[thedoc (OP)]

Kerry Ellis  asked the Naked Scientists:
   Hi.

Firstly, I want to say how enjoyable and informative both the Naked
Scientist and Naked Astronomy podcasts are.  Well done to all involved.

In a podcast earlier this year I believe there was a statement made about gravity promulgating at the speed of light.  As I understand it black holes attract matter inside their event horizon at a speed greater than light, which is why light cannot escape.  If the promulgation of gravity is limited to the speed limit and within a black hole the speed is greater than the speed of light, how does the gravitational force from the black hole escape?

Kerry Ellis

South Australia

What do you think?

[Janus]

When one speaks of "gravity" traveling at the speed of light, they really mean gravitational waves or radiation, and this is not the same as the gravitational field which causes the attraction between objects. Gravitational waves do not "carry" the gravitational force, they carry information about changes in gravity
So while information about changes to the field cannot escape from inside the event horizon to outside, gravity outside the event horizon remains (because it hasn't gotten any information to tell it otherwise. One old term for a black hole was a "frozen star", so called because from the outside, the black hole seemed to be "frozen" in the state it was in just before the event horizon formed.)
Gravity doesn't have to "catch up" to light, as the light is interacting with the gravity field that is already there. 

Another way to look at it is that gravity is a curvature of space-time, and that inside a black hole's event horizon, the curvature is so great that all paths lead back to the center. Light inside the event horizon is forced to follow a path that doesn't lead out of the event horizon.  Outside of the event horizon, the space-time curvature just isn't as severe. The event horizon just marks the boundary between all paths leading inward, and some paths leading away.

[evan_au]

how does the gravitational force from the black hole escape?

A black hole typically forms from the collapse of a massive star.
- The black hole has almost the same mass as the original star (some matter, neutrinos and light escape in the explosion)
- The gravitational field of the star has been present for millions of years while the star is burning, and has been felt for a distance of millions of light-years around the star.
- According to the General theory of Relativity, the mass of the star causes a curvature in spacetime, which will bend the path of light passing near the star.
- Imagine placing a bowling ball in the middle of a trampoline. The whole trampoline will drop a bit, with the most extreme curvature near the bowling ball. If you roll a marble across the trampoline, it's path will be deflected by the mass of the bowling ball.
- During a collapse to a black hole, the mass of the star cannot be held up by the internal pressure of the star, so the star collapses inwards, reaching speeds that are a significant fraction of the speed of light.
- When the mass of the star is in a smaller volume, passing light will be bent even more strongly by the stronger surface gravity.
- Imagine replacing the bowling ball by a small cannonball of the same mass. The curvature of the trampoline will be similar near the edges, but it will be more extreme near the very dense cannonball. A marble can now pass closer to the center of the depression, and be deflected even more.
- When the mass of the star falls within the diameter of a black hole (a few km across), any light passing close by will be bent in a circle, and never escape. Light passing even closer will pass inside the event horizon, never to be seen again.
- The gravitational waves reported early in 2016 were produced by two merging black holes. Each isolated black hole has it's own steady gravitational field. But when two black holes circle each other, they produce "ripples" on this steady gravitational field which is already felt outside the event horizon. These ripples travel at the speed of light.
- Imagine two cannonballs in the center of a trampoline, spinning around each other. This will produce ripples that extend out towards the edges of the trampoline (until they run into each other and stop).

So the gravitational field of the star does not need to "escape" from the black hole - it has always been there, outside the star and black hole.
- According to General Relativity, there are 3 things you can tell about a black hole from the outside:
- It's mass
- It's angular momentum (rotation)
- It's electric charge

We know that General Relativity is incomplete as a theory - it is not compatible with quantum-level effects near a black hole like Gravitons (the particles which are thought to carry gravitational attraction).
- Physicists are working to align Quantum Theory and General Relativity through avenues like "string theory" and "quantum loop gravity". But no major breakthroughs have been announced as yet.
- When there is a breakthrough, perhaps they will be able to explain how Gravitons can be exchanged between two orbiting black holes?

[yor_on]

No, gravity waves, if they exist 'propagate' at the speed of light. Gravity in itself should be more of 'all existent' everywhere, although maybe not measurably so in 'deep space', as I think of it at least. The reason light is 'bent' inside a event horizon is due to lights energy and momentum, giving it a equivalence to 'mass'. It follows the paths SpaceTime and gravity draws for it, to do otherwise would be to 'accelerate' intrinsically as some weird rocket (no influence by gravity in other words). ( A 'acceleration' when it comes to light is blue, respectively red shift, and will always need a observer to be defined. It's a proposition, or measurement, made by you between frames of reference. )

"Spacetime tells matter how to move; matter tells spacetime how to curve"

So a 'event horizon' becomes a limit from where those light paths down-under never will be able to reach you, you observing that black hole from a distance.
=

when it comes to how 'gravity escapes', then that is a result of the mass that 'black hole' represents. SpaceTime adapts to it, all the way to event horizon and beyond. Whatever measurements we can do though ends at that event horizon, as far as I know.

[jeffreyH]

An interesting question is what density of gravitons is required to stop one photon? Will it be 10 gravitons to 1 photon? A million to 1? If this always happens at the event horizon of any sized black hole then can the mathematics always produce the same density in that region or does the energy of an individual graviton need to increase with radial distance to the horizon?