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What Happens When Two Black Holes Collide?

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Illustration: Angelica Alzona (Gizmodo)

On September 14, 2015, signals from one of the Universe’s most mind-boggling, powerful events produced the tiniest signal in a pair of detectors, one in Louisiana and one in Washington state. They’d detected two already-wild objects, black holes, slamming into one another.

You’re probably familiar with black holes as cosmic vacuum cleaners, but they’re a little bit more complex than that. One core takeaway of Einstien’s theory of gravity is that heavy enough things actually change the shape of the space around them, and gravity is how we experience this warping. Black holes are regions of space so small and massive that they carry a point-of-no-return, an “event horizon” beyond which space is so warped that every path that anything could travel leads to the black hole’s middle. Nothing, not even light, can escape.


So, when two of these objects collide, you can imagine that something phenomenal happens, and indeed scientists have measured the result several times using the Laser Interferometer Gravitational-Wave Observatories, or LIGO, as well as the Virgo detector. For this week’s Giz Asks, we asked scientists to give us the nitty-gritty.

Imre Bartos

Physicist and Assistant professor at the University of Florida, Member of the LIGO Scientific Collaboration

When the black holes get close to each other, they fuse into one, bigger black hole. The new black hole’s radius will be roughly the sum of the two original black hole’s radii, making the new black hole encompass a much bigger volume. The fusion is somewhat like what two water droplets would do in space when they get close.

Also importantly, the black holes emit copious amounts of gravitational waves as they close in on each other. This can turn a few percent of their mass into pure energy radiated away as gravitational waves.

We detected the collision of two black holes for the first time not too long ago, in 2015, following the construction of the Advanced LIGO gravitational-wave observatories. With continued technological improvements, we will go from this first detection to a discovery every week in the next few years. While we observe these collisions, we don’t yet know what cosmic process brings the black holes close to each other so they can collide. Observing these collisions can also help us answer a range of outstanding questions, such as how black holes work as cosmic particle accelerators, or whether Einstein’s General Theory of Relativity is the correct description of nature. Black hole collisions can even help us better map how the Universe is expanding.


Sabine Hossenfelder

Theoretical physicist at the Frankfurt Institute for Advanced Studies in Germany, author, and blogger researching quantum gravity

The most remarkable thing about black holes is that they are immaterial.
They are pure space-time deformations, defined by the event horizon that
bounds the region from which nothing can escape.

In the simplest case, the horizon of a black hole is spherical. If two black holes get too close, these spheres will merge and form one larger sphere. After the merger, the sphere will wobble for some while until it settles down, which is called the “ringdown.” Both the merger and the ringdown produce gravitational waves. The gravitational wave signal does not only contain information about the black holes that merged, it also allows us to test whether we correctly understand how space-time bends in such extreme circumstances. For all we presently know, Einstein got it right.

This computer simulation shows the collision of two black holes, which produces gravitational waves.
This computer simulation shows the collision of two black holes, which produces gravitational waves.
Image: SXS (NASA)

Oliver Jennrich

Fundamental physics mission scientist at the European Space Agency who works on the upcoming Laser Interferometer Space Antenna gravitational wave experiment.

[Black holes] emit gravitational waves and merge to one larger black hole. But that is not the end of the story. The story typically begins with two stars orbiting each other, much like the Earth orbits the Sun. If the right conditions are met, the two stars will become black holes when their fuel is eventually spent and the remaining matter collapses into two black holes. The two black holes keep orbiting each other and for them to collide, their distance must become smaller. In other words: they need to lose energy. For black holes, essentially the only way to do that is by emitting gravitational waves. So, revolution by revolution, the system of two black holes emits gravitational waves and their orbit shrinks. The closer they get, the more efficient becomes the emission of gravitational waves, i.e. the orbit shrinks faster and faster while the amount of gravitational waves becomes bigger and bigger. This is called the inspiral phase.

At some point, the two black holes are so close to each other, that their mutual gravitational attraction starts to deform them, which brings them even closer until the two black holes merge and become one peanut-shaped object. Much like a very much elongated soap-bubble, this peanut-shaped black hole wobbles and oscillates and eventually regains a spherical shape. This is the post-merger or ring-down phase during which the new wobbling black hole emits very characteristic gravitational waves as well.

The freshly formed back hole’s mass is typically a few percent smaller than the sum of the masses of the two initial black holes—all the rest has been radiated away by gravitational waves, most of it during the merger phase. As the initial masses of the black holes can be huge (million times the mass of our Sun), even a few percent of that mass constitutes a very large amount of energy. In fact, the merger of two black holes is the most powerful event in the Universe, releasing more power than the rest of the Universe combined. [Ed note: “Power” here means the rate that energy is released.] However, the effects of that titanic amount of energy are very small—the gravitational waves from such events would change the distance between the Sun and the Earth by amount the diameter of a hydrogen atom.

Ground based gravitational wave detectors like the kilometer-sized LIGO and Virgo detectors are capable of measuring signals that are emitted from merging black hole that have as much as 30 times the mass of the Sun. In the final phase of the inspiral, the black holes move around with about 60 percent of the speed of light and the resulting gravitational waves are in the range of 100-300 Hz. To observe much heavier black holes, observations at lower frequencies are needed. On Earth, those signals are masked by noise caused by earthquakes, weather, and people. For this reason, LISA, an ESA-led mission, will be detecting gravitational waves from space, using three spacecraft 2 million kilometers apart to register gravitational waves in the frequency range of 30 mHz to 0.1 Hz.


Jillian Bellovary

Theoretical astrophysicist and assistant Professor at Queensborough Community College

When two black holes collide, they make one bigger black hole. However, the mass of the bigger black hole is NOT the sum of the masses of the two smaller ones. It’s a little bit less, because some of their mass is converted to energy and radiated away in gravitational waves. We know this is true because we’ve detected these ripples in the fabric of space-time with the LIGO detector.

Something we also think is true (but we haven’t observed yet) is that after the merger, the new big black hole gets a “kick” in velocity, and zooms off in a (seemingly random) direction. The amount of kick and the direction depend on the properties of the binary black hole system before it merged.

Part of my research on finding massive black holes in dwarf galaxies depends on how efficient this kick is; if the black hole gets kicked out of the galaxy (or even just kicked out of the center, making it wander around in the outskirts), it’s much harder to find, but I’m trying to think of ways to look for these wandering black holes.


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