on one black hole's event horizon and then some on the other resulting in the space craft and those inside of it to be ripped into two bits sans the atoms that fall along the razor edge.

However, if you were a 2d entity, you might be able to pull this off... But I'm not sure how a 2d entity can survive in a 3d world much less transport itself between two black holes.

Lastly it could be possible that the two black holes could be uneven in strength so that the even horizon is contantly shifting towards the lesser gravity as the larger consumes it so that the razors edge on the EH would be constantly being dragged towards the black hole.

I could be wrong about this though...

Re:something I always wondered

[PieSquared]

There is nothing magic about the event horizon of the black hole. When two things pass an equal distance from any two objects that are big enough that they would normaly fall toward one thing or the other, the net force is zero. This is not to say that the force on the object is zero however, and if something passed between two black holes that were close to each other, they would be ripped in half. Due to the massive forces involved, however, people invariably talk of a slice of atoms that are in the exact center that would go to neither one. This only happens because of an incorrect train of thought. Remember that any spaceship or body is made of atoms (as far as *we* know...). Each of these atoms is incredibly small, and only an atom that was exactly in the center where net force was zero would stay put. Also remember that the majority of the mass of an atom is in the nucleus and this only take a tiny fraction of the total volume, making it highly improbable that any given atom through which the line passes isn't closer to one black hole then the other. In the fraction of those atoms in the center plane who ARE exactly blanced, which no longer make up a sheet but instead scattered atoms, that these atoms are moving, some translational movement from when they arived, all rotational or at least electron movement. As such, they will eventually tend one way or another and pass into a black hole.

As for the black holes themselves merging, remember that the event horizon isn't magic. It only means that something, even light, that passes into it won't get out without passing light speed due to the amount of gravity involved. Now recall that this is only true because the NET force is such that a speed faster then light is needed to get out. With two black holes near each other, the net force at any point between them will be less, causing the event horizon to shrink away from the black holes. My only uncertainty comes when the actual masses come close to each other, and then only to wonder what happens to light that passes between them.... can light itself be ripped with 2 black holes a few miles apart?

In anwser, I think the event horizons should shrink as the black holes get to the point where they should touch, doing unplesent things to anything that passes between them, but allowing the black holes to accelerate toward each other in perdictable patterns until they touch and become one larger black hole.

I think that it would be interesting to plot the event horizons of two black holes near each other: if my thinking is correct, there would be a conical section from the point between the holes outward missing from the event horizon: in which light could travel and be observed if said system of black holes passed between a sun and us. (assuming it doesn't somehow get torn apart by the forces involved, and that we have telescopes that can see it with all that plasma and such flying about).

Re:something I always wondered

[Anonymous Coward]

The event horizon of a black hole can be thought of as the surface from which no information (particles, energy, whatever) can escape. It's the event horizon because it's where observable events (time) ends; you can't see what happens inside a black hole.

Now, merging black holes. If you're in the exact center (or maybe not the very exact center, since black holes drag space-time around them and other funky effects), then maybe you don't get "pulled" into either black hole before the merger. But you still can't escape the combined system, which is the point where the event horizon swallows you up.

To think of it another way, if the system were Newtonian, and consisted of point masses, then you could balance perfectly between the two. But at some point, the field becomes so strong that you can't escape that balance point; if you try to leave, no matter how powerful your engines, both black holes will act to pull you back. (Similar to what happens at the L1 Lagrange point.) At this point, that balance point has been enveloped by the event horizon.

The event horizon is a surface that encloses a volume that simply describes a region of space-time where events (which, as far as we know, are limited by the speed of light) can no longer observed. As such, it's not really a physical boundary, but a mathematical one. An object crossing the event horizon wouldn't notice until it tried to get out. Otherwise, there's nothing special about it.

Of course, this all assumes continuity, and we know the actual universe is quantum, and once you add quantum in, you get funky effects like Hawking radiation. But we haven't solved the quantum gravity problem yet, and Einstein's theory is the best we've got for now.

Re:something I always wondered

[BinaryOpty]

There's a difference between the strength of a gravitational field and a gravitational gradient. It's like at the center of the Earth. The gravitational gradient there (relative to the Earth's field) is zero, but the force of all that overhanging rock is pretty high. You wouldn't float there comfortably with no force acting on you. You'd be squished.

Doesn't Newton's shell theory state that when within a large spherical body of mass you can treat the mass as a shell of radius to where you are within it because the mass that's further out than you ends up cancelling out? As such, if the center of the earth was somehow hollow and you somehow got transported there or something, then you would float because by the shell theory all of the mass above you is cancelling out all the mass below you and thus you have a net gravitational force of 0 pulling on you. The "overhanging" rock wouldn't squish you because you'd have "underhanging" rock to counteract its pull (even though those terms mean nothing if you're at the center of the planet where essentially every direction is up).

Re:something I always wondered

[ScriptedReplay]

There's a difference between the strength of a gravitational field and a gravitational gradient. It's like at the center of the Earth. The gravitational gradient there (relative to the Earth's field) is zero, but the force of all that overhanging rock is pretty high. You wouldn't float there comfortably with no force acting on you. You'd be squished.

ouch! no. At least, not if you assume spherical symmetry. Baby analytical mech. example: the uniform sphere. Gravitational force is linear inside, going to zero.

You'd be squashed, alright, but not by gravity. It's the pressure in all that rock around you that you have to watch for. But if you manage to stabilize the hole you supposedly dug in the center of the Earth against the surrounding pressure, then you'd be floating quite comfortably.

Re:Why?

[mnemonic_]

Probably because you don't know how to punctuate quotations properly.

Conglomeration

[RealGrouchy]

I dunno, but here on Earth, mergers of Supermassive companies usually end up in additional service charges.

- RG>

If I was sufficiently advanced

[cnflctd]

Two supermassive black holes are spiraling closer and closer, leading to an inevitable merger.

But is it really inevitable, I ask myself? What would it take to pry them apart? Welcome to einstein's tractor pull!

Imaging the black holes 1 and 2 falling straight towards each other. (Trying to do this with them spinning makes my head hurt). You take a third supermassive BH, call it 3, and give it a large velocity relative to the other two. Send it thru the system at a slight angle.

As it hurtles by the hole 1, it drags it along -- has to come real close, but not too, noam sayin?

As 1 and 3 zip by 2, 1 gets slowed down some, but still has excape velocity from 2. See? No sweat. Now if DARPA will give me a grant, I'd hire a math major to solve orbiting BH case.

Re:Oh boy

[cnflctd]

Only if you speak both Russian and English. When a Russian says "YourAnus", he won't get the joke. Black hole on the other hand is russian slang for, well, your anus.

"Can only end badly"?

[ElMiguel]

They are black holes. How much worse can it get?

Re:"Can only end badly"?

[otis wildflower]

They could get stabbed...

Re:Oh boy

[chrismcdirty]

I propose we call them star holes, since they're in space and they suck up stars, among other things.

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