What Happens When Black Holes Collide?

My young friend Sam comes up with some great space questions, and he recently asked "what happens if a black hole collides with a black hole?". His mum sent me the question via a video in a chat message, but you can ask me questions too: just fill in this form.

Do black holes crash into each other?

Short answer: Yes.

Given that space is really big and that black holes are (relatively) small it'd be unlikely for two of them to smack into each other in a random head-on collision, but given how black holes are formed in the first place they tend to be part of a system (like our own solar system) with other stuff (like stars, planets and dust) orbiting them. Sometimes two black holes might form in the same system and orbit around each other. Over time, orbits can degrade which means that things orbiting each other gradually spiraling closer to each other until they eventually collide.

What happens when black holes collide?

The relatively boring part of the answer to this is that when two black holes get close enough to each other they fall inside each other's event horizon and become one bigger^[1] black hole. That's it, really. There won't even be a massive, big-budget, hollywood-movie-style explosion, because explosions blow stuff outwards and black holes can't do that because once any stuff is inside their event horizon it doesn't - it can't - come back out again.

In fact, scientists think that many of the supermassive black holes that exist at the centre of most galaxies probably started off as clusters of much smaller black holes that were orbiting around each other and eventually merged.

Except...

As we've already mentioned, black holes that crash into each other tend to be part of a system, and there's often a lot of other stuff in that system too, orbiting around (and falling into) those black holes. Some space scientists have suggested that the merging of two black holes might have an effect on some of the stuff nearby, potentially causing it to give off some light that we could detect.

The merging black holes don't cause this by physically touching it: instead, but their gravitational fields have an effect at a distance. As two objects that are orbiting each other get closer together, they both start to move faster and faster in their orbits. This might seem a little counter-intuitive, but you can find examples of the same kind of thing happening here on Earth: the classic demonstration is when you see an ice skater spinning, like in the video below:

The video shows an ice skater starting to spin. At first they have their arms and a leg stretched out wide, but as they bring their arms and leg closer in to their body, they start to spin faster and faster.

You could also experiment with the same effect next time you're in a playground: give the roundabout a push to get it started, and jump on. If you hold on tight but lean out as far as you can the roundabout will spin relatively slowly. Try pulling yourself inwards: the roundabout will spin faster and faster as you move closer to the middle without you or anyone else needing to push it any more.

As you move through space your own gravitational field affects it, but not much. Black holes have a lot of mass, so their gravity affects space more. As two black holes that are spinning around each other get closer together, they move faster and faster through space, and their combined gravitational fields cause ripples in the very fabric of space. These ripples spread out from them in a similar way to ripples in water when you drop a rock in, and just as those water ripples might cause a leaf on the surface of the water nearby to bob up and down, matter near to colliding black holes might feel the effects of the ripples in space.

In fact, it's not just nearby that the effects can be felt: these ripples - known as gravitational waves - spread out across the universe, causing space itself to expand and contract. By the time they reach us they're so small that we don't notice them. We know they're there because the equations of Einstein's theories of relativity tell us they are^[2]. This led scientists to think of ways to detect them (as a way of checking that Einstein's theories of relativity are correct).

Detecting ripples in space

I won't go too deeply into this here^[3] but it's worth mentioning that the scientists didn't only think of ways to detect gravitational waves, but actually built the equipment they thought of. Gravitational wave detectors involve firing a laser down two 4km tunnels that are at right-angles to each other. Lasers are cool.

Not only did they actually build such a detector, but in 2015 they actually detected gravitational waves. These ripples in space were caused by two black holes colliding with each other. This collision happened a long way away, and the resulting gravitational waves had been traveling through space for more than a billion years before being detected by the detector known as LIGO.

Here's a picture of the LIGO gravitational wave detector:

Aerial5 by Prachatai (CC BY-NC-ND 2.0)

Footnotes:

1. Strictly speaking, one more massive black hole: the "new" black hole will have the same mass as those of the two original black holes combined. Which makes sense, I think. [back]
2. This is just one of the amazing things about maths: it allows us to find out things about the universe that sound completely crazy and we would never even think to look for otherwise. [back]
3. If you'd like to know more, ask a question. [back]