How are gravitational waves created

Cosmic discovery Flash of light and gravitational waves at the same time

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Researchers have already measured gravitational waves several times, but now they have announced a spectacular new discovery: At the same time as a gravitational wave, they were able to observe a flash of gamma rays.

Status: October 16, 2017

On August 17, 2017, the gravitational wave detectors LIGO in the USA and Virgo in Italy registered a gravitational wave that came from a huge cosmic event: In a galaxy 130 million light years away, two neutron stars had collided and merged.

Neutron stars

A star that is significantly larger than the Sun, but not so big that it could become a black hole, will collapse into a neutron star at the end of its life. It has a diameter of around twenty kilometers, but is twice as heavy as the sun. In other words, it is incredibly compact: a teaspoon weighs a billion tons. That is why there are incredible gravitational forces on the surface of a neutron star, around a hundred billion times stronger than on Earth.

Not only gravitational wave detectors noticed the gigantic collision. At the same time, the FERMI space telescope registered a gamma-ray burst, a very bright, high-energy X-ray flash. Astronomers had already seen a few such flashes and suspected that they might have something to do with neutron star collisions. Now they could register a lightning bolt and a gravitational wave signal at the same time. That should serve as evidence of their acceptance. In addition, around seventy other telescopes around the globe in different wave ranges - light, radio waves, X-rays - observed the effects of this huge collision. They succeeded because the gravitational wave detectors were able to pinpoint where in the sky the neutron star collision had occurred. Because Virgo has also been in operation in Italy since August. This makes a total of three detectors that act together like three DF antennas.

The neutron star collision occurred in the galaxy NGC 4993.

Some researchers are now speaking of the dawn of a new era in astronomy. Perhaps rightly: The gravitational wave detectors could now let you know if something exciting happens in space, such as a supernova explosion. The experts are also working on continuously improving the detectors. This interplay between gravity wave detectors and telescopes could soon become routine.

Proof of one’s theory

100 years ago Albert Einstein presented a spectacular idea: gravitational waves, curvatures in space-time. They arise when masses are accelerated. Strong waves arise, for example when stars explode at the end of their lifespan. However, gravitational waves are very difficult to detect; researchers had tried in vain for decades. In February 2016, an international team confirmed that 99 percent of the existence of gravitational waves was confirmed. A sensation among astrophysicists.

"We have detected gravitational waves," said the head of the experiment, David Reitze. The researchers captured the crucial signal from space on September 14, 2015 with their measuring devices in Louisiana. The data have been evaluated since then. "It was created at a time when the first multicellular organisms were developing on earth," said Gabriela González from Ligo - the detector system in the USA that had received the signal. It comes from the collision of two black holes.

"The gravitational waves are opening a new era in gravitational-wave astronomy that promises groundbreaking new insights as 99 percent of the universe is dark."

Karsten Danzmann, 2017 Körber Prize Winner and Director of the MPI for Gravitational Physics in Hanover

New gravitational wave detection - this time in Europe

On August 14, 2017, not only were the two US LIGO detectors (Laser Interferometer Gravitational-Wave Observatory) able to detect gravitational waves, but also the Franco-Italian Virgo detector near Pisa for the first time. These waves of gravity also came from the merging of two black holes. This was the first detection by means of three detectors, on the one hand by the two LIGO detectors, which are very similar in design, and the Virgo detector, which is somewhat different in many details. With three detectors, LIGO and Virgo, the distance and the location of the gravitational wave source in the sky can be localized much better than with two.

Gravitational waves - QUESTION AND ANSWER

Why do such waves arise?

Gravity is actually a simple force. An apple always falls immediately from the tree, regardless of whether it is large or small, and it does so along a straight line. This is true on earth, but not in the universe: there is the gravitational field. It is not the same everywhere, but changes because there are large masses: double or neutron stars, supernova explosions, black holes. Whenever such masses are accelerated, gravitational waves arise, i.e. vibrations in space.

What do the gravitational waves do?

The gravitational waves compress and stretch space - similar to throwing a stone into a calm lake. Then the surface will ripple. The heavier the stone, the stronger it is. Similar to gravitational waves: the heavier a star is and the more it is accelerated, the more gravitational waves are created. In principle, even gravitational waves are created when the earth orbits the sun. But they are so weak that they cannot be measured.

Why has it not yet been discovered?

The vibrations that arise when the space is stretched and squeezed are incredibly tiny. The changes that gravitational waves create between the sun and earth are less than the diameter of a hydrogen atom. So far less than a billionth of a millimeter. Until 2015 there was only indirect evidence: In the 1970s, two US astronomers discovered two neutron stars that orbit each other closely. And the speed with which they turn around each other decreases, they are, so to speak, slowed down by their gravitational waves.

How do you measure gravitational waves directly?

You need a sophisticated experiment for that. The trick: laser beams that are reflected back and forth on two equally long stretches and meet again. Then you see a certain pattern, experts speak of interference, similar to the waves on the surface of the water that overlap when I throw two stones into it. If one of the laser beams is disturbed on its way by gravitational waves, then the interference pattern also changes.

When did the researchers find what they were looking for?

The experiment is called LIGO and is available in two versions in the USA - you need two to be sure that it is actually a signal from space. There are two tubes, each four kilometers long, through which laser beams are sent. There were rumors for weeks, and in February 2017 the researchers confirmed at a press conference: They measured the gravitational waves that were created when two massive black holes merged. The experts were excited - one physicist said she felt like Galileo when he looked through a telescope for the first time. 100 years after Einstein, the mysterious waves are proven.

Can the astrophysicists stop researching now?

If you refine the measurement method, a whole new window of observation opens up in space. So far, visible light, radio waves or gamma rays have been measured. In the future, one could "see" events that happened in the dark - that is, black holes or even the big bang. There are already plans for satellites in space that are supposed to do exactly what has succeeded on earth - that would be many times more precise.

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