What Hawking meant when he said "there are no black holes"

During a technical talk, Stephen Hawking made the next bold statement: "there are no black holes". It is quite easy to grasp that Hawking meant that there are no black holes in the Universe, that is, that the popular concept of black hole is an illusion, but some people [1] seems unable to accept that the champion of the black holes finally changed his mind.

In August 2013, Hawking gave a talk at The Kavli Institute for Theoretical Physics, the talk was given the uninspiring title "Information Preservation and Weather Forecasting for Black Holes" [2]. This is the relevant quote that has raised dust:

The absence of event horizons mean that there are no black holes - in the sense of regimes from which light can’t escape to infinity. There are however apparent horizons which persist for a period of time. This suggests that black holes should be redefined as metastable bound states of the gravitational field.

That is to say, Hawking affirms that black holes do not exist and proposes to redefine them introducing the concept of apparent horizon. Those words have not been well received by the academic community directly involved in black holes or by many journalists. This is easy to understand. Physicists and astronomers have claimed for decades that black holes are real objects, as real as a tree or an electron, and countless articles have been written by journalists for the general public. There are not only huge financial interests, but also egos involved here. Hawking literally told them that everything they have said and published about black holes was wrong.

Therefore, articles such as [2] are an attempt to make us believe that Hawking did not mean that there are no black holes. How could he tell? Rebecca Jacobson begins her PBS article by stating that an event horizon is the area believed to exist around a black hole from which nothing, not even light, can escape. This is not correct, an event horizon is not something that surrounds an black hole, the event horizon is part of the black hole. In fact, the event horizon is one of the fundamental features of a black hole (the other being the central singularity).

Jacobson then mentions Joseph Polchinski, who states that "Hawking was not claiming that black holes don’t exist. Astronomers have been observing black holes for decades". You see? This is just what I meant above when I wrote that Hawking’s talk had not been well received. Polchinski was a string theorist who worked on black holes, and we can say he is wrong twice. First, the quote from Hawking that I reproduced makes it clear that Hawking claimed there are no black holes. Second, it is false that astronomers have been observing black holes for decades. Black holes have never been observed! What astronomers have observed are massive objects that, according to general relativity should be black holes, but according to more fundamental theories they are not.

The only proof of the existence of a black hole would be the unequivocal detection of an event horizon, and this has not happened. All that astronomers have collected are very indirect observations about the existence of dark massive compact objects [3]. The Nobel committee seems well aware of this fact. Indeed, if you review the 2020 Nobel Prize in Physics announcement, you can see the careful wording used. Roger Penrose, a theorist, is awarded for "the discovery that black hole formation is a robust prediction of the general theory of relativity", but Reinhard Genzel and Andrea Ghez were awarded for "the discovery of a supermassive compact object at the centre of our galaxy". No black hole has been discovered, so it is good to see that the Nobel committee did not mention black holes for the empirical half of the Prize.

Black holes are a prediction of general relativity, but this is an approximated theory that ignores the quantum structure of matter and graviton effects, as general relativity is a classical theory. The concept of black hole arises when it is assumed that general relativity is the final theory of gravitation, in the sense that it works for arbitrary values of the gravitational interaction. However, just as classical electrodynamics stops working beyond a certain scale where the intensity of the electromagnetic field is too strong, general relativity also stops to work beyond a certain scale. Black holes assume that gravitation is infinitely strong and a singularity is created, but general relativity stops to work much before any physical object can reach that hypothetical regime of infinite gravity. When we add graviton corrections to the classical picture described by general relativity, both event horizons and singularities vanish, that is, black holes vanish. The black hole concept is an artifact of the classical theory, just as the concept of "Poincaré stresses" was an artifact of classical electrodynamics.

First order graviton correction to the classical gravitational force

Of course, I am aware that some physicists have worked on quantum versions of the concept of black hole. Polchinski is one of them, but he was a string theorist and we know that string theorists are wrong about general relativity and gravitation. The rest of the article [1] is a discussion about the information paradox, but since there are no black holes in nature, there is no event horizon and no information paradox.

It took Hawking more than four decades to understand that the concept of black holes is an artifact of assuming that a classical theory like general relativity is valid for arbitrary strengths of the gravitational interaction. Let us see how many more decades pass before the physics and astronomy communities accept that there are no black holes in the Universe.

NOTES

1. https://www.pbs.org/newshour/science/hawking-meant-black-holes
2. Information Preservation and Weather Forecasting for Black Holes. arXiv:1401.5761. Hawking S. W.
3. A list of indirect observations is provided in a volume of my series "Common misconceptions in physics" (in preparation).