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Troubled by Nova show on black holes

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#1 City Kid

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Posted 14 January 2018 - 01:40 AM

I was just watching the Nova show Black Hole Apocalypse. In it, Janna Levin (I think) made the comment that a black hole isn't an object, it's a hole in space where nothing exists. Am I misunderstanding something or is that flat out wrong? My understanding is a black hole is as much an object as a star, planet, or anything else. It just happens to have so much mass that not even light can escape beyond it's event horizon. Am I missing something? It's troubling to me that an astrophysicist would make a false statement on a show like this.


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#2 otocycle

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Posted 14 January 2018 - 01:55 AM

I watched the NOVA program, and when Levin said a black hole is not an object, she was holding  a solid black object (like a black billiard ball) that was graphically morphed into the black absence of an object.   What she was trying to demonstrate was a black hole's very different nature as a space time singularity that warps anything past the event horizon.   So in this way it is not an object in the conventional sense.

 

Accordingly, I don't think her statement was false or misleading.

 

Nat Geo produced a very similar black hole program years before NOVA, and in it the same thing is stated:  black holes are more like a region than an object:

 

https://www.youtube....h?v=FAyWr79pLog


Edited by otocycle, 14 January 2018 - 02:27 AM.

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#3 bobzeq25

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Posted 14 January 2018 - 02:55 AM

The edge of a black hole is not like the edge of an object.  The event horizon is simply a region where the mass inside (which may be essentially a point) is generating enough gravity to stop light from escaping.  So what could be "seen" as the edge of a black hole is very much not an object.


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#4 Codbear

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Posted 14 January 2018 - 03:43 AM

I did not see the Nova episode but the information is correct concerning black holes. 

 

As otocycle said, think of a black hole as a region of space. The sperical "edge" of the black hole is not a physical substance like a bubble, but it defines the outer limits of the black hole. At the center of the black hole is a singularity, which is defined as an infinitely dense, therefore volumeless, point in space where all the matter that created the black hole has been crushed into by its own gravity.

 

The reason the region is black is due to the escape velocity of the black hole being greater than the speed of light, so no visible light escapes from the black hole . To put it in perspective, the escape velocity of earth is about 7 miles a second, whereas at the event horizon of a black hole it would be the speed of light, or about 186,000 miles per second.


Edited by Codbear, 14 January 2018 - 04:01 AM.


#5 ishorx

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Posted 14 January 2018 - 05:21 AM

hi,acording to the scientists all known laws of physics breakdown with a blackhole and they can't really explain it but yet there is something there....immense gravity,they infer it exists because of the effect it has on objects near it so it is something



#6 Nikonuser

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Posted 14 January 2018 - 05:23 AM

I was just watching the Nova show Black Hole Apocalypse. In it, Janna Levin (I think) made the comment that a black hole isn't an object, it's a hole in space where nothing exists. Am I misunderstanding something or is that flat out wrong? My understanding is a black hole is as much an object as a star, planet, or anything else. It just happens to have so much mass that not even light can escape beyond it's event horizon. Am I missing something? It's troubling to me that an astrophysicist would make a false statement on a show like this.

 

 

No one knows


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#7 B l a k S t a r

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Posted 14 January 2018 - 08:06 AM

I've heard physicists refer to a neutron star as being the densest objects known. That had me scratching my head as black holes are made of matter which has mass far greater than any other object known as evidenced by their gravitational fields. They also come in different sizes which is super.

 

This singularity concept seems to be a mathematical construct that may not reflect reality. No one knows, seems to be the current correct answer. Newton-> Einstein-> Black hole / Dark matter-energy / quantum. 


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#8 HarryRik9

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Posted 14 January 2018 - 08:20 AM

Black Holes are a theoretical speculation.Opinions differ as to whether or not the theory correctly predicts them. 



#9 earlyriser

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Posted 14 January 2018 - 08:42 AM

From our reference frame, time dilation causes everything to stop at the event horizon. So, I have a hard time getting my head around how anything gets past this barrier. It would seem that matter will just pile up at the event horizon until the additional mass causes the Swartzchild radius to extend past the matter, at which point everything breaks down and who knows what happens then.



#10 Jim Davis

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Posted 14 January 2018 - 08:47 AM

From our reference frame, time dilation causes everything to stop at the event horizon. So, I have a hard time getting my head around how anything gets past this barrier. It would seem that matter will just pile up at the event horizon until the additional mass causes the Swartzchild radius to extend past the matter, at which point everything breaks down and who knows what happens then.

