black hole presence

[phyti]

My brother asks me this question for which I have no answer.

If gravitational effects are propagated as waves at light speed, and light cannot escape from a black hole, how do the g-waves escape?

 

[xyzt]

My brother asks me this question for which I have no answer.

If gravitational effects are propagated as waves at light speed, and light cannot escape from a black hole, how do the g-waves escape?

 

Nothing can escape from INSIDE the event horizon, g-waves are not coming from within the even horizon.

[Janus]

My brother asks me this question for which I have no answer.

If gravitational effects are propagated as waves at light speed, and light cannot escape from a black hole, how do the g-waves escape?

It is changes in the gravitational field that propagates as waves. The field which is responsible for gravity is already there. In other words, anything that happens inside the event horizon has no effect on the gravity field outside the horizon, and since the gravity field was in existence before the star collapsed into a black hole it remains afterward. This is sometimes called the "frozen star" model for a black hole.

[phyti]

It is changes in the gravitational field that propagates as waves. The field which is responsible for gravity is already there. In other words, anything that happens inside the event horizon has no effect on the gravity field outside the horizon, and since the gravity field was in existence before the star collapsed into a black hole it remains afterward. This is sometimes called the "frozen star" model for a black hole.

Does the g-field require energy to persist (after collapse), and if so, what is the source?

[md65536]

Does the g-field require energy to persist (after collapse), and if so, what is the source?

Perhaps a different perspective might help. All of the properties of a black hole (which by the no-hair theorem are only mass, charge, angular momentum, linear momentum, and location) are properties of (or available at?) the surface of the black hole. So while information about anything inside the surface is unavailable, these properties do not disappear.

 

I'm not sure how this relates to the other answers (are the properties available only because they're "frozen" at the horizon?). But if you think of a black hole in terms of its surface, it is a thing with measurable mass etc... only the inside is unknowable to us.

[Delta1212]

Does the g-field require energy to persist (after collapse), and if so, what is the source?

Maintaining a gravitational field does not consume any energy.

[MigL]

My understanding is the same as md65536 has posted. The measurable quantities are conserved by the event horizon, as GR predicts there is nothing inside the event horizon other than a possible singularity at the center.

 

And as Delta1212 has stated a static gravitational field doesn't consume energy, however it is possible to extract energy from a BH's gravitational field by taking advantage of its conservative nature, or by manipulating the frame drag effect of a rotating BH ( the only kind which can exist ). This energy extraction will manifest itself as an areal reduction of the event horizon.

[phyti]

My understanding is the same as md65536 has posted. The measurable quantities are conserved by the event horizon, as GR predicts there is nothing inside the event horizon other than a possible singularity at the center.

 

And as Delta1212 has stated a static gravitational field doesn't consume energy, however it is possible to extract energy from a BH's gravitational field by taking advantage of its conservative nature, or by manipulating the frame drag effect of a rotating BH ( the only kind which can exist ). This energy extraction will manifest itself as an areal reduction of the event .

I didn't believe you could get free energy to accelerate toward a larger mass, and that a typical mass supplied the energy for a typical g-field. Is it possible to consider the newly captured mass as contributing energy to the BH g-field? (still thinking conservation of energy)

[md65536]

I didn't believe you could get free energy to accelerate toward a larger mass, and that a typical mass supplied the energy for a typical g-field. Is it possible to consider the newly captured mass as contributing energy to the BH g-field? (still thinking conservation of energy)

There's no transfer of energy. Acceleration toward a mass would be converting gravitational potential energy into kinetic energy, and is conserved.

 

The mass of a BH is "mass-energy", and it is conserved.

 

 

Gravitational energy can be transferred in the form of gravitational waves but that's not applicable to acceleration of masses directly toward each other. If I understand it correctly, you can think of the two masses in terms of two independent static gravitational fields, and (I think) they don't change if the masses are accelerating at a constant rate. However if you change their direction (like if the BH is orbiting the other mass or some other mass) then you have changes to the field that need to be propagated in the form of gravitational waves (emitted energy in all directions which is mostly lost, rather than transferred to the other mass). BUT I think I'm missing something because the acceleration toward a BH depends on the distance to it, so how is that change in acceleration handled?

Edited June 1, 2014 by md65536

[MigL]

Extracting and using energy from a black hole is discussed in several books. The most readily available are Hawking's "A Brief History of Time" ( IIRC ) and Thorne's " Black Holes and Curved SpaceTime: Einstein's outrageous legacy".

[phyti]

Thanks for all responses. I'll have to read suggested sources to learn more.