[TommyJ]

'Black' means darkest observable space (even light cannot escape), 'whole' is actually really a well limited by an event horizon, under which everything goes to spaghettification.
Currently there is no means to tell what is inside the event horizon of a black whole. It's roundness is conditioned by Stable circular orbits in higher-dimensional multi-black hole spacetimes. Beyond this visible orbits nothing falls into a static black hole. Our galaxy with all matter has it's orbit = 3R(event horison), light comes closer = 1.5R(event horizon).

[Eternal Student]

Hi.

It's roundness is conditioned by Stable circular orbits in higher-dimensional multi-black hole spacetimes

    I'm not sure what this means.

[Professor Mega-Mind]

I think he means that "singularity" is the face that black-holes put on , when they get up in the morning ! .😄

[TommyJ]

Stable circular orbits in higher-dimensional multi-black hole spacetimes

Let me put some stand here.

The innermost stable circular orbit, or ISCO. In Newtonian gravity, all circular orbits are stable. This means that if such an orbit were perturbed slightly, the orbit would just become somewhat elliptical but nothing else would happen. But in general relativity there is an innermost stable circular orbit. For the Schwarzschild geometry in Schwarzschild coordinates, Rs that innermost radius. Inside Rs, a circular orbit is unstable; a slight perturbation inward leads to a rapidly-opening inspiral to the hole. Thus it is common to assume that accretion disks of gas spiraling onto a black hole are effectively truncated at the ISCO; there is gas inside the ISCO, but no longer is there a disk which moves with slow inward radial motion. Gas pressure gradients change somewhat the location of the ISCO, but this is a decent approximation that allows you to compute approximately the radiative efficiency of a geometrically thin disk.

Shep Doeleman:
“There really are no deeper questions in the universe than how black holes work. Because we know that at their heart they contain this mystery of how do gravity and quantum mechanics work together, which is the deepest question there is right now.”

So, there are lot's of work through different approach has being done and on-going after Einstein predictions.
The greatest breakthrough and fascination is the orbit observation.

[Halc]

But in general relativity there is an innermost stable circular orbit. For the Schwarzschild geometry in Schwarzschild coordinates, Rs that innermost radius. Inside Rs, a circular orbit is unstable

No.  Even immediately outside Rs, no orbits are possible. The nearest possible orbit is outside the photon sphere which is 1.5 Rs. Orbits outside that radius are stable, and those at or inside are impossible. The only unstable orbit is a massless thing exactly at 1.5 Rs.
It gets more complicated with something like a Kerr black hole where orbits become non-circular.

[Eternal Student]

Hi @TommyJ

   Thanks for taking the time to reply.
   The only reasonable possibility is that your  R_s   is not the same notation that most others use.  I concur with Halc where  R_s = the Schwarzschild radius.
   What you've said about the truncation of accretion discs sounds reasonable.

Best wishes.

[evan_au]

'Black' means darkest observable space (even light cannot escape)

Steven Hawking identified a way that light could escape from a black hole, through the eponymous Hawking Radiation.
- The light has a "black body" spectrum
- Although, with a stellar-mass black hole having a black body "temperature" of nanoKelvins, you would be hard-pressed to detect it.
- The peak emission would be in long-wave radio spectrum, rather than in visible light
See: https://en.wikipedia.org/wiki/Hawking_radiation

It gets more complicated with something like a Kerr black hole where orbits become non-circular.

Due to the formation of stellar-mass black holes from the collapse of a rotating star,
- or their mechanism of growth, fed by a spinning accretion disk,
- you would have to assume that most black holes are "rotating" (ie have angular momentum), ie are a Kerr black hole.
See: https://en.wikipedia.org/wiki/Rotating_black_hole

[TommyJ]

Truly, the most of the black hole should be rotating, since they are formed from collapsing stars, and all stars known to be rotating.
 
Rotating black hole is measured by mass and spin (0 to mod [1 - max])).
 
Additional matter, falling into a black hole also contributes to its rotation.
The mass of a black hole is estimated by its influence on other far bodies. Spin is harder to measure than mass, as it can be measured only with the closest particles to the black hole.
Innermost Stable Circular Orbit (ISCO), closer than this no orbits are stable, and falling into a black hole. The faster it spins the smaller the ISCO becomes. The spin is supporting the particles against the gravity.
 
The greater the spin, the closer the ISCO shrinks, up to the event horizon. ISCO’s less than event horizon haven’t been seen yet.
 
So, ISCO and black hole spin are related. One of the closest to the maximum black hole spin detected is 0.98 (GRS 1915+105).
The radii of ISCO discs are measured in the same way as the stars are measured.

[yor_on]

No, they are not holes, they are singularities.

[TommyJ]

No.  Even immediately outside Rs, no orbits are possible. The nearest possible orbit is outside the photon sphere which is 1.5 Rs. Orbits outside that radius are stable, and those at or inside are impossible. The only unstable orbit is a massless thing exactly at 1.5 Rs.
It gets more complicated with something like a Kerr black hole where orbits become non-circular.

Also, no-hair theorem states that all black hole solutions of the Einstein–Maxwell equations of gravitation and electromagnetism in general relativity can be completely characterized by only three externally observable classical parameters: mass, electric charge, and angular momentum.
This metaphor means that during black hole formation the data of the collapsing source is lost [under event horizon].
A black hole is one of the rare cases when the theory was elaborated within math long before any evidence gained. In this ‘singularity’ situation physics laws and their prediction ability are lost.
Lets us keep hoping for a telescope, that is able to have a glimpse inside an event horizon.

[Professor Mega-Mind]

Using what exactly , magnetic-field lines of force ? .🤓

[Origin]

Lets us keep hoping for a telescope, that is able to have a glimpse inside an event horizon.

Why hope for something that is impossible?

[Halc]

Lets us keep hoping for a telescope, that is able to have a glimpse inside an event horizon.

Kind of contradicts the rest of what you said. The only way to have a glimpse inside is simply to jump in yourself. Things will look disappointingly normal (at first) if you choose a large one.
We already know this, so I don't see the point in the experiment. You can't see the singularity any more than I can see 2022 now.

[TommyJ]

Why hope for something that is impossible?

..tangling, right.
What is fascinating (both those who are participating and those who are tracing), that current EHT success is already stepping forward for more data to be extracted (e.g. low-energy radio waves to high-energy gamma rays).
Cooperated group announcing planned works to be proceeded with.

https://www.nasa.gov/mission_pages/chandra/news/black-hole-image-makes-history