Time only appears to stop at the event horizon to an outside observer. To the object falling in, time continues as normal. Another example: light moves at the speed of light, so to an observer, its time has stopped. That doesn't mean it stops moving.


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#11 MG1692

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Posted 14 January 2018 - 09:11 AM

hi,acording to the scientists all known laws of physics breakdown with a blackhole and they can't really explain it but yet there is something there....immense gravity,they infer it exists because of the effect it has on objects near it so it is something

A friend of mine in the business once described a black hole as the place Phds go to die :)


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#12 shawnhar

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Posted 14 January 2018 - 10:07 AM

 

From our reference frame, time dilation causes everything to stop at the event horizon. So, I have a hard time getting my head around how anything gets past this barrier. It would seem that matter will just pile up at the event horizon until the additional mass causes the Swartzchild radius to extend past the matter, at which point everything breaks down and who knows what happens then.

Time only appears to stop at the event horizon to an outside observer. To the object falling in, time continues as normal. Another example: light moves at the speed of light, so to an observer, its time has stopped. That doesn't mean it stops moving.

 

I really hate this argument.

 - So if I existed before the "black hole", and watched the star die, I could watch it until the end of time, literally the end of time, and nothing would ever fall into it, not a single particle, nothing, ever, for all of time.

 

 "But that doesn't mean it stops moving" doesn't make sense, since I can watch it forever and it's clearly not moving.



#13 Jim Davis

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Posted 14 January 2018 - 10:36 AM

 

 

From our reference frame, time dilation causes everything to stop at the event horizon. So, I have a hard time getting my head around how anything gets past this barrier. It would seem that matter will just pile up at the event horizon until the additional mass causes the Swartzchild radius to extend past the matter, at which point everything breaks down and who knows what happens then.

Time only appears to stop at the event horizon to an outside observer. To the object falling in, time continues as normal. Another example: light moves at the speed of light, so to an observer, its time has stopped. That doesn't mean it stops moving.

 

I really hate this argument.

 - So if I existed before the "black hole", and watched the star die, I could watch it until the end of time, literally the end of time, and nothing would ever fall into it, not a single particle, nothing, ever, for all of time.

 

 "But that doesn't mean it stops moving" doesn't make sense, since I can watch it forever and it's clearly not moving.

 

No, that's not what happens. You would see the star fall in, and stop. Then fade away, since every bit of light that will ever escape has been created, and there will never be any more. It seems to fade since the last light has to climb a very steep gravity well, so from your perspective, earlier events will appear first, and latter ones stretched out as they climb the gravity well. Eventually there will be no more, since no more will be created. Light from outside cannot shine on it and bounce off to illuminate it for you, its not there any more.



#14 llanitedave

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Posted 14 January 2018 - 11:45 AM

 

 

From our reference frame, time dilation causes everything to stop at the event horizon. So, I have a hard time getting my head around how anything gets past this barrier. It would seem that matter will just pile up at the event horizon until the additional mass causes the Swartzchild radius to extend past the matter, at which point everything breaks down and who knows what happens then.

Time only appears to stop at the event horizon to an outside observer. To the object falling in, time continues as normal. Another example: light moves at the speed of light, so to an observer, its time has stopped. That doesn't mean it stops moving.

 

I really hate this argument.

 - So if I existed before the "black hole", and watched the star die, I could watch it until the end of time, literally the end of time, and nothing would ever fall into it, not a single particle, nothing, ever, for all of time.

 

 "But that doesn't mean it stops moving" doesn't make sense, since I can watch it forever and it's clearly not moving.

 

It's red shifting, and every object that approaches and then arrives at the event horizon experiences, to the outside observer, a gravitational redshift that asymptotically lengthens towards infinity.  You don't see it "fall in", because no light will escape from that point.  But between the relativistic slowdown of time and the gravitational red shift, things would appear to gradually fade away rather than wink out.


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#15 HarryRik9

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Posted 14 January 2018 - 01:05 PM

 

From our reference frame, time dilation causes everything to stop at the event horizon. So, I have a hard time getting my head around how anything gets past this barrier. It would seem that matter will just pile up at the event horizon until the additional mass causes the Swartzchild radius to extend past the matter, at which point everything breaks down and who knows what happens then.

Time only appears to stop at the event horizon to an outside observer. To the object falling in, time continues as normal. Another example: light moves at the speed of light, so to an observer, its time has stopped. That doesn't mean it stops moving.

