Beyond darkness: Why India needs to unravel the mystery of black holes

In the universe we know, nothing is more powerful, dangerous, and mysterious than the mighty black holes. One can imagine them as places where gravity is so powerful that not even the fastest-moving particles of our universe — light — can escape.

And yet, despite their name, black holes are far from "empty." They are filled with vast amounts of matter compressed into an incredibly small area.

The Indian Space Research Organisation has launched the X-ray Polarimeter Satellite, or XPoSat. This 480 kg satellite is tasked to delve into the mysteries of black holes using X-rays. With this, India has become the second country after the United States to have such a specialized 'observatory' of this kind to study black holes.

WHAT IS A BLACK HOLE?

If you were to take all of Earth and somehow find a way to shrink it — down to the size of a playing marble — the density of this tiny but heavy Earth would be enough to turn it into a black hole.

The universe is naturally filled with billions of black holes that are custodians of gravity and time, holding the universe together as well as devouring the biggest of stars.

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Nothing can escape the gravity of a black hole — not any unfortunate astronaut, nor even planets and the biggest of stars — can escape its pull. That is the essence of a black hole; it is filled with mass and sucks in even light, thereby in an eternal state of darkness.

The universe abounds with a diverse array of black holes, each differing in size, mass, origin, and lifespan.

HOW MANY BLACK HOLES EXIST?

The precise number remains unknown, but the estimates are staggering. It is thought that there are 40 quintillion black holes in the observable universe — that is 40 with 18 zeros following it. To put this into perspective, this number far exceeds the total grains of sand on all the beaches on Earth, by a factor of six.

And they occur in different types; more precisely, four variants of black holes exist - Stellar, Supermassive, Intermediate, and Primordial.

Sagittarius A*, the black hole at the centre of the Milky Way, is 4 million times the mass of the Sun. The gravity of the Supermassive Black Holes is the reason why the galaxy is kept together.

Black holes are incredibly difficult to study because they do not emit light. Also, they are located far away from Earth. The nearest black hole to Earth is located at a distance of 1,560 light-years, which is 14,750 trillion kilometers (thankfully!). This is the reason they look tiny when observed from Earth, making them exceedingly difficult to study.

HOW DO WE 'SEE' BLACK HOLES?

Interestingly, we observe the light from materials that are falling into the black hole. As these materials spin closer to the black hole, they get squeezed into increasingly tiny spaces. This results in the materials becoming extremely hot and energetic, eventually emitting very bright light just before being consumed by the black hole forever.

This is the light that we detect from the region just outside the black hole. This emitted light, particularly in the X-ray spectrum, allows us to map and understand the characteristics of a black hole.

XPOSAT TO STUDY BLACK HOLES

The XPoSat mission launched by India will orbit the Earth and is equipped with two key scientific instruments. The primary instrument, POLIX (Polarimeter Instrument in X-rays), will focus on measuring the Polarisation of X-rays. It will specifically examine X-rays in the medium energy range of 8-30 keV (kilo-electron volts) that originate from astronomical sources. This will help scientists understand the direction and twist of these X-rays.

The second instrument, XSPECT (X-ray Spectroscopy and Timing), will provide detailed insights into the lower energy range X-rays.

WHY IS X-RAY POLARISATION CRUCIAL?

Light is a type of electromagnetic wave composed of oscillating electric and magnetic fields. These fields oscillate perpendicular, or at a 90-degree angle, to each other. In a typical light wave, these oscillations occur in multiple, random directions. However, when light is polarized, these oscillations are aligned in a specific direction.

Polarisation of X-rays, a high-energy form of light, is particularly important as it offers deep insights into the nature and environment of cosmic phenomena. The process of Polarisation occurs when the orientation of the electric field of these X-rays becomes ordered in a particular direction, rather than being randomly oriented.

This ordering can happen through various interactions. For instance, when X-rays travel through strong magnetic fields, like those near black holes, their electric field aligns with the direction of the magnetic field, resulting in polarisation.

Analyzing the Polarisation of X-rays provides scientists with crucial information about the source and surroundings of these rays. In extreme environments where high-energy X-rays are generated, such as around black holes, the Polarisation can reveal details about the structure of these regions. For example, it can help determine the direction and strength of magnetic fields, the distribution of materials around the source, and even the rate of spin of black holes.

WHY IS THE STUDY OF BLACK HOLES SO IMPORTANT?

Black holes serve as extraordinary laboratories for scientists. Their extreme nature makes them ideal for pushing the boundaries of our understanding of the fundamental laws of physics.

Studying black holes has led to significant breakthroughs in our understanding of the cosmos. For instance, observing the collisions of black holes has provided crucial tests for Einstein's Theory of General Relativity, which describes the relationship between mass, space, and time.

One crucial area where black holes contribute to our knowledge is in understanding the growth of celestial bodies. This is especially true for supermassive black holes, which are often found in highly active states. These giants are not just silent observers in their galaxies; they actively consume stars and other matter. Observing their processes provides a window into the science of high-energy environments. It also offers clues about the conditions that lead to the birth, evolution, and eventual demise of stars, planets, and galaxies.

Moreover, black holes, particularly the supermassive one at the center of our Milky Way, have a more personal connection to us. They likely had a significant influence on the development and structure of our galaxy. This, in turn, has implications for how our solar system, and eventually Earth, came into existence.

Thus, black holes are not just fascinating objects; they are keys to understanding the broader narrative of the universe, from the most fundamental forces to the grand scale of galactic evolution. They help us piece together the cosmic puzzle, making them essential to our quest to know our place in the vastness of space.

(This is an authored article by Srijan Pal Singh. He is an author and an IIM Ahmedabad graduate, who was the Advisor for Policy and Technology to Dr. APJ Abdul Kalam, 11th President of India. He is the Founder and CEO of Dr. Kalam Centre and Homi Lab)

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