© Elena E. Giorgi |

The concept of a “black hole” — a celestial body so dense and massive that not even light can escape its gravitational field — dates back to the 18th century, with the theoretical work of Pierre-Simon Laplace and John Michell. But it wasn’t until the early 20th century that these mysterious dark objects were first described mathematically by German physicist Karl Schwarzschild. Schwarzschild’s work predicted the existence of a finite distance around the black hole (called the “event horizon”) from which light cannot escape.

Emil Mottola, a physicist in the Theoretical Division at Los Alamos National Laboratory, laughs as he explains this bit of history behind black holes. “Would black holes have captured the popular imagination if they were still known as Schwarzschild’s solution?” he quips. Mottola has a point. The name “black hole” was coined by the American physicist John Wheeler in the 1960s, when these objects became the subject of serious study and first entered the popular vocabulary.

“And then of course, Stephen Hawking made black holes very popular with his own research and theory of black hole radiation,” Mottola adds. “To this day,” he explains, “black holes are far from being understood, and science fiction may have taken over from science fact. We can’t answer many of the most important questions without knowing what the internal states of a black hole are, but no one has ever been inside a black hole, so no one actually knows what is inside.”

One particularly vexing feature of black holes is the so-called “information paradox.” In 1974, Stephen Hawking theorized that black holes emit small amounts of radiation (called Hawking radiation). However, if this is true, black holes should eventually evaporate due to the loss of mass, leaving no way—not even in principle—to recover the information that was originally enclosed in it. This question alone has generated hundreds of research papers with still no completely satisfactory resolution.

In 2001, Mottola and his colleague Pawel O. Mazur proposed an alternative to Hawking’s black hole theory that eliminates the paradox. “Think of a black hole as having a physical surface,” Mottola says. He imagines this surface to be much like a soap bubble that bends and fluctuates in space.

“Our idea is that quantum effects build up right at the event horizon (the bubble’s surface), leading to a phase transition. This in turn creates a gravitational repulsive force inside the “bubble” that prevents the surface from collapsing. This repulsive force is the same ‘dark energy’ force believed to cause the expansion of the universe. We call these objects Gravitational Condensate Stars or ‘Gravastars’— celestial objects that would be compact, cold and dark, and look to astrophysicists just like ‘black holes,’ although they are not ‘holes’ at all. Our hypothesis does not contradict the conservation of information because there is no infinite crushing of space and time inside a Gravastar, and information is never destroyed.”

According to Mottola, the mathematical equations Hawking used to describe the temperature of a black hole are in reality describing the surface tension of a Gravastar. “If we assume that black holes have a temperature, then they need to have an enormous entropy too, but we can’t easily explain that enormous black hole entropy. In our theory, black holes don’t have a temperature, they have surface tension, like soap bubbles. In 2015 we showed that this possibility of a surface and surface tension was already inherent in Schwarzschild’s original formulation of black hole interiors in 1916, and so is consistent with both Einstein’s General Relativity and Quantum Mechanics.”

As I look over my notes, I pose Dr. Mottola one final question: “Is there any way to find out who’s right, you or Stephen Hawking?”

He smiles because he knows that whatever Hawking says these days carries a lot of weight, including when he proposes that black holes could be mysterious portals to other universes.

“I believe we may well find out the answer in the next five to ten years,” Mottola says. “If ‘black holes’ actually are Gravastars with a surface, their surface oscillations would cause them to emit gravitational waves at certain frequencies, which is a substantially different signal than that expected from the black holes that Hawking and colleagues theorize. LIGO directly detected gravitational waves for the first time in 2015, so we have just entered a new era of gravitational wave astronomy. In a few years, we may have enough data from the gravitational waves detected by LIGO and its sister observatories to be able to resolve the conundrum.”

Needless to say, the Los Alamos scientist is very excited at that prospect.

**References**

[1] Mazur, P., & Mottola, E. (2004). Gravitational vacuum condensate stars Proceedings of the National Academy of Sciences, 101 (26), 9545-9550 DOI: 10.1073/pnas.0402717101

[2] Emil Mottola (2010). New Horizons in Gravity: The Trace Anomaly, Dark Energy and Condensate

Stars Acta Physica Polonica B (2010) Vol.41, iss.9, p.2031-2162 arXiv: 1008.5006v1

[3] Mazur, P., & Mottola, E. (2015). Surface tension and negative pressure interior of a non-singular ‘black hole’ Classical and Quantum Gravity, 32 (21) DOI: 10.1088/0264-9381/32/21/215024