 

The claim that time only appears to stop at the event horizon is disproved by the fact that we know that in the GPS system the clocks on the ground run slower than the clocks in the satellites. So it is not correct to say that time runs normally for objects falling into the Black Hole, that claim is not in accordance with experiments. 



#16 earlyriser

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Posted 14 January 2018 - 01:14 PM

Could matter falling into a black hole form a dense shell around the Swartzchild radius rather than a singularity? I don't think there is any way to form a singularity without violating Relativity, and so far Relativity has held up to every test thrown at it. 



#17 Jim Davis

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Posted 14 January 2018 - 01:15 PM

 

 

From our reference frame, time dilation causes everything to stop at the event horizon. So, I have a hard time getting my head around how anything gets past this barrier. It would seem that matter will just pile up at the event horizon until the additional mass causes the Swartzchild radius to extend past the matter, at which point everything breaks down and who knows what happens then.

Time only appears to stop at the event horizon to an outside observer. To the object falling in, time continues as normal. Another example: light moves at the speed of light, so to an observer, its time has stopped. That doesn't mean it stops moving.

 

The claim that time only appears to stop at the event horizon is disproved by the fact that we know that in the GPS system the clocks on the ground run slower than the clocks in the satellites. So it is not correct to say that time runs normally for objects falling into the Black Hole, that claim is not in accordance with experiments. 

 

GPS clocks on the ground run slower than GPS clocks MOVING in orbit, from the point of view of someone on the ground. This is exactly what the theory of relativity of predicts. If you were on the satellite, looking at the GPS on the ground, to you the clock on the ground would be running slower.



#18 earlyriser

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Posted 14 January 2018 - 01:19 PM

 

From our reference frame, time dilation causes everything to stop at the event horizon. So, I have a hard time getting my head around how anything gets past this barrier. It would seem that matter will just pile up at the event horizon until the additional mass causes the Swartzchild radius to extend past the matter, at which point everything breaks down and who knows what happens then.

Time only appears to stop at the event horizon to an outside observer. To the object falling in, time continues as normal. Another example: light moves at the speed of light, so to an observer, its time has stopped. That doesn't mean it stops moving.

 

Observed from our frame of reference, it doesn't just appear to stop, it does. Time dilation is a real effect. The occupants of a space ship that ventured close to a black hole for say an hour ship time would return millions or billions of years after they left in our time.


Edited by earlyriser, 14 January 2018 - 01:22 PM.


#19 EJN

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Posted 14 January 2018 - 01:20 PM

There was a recent similar thread here, I will repost what I said there rather than re-type it all.
 
It is true that the event horizon is not a physical object, but a boundary between spacetime
regions where light can escape, and where light cannot.

 

_________________________________________________________________________________

 

Quoting myself:

 

In the classical theory of general relativity,
a singularity is not a physical object, but a coordinate singularity, a point at which the spacetime
metric becomes undefined. The north (and south) poles of the earth are coordinate singularities
in the system we use to define longitude, since all longitude lines converge at the poles, the
longitude of the poles is undefined.

Taken literally, the singularity is a point of infinite spacetime curvature. As a physical object, it
is almost certain that quantum effects would become dominant, but since no reasonable theory
of quantum gravity exists, the true nature of the "singularity" remains conjecture. Also, the formation
of the event horizon which defines the black hole is well described by the classical theory of GR,
and whether or not singularities exist has no bearing on whether or not black holes themselves exist.

 

In the classical theory of GR that is actually true. Gravitational collapse always leads to a singularity
if there is no stable state which can resist further collapse, with neutron degenerate matter being
the most dense stable state known to exist, and beyond 3 solar masses collapse to a black hole
appears to be inevitable.

In fact there developed the principle of "cosmic censorship" which stated that all singularities
are hidden by event horizons. However, in classical theory, the big bang is a "naked" singularity.

The problem being at some point quantum mechanics must be factored in. In which case singularities
in the classical sense do not exist. But there is still really no working theory of quantum gravity, just toy
theories like LQG (loop quantum gravity), so what really happens is still conjecture.

What I mean by having nothing to do with one another is that although we don't know the nature
of singularities or even if they exist in the classical sense, the mathematics of the formation
of an event horizon are well understood, and there is observational evidence that black holes
(defined as an object cloaked in an event horizon) really exist, such as Cygnus X-1.

In non technical accounts the terms event horizon and singularity sometimes get mixed up, but refer
to entirely different things.

The definition of event horizon is the boundary beyond which nothing can escape, including light.

