The merger of two black holes

If we cannot get any signal from behind the event horizon, how can two equal-mass nonrotating black holes merge into one spherical black hole? How do we know that the masses were combined into a spherically symmetric system?

           background stars

      ● --->                  <--- ●

    -----------------O--------------> x

                  observer

Let us imagine a head-on collision of two identical nonrotating black holes. They move towards each other on the x axis and will meet at the origin. Our observer is some distance away on the negative y axis.

Both black holes act as powerful gravitational lenses. Long before the collision, they start to distort each other's images. They start to form the Einstein ring in the lens and change shape to a half moon plus an extra image on the other side of the lens.

 

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The main images stretch vertically and are distorted into a crescent shape.

When the black holes are close enough, maybe the half moon images of the black holes combine into a full moon? We would need a computer simulation to calculate the image.

The observer will see many images of each object which fell into the black holes.

The spacetime geometry is determined by the path of light rays. If something looks spherical, then it has distorted the geometry like a spherical object.

Let us ignore the fact that the light coming from an event horizon dies off at a very quick rate.

Because of a mirror symmetry, when looking to the direction of the y axis, the observer will always see light which originated from the background stars. Geodesic lines dictate that at one direction, he can only see light from one object, and because of the symmetry, it cannot be an object which fell into either of the black holes.

Our observer will see the half moons of the black holes glued together but split in the middle where he will see background light arriving.

An object falling into a spherical black hole

Suppose that we have a large black hole at the origin of the x axis. Our observer throws a 1 kilogram weight straight towards it.

We assume that the black hole was built from a collapse of a shell of 1 kilogram weights.

Our observer sees the images of those very many weights floating at the event horizon.

When the extra 1 kg weight moves towards the event horizon, it acts as a gravitational lens. It magnifies the image of the entire black hole slightly. Close to the position where it will hit the horizon, it magnifies more, kind of "making room" for itself to land.

Does the weight leave a bump in the gravitational field of the black hole? Or does the optical magnification even out the apparent mass distribution on the event horizon, so that the gravitational field stays spherically symmetric?

We may imagine a spherical metal ball holding a negative charge, and adding an extra electron to it. The electron will make room for itself and even out the charge density on the surface. Does the same happen if we drop the 1 kg weight to the black hole?