The Mystery of Black Holes

“We have taken the first picture of a black hole. This is an extraordinary scientific feat accomplished by a team of more than 200 researchers.” said Sheperd S. Doeleman, Event Horizon Telescope (EHT) Project Director at Harvard-Smithsonian Center for Astrophysics, on 10th of April.

The picture is the product of observations by EHT-an international consortium that linked eight radio observatories around the world to create a single, earth-size telescope.

It is the first-ever direct visual evidence of a black hole which shows a glowing orange-yellow ring of light surrounding a dark silhouette, at the centre of Messier 87-a galaxy 55-million light-years from Earth.

It is a super massive black hole, 6.5-billion times more massive than our sun and measures 40-billion km across (three million times the size of the Earth), described by scientists, a monster.  The event was marked with simultaneous six press conferences held in Washington, Brussels, Santiago, Shanghai, Taipei and Tokyo. Indeed it was a marvelous moment for scientists to disclose this groundbreaking result. The findings will certainly provide insight into the extremely dense pockets of matter of this object and understating of gravity. This marks the beginning of a new era in astronomy and astrophysics.

The term “black hole” has been a fodder for wild ideas in science fictions stories, history, films and frequently used in day-to-day parleys to refer to an imaginary place where things are lost for no apparent reason.

The term “black hole” was coined in 1967 by John Wheeler, an American Astronomer and introduced in Physics for the description of that region where the gravitational field is so strong that not to speak of material particles, even the light photons are caught and held in its grip. Earlier, John Michell English scientist and clergyman was the first to postulate the existence of black hole in 1784. He gave talks, used thought experiments to explain that light would not leave the surface of a very massive star if the gravitation was sufficiently large.

The French mathematician/physicist/astronomer Laplace in 1795 described the idea of massive stars from which no light could escape as dark body. The area was left with no interest until 1916 when Einstein published his general theory of relativity, wherein the field equations predicted the existence of black holes.

Einstein did not believe in black holes and he resisted the idea but black holes became the most intriguing objects for astronomers and astrophysicists. They are perhaps the most enigmatic and strange objects known to exist in the universe.

The original label of a black hole is “gravitationally collapsed star” which is based on Newtonian Physics where we describe Gravity, as a force-like the one we experience on earth.

Einstein in his general relativity showed that gravity is not a force but a result of curvature of space-time. The matter forms the shape of the space-time and space-time tells the matter how to move. The black holes are extremely dense pockets of matter with incredible mass and minuscule volume that can exist from the size of a human cell to the one in M87.

The space-time around it is so drastically warped that anything that passes too close gets sucked into it, be it a material particle or a photon of light. The black hole being a bottomless pit in space-time, how can we discover its presence? The only way we can see its presence is how it distorts space and affects matter nearby.

If there are surrounding stars, the black hole pulls the matter towards itself which move with tremendous kinetic energies, getting enormously hot, emitting X-rays and other radiations. The Cygnus X-1, the first X-ray source accepted as a black hole is about 10-15 times more massive than the sun. Scientists are able to detect stars, gas and swirling material orbiting black holes forming an “accretion disc” which gives off radio waves that can be captured by high-powered telescopes.

How can we visualize the center of a black hole?  At the center of the black hole lies the gravitational singularity, where the space-time curvature becomes infinite as described by general relativity. We may call it the laws of Physics have broken down or the mathematical model has gone berserk to describe anything.

Event horizon is a mathematically defined demarcation at which light cannot escape the black hole’s gravity. The radius of event horizon surrounding a non-rotating black hole is called Schwarzschild radius named after the German astronomer/physicist Karl Schwarzschild who provided the exact solution to Einstein’s field equations.

Nothing inside the event horizon can ever cross the boundary and escape the black hole and nothing that enters a black hole can be observed from outside the event horizon. Once an object crosses the event horizon, the date with the singularity is certain.

However, outside the event horizon, escape remains theoretically possible. Black holes themselves do not radiate any energy or radiation, but the same happens outside the event horizon, what we call Hawking radiation, theorized by Stephen Hawking in 1974. This Radiation according to him weakens a black hole over time and finally kills it.

How does a black hole form? Subramanian Chandrasekhar Indian-American scientist at the University of Chicago did a commendable work on evolutionary stages of massive stars and black holes and received Nobel Prize in Physics in 1983.

For a long time, it was believed that all the stars would collapse into white dwarfs-the sun like stars that would eventually die.  In 1931 Chandrasekhar pointed out that the life-history of a star of small mass must essentially be different from that of a star of large mass. The Chandrasekhar’s Limit is now accepted to be approximately 1.4 times the mass of the sun.

While a white dwarfs star is the end stage for the vast majority of stars in the universe, stars that exceeds this mass is destined to end its life in that most violent of explosions, we call supernova and it could become a neutron star or even a black hole.

The stars with original mass greater than 20 solar masses, the core will be more than 3 solar masses, then even neutron degeneracy pressure can’t hold up against the gravity of the core and it will collapse even further to become a black hole. 

Now that black holes are real entities in interstellar space, their study is going to further increase our understanding about the universe. The observations and the data already collected could reveal more secrets about this elusive object.

The big question is to understand the nature of gravity. The power of gravity is such that without it, the creation of the universe would not have been possible.

The extreme cases of gravity near black hole will enable scientists to test physical laws and validity of their theories that may open up multiple pathways to higher levels of understanding as to what is actually happening in the universe.

We can understand space-time curvature, the slow-down of time and off course the excitement about the possibility of time-travel. Black holes have the power to destroy anything around and there could be unseen tiny black holes nearby in the solar system. The scientists could build a telescope even larger than earth, by adding space telescopes to the EHT array, to locate these black holes. The research can aid our existence on earth. No doubt, exploring nature is a part of human endeavor.

Dr Mohmad Amin Malik IS Associate Professor, Higher Education Deptt,J&K