black hole singularity gravity attraction

[hewj11]

Hello,

Thought. When black holes are attracted to each others and start there dance to merging, is it the singularities that are first merging and the event horizons following or are the event horizon and singularities move as one? Or am i missing it entirely?

Also if the universe is curved would the centers (singularities) of black holes all meet in a certain "depth" or point? Maybe in cosmic local groups have of galaxies would there be a point where all come together?

Thanks

[Spyman]

We don't know if there really exists true pure singularities or what would happen if two such exotic things get close together. Singularities often indicate that a theory or mathematical concept is used beyond its capability or is lacking some information, therefor it is thought that our current theory of gravity can't correctly describe the center of a Black Hole.

 

However when two Black Holes gets close enough their mutual gravitation will create a new Event Horizon enclosing both masses. The Event Horizon is not a physical object, it's a limit where gravity gets strong enough to hold back light, and as such it will always be surrounding the mass creating it.

 

According to current observations Dark Energy is forcing the Universe to expand with an accelerating rate, leading to already existing Black Holes to get further and further apart, whether the Universe is curved or not.

[lemur]

I started to hypothesize that if light traveling at C gets caught in a black hole, then so would gravitation, since that travels at C. Then I wondered how gravitation escapes a black hole in the first place if light can't.

[ajb]

I started to hypothesize that if light traveling at C gets caught in a black hole, then so would gravitation, since that travels at C. Then I wondered how gravitation escapes a black hole in the first place if light can't.

 

Classically the force of gravity due to a black hole is the curvature of space-time surrounding the black hole. That is outside the horizon. So there is no course for concern.

 

You could now ask about gravitational waves course by fluctuations (or similar) in the even horizon. You can course such ripples in the horizon by chucking massive objects in. Anyway, as these are ripples in the horizon rather than ripples behind the horizon they are free to escape.

 

So, what about gravitions? These are the quanta of fluctuations in the curvature of space-time. Again, these originate at the horizon and are free to escape. (Mod all usual warnings about quantum GR.)

 

In all gravity is not trapped inside the horizon.

[lemur]

Classically the force of gravity due to a black hole is the curvature of space-time surrounding the black hole. That is outside the horizon. So there is no course for concern.

 

You could now ask about gravitational waves course by fluctuations (or similar) in the even horizon. You can course such ripples in the horizon by chucking massive objects in. Anyway, as these are ripples in the horizon rather than ripples behind the horizon they are free to escape.

 

So, what about gravitions? These are the quanta of fluctuations in the curvature of space-time. Again, these originate at the horizon and are free to escape. (Mod all usual warnings about quantum GR.)

 

In all gravity is not trapped inside the horizon.

 

This is where there seems to be a conflict between these two models of gravitation. If gravity consists of particles or waves that are mobile (at C), then how can gravity exist as spacetime curvature? My impression, anyway, is that when you talk about "space-time surrounding the black hole," you're implying that such space-time exists outside the event horizon without being emitted by it. But how can this be if gravitation travels (at C)?

[ajb]

This is where there seems to be a conflict between these two models of gravitation. If gravity consists of particles or waves that are mobile (at C), then how can gravity exist as spacetime curvature? My impression, anyway, is that when you talk about "space-time surrounding the black hole," you're implying that such space-time exists outside the event horizon without being emitted by it. But how can this be if gravitation travels (at C)?

 

 

Gravitational waves travel at the speed of light. Small changes in an objects mass/energy radiate out as gravitational waves. In this sense, we mean that gravity travels at c. That is, any change in an object will not instantaneous be detected, but rather this change takes time to propagate.

[imatfaal]

AJB / Lemur - how are neutrinos affected by black holes? I presume that they would follow the same path as light and be trapped. Gravitons? If they follow a simple straight geodesic then they are gonna be trapped - but then how do they mediate the force, if they exist?

[ajb]

AJB / Lemur - how are neutrinos affected by black holes? I presume that they would follow the same path as light and be trapped.

 

Neutrinos have a small but non-zero mass, so they would not follow the same paths as light assuming they are under free-fall.

 

 

Gravitons? If they follow a simple straight geodesic then they are gonna be trapped - but then how do they mediate the force, if they exist?

 

Gravitons according to naive quantisation of general relativity are massless. There are theories with massive gravitons, but I do not think they are generally accepted as being of real phenomenological application. Never the less they deserve study.

 

Anyway, by a black hole let us fix this to mean the Schwarzschild metric. This is a good approximation for low charge slowly rotating black holes in nature.

 

Assuming no other forces, test particles will move along geodesics. These are the straightest possible paths. For the Schwartzchild metric, which we take to describe the space time around a black hole there are geodesics that do not hit the horizon. So, not every test particle will necessarily fall into the black hole.

 

So really there is no conflict here.