by Prasad Ganti
An episode of Nova titled “Black Holes” was aired recently on PBS (pbs.org). The latest issue of Scientific American carried an article by Priyamvada Natarajan on the same subject. The author was also featured on the Nova program. While the Nova program started off with the black holes as a prediction of Albert Einstein’s theory of General Relativity from the early 1900s to the present day discoveries, the Scientific American article focuses only on the recent discoveries.
Albert Einstein came up with the general theory of relativity in 1915 to describe the effect of gravity on space and time. Karl Schwarzschild, the Jewish Physicist and an Astronomer fighting for Germany in World War I, solved Einstein’s equations and came up with the concept of an object of immense mass packed into a very small volume, which warps the space and time so much that even light cannot escape once it is trapped. It remained a concept for several decades. Schwarzschild died immediately after the war due to a disease he had contracted on the war front.
Lot of debate ensued as to the existence of black holes and the process of trying to find them. Any object cannot be a black hole. Schwarzschild had come up with a radius for a given mass. Unless the mass is packed in a very small radius (for those who are mathematically inclined, R=2*G*M/c**2, G is the gravitational constant, M is the mass of the object, c is the speed of light). Since the speed of light is huge and occurs in the denominator of the equation and gets squared, the radius is very small indeed.
While the black hole itself will only devour mass and not give out anything, its surrounding halo can emit radiation. The gas and dust swirling around the Black Hole gets heated to very high temperatures, giving off radiation. X-rays were observed in 1964 to be coming out of a source in Cygnus constellation. Speculation arose that there could be a Black Hole resulting from the collapse of the corresponding star. Betting took place between Stephen Hawking, the famous contemporary astronomer and Kip Thorne the Nobel prize winner from 2016. Hawking was skeptical that the X-ray source was a black hole. Thorne finally won the bet in 1990 when data overwhelmingly conferred the honor of the first black hole to be discovered on Cygnus X-1.
Such black holes which form from the death of a massive star are known as stellar black hole. Stars need to be much heavier (several times at least) than our Sun to die and result in a black hole. There was a growing evidence of a massive black hole in the center of our Milky Way galaxy. Other analysis of hundreds and thousands of galaxies revealed supermassive black holes in their centers. They grow by accumulating dying stars in the galaxy like a big corporation forming from the mergers and acquisitions of smaller companies!
In the 1960s, very bright but distant objects were found. Christened as Quasars (Quasi Stellar Radio Sources), these objects were the brightest ones found to date in our universe. But being the furthest objects to be discovered, quasars were mysterious. What powered their luminosity? They are certainly not massive stars. Since looking far away in universe always means looking back into the past, quasars are the oldest objects in the universe, formed when the universe was still in its infancy.
Now it is agreed that quasars are powered by black holes. But how did black holes form in such early stages of the universe. Stars were just forming, so there could not be corpses of stars. Latest theory is that huge masses of cloud and dust directly collapsed into black holes, instead of going through the star formation process.
Black holes keep devouring matter in their neighborhood. They grow in size. Mergers of black holes and other dense objects produce gravitational waves, which have been detected in the last couple of years. With the launch of James Webb space telescope in 2019, more is expected to be known about quasar formation. Astronomy is getting more exciting with these discoveries and there is endless potential for more. Our universe is a very interesting place indeed!