Thanks, I'll look that up

I guess I'm getting used to the idea that the result of two blackholes merging is not spherical. If you imagine one mass falling into the other it never gets there, just slows. So although the event horizon has coalesced, the two centre masses have not physically met (from our point of view). So the merged blackholes event horizon should really be like a very oblate 8 shape?

To add to this, anything that crosses the "horizon" will appear to stay there, from an outside perspective, like a still photograph. Eventually that image of it will fade away as time goes on. Meanwhile that object has long since been violently destroyed and sucked into the center of the black hole. Nothing but particles survive this.

The only thing that comes out of a black hole is Hawking Radiation Which is basically the black hole spewing out radiation from the crushed mass.

Here's a good introduction to this idea by PBS Spacetime They're a great entry point into a lot of very complicated science, described in an easy to understand way.

Edit: The ending of Interstellar is just story telling. Cooper would have been stretched apart instantly and destroyed at a fundamental level when he got anywhere near the black hole due to the insanely extreme gravitational force.

So.. Let's think that through.. say a planet was falling into the blackhole. With respect to visible light, yes, makes sense to me it's progressively red shifted and you never see it cross the horizon. But if this applies to the gravity of that planet too, then its gravity is forever detectable at that point where it appears to be crossing. The blackhole therefore appears to have a 'bobbled' structure. The main mass of the blackhole, and a minor mass 'glued' to its side. It means the gravity of mass falling into the blackhole never joins it at the centre but is forever detectable at its surface? And if mass fell into the blackhole in an uneven way then gravitationally the blackhole would forever appear 'uneven' rather than perfectly smooth?

Now let's do the same for two blackholes colliding. They each get progressively 'red shifted' and never cross they others event horizon

So are we saying if we were close enough to two black holes that merged, we'd detect, gravitationally, that they actually have not merged, but are sort of frozen at each others horizon??

I mean, this may well be the case, but as far as data LIGO detected goes (there are places where you can hear a sound representation of merging blackholes circling closer and closer until they merge). The impression is that their orbit round each other gets faster and faster until there's a single abrupt moment they collide.

But this seems at odds with them 'red shifting' and getting stuck at the event horizon. Surely that signal would look like gravitational oscillations (of their mutual orbit) getting slower and slower the closer to the horizon they got? Doesn't seem to add up

my theory is that nothing ever crosses the event horizon. as you approach a black hole time dilates so the event horizon should recede from you as you approach it until the singularity evaporates. the idea that reality decouples never made sense to me but I'm sure my idea has been disproved by someone with an above room temperature IQ.

So how does mass falling into a blackhole ever appear to add to its mass at the centre? If we're saying even gravitational waves can't escape. This sounds like two merged blackholes don't actually ever merge, they get forever 'stuck' at each others event horizon.

If that were the case it feels like the rapid oscillation of gravitational waves that LIGO detected should actually slow down as the blackholes horizons touch. But as far as I've seen this isn't the case. The waves increase rapidly in frequency until there's an abrupt moment the blackholes merge and the waves abruptly stop?

But that makes no sense if each blackhole is progressively 'red shifted' at the others horizon

Yup, am visualising it as a distinct 'hard line' barrier.

Nothing that's crossed the horizon can casually affect anything outside the horizon. But that means when our super massive probe approaches the horizon and is gradually red shifted, never actually appearing to cross, then it's mass is also forever detectable at that point. That means it's mass never joins the main black hole at the centre. The blackhole has a 'bobbled' appearance, gravitationally speaking, with uneven blobs of mass falling into it forever stuck at its surface. But if this is the case then how do blackholes get bigger? If any mass that falls into them after they first form is stuck at the surface. Does that mean the singularity is forever the mass of the instant the blackhole first formed and ALL additional mass consists of mass frozen at the surface of the event horizon?

What does that mean for merging blackholes? They never actually merge? They're just forever frozen at each others even horizon?

That could well be the case, it just seems weird. And it doesn't seem to be what the LIGO gravitational wave signal seems to imply (from my completely unprofessional, uncredentialled, uninformed view)

Yes, mass has to be thrown out as rocket fuel and momentum conserved. But this doesn't mean the distribution of mass has to be equal. Think of a spacecraft (single concentrated area of mass) and the gaseous trail of its spent fuel (indistinct tail of low density mass spread over a much wider area). It would seem in that circumstance the single area of mass of the spacecraft is detectable as having a different profile to the much wider spread out exhaust. (Or to make the comparison more extreme, it's the gravity of a mountain sized 'probe' versus a trail of propellant the consistency of cloud stretching thousands of miles. Seems like these would have different gravitational profiles)