Black Holes: Why research about them? What makes them so intriguing?

“Gravity is the oldest known, but the least understood force in nature,” Dr. Gaurav Khanna (Professor in the Department of Physics at the University of Rhode Island)told Universe Today. “For students of gravity, black holes are amongst the most interesting objects to study because gravity is the dominant force there — in fact, it is infinitely strong! Then there are astrophysical reasons of interest in black holes. They play important roles in galaxies, perhaps even in the large-scale behavior of the universe and more. The other thing to note about black holes is that they are very ‘simple’ especially when compared to stars and other astrophysical objects. This is a consequence of the so-called ‘no hair’ theorem that states that black holes can be fully characterized by only 3 attributes — their mass, charge and spin. That simplicity makes them particularly appealing to study and research.”

Black holes are known for exerting gravity so strong that light can’t even escape, and while Albert Einstein’s 1915 theory of general relativity is often credited with first propositioning the concept of black holes, the concept of an object whose size and gravity would not let light to escape was first proposed in a 1784 letter by English clergymanand philosopher, John Mitchell. In this letter, Mitchell called these objects as “dark stars” since he hypothesized that stars whose diameters exceeded 500 times that of our Sun’s diameter would cause the formation of these objects. Furthermore, he suggested that gravitational waves influencing proximate celestial bodies would facilitate detection of these objects.

Fast forward to Einstein’s theory of general relativity, which also hypothesized both the existence of black holes and gravitational waves, both of which continued to be studied throughout the 20th century, which includes what’s termed the “golden age of general relativity” during the 1960s and 1970s. This includes the very first object accepted by the scientific community as a black hole, called Cygnus X-1, which was discovered in the year 1964. However, it took additional 52 years for the existence of gravitational waves to be established through a black hole merger, which was accomplished by the LIGO Scientific Collaboration.

Given the history combined with vital discoveries only happening within the past few years, what are some of the benefits and challenges of studying black holes?Dr. Khanna divulges to Universe Today, “As I stated above, studying black holes, which are a consequence of Einstein’s relativity theory, offers insight on the nature of gravity, space and time at the most fundamental levels. As physicists, we are yet to develop a complete understanding of the quantum nature of gravity, and black holes are the key to unlocking that mystery. On the challenges, I’d say that the clearest one perhaps is that black holes can only be observed indirectly. Unlike stars, since they don’t emit radiation themselves, it is difficult for astronomers to collect data on them. At best, we can observe their influence on their environment (like gas, stars, etc.) and infer their properties and behavior. On the theoretical side, while it is indeed true that black holes are very “simple” compared to stars, there are still challenges. The mathematics and physics that describe them is fairly advanced and even computer simulations involving them are challenging requiring massive processing power and memory.”