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I've read that an observer watching an object fall toward a black hole will never see it cross the event horizon. For example, see the following Stack Exchange question.

How can anything ever fall into a black hole as seen from an outside observer?

I also know that a black hole merger is characterized by 3 phases: inspiral, merger, and ringdown as discussed at the following link.

https://en.wikipedia.org/wiki/Binary_black_hole

That site describes the ringdown as follows:

Immediately following the merger, the now single black hole will “ring” – oscillating in shape between a distorted, elongated spheroid and a flattened spheroid. This ringing is damped in the next stage, called the ringdown, by the emission of gravitational waves. The distortions from the spherical shape rapidly reduce until the final stable sphere is present, with a possible slight distortion due to remaining spin.

To me, this seems to contradict the idea that we never see an object cross the event horizon.

If we never see them merge due to time dilation, then how can we detect gravitational waves of merging black holes?

I know that a black hole might not qualify as an "object", and that the merger of two event horizons might be a special case. If so, then please also explain how we could witness the merger of a black hole with a neutron star.

Thank you.

EDIT: Prompted by an answer to my question, I want to focus my question on how we can observe the RINGDOWN that occurs after the merger.

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The gravitational waves that make it to the detector were never inside the event horizon, so there's no contradiction here. You seem to be under the mistaken impression that somehow the gravitational waves start in or pass through the black hole.

The gravitational radiation that makes it out to the detector is generated in the strong field region near (but outside) the black hole. At some infinitesimal level of precision, even regular objects can generate gravitational waves in theory. What's special about the BH-BH merger or the neutron star-BH merger is that the interaction is so strong that the waves are detectable even at astronomical distances.

Some gravitational radiation generated by the merger does fall into the black hole, but we obviously don't detect that radiation.

Using the visual analogy that you implied at the beginning of the question, this is something like noticing that you can see the object when it's near but outside the event horizon. The fact that you won't see it cross the event horizon has nothing to do with that.

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  • $\begingroup$ Thank you, your answer has helped my understanding. I'm still confused by the "ringdown" description. These are waves that we detect AFTER the merger. But if the merger never occurs in our timeline, then how can we detect the after-effects of a merger? $\endgroup$ – James Jan 8 '20 at 19:15
  • $\begingroup$ The distorted BH that's ringing down still has a strong-field region that's outside the event horizon. Answer still applies. The event horizon is a global feature of the spacetime (something like "two dimensions of space crossed with one dimension of time"). You can take a spacelike slice through that spacetime where you'd say the BH have merged. That again has nothing to do with getting signals from inside the BH. The only problem about seeing the object fall through the EH is that it would require a signal of some sort getting past the EH to you at a large distance away. $\endgroup$ – Brick Jan 8 '20 at 19:20
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Although it is true that you can never see an object cross an event horizon, the interpretation of this as meaning that it "never occuring our timeline" is unhelpful in understanding what is going on.

A more useful way of looking at it, is that the closer an event occurs to the event horizon the longer it takes for a signal from this event to arrive at a distant observer with this signal time approaching infinity as you place the event closer to the event horizon. There are perfectly good local coordinates in which there merger of two black holes (the formation of a common apparent horizon) happens at a finite time.

In such coordinates, the ringdown signal does not originate from the horizon, but rather from the gravitational degrees of freedom around it (peaked roughly at the photonsphere). This is then fully consistent with the signal reaching us at a finite time.

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