Is the recently discovered "black hole" black at all wavelengths or just at radio wavelengths?
The black hole "picture" in M87 is actually a pictorial representation of data received at radio wavelengths and then heavily processed. Viewed from Earth, the hole would be 42 microarcseconds across. The Hubble space telescope can resolve optical images at about 50,000 microarcseconds. Hence, the need for a high resolution technique based on radio waves.
We can't really see any structures inside M87. So is this an actual black hole at all wavelengths, or only a theoretical black hole? And how do we know?
Calling this thing a black hole seems to be premature.
Presently only VLBI radioastronomical interferometry can offer such a high angular resolution. If we would built an optical interferometer with comparable resolution - why not ? BTW, I can imagine a great disappointment of Spektr-R/ Radioastron team who sent a huge radioastronomical spacecraft to get much higher resolution with VLBI base 350 000 km - they also had an ambition to image event horizon of M87, but they missed this achievement: https://www.youtube.com/watch?v=OB6fRkYPoyc&list=PLnAqiQM6RrHGxcpNKNxjwA9yKeWKNIsze&index=38&t=461s
BF> "We can't really see any structures inside M87. So is this an actual black hole at all wavelengths, or only a theoretical black hole?"
"And it wasn’t until 1967, 12 years after Einstein’s death in 1955, that these astrophysical singularities became known as “black holes”—a term coined by American physicist John A. Wheeler during a conference in New York." https://www.history.com/news/black-holes-albert-einstein-theory-relativity-space-time
On the question of "Black Hole", Albert Einstein (in whose name this big fraud is promoted) himself in a published a paper (link Below) emphatically dismissed the possibility of "Back Holes" even in theory! The following is a copy of his conclusion at the end of this mathematically rich and lengthy publication: "The essential result of this investigation is a clear understanding as to why the "Schwarzschild singularities" do not exist in physical reality. Although the theory given here treats only clusters whose particles move along circular paths it does not seem to be subject to reasonable doubt that mote general cases will have analogous results. The "Schwarzschild singularity" does not appear for the reason that matter cannot be concentrated arbitrarily. And this is due to the fact that otherwise the constituting particles would reach the velocity of light.
This investigation arose out of discussions the author conducted with Professor H. P. Robertson and with Drs. V. Bargmann and P. Bergmann on the mathematical and physical significance of the Schwarzschild singularity. The problem quite naturally leads to the question, answered by this paper in the negative, as to whether physical models are capable of exhibiting such a singularity." Albert Einstein. https://www.cscamm.umd.edu/people/faculty/tiglio/GR2012/Syllabus_files/EinsteinSchwarzschild.pdf
So, what was "discovered" recently is probably the "smile" (left behind) of a Black Cheshire Cat while the Cat itself vanished (if it existed at all)!
Please note: This link to Einstein's work was accessible before when I discussed and referred to this publication by Einstein in other RG forums. But it now seems that any online access is not available or blocked!
AM: Please note: This link to Einstein's work was accessible before when I discussed and referred to this publication by Einstein in other RG forums. But it now seems that any online access is not available or blocked!
If you delete the end of the URL step by step, it keeps saying the file isn't found until you reach this which tells you the page has been moved:
https://www.cscamm.umd.edu/people/faculty/
The file was probably in the student material for a course given in 2012 according to the directory names and has been deleted. I think the lecturer who gave it may have moved on.
Anyway, note that Einstein in what you quoted was only saying it was the singularity that is questionable, the term "black hole" usually refers to the region bounded by the event horizon, and the image is of the photon sphere which is farther out still.
Brian Fraser "Is the recently discovered "black hole" black at all wavelengths or just at radio wavelengths?"
It's black at all wavelengths. No electromagnetic radiation, of any wavelength, can escape from inside the event horizon, since the escape velocity would have to exceed the speed of light. This is the very definition of a black hole.
Brian Fraser "Calling this thing a black hole seems to be premature."
You seem to be focused only on its external appearance, in a single observation. But before it was ever observed in such incredible angular resolution, its black hole nature was already known - from multiple observations of its gravitational signature on the stars and gas orbiting it, the relativistic polar jets streaming from it, and the rapidity of the temporal variability in their emissions. In short, astronomers had measured and constrained that a mass of ~ 2 billion Suns was contained in a volume of space not a lot bigger than our Solar System, and that it was not actively emitting its own electromagnetic energy. The only thing that fits those properties is a black hole.