This article is accurate, and has a nice level of detail for an encyclopedia entry. I only have a couple minor comments.
I would suggest adding a missing logical step in the statement, "Collapsing gas clouds larger than 100 Msun are expected to dissipate from radiation pressure, so this is the approximate upper limit for stellar-mass black holes." What is missing is that 100 Msun is the approximate upper limit to the mass of a star. Perhaps this could be re-phrased as, ". . . radiation pressure, which would prevent more massive stars from forming, and consequently set an approximate upper limit for stellar mass black holes."
>>This is a nice clarification; we have made the suggested change in wording. --RHP
The brief reference to "intermediate-mass black holes" is a bit out of place in an article that otherwise contains very well- established ideas and facts. There is still a significant amount of debate as to whether astronomers have actually identified intermediate-mass black holes. The dynamical measurements of objects in the centers of two globular clusters are not of the same quality as the binary mass functions used to measure the masses of stellar-mass black holes, or the orbits of stars and maser-emitting gas around supermassive black holes. The claim that ultra-luminous X-ray sources are intermediate-mass black holes is a matter of active debate, because there are at least two reasonable alternatives (the X-ray emission is beamed, or the Eddington limit can be violated in an accreting system). If intermediate-mass black holes are to be mentioned, I would suggest saying that, if they are confirmed, they could be the missing step in growing supermassive black holes.
>>The text has been modified in accordance with the reviewer's comment --RHP
Report from referee B
The article "Black Holes" provides an excellent accessible description of the physical principles of black holes and their relevance for astrophysics and fundamental physics.
I merely have the following few comments I would like to bring to the authors attention.
1) In the very first paragraph, the authors mention the
event horizon as the "no-escape region" from which light cannot reach outside observers. Strictly speaking, the event horizon is the boundary from inside of which light cannot escape to infinity. The notion of escaping to the "outside" is more remniscent of the apparent horizon (vanisihng expansion of the null geodesics). I am aware that the notion of an apparent horizon is rather technical and potentially unsuitable for the present report, but the author's may consider describing the observer as "far away from the black hole".
>>Although the original wording was technically correct, the reviewer's comment has led to a minor change and to a somewhat clearer wording. -- RHP
2) Fourth line of the section titled "Classical vs. Relativistic Black Holes"
This point is similar to the previous item. The authors state that "light cannot escape from an object unless the escape velovity is matched. Again, a qualifier "to infinity" appears to be helpful to me as light can escape the surface of the object up to finite distances if 0 < v < v_esc.
>>A change in wording has been made following the reviewer's suggestion. --RHP
3) Last but one paragraph of the section titled "Stationary Black Holes".
The paragraph starts "All of these spacetimes,..."
I had some difficulty following this paragraph, in particular the sentence starting "But near the hole the rate of coordinate time for a fixed observer's hypothetical clock must be adjusted..." I do not understand why the coordinate time needs to be adjusted. As the coordinate time is merely a gauge quantity, it is essentially arbitrary. I believe, the statement is that the proper time of a close observer advances more slowly than that of a distant observer and that the "standard coordinate time" is identified with the proper time of a distant observer. It is possible that I am missing some point here. Similarly, is the coordinate time in Fig.2 the proper time of a distant observer when he receives signals from the falling particle?
>> This has been the most challenging part of the article to write.
We have made a major change in the wording, and are now happy with it. It is slightly more technical than we had hoped, but there appears to be no way to be both very simple and correct. --RHP
4) Is it possible that an explicit reference to Fig.3 is missing?
>> An oversight. We have added explicit references in the text to
Figs 3, 4, and 5. --RHP
5) Near the end, it might be worth mentioning the Hulse-Taylor pulsar as indirect evidence for gravitational waves.
>> The authors feel that a reference to Hulse-Taylor would be suitable for a
general article on gravitational waves, but not in this black hole article, since there is no serious argument that either component of the binary is a black hole. -- RHP