Comments about: Black holes up close

Comments about the Perspective in Nature: Black holes up close

Following is a discussion about this article in Nature Vol 615 23 March 2023, by Ramesh Narayan & Eliot Quataert
To study the full text select this link: https://www.nature.com/articles/s41586-023-05768-4

The text in italics is copied from the article
Immediate followed by some comments

In the last paragraph I explain my own opinion.

Introduction

Einstein’s theory of general relativity (GR) radically altered our understanding of the nature of space and time.
From a physical point of view space and time are two complete different concepts. Spacetime is a mathematical concept.: In GR, space and time are dynamical quantities, leading to the existence of gravitational waves and the expanding Universe.
Gravitational waves come from revolving objects. The stars orbiting Sagittarius A* also cause gravitational waves. The expanding Universe is an issue at his own right.: But perhaps most remarkably, GR predicts a fundamentally new type of object, the black hole (BH).
A black hole is a very small, very heavy object situated at the center of each galaxy. The most important physical characteristic is that a black hole can not be observed, is not vissible from a human point of view. This is partly caused by the physical fact that all objects try to attract what is in their neighbourhood. From a mechanical point this is called the force of gravity. At the same time all objects can also emit energy i.e. light. When the attracting force of gravity is greater than the force, with causes the emmision of light, no light will be observed.: Unlike normal stars and planets that have surfaces, BHs in GR are defined by the presence of an event horizon, a region within which gravity is so strong that nothing can escape.
A Black hole also has a surface. The concept of an event horizon as a region from with nothing can escape is tricky. What means nothing?: (There were Newtonian physics-based predictions of BH-like objects in the late 1700s by Michell and Laplace, but their prescient ideas were too far ahead of the times and were not subsequently developed).
Newton assumed that all objects are points in space. Using that concept, most probably, the mass of Sgr A* is calculated based on the observations of stars revolving around Sgr A*.: In physics, BHs have played a central role in the theoretical quest for a quantum theory of gravity, in part because GR predicts its own failure in the interior of BHs: all of the matter out of which the BH is made collapses to a singularity of infinite density where the equations of GR break down.
The evolution of a star (the collapse) in a small BH is a physical process. Most probably it is a mixture of elementary particle physics and the force of gravity. A singularity is a mathematical concept.: The most traction in elucidating this tension has occurred in the context of understanding Hawking’s famous prediction that in quantum mechanics BHs should actually produce a small amount of radiation (which, we note, is completely unobservable for any BH that has been discovered).
The first issue to understand what means produce. Nobody knows what is happening in the region, so called event horizon. In that region particles or radiation can be produced which are ejected, based on infalling matter. See fig 3.: In astronomy, a 60-year quest has led to the amazing realization that BHs are not only real but also commonplace.
Okay.: There are tens of millions of stellar-mass BHs per galaxy: a few of these happen to shine as bright X-ray sources, and gravitational-wave measurements have detected dozens of mergers of stellar-mass BHs.
That means that there are also millions of stellar-mass BH's in the Milky Way Galaxy. . I have my doubt. For a list of stars around Sgr A* see this url: https://en.wikipedia.org/wiki/Sagittarius_A*_cluster

1. Black-hole accretion and gravity

: The most widely used technique to study BHs is seemingly paradoxical: contrary to their name, BHs are the central engines for some of the brightest and most unusual sources of radiation in the Universe.
The most probable explanation that a certain type of conversion takes place between infalling matter and outflowing matter.: The radiation is produced by gas (outside the event horizon) that is spiralling into the BH via an accretion disk.
That is a physical description.: Accretion converts gravitational potential energy into heat, which in turn can be converted into radiation and outflows of gas; BHs power the most spectacular observational sources via accretion because they have the strongest gravity of any object in the Universe.
I expect that part of the explanation is that during accretion there is an increase in speed.: The birth and growth of supermassive BHs in the first billion years after the Big Bang remains a major puzzle; new observational facilities, such as the James Webb Space Telescope, are likely to shed light on this longstanding problem.
There are two major issues: The birth of small BH's, during the first billion years, after the Big Bang and the growth of these BH's, during the next 10 billion years, into the supermassive BH's we see today. Probably, during this period, many of the small BH's merged with the larger BH's.: Much of the radiation we observe originates close to the event horizon, where most of the gravitational potential energy of the inflowing matter is released.
The region of what is called the event horizon could be the birth place of radiation, but the most important aspect are the actual reactions that take place. The concept 'gravitational potential energy' is of less importance.: A large fraction of the gravitational potential energy released by accretion is converted to heat (this is inevitable because of the ‘friction’ associated with angular momentum transfer), and thence to the radiation we observe (this last step is more tricky than one might think).
As said the most important part are the actual reactions that take place. These are not tricky.

2. Hot accretion

: Unfortunately, it has a fatal flaw: it is thermally unstable, which means that the system cannot survive for any length of time.
This requires some thoughts.
: Although the hot accretion model is consistent with available observations, we should bear in mind that it is not necessarily correct.
This requires a clear understanding what hot, high temperature versus low temperature, means in a physical context.: As we describe in the next section, recent observations have provided further support for the two-temperature hot accretion model and have allayed some of the doubts.
Let us wait.

3. Inward bound

4. Testing gravity with EHT

5. Energy extraction from spinning BH's