What happens if two black holes merge?

In one line

The rule

When two black holes merge, their masses combine (minus the energy lost as gravitational waves), forming a single, larger black hole. The process releases energy as gravitational waves, as predicted by Einstein's General Theory of Relativity.

There is no single formula for the entire process, but the energy radiated as gravitational waves (E) can be estimated: E = (m₁ + m₂ - M_final) × c² where:

• m₁ = mass of first black hole
• m₂ = mass of second black hole
• M_final = mass of the new black hole after merger
• c = speed of light

Let's understand it simply

Imagine two giant whirlpools in a river. Each one pulls in water strongly. If they come close, they start spinning around each other. Finally, they crash together, making one even bigger whirlpool. When this happens, the water shakes and sends out waves far away.

Black holes do something similar in space. They pull on each other, spin around, and finally join. When they merge, they shake space itself, sending out invisible waves called gravitational waves. These waves travel across the universe, like ripples on a pond after you throw a stone.

How it works

1. Two black holes come close because they lose energy by sending out weak gravitational waves.
2. They start orbiting each other faster and faster, getting closer as they lose more energy.
3. At the last moment, they spiral in and crash together, forming one new black hole.
4. The new black hole is not calm at first—it wobbles and vibrates, sending out a final burst of strong gravitational waves.
5. After a short time, the new black hole settles down, and the waves move outwards at the speed of light.

The story behind it

Albert Einstein predicted in 1916 that moving massive objects, like black holes, would create ripples in space called gravitational waves. For many years, no one could detect these waves because they are very faint. In 2015, scientists using the LIGO observatory in the USA finally detected gravitational waves from two black holes merging, proving Einstein right. The two black holes were each about 30 times the mass of our Sun, and their merger made headlines around the world.

Where you see it in real life

• LIGO and Virgo detectors: Scientists use huge machines on Earth to catch gravitational waves from black hole mergers.
• Space news: When a new black hole merger is detected, it often appears in science news.
• Simulations and movies: You might see animations of black holes merging in documentaries or movies like 'Interstellar.'
• Astrophysics research: Scientists study these mergers to learn about the universe’s history and how black holes grow.
• Testing Einstein’s theory: Each merger helps check if Einstein’s ideas about gravity are correct.
• Understanding heavy elements: Some mergers (with neutron stars) help explain where gold and platinum come from in the universe.

Why it matters

• Proves Einstein’s theory: Detecting gravitational waves shows that space really can ripple, just as Einstein said.
• Lets us see invisible things: Black holes cannot be seen directly, but their mergers can be 'heard' through gravitational waves.
• Tells us about the universe: By studying these waves, we learn how many black holes there are, and how big they can get.
• Opens a new way to study space: Gravitational waves are like a new sense, letting us 'listen' to the universe, not just see it.

Worked example

Let’s say two black holes merge:

• Black hole 1: 30 times the mass of the Sun (m₁ = 30 M☉)
• Black hole 2: 35 times the mass of the Sun (m₂ = 35 M☉)
• After merging, the new black hole is 62 times the mass of the Sun (M_final = 62 M☉)

How much mass was lost as energy?

Given: m₁ = 30 M☉, m₂ = 35 M☉, M_final = 62 M☉
Step 1: Add the two masses: 30 + 35 = 65 M☉
Step 2: Find the difference: 65 - 62 = 3 M☉
Step 3: This 3 M☉ was turned into energy (gravitational waves)
Step 4: Energy released = 3 × (mass of Sun) × c²

So, 3 times the Sun’s mass was turned into pure energy and sent out as gravitational waves!

Common mistakes

• Thinking black holes 'explode': They do not explode; they merge quietly (but powerfully), sending out waves, not light.
• Believing black holes disappear: The black holes do not vanish; they become one bigger black hole.
• Assuming all mass is kept: Some mass is lost as energy in the form of gravitational waves.
• Confusing gravitational waves with sound: Gravitational waves are ripples in space, not sound waves in air.

Remember it

When two black holes merge, they make a bigger black hole and send out ripples (gravitational waves) across space.