the extra mass is converted into energy in the form of gravitational waves (maybe other forms too idk but this is part of it)

Rather confused. 225 solar masses isn’t gigantic by any means

They can merge event horizons on a finite timescale from the outside and still take an infinite amount of time to merge singularities or whatever it is that singularities do when they meet up in the privacy of an event horizon.

An observer falling into the black hole would not observe any distortion in time. They would simply fall in, under the influence of gravity. From the perspective of a far-away observer it would look as if time is slowing down as the photons would take increasingly longer to escape. At the event horizon the photons would effectively be held in place. Eventually though, the last photon will have escaped and you will just observe a slightly larger black hole.

So the merger definitely happens from the point of view of the black holes. We might observe odd artifacts but they would eventually fade away.

Black holes' radius is linear in proportional to their mass, they are not constant density, larger ones are less dense. Therefore, when a 5 solar mass BH merges with another 5 solar mass, the radius doubles, surface area quadruples, and volume goes up 8x. If they "never actually merge", SMBHs would not exist.

Collision is when they approach each other, then round down to one black hole.

Large mass stars are very, very few in number, and have short lifetimes, so collapse into black holes (if it happens), and then mergers are vanishingly small. 225 solar masses black holes are in the realms of unlikely if only had that information.

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

Then again, the universe is really big and to paraphrase Pratchett, million-to-one chances happen nine times out of ten.

https://en.wikipedia.org/wiki/Phoenix_Cluster#Supermassive_b...

"Such a high mass may place it into a proposed category of stupendously large black holes (SLABs), black holes that may have been seeded by primordial black holes with masses that may reach 100 billion M or more, larger than the upper maximum limit for at least luminous accreting black holes hosted by disc galaxies of about 50 billion M[12]"

(which comes from a paper from one of my old lecturers).

If we accept that primordial black holes do indeed exist (the evidence against, is, I believe that they should be evaporating about now, but are as yet unobserved), there must therefore be a weight distribution of those, which means there are probably other 'solar mass' 'seeded' black holes lurking, that were never created by stellar collapse.

There are no medium sized bkack holes. As far as we look back in time with james webb,the largest are already there.

Just guessing, but maybe the common situation is that one ever growing black hole absorbes small ones? But that two of these large ones merging "should not happen"?

It wouldn't do anything special actually. A black hole from a distance does nothing a sun can't do.

Black holes only become destructive/powerful when you are very close to them.

To elaborate: A black hole is mass, a sun is mass. From a distance there's no difference. The only difference is up close - you can get a lot closer to a black hole dramatically increasing the gravitational force.

But from a distance? Nothing special.

The models are constrained by what is known at the time they are built - those that clash with existing observations are discarded while those that don't, get published.

There just isn't a way to make black holes that big from the collapse of stars when they go supernova.

So maybe both of these black holes formed from earlier mergers of smaller black holes. Or maybe there are other ways to make larger black holes we don't know about. They are in a range of mass we don't really expect to see theoretically.

Thanks – I changed it (earlier today) to:

Black hole merger challenges our understanding of black hole formation

“Black hole merger detected that defies theoretical boundaries.”