(Brian) Q: “What happens to the event horizons during the early stages of the merger process? Would they extend outwards towards the other mass?”
(Tom) A: The event horizons are not physical objects so there should be no effects similar to what happens when two physical objects merge (e.g., turbulence, etc.).

Brian’s question is similar to the one that I wanted to ask. I don’t know if you are correct Tom, that there would be no effect to the Even Horizons as the two singularities approach each other in a merger.

Even though both objects are super massive, the pull they exert on each other should still create an area of zero gravity at a specific location between the objects relative to their masses. For example, the SOHO (Solar and Heliospheric Observatory) Satellite orbits at a distance between the Earth and the Sun where the two body’s gravities “balance each other out” so that the pull on the satellite is equal in both directions.

Regardless of their masses, there should be a similar zone of zero gravity that exists between these two black holes where the pull from each black hole is equal but in opposite directions.
Considering that the Event Horizon is simply a threshold where the gravitational pull of a black hole becomes so great that nothing can escape, as these two monsters approach each other, shouldn’t the event horizons retreat inward towards their centers by this zero G zone between the two masses? If there is no gravity, relatively then there can be no Event Horizon, right? I understand that this zone would become increasingly narrow or small as the two masses move inward towards each other, but it should still be there regardless of it’s size.

I know I must be wrong in this conclusion but it is something that I have been thinking about as I’ve been considering whether Newton’s Shell Theorem, should also be applied to Black Holes. Just as there is zero gravity at the center of the Earth and Sun, because all the mass of these objects goes out in all directions away from the center, shouldn’t the same be true for a Black Hole and doesn’t that imply that there would be no singularity at the center of a black hole?

Good question. From the point of view of a “clock” maintained at either black hole singularity they will. From the point of view of a remote observer watching the merger it will take an infinite amount of time.

“What happens to the event horizons during the early stages of the merger process? Would they extend outwards towards the other mass?”

The event horizons are not physical objects so there should be no effects similar to what happens when two physical objects merge (e.g., turbulence, etc.).

“If orbiting black holes generate gravity waves and waves are a form of energy, wouldn’t those gravity waves be getting their energy directly from the masses of the black holes? Is this a way for a black hole to lose mass, like Hawking radiation?”

It’s my understanding that the energy contained in the gravitational waves comes from the initial kinectic energy of the two black holes.

“If so, is this a significant energy loss to the black holes?”

Probably not, in this case the kinetic energy of the smaller of the black holes is 7.2E50 joules but its rest energy is 3.6E54, 5000 times greater.

“Would the merged black hole rotate, having gotten a big kick from the orbital motions of the original pair?”

They are probably both rotation to begin with. After the merger the resulting angular momentum vector is the vector sum of the initial angular momentum vectors (using the parallelogram rule).

“I seem to recall that a rotating hole had a singularity that was a ring, rather than a point (or was it a double event horizon?).”

I think that’s probably true. Of course any attempts to go inside the event horizon and verify this are likely to be fruitless.

“Wasn’t there some idea that an energetic phenomenon (Gamma Ray Bursts?) was the result of black hole mergers? Or was it simply black holes consuming large masses like stars?”

GRBs are thought to be coming from massive stars during black hole formation, not from swallowing stars although that probably would be an energetic event also.

Eki-eki-eki-*what-is-this-last-sound?*

NI!

NI!

Is that the correct translation?

But the movie is absolutly… well…. great. Even the German synchronization.

Ni!

Click on my name to find out.

“Zieht den Bayern die Lederhosen aus!”

(probably a good translation is “take the leather trousers off the Bavarians”, but while I read it, I think this sounds rather stupid and wrong ) when we have a match against “Bayern München” (Munich).

So, I should have known!!

Btw: in about 13min (9:00 pm MET) they show “Monty Python and the holy grail” on TV. Maybe I watch my DvD.

… so I am not the other German stereotype as a fat, wheat bear drinking bavarian, only eating “bratwurst and sauerkraut”.

You left out the “lederhosen wearing” bit.

No, I wasn’t referring to anti-gravity when I said “opposing gravitational fields”. I merely extended the concept of a Lagrange point between the Earth and the Sun to this (putative) binary black hole system. There ought to be a point somewhere between the 2 holes where their gravity cancels out.

