# B Black hole ringdown

#### Lord Crc

I thought I'd create a new thread rather than derail.

In https://www.physicsforums.com/threads/if-quark-stars-exist-why-do-neutron-stars-become-black-holes.937452/#post-6143231 it was explained to me that gravitational waves do not escape the interior.

The reason I thought they did is that we have observed black hole ringdown, and that the ringdown was gravitational waves from the interior mass settling down.

If gravitational waves can't escape from the interior, what is the source/driver of the ringdown?

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#### Drakkith

Staff Emeritus
2018 Award
I believe the event horizon itself (or spacetime at the event horizon rather) is 'misshapen' and the ringdown is the result of the change in the EH as it settles into a stable state. Someone correct me if I'm wrong please.

#### Lord Crc

Right, but how does that happen, after all the EH isn't a "thing".

#### Drakkith

Staff Emeritus
2018 Award
Couldn't tell you. Unfortunately I know very little details about this topic.

#### phyzguy

Whether the EH is a "thing" or not we could debate, but the fact is that the space-time metric obeys the Einstein Field Equations. When the merged BH is first formed, the metric around it is distorted from the long-time solution, which is a Kerr metric. It quickly relaxes to the Kerr solution, radiating gravitational waves as it relaxes. The gravitational waves are not coming from the interior of the BH, but from the distorted space-time region around the BH. The simulations of this happening (which I'm sure you've seen) are obtained by solving the Einstein Field Equations given the initial conditions of two separate BHs spiraling in.

#### Lord Crc

So I should imagine the EH as a (Neumann or something) boundary condition as far as the external spacetime is concerned? And the ringdown is the reconfiguration (relaxation) of the external spacetime happening when the two boundaries become one?

#### Nugatory

Mentor
If gravitational waves can't escape from the interior, what is the source/driver of the ringdown?
The gravitational waves come from the rapidly changing curvature of spacetime near but outside of the two black holes. The event horizons are irrelevant here; in fact two colliding neutron stars will also produce gravitational waves even though they have no event horizons.

#### Lord Crc

I can take that as a confirmation of my post #6 Nugatory?

In that case I get the ringdown, thanks for the replies.

#### Lord Crc

So I managed to confuse myself by thinking more about this.

I thought gravitational waves were how changes in the gravitational field was propagated. The Insight https://www.physicsforums.com/insights/how-fast-do-changes-in-the-gravitational-field-propagate/ says so as well.

What got me confused was the following scenario: take a stationary black hole of suitable size (tens of solar masses?) with an orbiting companion body, a neutron star for example. Then shoot another suitably sized black hole into the system with such a velocity that it grazes the EH of the stationary black hole and escapes the system.

The intruder BH should cause the stationary BH to move no? If it does, the gravitational field felt by the companion mass would change too no? How is this change propagated?

#### Lord Crc

Maybe this should have been a new thread, I see how ringdown is spacetime settling down. This question was more about gravitational waves escaping EH.

#### Lord Crc

I posted a new thread in the GR forum with the follow up question.

#### PeterDonis

Mentor
I believe the event horizon itself (or spacetime at the event horizon rather) is 'misshapen'
This is how it's often described in pop science articles, but IMO it's misleading. Since nothing can escape from the horizon, the gravitational waves emitted by the merger can't be coming from the horizon, or inside it. The waves are coming from a region of fluctuating spacetime curvature outside the horizon: basically they carry away that fluctuating spacetime curvature so that what remains is the horizon of the new merged black hole.

It is true that the horizon's shape changes as the waves are emitted; but you have to be very careful how you interpret that piece of ordinary language.

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