Wednesday , May 12 2021

What happens when two black holes collide?

Illustration: Angelica Alzona (Gizmodo)
Giz asksIn this series Gizmodo, we ask questions about everything from space to rifles and we get answers from different experts.

On September 14, 2015, signals from one of the most powerful, most powerful events in the universe created the tiniest signal in both detectors, one in Louisiana and one in Washington State. They found two already wild objects, black holes, shaking each other.

You're probably familiar with black holes like cosmic vacuum cleaners, but they're a bit more complicated than that. One basic yield of the theory of gravity of Annehen is that hard enough things actually change the shape of the space around them, and gravity is how we experience this distortion. Black holes are areas of space so small and massive that they carry a point of return, a "horizon of events," above which space is so warped that any path that can travel leads to the middle of a black hole. Nothing, even a light, can not escape.

So, when two of these objects collide, you can imagine that something phenomenal happens, and indeed scientists measure the result several times using Laser Interferometric Gravitational-Wave Observatories, or LIGO, as well as Devica Detector. For this week, Giz Asks, we asked scientists to give us a mouthpiece.

Imre Bartos

Physicist and docent at the University of Florida, member of the Scientific Research Collaboration of LIGO

When black holes are close to each other, they merge into one, a larger black hole. The new radius of the black hole will be approximately the sum of the radii of two original black holes, making the new black hole cover much larger volume. Fusion is something similar to making two drops of water in space when they get closer.

Also, more importantly, black holes emit abundant quantities of gravitational waves as they close to each other. This can turn several percent of their mass into clean energy radiated as gravitational waves.

Initially we did not find a clash of two black holes, not recently, in 2015, after the construction of observatories for advanced slim gravitational waves. With continuous technological improvements, we will go from this first detection to discovery every week in the next few years. As we observe these clashes, we still do not know what the cosmic process is bringing black holes closer to each other so they can collide. Observing these clashes can also help us answer a number of exceptional questions, such as how black holes work as cosmic particle accelerators or whether the general theory of Einstein's relativity is an accurate description of nature. Black holes may even help us better out the way of expanding the universe.

Sabine Hosenfelder

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

The most striking thing about black holes is that they are immaterial.
They are purely space-time deformations, defined by the horizon of the event it
it limits the region from which nothing can escape.

In the simplest case, the black hole horizon is spherical. If two black holes are too close, these spheres will merge and form a larger sphere. After the merger, the sphere will get stuck for a while until it calms down, which is called the "ring". Both the fusion and the ring create gravitational waves. The gravitational wave signal does not only contain information about the black holes that merge, but it also allows us to check whether we correctly understand how space-time bends in such extreme circumstances. For everyone at the moment we know, Einstein got it right.

This computer simulation shows a collision of two black holes that produce gravitational waves.
Image: SXS (NASA)

Oliver Jenrich

A scientist on fundamental physics at the European Space Agency working on the upcoming gravity wave experiment Laser Interferometer Space Antenna.

[Black holes] they 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 that orbit one another, just as the Earth orbits the Sun. If the right conditions are met, the two stars will become black holes when their fuel is finally spent, and the remaining matter will fall into two black holes. The two black holes keep orbiting themselves and in order to collide, their distance must be smaller. In other words: they need to lose energy. For black holes, in essence the only way to do this is through the release of gravitational waves. So, a revolution with a revolution, the system of two black holes emits gravitational waves and their orbit decreases. The closer it becomes, the more efficient the emission of gravitational waves becomes, the orbit decreases faster and faster, while the amount of gravitational waves becomes bigger and bigger. This is called the inspiration phase.

At some point, the two black holes are so close together that their mutual gravitational attraction begins to deform, which brings them closer as long as the two black holes do not merge and become a kind of peanuts. Similar to the much elongated soap bubble, this red hole in the form of peanuts turns and oscillates and eventually returns a spherical shape. This is a post-coupling or ring-down phase, with the new red hole emitting many characteristic gravitational waves.

The mass of the newly opened hole is usually several percent smaller than the sum of the mass of the two initial black holes-the rest is radiated by gravitational waves, most of them during the fusion phase. Because the initial masses of black holes can be huge (millions of times more than the mass of our Sun), even a few percent of that mass represents a very large amount of energy. In fact, the fusion of two black holes is the most powerful event in the universe, releasing more power than the rest of the Universe combined.[[[[One note: "Power" here means a rate that the energy is released.]However, the effects of that titanium amount of energy are very small – gravitational waves of such events will change the distance between the Sun and the Earth by measuring the diameter of the hydrogen atom.

Ground-based ground-based gravity detectors, such as the LIGO and Virgo-kilometer-kilometers detectors, are capable of measuring signals emitted from the black hole coupling, which has as much as 30 times the mass of the Sun. In the final phase of the inspiration, black holes move about 60 percent of the speed of light, and the results of gravitational waves range from 100-300 Hz. To observe much heavier black holes, observations at lower frequencies are required. On Earth, these signals are masked by noise caused by earthquakes, time, and humans. For this reason, LISA, an ESA-led mission, will detect gravity waves from space using three spacecraft at a distance of 2 million kilometers to register gravity waves in the frequency range of 30 MHz to 0.1 Hz.

Gillian Belovri

Theoretical astrophysicist and assistant professor at the Queensboro Community College

When two black holes collide, they make a bigger black hole. However, the mass of the larger black hole is NOT the sum of the mass of the two smaller ones. It's a little less, because part of their mass is converted into energy and radiated into gravitational waves. We know this is true, because we detected these waves in the space-time structure with the LIGO detector.

Something that we also think is true (but not yet noticed) is that after the merger, the new big black hole gets a "punch" at a speed and zooms in (seemingly random) direction. The amount of impact and direction depend on the properties of the binary black hole before they merge.

Part of my research to find large black holes in dwarf galaxies depends on how effective this blow is; if a black hole is thrown out of the galaxy (or even just thrown out of the center, making it wander around in the periphery), 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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