Bengaluru: 

Black holes are some of the most intriguing and least understood phenomena in the universe. Formed when massive amounts of matter collapse into an extremely small space, black holes create a region with gravity so intense that nothing—not even light—can escape. At their core, the laws of gravity, quantum mechanics, and thermodynamics intersect and often conflict. Gravity, the dominant force across the cosmos, plays a central role in black hole formation.

What exactly is a black hole?

The mass of a black hole directly determines the size of its event horizon. Black hole masses are not uniformly distributed—astrophysically, they typically range from 8 to 60 solar masses (stellar) to billions of solar masses (supermassive).

A black hole is a dense concentration of mass packed into a minute space. The gravity just outside a black hole—at its event horizon (a boundary, not a solid surface)—is so strong that nothing can escape. At the very centre of the black hole lies the singularity, a point of infinite density where space and time cease to behave normally and the known laws of physics break down. Black holes can form from the gravitational collapse of massive stars (stellar), mergers of smaller black holes, the collapse of gas clouds in early galaxies (supermassive), or quantum fluctuations in the early universe (primordial).

Physical properties and classification

Three fundamental physical quantities define a black hole: mass (M), charge (Q), and angular momentum/spin (J).

From these, we derive four basic types of black holes:

Spinless, chargeless – Schwarzschild black hole

Spinning, chargeless – Kerr black hole

Non-spinning, charged – Reissner–Nordström black hole

Spinning, charged – Kerr–Newman black hole

Black hole formation:

He explained that nearly every galaxy has a black hole at its centre. When a star exhausts its nuclear fuel, it collapses under its gravity, forming a black hole where space-time is shaped by a highly concentrated mass. Space-time is the four-dimensional framework combining space and time, where events happen and gravity operates.

Prof Arun Mangalam, a senior professor at the Indian Institute of Astrophysics (IIA), shared detailed insights. Mangalam conducts research in theoretical astrophysics, gravitational dynamics, relativistic astrophysics, gas dynamics, stellar dynamics, and hydromagnetic processes concerning black hole systems, galaxies, and magnetic fields.

The professor further said that in Einstein's theory, gravity is mass bending space-time, causing objects to fall. A black hole is a region that can’t communicate with the rest of the universe, where no information can escape or reach the outside universe. Einstein's General Relativity (1915) allows for such objects, and a year later, Karl Schwarzschild – a soldier of WWI – solved Einstein's equations for a point mass, introducing the first theoretical black hole model.

Einstein’s theory of relativistic gravity:

Discussing Einstein's theory of relativity, Mangalam said that general relativity fundamentally alters our understanding of inertia by linking it to the curvature of space-time caused by gravity. In general relativity, inertia is not simply a resistance to acceleration against a fixed background space, but rather a body's tendency to follow the "straightest possible path" (a geodesic) through the curved space-time.

Black holes arise from Einstein's general theory of relativity. If the Sun were compressed to a radius of just 3 kilometres—about four millionths of its current size—it would become a black hole. For Earth, that threshold is a mere 9 millimetres, a billionth of its present size. According to relativity, anything that falls into a black hole is lost forever, with no information escaping.

He also highlighted that Earth feels gravity from the Sun, the solar system moves within the galaxy, and galaxies move relative to each other—raising the question of a "correct inertial frame" of reference. An inertial frame is one where there is no acceleration. Einstein showed that all inertial frames are equally valid; there’s no absolute motion, only relative motion. This idea forms the basis of universal space-time, where the laws of physics hold true in any inertial frame.

Prof Mangalam explained that due to the light speed limit, even far-apart objects can influence each other gravitationally. Relativity links space-time to all forms of energy, not just mass. Each energy type has an associated force, and gravity—the long-range force—connects and governs the entire universe.

Hawking radiation and black hole evaporation:

Mangalam explained that Hawking radiation is a quantum phenomenon. It addresses how the mass of a black hole might evaporate over time. Proposed by Stephen Hawking, it describes how black holes can slowly lose mass and energy through quantum effects near the event horizon—the boundary beyond which nothing can escape. It’s essential to note that the ‘surface’ of the black hole is the region where the gravitational pull is so strong that the escape velocity exceeds the speed of light.

