matter bettween 2 event horizons

[scuba cat (Sam)]

Theoretically, say some dense stable matter was between two cross sections of two event horizons, like a ven-diagram.
Now say that the black holes forming the event horizons are moving away from each other. What would happen to the matter when the event horizons no longer cross?

Sam

[Robh (Rob)]

Interesting question. Here's what I think.
I would assume that any matter inside an event horizon is already lost to the Black Hole. Thus, there would be no "normal" matter in the intersection of the event horizons. In fact, the combined event horizons would be distorted at the contact face, initially perhaps the shape of a dumbbell. I would also assume that with the event horizons already merging, it would be impossible for the black holes to separate i.e. they would merge into a black hole with a larger event horizon.
Someone else might have a different take on the scenario.

Regards, Rob

[sjastro]

In the observer's frame of reference overlapping event horizons cannot occur.

Consider an object emitting photons near the edge of the event horizon of black hole A. Suppose black hole A is brought into close proximately of black hole B. Now according to GR the object cannot pass inside the event horizon of black hole B as the photons from the object become infinitely red shifted in the observer's frame of reference. The "closest" the object can approach is the edge of the event horizon of black hole B.
In other words the horizons cannot overlap. In fact if the horizons "touch" this is equivalent to the black holes merging with the event horizon expanding out to the sum of the combined black hole masses.

Regards

Steven

[CraigS]

Ok … interesting …

So in our holographic universe, it seems overlapping 'Cosmic Horizons' can't occur either (for the same reason) …

(Just thought I'd say that … even though the term 'holographic universe' appears to arouse concerns).


Cheers

[Robh (Rob)]

Originally Posted by CraigS

Ok … interesting …

So in our holographic universe, it seems overlapping 'Cosmic Horizons' can't occur either (for the same reason) …

(Just thought I'd say that … even though the term 'holographic universe' appears to arouse concerns).


Cheers

There are similarities between a black hole's event horizon and a cosmic event horizon. To an observer at distance from the black hole or an observer at the centre of the cosmic horizon, velocity = c and change in time = 0 at the horizon.

However, I think there are differences.
With black holes, the event horizon is created by mass "compressed" into a critical volume by gravity. Light does not have enough energy to escape from the horizon's surface. Combining two black holes increases the mass and the size of the event horizon but the effect is the same. I'm not sure if Steven is suggesting that the merger would be instantaneous if the horizons touch. I wonder if this is the case, as the event horizons would be gravitationally distorted first with perhaps an ill-defined contact point.

Cosmic event horizons are brought about by objects exceeding the speed of light due to expansion away from the observer's location. I don't see any reason that these event horizons could not overlap (but not merge) for observers in two separate frames of reference. If each observer's universe can be defined by their cosmological horizon, as in the holographic principle, then it is still possible for information to be passed from one observer's universe to the other where they intersect.

Regards, Rob

[sjastro]

With black holes, the event horizon is created by mass "compressed" into a critical volume by gravity. Light does not have enough energy to escape from the horizon's surface. Combining two black holes increases the mass and the size of the event horizon but the effect is the same. I'm not sure if Steven is suggesting that the merger would be instantaneous if the horizons touch. I wonder if this is the case, as the event horizons would be gravitationally distorted first with perhaps an ill-defined contact point.

Rob,

The event horizon is independant of space-time distortion. It is a property of the time component of the Schwarzchild metric. You can create an event horizon by accelerating a clock in flat Minkowski space. The horizon itself doesn't undergo "distortion" by gravity.

If an object is subjected to tidal forces at the event horizon, it's due to a small radius event horizon being close to the singularity where space time curvature is more pronounced. The horizon itself doesn't cause the distortion nor is the horizon itself distorted.

Regards

Steven

[Robh (Rob)]

Originally Posted by sjastro

Rob,

The event horizon is independant of space-time distortion. It is a property of the time component of the Schwarzchild metric. You can create an event horizon by accelerating a clock in flat Minkowski space. The horizon itself doesn't undergo "distortion" by gravity.


Regards

Steven

Steven,

Not according to this ...
http://www.scholarpedia.org/article/Black_holes
Read section on Dynamical black holes. Interesting movie attached.

Regards, Rob

[sjastro]

Originally Posted by Robh

Steven,

Not according to this ...
http://www.scholarpedia.org/article/Black_holes
Read section on Dynamical black holes. Interesting movie attached.

Regards, Rob

Rob,

The distortion of the event horizon is a property of angular momentum not gravity. In the case of dynamical black holes one needs to consider the angular momentum of the black holes rotating around a centre of mass.

Consider the effects of intrinsic angular momentum on horizons.
A Kerr black hole is a rotating black hole and has 2 horizons, one is similar to the Schwarzschild event horizon, the other is is a flattened sphere that extends out from the equatorial region of the black hole. The faster the black hole rotates the greater the dimensional change effects on each horizon.

A general statement about the effect of angular momentum on horizons is also found in your reference under "Black hole parameters".

Regards

Steven

[CraigS]

As an aside, here's a really cool video taken from some modelling they're doing of black hole collisions, (dated June 4, 2010) !

If interested, the accompanying paper is here

They analyse the deformations, recoil 'kicks' and geometries surrounding the collision, shown in the video.

Cheers

[Robh (Rob)]

Originally Posted by sjastro

Rob,

The distortion of the event horizon is a property of angular momentum not gravity. In the case of dynamical black holes one needs to consider the angular momentum of the black holes rotating around a centre of mass.

