In response to:

Gravity’s Black Rainbow from the September 29, 2016 issue

To the Editors:

In his review of a book entitled Black Hole Blues by Janna Levin [NYR, September 29], Lawrence Krauss mentions the collision of two black holes but he never tells us what a black hole is. During the last couple of months I have been surveying some of my colleagues who work in this field to supply a definition. The most succinct is that it is a body of matter that is self-gravitational, surrounded by an event horizon, and with a singularity in the gravitational field at an interior point.

To adumbrate: “self-gravitational” means that it is held together by its own gravitating masses and not some external force. An “event horizon” is something like a membrane wrapped around the object that allows the passage of, say, radiation from the outside to the inside but not vice versa. A “singularity” occurs at the point in the gravitational field where the concentration of matter is so great that the effects of gravity are infinite.

But we have known since the work of Stephen Hawking that what we call black holes have a temperature and hence give off radiation, which Hawking summarized by noting that “black holes ain’t so black.” For black holes more massive than our sun this radiation is tiny, but it exists nonetheless. There are two schools of thought. One says that the singularity is real and the other, to which I belong, says it is an artifact of our present theory and will go away when gravitation is successfully melded with quantum mechanics.

Have black holes been observed? Literally and by definition, no. One observes the effect of something invisible on objects visible and asks what else could it be. The same is true of the LIGO experiment described by Levin, except here one cannot even determine where in the heavens this alleged collision of two black holes took place. The same question is asked—if it is not the collision of two black holes, what else could it be? If there has been a plausible answer to that question I have not seen it, but we might keep an open mind.

Jeremy Bernstein
Aspen, Colorado

Lawrence Krauss replies:

Jeremy Bernstein makes a good point, namely that what was detected at LIGO is something that agrees with the predications for the collision and merger of two black holes (which he nicely summarizes the properties of). But of course we cannot see the collision directly, and cannot therefore prove that it was black holes that actually were the source of the signal. However, based on our present understanding of gravity, there are no other realistic candidates. So, if it “walks” like a black hole and “quacks” like a black hole, we tend to assume that it probably is a black hole.