I feel the obvious answer should be “no” but help me think this through. It came from the previous Q on blackholes and am posting here for more visibility.
So considering two blackholes rotating about each other and eventually combining. It’s in this situation that we get gravitational waves which we can detect (LIGO experiments). But what happens in the closing moments when the blackholes are within each others event horizon but not yet combined (and so still rotating rapidly about each other). Do the gravitational waves abruptly stop? Or are we privy to this “information” about what’s going on inside an event horizon.
Thinking more generally, if the distribution of mass inside an event horizon can affect spacetime outside of the horizon then what happens in the following situation:
imagine a gigantic blackhole, one that allows a long time between passing the horizon and being crushed. You approach the horizon in a giant spacecraft and hover at a safe distance. You release a supermassive probe to descend past the horizon. The probe is supermassive in the way a mountain is supermassive. The intention is to be able to detect it’s location via perturbation in the gravity field alone. Similar to how an actual mountain causes a pendulum to hang a miniscule yet measurable distance off the vertical.
Say the probe now descends down past the horizon, at some distance off the normal. Say a quarter mile to the ‘left’ if you consider the direction of the blackholes gravitational pull.
Let’s say you had set the probes computer to perform some experiment, and a simple “yay/nay” indicated by it either staying on its current course down (yay) or it firing it’s rockets laterally so that it approaches the direct line been you and the singularity and ends up about a quarter mile ‘right’ (to indicate nay).
The question is, is the relative position of the mass of this probe detectable by examining the resultant gravitational force exerted on your spaceship? Had it remained just off of centre minutely to the ‘left’ where it started to indicate the probe communicating ‘yay’ to you, or has it now deflected minutely to the right indicating ‘nay’?
Whether the answer to this is yes or no, I’m confused what would happen in real life?
If the probes relative location is not detectable via gravity once it crosses the horizon, what happens as it approaches? Your very sensitive gravity equipment originally had a slight deviation to the left when both you and probe were outside the horizon. Does it abruptly disappear when it crosses the horizon? If so where does it go? The mass of the probe will eventually join with the mass of the singularity to make the blackhole slightly more massive. But does the gravitational pull of its mass instantly change from the location in the horizon where it crossed (about a quarter mile to the ‘left’) to now being at the singularity directly below. Anything “instant” doesn’t seem right.
Or… it’s relative position within the horizon is detectable based on you examining the very slight deviations of your super sensitive pendulum equipment on board your space craft. And you’re able to track it’s relative position as it descends, until it’s minute contribution to gravity has coalesced with the main blackhole.
But if this is the case then aren’t we now getting information from within the horizon? Couldn’t you set your probe to do experiments and then pass information back to you by it performing some rudimentary dance of manoeuvres? Which also seems crazy?
So both options seem crazy? Which is it?
(Note, this is a thought experiment. The probe is supermassive using some sort of future tech that’s imaginable but far from possible by today’s standards. Think a small planet with fusion powered engines or whatever. The point is, in principle, mass is detectable, and mass is moveable. Is this a way to peek inside a blackhole??)
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
Gravity is a vector field without distinguishing differences between one source and another. The gravity of the falling mass always was and always will be “joined” with that of the black hole, and every other piece of surrounding matter. It’s not like light where two nearby sources remain distinguishable. There’s no “bobbled” structure, just a very very slight shift in the location of the center of mass which gets smaller as the falling object gets closer.
As for the faster rotation of colliding black holes, event horizons aren’t objects, they’re regions of spacetime, and larger than the actual “surface” of a black hole. They combine into a single event horizon long before they ever actually “touch” each other.
Sure. But if we’re observing this “slight shift in the centre of mass” outside the event horizon then that suggests one could message from inside the horizon to outside by moving a suitably heavy mass side to side.
Well, no. Because the “event” of that movement never makes it outside. Hence event horizon. Causality itself cannot traverse it.
edit: Put differently, the propagation of gravity still happens at the local speed of light and is constrained by curved spacetime, just like everything else. Gravity itself is, in a sense, affected by gravity.
So when LIGO is detecting gravitational waves of two blackholes rapidly orbiting each other until they collide. Do the gravitational waves gradually slow and then stop as the blackholes approach each others event horizon? Are we ever actually observing the blackholes merge? Because if we are then doesn’t that suggest we’re observing something via gravitational waves inside the event horizon? And if we’re aren’t, doesn’t that suggest the black holes never actually merge (from our frame of reference) they just get infinitely close to each others event horizon without us observing them crossing it?
I already addressed this in an earlier comment. Event horizons aren’t objects, they’re regions. Your final suggestion here is roughly correct, the black holes proper never merge from our frame of reference but their event horizons do long before that would happen.
Yes, I was confusing the picture there a bit… I meant would we ever actually see the result of one black hole crossing where the others event horizon would have been… but that was a bit of a mouthful.
Thanks, yes, understand that the event horizon becomes one big region roughly like a figure 8 on its side as the blackholes approach each other
But it still feels like we detect things happening after this merging of event horizons? Is that not so?
Either we get gravitational waves of the two singularities continuing to orbit each other within this one big event horizon, or we don’t. If we do that’s information from inside an event horizon. And if we don’t it suggests that that (from our reference) the two singularities remain apart, the two blackholes now having this one 8-shaped event horizon, which forever remains in this shape.
Neither seem right to me to be honest lol…
We don’t. That’s why the indicator of it happening is a rapidly elevating frequency as they get closer that suddenly drops to nothing once the event horizons converge.
https://www.youtube.com/watch?v=1agm33iEAuo