Can black holes merge more than once?

In one line

The rule

When two black holes come close, they can merge into a single, larger black hole. This new black hole can later merge with other black holes if it meets them.

There is no strict limit to how many times a black hole can merge, as long as it finds another black hole nearby.

Let's understand it simply

Imagine you are playing marbles. If two marbles roll together and stick, they become a bigger marble. Now, if this bigger marble rolls and meets another, they can join and make an even bigger one. Black holes are like these marbles, but with gravity (the force that pulls things together).

In space, black holes pull on each other. If two come close, they spin around and finally crash together, making a single, bigger black hole. This new black hole can meet others and merge again. There is no rule that says it can only happen once.

How it works

1. Two black holes come close because of gravity.
2. They start spinning around each other, losing energy as they do so.
3. They get closer and closer, finally merging into one bigger black hole.
4. This new black hole can move through space and, if it finds another black hole, repeat the process.
5. Each time, the black hole gets bigger and heavier.
6. This can happen many times, especially in crowded places like the centers of galaxies or star clusters.

The story behind it

The idea of black holes merging became famous after scientists detected gravitational waves (ripples in space) in 2015. This was the first proof that black holes really do crash together. The experiment was called LIGO. Since then, scientists have found many such events. Some black holes found by LIGO are so big, they must have formed from earlier mergers.

Where you see it in real life

• In globular clusters (crowded groups of stars): Many black holes can be close together, so mergers can happen again and again.
• In the center of galaxies: Supermassive black holes can eat up smaller black holes over time.
• In merging galaxies: When two galaxies crash, their black holes can find each other and merge.
• In gravitational wave detections: Some black holes seen by LIGO are so massive, they likely formed from earlier mergers.
• In computer simulations: Scientists use computers to show how black holes can merge multiple times.
• In the growth of supermassive black holes: The biggest black holes in the universe probably grew by merging many times.

Why it matters

• Helps us understand how the biggest black holes in the universe are formed.
• Explains why some black holes are much heavier than a single star could make.
• Shows how galaxies change and grow over time.
• Lets us study gravitational waves, which tell us about the universe in a new way.

Worked example

Let's imagine a simple story:

Given: Black hole A (10 times the Sun's mass), Black hole B (15 times the Sun's mass)
Step 1: A and B merge → New black hole C (about 24 times the Sun's mass; some mass is lost as energy)
Step 2: Black hole C meets black hole D (20 times the Sun's mass)
Step 3: C and D merge → New black hole E (about 42 times the Sun's mass)
Step 4: This can continue if E meets another black hole

Common mistakes

• Thinking a black hole can only merge once: In reality, it can merge again if it finds another black hole.
• Believing black holes "eat" everything around them: They only merge if they come close enough; most black holes are far apart.
• Assuming all black holes are the same size: Mergers make bigger and bigger black holes.
• Forgetting that some mass is lost as energy (gravitational waves) during each merger.

Remember it

Black holes are like marbles that can join together again and again, making bigger marbles each time.