Regards

Steven

Steven,

Interesting debate. I would suggest that the distortion of the event horizon would be a combination of both angular momentum and gravity. In my mind, it is reasonable to assume that two close black holes would create immense mutual gravitational disturbances that would deform the event horizon. After all, it is mass and hence gravity that defines the black hole's event horizon in the first place. The upper diagram in Craig's reference (previous post) indicates this distortion as the two black holes approach each in linear fashion (no orbital physics involved).

Regards, Rob

[CraigS]

Wow .. interesting exercise !

I notice from the paper I cited, the two approaching holes are of different sizes (radii) and thus, differences in the tidal forces because of this (?)

I guess the more interesting case would be for two approaching holes of the same radii.

The paper deals more with the effects of the momentum transfers and recoils, upon impact.

Cheers

[Robh (Rob)]

Interesting for its simplicity ...

The Schwarzschild radius is the radius of the event horizon of a black hole.
If the mass of two separate black holes is m1 and m2, then the Schwarzschild radii are:
r1 = 2G m1/c^2 and r2 = 2G m2/c^2

The combined mass of the two merged black holes is m1+m2.
Its Schwarzschild radius r = 2G (m1+m2)/c^2 = 2G m1/c^2 + 2G m2/c^2
which is simply r = r1 + r2

Very nice!

Rob

[CraigS]

Yes .. very simple, Rob ! Neat ! Hides the complexity very nicely !

I'm still thinking about the cosmic horizon side of things (I notice Steven's steering clear of this …that's OK … those String Theorists hijacked it all from quantum mechanics, any way )

I think its actually simpler for me to think of all this from holographic/cosmic horizons perspective, as long as gravity isn't a factor.

Amazing.

Cheers

[sjastro]

Originally Posted by Robh

Steven,

Interesting debate. I would suggest that the distortion of the event horizon would be a combination of both angular momentum and gravity. In my mind, it is reasonable to assume that two close black holes would create immense mutual gravitational disturbances that would deform the event horizon. After all, it is mass and hence gravity that defines the black hole's event horizon in the first place. The upper diagram in Craig's reference (previous post) indicates this distortion as the two black holes approach each in linear fashion (no orbital physics involved).

Regards, Rob

Rob,

The event horizon has no physical reality in the black hole's frame of reference. It only exists to an observer outside the horizon. It cannot be distorted by gravity because it doesn't exist to the black hole.

For a nonrotating black hole, it takes an infinite amount of time for an object moving in a radial direction to cross the horizon (hence the term event horizon). If the event horizon is well away from the singularity the tidal forces on the object are small. The object appears to come to a grinding stop at the horizon but remains intact. In the black hole's frame of reference the object is perfectly free to travel towards the singularity to be eventually destroyed by tidal forces.

For a rotating black hole if the object approaches at an angle except along the axis of rotation, the object will appear to spiral towards the flattened horizon. At this horizon it will appear to orbit forever.
However like the nonrotating black hole, the object will actually pass through the horizon along a radial path and be destroyed by tidal forces.

The role of angular momentum as can be seen plays a very unfamiliar part when it come to event horizons.

Regards

Steven

[CraigS]

Ok .. so time for a question …

What happens to the matter inside the smaller radius BH as it approaches one of a larger size ?
(Hypothetical, of course). Surely the matter is distorted by tidal forces ..(?)

If this is the case, then can this distort its event horizon before collision ?
(I guess this is very hypothetical).

Cheers

[Robh (Rob)]

Originally Posted by sjastro

Rob,

The event horizon has no physical reality in the black hole's frame of reference. It only exists to an observer outside the horizon. It cannot be distorted by gravity because it doesn't exist to the black hole.


Regards

Steven

Steven,

Looks like we need some observational evidence.
It is the reality of the event horizon relative to an outside observer that is relevant here. In my opinion, the event horizon will appear to be distorted by the gravity of the interacting black holes to an observer outside the horizon.

Regards, Rob.

[sjastro]

Originally Posted by Robh

Steven,

Looks like we need some observational evidence.
It is the reality of the event horizon relative to an outside observer that is relevant here. In my opinion, the event horizon will appear to be distorted by the gravity of the interacting black holes to an observer outside the horizon.

Regards, Rob.

Rob,

The search for observational evidence by definition contradicts GR.
If gravity does in fact distort the event horizon, then one can argue it is impossible to define the Schwarzschild radius for low mass black holes as the event horizon is well within the tidal forces of a black hole and is therefore distorted.

Regards

Steven

[avandonk]

You may as well be arguing about how many angels can dance on a pin!
At best hypothetical at worst absurd!

If you fall into a giant black hole you will not be turned into spaghetti but just float as you have orgasmic revelations just before you die!


Climbing dangerous mountains is just so passe.


Bert

[cwjohn (Chris)]

Originally Posted by avandonk

You may as well be arguing about how many angels can dance on a pin!
At best hypothetical at worst absurd!

If you fall into a giant black hole you will not be turned into spaghetti but just float as you have orgasmic revelations just before you die!


Climbing dangerous mountains is just so passe.


Bert

Agreed. Bit like a discussion when everyone has overly imbibed. Immensely meaningful at the time but complete nonsense when you sober up.

[CraigS]

Sorry Bert and Chris;

I have found this thread to contain a lot of very useful information.

I would like to thank Steven and Rob, as I have learnt heaps, and so too will others, particularly if they read the attachments and then review the posts.

Next time I hope both Bert and Chris contribute constructively as I know you both can.

Cheers and Regards.