*Originally posted by OdBod*

**We are told this happens,especially when galaxies collide.If an outside observer perceives time progressivly slowing for something falling into a Black Hole, then what will an outside observer see when two Black Holes come togeather?**

If you were floating in space looking at an object falling into a black hole then you would see the

object slowing and reddening as it got closer and closer to the event horizon.

This happens because the closer it gets to the black hole event horizon the more stretched space

becomes and so the longer it takes for a photon to climb out of the gravity well and reach your eye/camera.

Light takes longer and longer to escape the combined gravities of the object and the black hole until

the object reaches the event horizon at which point the light takes infinity time to reach you.

So you never "see" an object reach or cross the event horizon.

However for two black holes merging, what you would see is incredibly complicated.

Space would be incredibly distorted as the two black holes spiral in towards each other leading to all kinds

of lensing effects and distortions with the event horizons of the black holes wobbling and distorting and

eventually merging.

To see what it would look like would require enormous amounts of computing power to simulate.

However it would definitely not take forever.

http://www.astro.cardiff.ac.uk/research/gravity/tutorial/?page=4blackholecollisions

Has a little bit on the topic.