However, when quantum effects are taken into account, Hawking found that particles can "tunnel"
back out, thus Hawking radiation.

So to summarize, General Relativity is a classical theory which is background independent in that
it defines the background, which is spacetime. It ignores quantum effects (h = 0)

Quantum mechanics is background dependent, and when doing calculations the background is
assumed to be flat spacetime (g = 0).

As of yet there is no workable theory of QM with curved spacetime (h > 0 and g > 0), and until there is
one the final state of matter, and spacetime itself inside a black hole remains conjecture.


If greater than 3 solar masses, neutron degeneracy pressure cannot stop collapse, in fact it
accelerates it. This was shown in a 1939 paper by J. Robert Oppenheimer and Hartland Snyder.
The reasons are rather arcane and are a consequence of the mathematics of general relativity,
but here goes: In the field equation of GR, the left side contains the spacetime curvature
(gravitation) and the right side the state of matter:

Gij = kTij

or in its expanded form,

Rij - 1/2 gijR = kTij

Ignoring the left side, on the right side the term Tij is called the energy-momentum or stress-energy
tensor (k is a constant). The important point is that it encapsulates the mass-energy density, and
the pressure. It can be written as a 4x4 matrix, and the upper left to lower left diagonal terms (called
the trace) contains the mass-energy density (a scalar value) and the 3 components of pressure
(a 3-vector value). Pressure is a form of energy, and from special relativity we know E = mc2, but this
can also be expressed as m = E/c2, so all energy has an equivalent mass and contributes to the
total gravitational field. Normally the contribution from pressure is negligible.

What Oppenheimer-Snyder showed is that over 3 solar masses, the pressure trying to stop the
collapse instead becomes the dominant force in accelerating collapse. And it won't stop until
it reaches the dreaded singularity and the equations blow up (technically, the metric becomes
undefined). That is in the classical theory of GR. Quantum effects probably would not become
important until the collapsing mass was about the diameter of an atomic nucleus.

The other flaw is that the interior of the black hole, due to strong spacetime curvature, has a
significant volume, greater than the simple Euclidian formula for the volume of a sphere. It
would take finite time for an infalling object to reach the center. Furthermore, inside the
event horizon all possible trajectories of objects lead to the singularity, there is no possible way
of avoiding it, whether it is a point singularity of the Schwarzschild metric, or the ring singularity
of the Kerr-Newman metric of rotating black holes.

Again this in the classical theory of GR. In a theory of quantum gravity, a true singularity
probably wouldn't exist. But we don't have a theory of quantum gravity which works at this point
(and I feel very strongly that string theory is a detour down a dead-end path).

So to summarize, the final state of matter in a gravitational field is:

Under ~ 0.1 solar mass, ordinary solid matter.

For stars ~ 0.1 to 1.4 solar masses, once thermonuclear reactions run out of fuel,
electron degenerate matter (white dwarf).

For 1.4 to ~3 solar masses, neutron degenerate matter.

> ~3 solar masses, black hole.


Edited by EJN, 14 January 2018 - 08:34 PM.

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#20 EJN

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Posted 14 January 2018 - 01:22 PM

The claim that time only appears to stop at the event horizon is disproved by the fact that we know that in the GPS system the clocks on the ground run slower than the clocks in the satellites. So it is not correct to say that time runs normally for objects falling into the Black Hole, that claim is not in accordance with experiments.



GPS clocks on the ground run slower than GPS clocks MOVING in orbit, from the point of view of someone on the ground. This is exactly what the theory of relativity of predicts. If you were on the satellite, looking at the GPS on the ground, to you the clock on the ground would be running slower.

 

That is correct.



#21 EJN

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Posted 14 January 2018 - 01:27 PM

As for time dilation and redshifting of an object falling into a black hole, see this excellent
article by John Baez (who happens to be a cousin of Joan Baez the folk singer).
 
 
http://math.ucr.edu/...es/fall_in.html
 
 
 

So if you, watching from a safe distance, attempt to witness my fall into the hole, you'll see me fall more and more slowly as the light delay increases. You'll never see me actually get to the event horizon. My watch, to you, will tick more and more slowly, but will never reach the time that I see as I fall into the black hole. Notice that this is really an optical effect caused by the paths of the light rays.

 

This is also true for the dying star itself.  If you attempt to witness the black hole's formation, you'll see the star collapse more and more slowly, never precisely reaching the Schwarzschild radius.
 
