
How Supermassive Black Holes Were Discovered - dnetesn
http://nautil.us/issue/69/patterns/how-supermassive-black-holes-were-discovered
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ars
Note that the data is still not definitive proof that black holes exist. The
observed density is above that of a black hole. If there exists something that
prevents black holes from forming, we would not be able to tell.

So despite all the data black holes have still never been observed.

What might prevent a black hole from forming? Time dilation is a top candidate
(leading to what's called "frozen stars", or "collapsed stars"). From our POV
the black hole never quite forms. (And yes I know, infalling matter sees a
black hole - but we don't.)

Other options would be quark degeneracy leading to quark stars (similar to
neutron stars). Is that strong enough to stop a black hole? It's unknown.

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raattgift
> I know, infalling matter sees a black hole - but we don't

Sure we do; a black hole is defined by the presence of a horizon, and we will
certainly see infallers suffering one-way dimming and redshifting into
undetectability. Nobody outside will see what has become of any of the
original collapsed matter, either, barring failure of the censorship
conjectures and ignoring evaporation. (Infalling matter, however, will collide
with whatever state matter inside takes, whether that's classical GR's
gravitational singularity or some degenerate form of matter that avoids
ultimate collapse.)

> "frozen stars" ... "collapsed stars"

Right: collapsed to the point where all the material has become too dim and
redshifted to observe; frozen to the point where outside observers cannot
track the evolution of the material inside the horizon.

> quark stars (similar to neutron stars)

Similar in that the critical radius is inside the object, rather than the
reverse, and that the critical density is not reached within that radius
because of terms offsetting internal pressure. Earth is similar to neutron
stars in that way too.

An ultrarelativistic fermion gas of quarks _inside_ a horizon is still a black
hole to any outside observer; the observables from abolishing the
gravitational singularity using such a gas (or anything similar; cf. preon
stars etc.) are only revealed, if at all, during evaporation.

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hnuser123456
Wow, thanks for sharing. I've never seen a real photo of a black hole before.

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cilefen
No actual photos yet exist of the environment immediately surrounding a black
hole. But here is a time lapse video of stars orbiting Sag A:
[https://m.youtube.com/watch?v=TF8THY5spmo](https://m.youtube.com/watch?v=TF8THY5spmo).
The Event Horizon Telescope project is trying to image some black hole
surroundings
[https://en.m.wikipedia.org/wiki/Event_Horizon_Telescope](https://en.m.wikipedia.org/wiki/Event_Horizon_Telescope)

~~~
hnuser123456
Believe me, I'm constantly checking for updates from EHT. And there is a radio
photo of Sgr A* in the article, near the end.

