Can black holes merge to form bigger black holes?

In one line

The rule

When two black holes come close, their gravity can pull them together, causing them to merge into a single, larger black hole.

There is no simple formula for this, but the process follows the rules of gravity and Einstein’s general relativity.

Let's understand it simply

Imagine you have two big balls of dough (like when making roti). If you press them together, they become one bigger ball. Black holes are like these balls, but made of gravity so strong that nothing, not even light, can escape. When two black holes get close, their gravity pulls them together. They spin around each other, get closer, and finally become one huge black hole. Some of their mass is lost as energy, like heat when you clap your hands hard.

How it works

1. Two black holes move near each other in space.
2. Their gravity pulls them closer, and they start orbiting (spinning) around each other.
3. As they spin, they give off energy as ripples in space called gravitational waves (like waves in a pond).
4. These waves carry away energy, so the black holes get closer and faster.
5. Finally, they crash into each other and merge, forming a single, bigger black hole.
6. The new black hole is a bit lighter than the total of the two, because some mass turned into energy (E = mc²).

The story behind it

The idea that black holes could merge comes from Einstein’s theory of general relativity, published in 1915. For many years, this was just a theory. In 2015, scientists using LIGO (Laser Interferometer Gravitational-Wave Observatory) in the USA detected gravitational waves for the first time. These waves came from two black holes merging over a billion light years away. This discovery won the Nobel Prize in Physics in 2017. The scientists who built LIGO, like Kip Thorne, Rainer Weiss, and Barry Barish, made this possible after decades of hard work.

Where you see it in real life

• LIGO experiment (USA): Detected gravitational waves from merging black holes in 2015.
• Virgo experiment (Italy): Also detects black hole mergers.
• Space: In the centre of galaxies, supermassive black holes may merge when galaxies collide.
• Computer simulations: Scientists use supercomputers to model black hole mergers.
• Science news: You may read about new black hole mergers being discovered every year.
• Movies: Films like "Interstellar" show black holes and sometimes merging events.

Why it matters

• Proves Einstein’s theory of gravity is correct, even in extreme cases.
• Helps us understand how the biggest black holes in the universe grow.
• Gravitational waves give us a new way to study the universe, not just with light.
• Black hole mergers are used in advanced research and may appear in competitive exams and future science careers.

Worked example

Let’s say two black holes, one with mass 30 times the Sun (30 M☉) and one with 35 M☉, merge. About 3 M☉ turns into energy.

Given:  Black hole 1 = 30 M☉, Black hole 2 = 35 M☉
Step 1: Add their masses: 30 + 35 = 65 M☉
Step 2: Subtract mass lost as energy: 65 − 3 = 62 M☉
Step 3: The new black hole has mass 62 M☉
Step 4: 3 M☉ turned into gravitational wave energy

Common mistakes

• Thinking black holes "explode" when they meet. They actually merge quietly, but send out strong gravitational waves.
• Believing all the mass adds up. Some mass always turns into energy (E = mc²).
• Imagining black holes "suck" everything in like a vacuum cleaner. They only pull in things very close to them.
• Confusing gravitational waves with sound waves. Gravitational waves are ripples in space itself, not air.

Remember it

Black holes can join to make bigger black holes, sending out ripples in space called gravitational waves.