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What happens when two black holes collide?



Illustration: Angelica Alzona (Gizmodo)
Giz asks In this Gizmodo series, we ask questions about everything, from the universe to the Po, and get answers from various experts.

On September 1

4, 2015, signals from one of the most overwhelming and powerful events in the universe produced the smallest signal in a detector pair, one in Louisiana and one in Washington state. They had detected two already wild objects, black holes that had collided.

You probably know Black Holes as cosmic vacuum cleaners, but they are a bit more complex. A core theme of Einstein's Theory of Gravity is that hard enough things change the shape of the space around them and gravity is how we experience this rejection. Black holes are such small and massive regions of space that they have a point-of-no-return, an "event horizon" beyond which space is so distorted that every path that everything can cover goes to the center of the black hole leads. Nothing, not even light, can escape.

When two of these objects collide, you can imagine something phenomenal happening. Scientists have measured the result several times with the Laser Interferometer Gravitational Wave Observatory or LIGO and the Virgo Detector. For this week, giz questions have asked scientists to give us the essence.

Imre Bartos

Physicist and assistant professor at the University of Florida, member of the LIGO Scientific Collaboration

As the black holes approach each other, they fuse into a larger black hole. The radius of the new black hole is about the sum of the radii of the two original black holes, so the new black hole has a much larger volume. The merger is much like two drops of water in space would act as they approach.

It is also important that the black holes emit a tremendous amount of gravitational waves as they close together. As a result, a few percent of their mass can be converted into pure energy, which is emitted as gravitational waves.

It was only recently, in 2015, after the construction of the Advanced LIGO Gravitational Wave Observatory, that we first discovered a collision of two black holes. With constant technical improvements, we will come to a discovery every week from this first discovery over the next few years. As we observe these collisions, we still do not know which cosmic process brings the black holes close together so they can collide. Observing these collisions can also help us to answer a number of unanswered questions, such as how black holes act as accelerators for cosmic particles, or whether Einstein's General Theory of Relativity is the right description of nature. Black hole collisions can even help to better represent the spread of the universe.

Sabine Hossenfelder

Theoretical Physicist at the Frankfurt Institute for Advanced Studies in Germany, author and blogger on Quantum Gravity Research

The most notable feature of black holes is that they are insignificant.
They are pure space-time deformations, defined by the event horizon at which
limits the region from which nothing escapes.

In the simplest case, the horizon of a black hole is spherical. If two black holes come too close, these balls merge into a larger ball. After the merger, the ball will sway for a while until it calms down, which is called "ringdown". Both fusion and ringdown create gravitational waves. Not only does the gravitational wave signal contain information about the fused black holes, it also allows us to test whether we properly understand how space-time bends under such extreme circumstances. As far as we know, Einstein did everything right.

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

Oliver Jennrich

Fundamental physics mission Scientist of the European Space Agency working on the upcoming Laser Interferometer Experiment for Gravitational Wave Space Antennas.

[Black holes] emit gravitational waves and merge into a larger black hole. But that's not the end of the story. The story usually begins with two stars orbiting each other, much as the earth orbits the sun. If the right conditions are met, the two stars become black holes when the fuel is finally consumed and the remaining matter collapses into two black holes. The two black holes are constantly circling each other, and to collide, their distance must be reduced. In other words, you have to lose energy. For black holes, this is essentially the only way to emit gravitational waves. Revolution for revolution: The system of two black holes radiates gravitational waves and their orbit shrinks. The closer they get, the more efficient is the emission of gravitational waves, i. H. The orbit shrinks faster and faster as the amount of gravitational waves increases. This is called inspiratory phase.

At some point, the two black holes are so close together that their mutual attraction deforms, bringing them even closer until the two black holes merge into a peanut-shaped object. Similar to a very elongated soap bubble, this peanut-shaped black hole wobbles and sways and eventually gains a spherical shape. This is the post-merger or ring-down phase where the new wobbly black hole also emits very characteristic gravitational waves.

The mass of the newly formed back hole is typically a few percent smaller than the sum of the masses of the two initial black holes – the remainder was emitted by gravitational waves, most of them during the fusion phase. Since the initial masses of black holes can be very large (millions of times the mass of our Sun), even a few percent of this mass make up 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. Editor's note: "Performance" here means the rate at which energy is released. ] However, the effects of this gigantic amount of energy are very small – the gravitational waves due to such events would change the distance between the sun and the Earth is the diameter of a hydrogen atom.

Ground-based gravitational wave detectors such as the kilometer-sized LIGO and Virgo detectors are capable of measuring signals emitted by the merging of a black hole with up to 30 times the mass of the sun. In the final phase of inspiration, the black holes move at about 60 percent of the speed of light and the resulting gravitational waves range from 100 to 300 Hz. To observe much heavier black holes, observations at lower frequencies are required. On earth, these signals are masked by noise caused by earthquakes, weather and people. For this reason, LISA, an ESA-led mission, will detect gravitational waves from space and use three spacecraft spaced 2 million kilometers to record 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 form a larger black hole. However, the mass of the larger black hole is NOT the sum of the masses of the two smaller ones. It is a little less, because part of their mass is converted into energy and emitted in gravitational waves. We know that this is true because we discovered these waves in space-time with the LIGO detector.

Something we also believe to be true (which we have not yet observed) is that the new big black hole gets a "kick" in speed after the merger and zooms in a (seemingly random) direction. The height of the kick and the direction depend on the characteristics of the binary black hole system before it is merged.

Part of my research on finding massive black holes in dwarf galaxies depends on how efficient this step is. If the black hole is thrown out of the galaxy (or even kicked out of the center so it can roam the outskirts), it's much harder to find, but I'm trying to find ways to look for those wandering blacks to keep holes.

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