These two holes are rather mismatched in terms of their mass. It occurred to me that the gravity cancellation point could actually be inside the “normal” event horizon of the smaller hole. Thus a conceptual reason for a dimple in an otherwise spherical event horizon.

On the other hand, if gravity is an attractive force (I know, I know, that’s not 4-D S-T thinking…) shouldn’t the 2 masses distend and distort towards one another, like the Earth and Moon? And, since there’s nothing left of the masses except a pair of singularities and gravity wells, won’t that event horizon bulge outwards?

However I’m having trouble choosing. I think the reason is that I’m shifting conceptual frames of reference, between Newtonian ideas and Relativistic ideas. When I use full-on field thinking, I talk myself into the idea that Lagrange points don’t exist at all, anywhere, which I know is not true!

The essential question remains: Is the merging event horizon an Innie, or an Outie? Or is it completely unmoved?

Btw: humourless Germans? Well, the British humour is, say, legendary here in Germany, too. The same prejudices everywhere….

Oh, just stereotypical British viewpoint of Germans as hardworking, but humourless, and of the French as riding around on bicycles, wearing a black beret, a black & white striped jersey, and with onions around their necks.

@ DrFlimmer,

The rotation has distorted the central singularity from a point into a flat ring, and the event horizon has assumed the shape of an ellipsoid. The figure [in the book] also shows additional features caused by the rotation. As a massive object spins, it induces a rotation in the surrounding space-time, a phenomenon known as frame dragging.

Interesting! Wikipedia has an article on Ergosphere (click on my name for the link) that refers to “frame-dragging” and an illustration of an oblate spheroid (ellipsoid) ergosphere around the event horizon of a rotating black hole.

@ Ivan3man AND also @ Brian
After a few hours of internet research I finally took the book “An Introduction to modern Astrophysics” (2nd edition) by Carroll & Ostlie (probably THE standard literature for astrophysics students). Page 640 is quite interesting:

“There is a firm upper limit for a rotating black hole’s angular momentum given by L(max)=GM^2/c.” (where G is the gravitational constant, M the mass and c the speed of light)
“If the angular momentum of a rotating black hole were to exceed this limit, there would be no event horizon and a naked singularity would appear, in violation of the Law of Cosmic Censorship.”

So there is, indeed, that upper limit. Sadly the book doesn’t give an explanation why the event horizon would vanish. Probably it has to do with complicate mathematical stuff of the Kerr solution of the Einstein equation of GR. But I don’t know that – probably someone else? Maybe I check Carroll’s “Spacetime and Geometry” (this is Sean Carroll, not the same Carroll as above; he is named Bradley ), but not now.

But the text goes on about “frame dragging”:

“The rotation has distorted the central singularity from a point into a flat ring, and the event horizon has assumed the shape of an ellipsoid. The figure [in the book] also shows additional features caused by the rotation. As a massive object spins, it induces a rotation in the sorrounding spacetime, a phenomenon known as fram dragging.
[...]
Near a rotating black hole, frame dragging is so severe that there is a nonspherical region outside the event horizon called the ergosphere where any particle MUST move in the same direction that the black hole rotates. Spacetime within the ergosphere is rotating so rapidly that a particle would have to travel faster than the speed of light to remain at the same angular coordinate (viewed from the outside).
[...]
The details of the singularity cannot be fully described until a theory of quantum gravity is found. The presence of a singularity seems assured, however. In 1965 an English mathematician, Roger Penrose, proved that EVERY complete gravitational collapse must form a singularity.”

Heh. So, you Germans do have a sense of humour, then?!

DrFlimmer: “But I wonder why there should be an upper limit for the rotational period of a black hole since it should not “rip itself apart” like a neutron star[?]“

Remember that the Universal speed limit is c — 299,792,458 metres/second (exactly). So, let’s assume a spherical cow; if a reference point on the equator of the ‘spherical cow’ in question is travelling close to the speed of light, as it is rotating at 1,150 times per second around its axis, the equatorial circumference of the ‘spherical cow’ would be limited to:
299,792,458 / 1,150 = 260.689 km maximum.

Therefore, the maximum diameter of the ‘spherical cow’ would be:
260.689 / π = 82.980 km — about the size of Pandora, a satellite of Saturn.

Q.E.D.