Hawking's work advanced the quest for a quantum theory of gravity. He proposed that black holes fully evaporate, erasing information and challenging the core principles of quantum mechanics.

In the vacuum of space, particle-antiparticle pairs constantly form and vanish. Near a black hole’s event horizon, one particle may fall into the black hole while the other escapes as radiation. To conserve energy, the black hole loses a tiny amount of mass for each escaping particle. Over time, this leads to black hole evaporation. This phenomenon connects quantum mechanics with general relativity.

Mangalam explained that black holes are found in the early universe. The ones known as quantum black holes may have formed shortly after the Big Bang, though they remain a debated topic in scientific literature. Star clusters, under the influence of gravity and mass distribution, can collapse to form black holes. In a black hole, its radius and mass are proportional—more massive black holes have lower density, while less massive ones are denser. Black holes come in various sizes, and a black hole with a mass of 10^A (where A is typically 6, 7, 8, or higher) is considered a supermassive black hole. The size of a black hole is determined by its mass.

Types of black holes:

Black holes exist in various forms based on their size and formation.

Stellar-mass black holes form from collapsing stars and typically have masses a few times that of the Sun. Some black holes originate from star clusters or collapsing gas clouds, reaching millions of solar masses. These are highly energetic and visible due to the intense radiation emitted as gas falls into them. Hence, these black holes are considered massive objects. Nearly all the stellar-mass black holes observed so far have been found because they’re paired with stars.

Intermediate-mass black holes, around 10^5 solar masses, also form from collapsing clouds, stellar collapse, or clusters, though on a smaller scale. The high densities of the early universe were a prerequisite for the formation of primordial black holes, but did not guarantee it.

When the universe expands from the Big Bang, black holes can be created approximately 10^–35 metres across, which is known as the Planck length, and with masses of 10^8 kg, much smaller than stellar-mass black holes or one solar mass. These remain speculative, with no experimental evidence yet. One solar mass is 2 × 10^33 g—a comparatively very large mass—and represents the approximate mass of the Sun.

Quantum black holes are theoretical, microscopic objects possibly formed just after the Big Bang due to quantum fluctuations in extremely dense space-time. These are tiny or the lightest black holes called quantum black holes. As the universe expands, the average density of matter decreases; therefore, the density was much higher in the past, in particular exceeding nuclear levels within the first microsecond of the Big Bang.

Supermassive black holes, ranging from millions to billions of solar masses (10^6–10^9), lie at galaxy centres, formed possibly from gas cloud collapses during the early universe. At the Milky Way’s centre is Sagittarius A*, about 4 million solar masses, a major subject of study. NASA’s James Webb Space Telescope has recently provided the most detailed observations of this black hole, showing constant flaring activity from its accretion disk—a swirling mass of gas and dust that emits radiation across the spectrum.

Do black holes last forever?

Not necessarily. According to Hawking’s theory, black holes slowly emit radiation and evaporate over trillions of years. The more massive a black hole is, the longer it takes to evaporate. While no black hole has been observed evaporating yet, in theory, none will last forever. The timescale of evaporation through this process is very slow and depends on the mass.

Energy efficiency and radiation:

In terms of energy efficiency, Prof Mangalam noted that nuclear fusion in the Sun converts only about 0.07 per cent of mass into energy. Black holes, however, can convert up to 40 per cent of matter into energy, especially in the accretion disk.

Radiation from infalling gas provides vital information about its mass, luminosity, angular momentum, magnetic fields, and spin. Black holes are surrounded by orbiting stars and matter, giving astronomers clues to study their hidden properties.

What is inside a black hole?

At the event horizon, the space-like vector inside connects to the time-like vector outside, reversing the structure of space-time. What happens inside can’t be seen from the outside. A black hole is essentially a different Universe, said Professor Mangalam.

In the end, a black hole is a region cut off from the universe—from the singularity at its heart to the glowing disks of matter around it, black holes remain one of the most awe-inspiring frontiers in science.

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