Now, this led early on to an image of a black hole as a strange sort of suspended-animation object, a "frozen star" with immobilized falling debris and gedankenexperiment astronauts hanging above it in eternally slowing precipitation.  This is, however, not what you'd see.  The reason is that as things get closer to the event horizon, they also get dimmer.  Light from them is redshifted and dimmed, and if one considers that light is actually made up of discrete photons, the time of escape of the last photon is actually finite, and not very large.  So things would wink out as they got close, including the dying star, and the name "black hole" is justified.
 
As an example, take the eight-solar-mass black hole I mentioned before.  If you start timing from the moment the you see the object half a Schwarzschild radius away from the event horizon, the light will dim exponentially from that point on with a characteristic time of about 0.2 milliseconds, and the time of the last photon is about a hundredth of a second later.  The times scale proportionally to the mass of the black hole.  If I jump into a black hole, I don't remain visible for long.


Edited by EJN, 14 January 2018 - 01:49 PM.


#22 Jim Davis

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Posted 14 January 2018 - 01:29 PM

An interesting explanation of how time and space work inside and outside of the event horizon: https://www.youtube....NhUJ2reI&t=685s



#23 shawnhar

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Posted 15 January 2018 - 11:02 AM

 

 

 

From our reference frame, time dilation causes everything to stop at the event horizon. So, I have a hard time getting my head around how anything gets past this barrier. It would seem that matter will just pile up at the event horizon until the additional mass causes the Swartzchild radius to extend past the matter, at which point everything breaks down and who knows what happens then.

Time only appears to stop at the event horizon to an outside observer. To the object falling in, time continues as normal. Another example: light moves at the speed of light, so to an observer, its time has stopped. That doesn't mean it stops moving.

 

I really hate this argument.

 - So if I existed before the "black hole", and watched the star die, I could watch it until the end of time, literally the end of time, and nothing would ever fall into it, not a single particle, nothing, ever, for all of time.

 

 "But that doesn't mean it stops moving" doesn't make sense, since I can watch it forever and it's clearly not moving.

 

It's red shifting, and every object that approaches and then arrives at the event horizon experiences, to the outside observer, a gravitational redshift that asymptotically lengthens towards infinity.  You don't see it "fall in", because no light will escape from that point.  But between the relativistic slowdown of time and the gravitational red shift, things would appear to gradually fade away rather than wink out.

 

I forgot about the spaghettification red shift extinction aspect.

Assuming you had a magic telescope that could see every last photon emitted, would you ever see the object disappear or would that last 50 feet take billions of years?

 

I always imagined this scenario the same as pushing an object with mass to the speed of light where the closer you get the more energy it takes and basically takes an infinite amount of energy to get that last little bit of speed. So it is wrong to think of the last 50 feet (or some small arbitrary distance) an object moves towards a black hole lasting 10's of billions of years to an outside observer?


Edited by shawnhar, 15 January 2018 - 11:03 AM.

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#24 PeterR280

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Posted 15 January 2018 - 11:55 AM

For an external reference frame, one that is away from the black hole, time slows down for objects as they approach the event horizon and goes to zero. But, it will take an infinite amount of time in the external reference frame. Everything will bunch up at the event horizon and never cross the event horizon. You can wait until the end of the universe for it to happen.

 

This raises interesting questions. It is true that in the proper reference frame; the reference frame of the falling object, time will march on and the object will continue to fall in. However, an infinite amount of time will have passed for the universe. Time will appear to speed up for the rest of the universe until it approaches infinitely fast time. Will the black hole still be there? Will the universe still be there? We do know that the black hole will slowly evaporate due to Hawking radiation for example. There are also theories that the universe will eventually break down to nothing.

 

We really don't understand what happens in extreme cases like black holes.



#25 David Knisely

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Posted 16 January 2018 - 08:18 PM

Black Holes are a theoretical speculation.Opinions differ as to whether or not the theory correctly predicts them. 

 

Well, the basic concept of an object that has a gravitational field strong enough to prevent light from ever leaving it is far from theoretical.  We have enough observational evidence for the existence of compact extremely massive objects in the universe that do not radiate visible light to make the current model of a black hole at least reasonably possible.  While not firm fact yet, they are far more than mere speculation.  In fact, hopefully soon, an array of radio telescopes may give us our first high-resolution look at the actual accretion disk of the dark object at the center of our galaxy and at the one in M87: 

 

http://eventhorizontelescope.org/

 

Clear skies to you.


Edited by David Knisely, 16 January 2018 - 08:19 PM.

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