# How does gravity arise

## How do gravitational waves arise?

Postulated almost a hundred years ago, they were searched for over fifty years: Gravitational waves are a prediction from Albert Einstein's general theory of relativity. They promise new insights into the structure and origin of the universe. While theorists track down the waves through simulation and modeling, experimental physicists and analysts scour the data from special detectors for their signals.

Gravitational waves are different from all waves known to us, such as light or sound waves. “Gravitational waves simply pass through stars and can also propagate in a vacuum. These waves are a distortion of the geometry of the space itself, ”says Roland Haas from the Max Planck Institute for Gravitational Physics in Potsdam.

### Gravitational waves follow from general relativity

Simulated wave propagation

Albert Einstein postulated these so-called gravitational waves a year after he developed the general theory of relativity in 1915. The waves result as a direct result of the field equations of Einstein's theory and were often mistaken for a mathematical artifact at the beginning - until they could be measured directly for the first time in 2015. “In the meantime, no serious scientist doubts that they exist,” says Karsten Danzmann, who conducts research at the same institute as Haas, albeit at the Hanover site.

While gravitational waves move through the universe, they compress and stretch the four-dimensional space-time - this structure consists of the three spatial directions and time as the fourth dimension. Einstein's revolutionary idea: Gravitation is not a force as with Newton, but a geometric property of this space-time. Every mass in it bends or deforms it and thus changes the trajectories of other bodies or particles.

Masses bend space-time

A four-dimensional curved space is necessary for the theory of relativity, it is not conceivable. The behavior of spacetime can be illustrated by reducing it by two dimensions: In a lattice structure, balls of different weights cause dents of different depths due to their mass - analogous to the curvature of spacetime.

### Accelerated masses create distortions in spacetime

When masses move at an accelerated rate, they not only bend space, but also send out gravitational waves. The theorist Haas describes it as follows: “Gravitational waves are physical distortions of space-time that move at the speed of light. Such distortions are generated as soon as objects move at an accelerated rate. "

Space points change their distance

Space points change their distance

Distortion of spacetime means that the passing wave changes the distances between objects in space. “An apple, which was originally approximately round, would be compressed by a gravitational wave in one direction and pulled apart in the other - a bit egg-shaped. And then it becomes round again. That would be the effect of a gravitational wave, ”says Haas. However, this distortion is so minimal and happens for such a brief moment that the process cannot be observed. You only know theoretically that it will happen.

The frequencies of gravitational waves can be between 10-18 and 104 Hertz lie. For comparison: audible sound frequencies are 20 hertz to 20 kilohertz, those of visible light a few hundred terahertz - one terahertz being 1012 Hertz corresponds. Gravitational waves have in common with light that they are also transverse waves that can propagate in a vacuum. However, gravitational waves are not absorbed by dust or gas in space, as is the case with light. Their propagation behavior is more like that of sound: “Sound squeezes and expands air and a gravitational wave just squeezes space,” says Danzmann.

### Measure the "huge bang" of merging black holes

Merging black holes

Merging black holes

The amount of energy transported by gravitational waves can be considerable: A supernova in our galaxy generates gravitational waves with an energy flow of one kilowatt per square meter, which is as much as the sun sends to us. Nevertheless, the waves are difficult to detect, because "the room is extremely stiff and it takes a lot of energy to bend it even a little," says Danzmann.

Measuring devices such as gravitational wave detectors therefore concentrate on major cosmic events - from luminous star explosions to rapidly circling neutron stars to super-heavy black holes. Danzmann describes it as follows: "Whenever galaxies collide, and that is what they do in cosmic expanses, at some point the two black holes in their centers merge and that gives a huge bang - and that is exactly what we measure."

In principle, any mass that does not move uniformly through space sends out gravitational waves. This applies equally to a long-distance runner starting out as well as to planets orbiting their star. What differs is the strength and frequency of the resulting gravitational waves. “The heavier the object, the more and the faster the position changes, the stronger the gravitational waves. Heavy celestial bodies simply have more mass and this creates a stronger gravitational field, ”says Haas.

### Sources determine the wave frequency

Two stars crash into each other

It also depends on the sources what frequency the gravitational waves have. “In principle, the heavier the body, the lower the frequency of the radiation that it emits,” says Danzmann. With emissions of up to one kilohertz, supernovae are among the more high-frequency gravitational sources. In these extremely luminous star explosions, massive stars, which have exhausted their nuclear energy supply, repel the outer layers in a huge explosion. The interior then collapses into a neutron star or black hole. They emit part of the energy released in the process as gravitational waves. Fast rotating neutron stars that are not rotationally symmetrical also emit gravitational waves. Here the typical frequencies are between ten and a thousand Hertz.

With close binary star systems of neutron stars or black holes that orbit each other and lose energy in the process, the frequencies of the transmitted gravitational waves can be lower. The period of oscillation of the wave corresponds to half of the period of revolution. This means that the closer the celestial bodies come, the lower the wavelength and the higher the frequency: "Merging double neutron stars can start at very low frequencies and then chirp up to a few hundred Hertz in the last second," says Danzmann, " Black holes, of course, come in all shapes and colors. "

Super massive black holes, which are 100,000 to several billion times as massive as our sun, emit frequencies in the millihertz range when they merge with one another. The waves emitted by merging black holes with a few dozen solar masses have frequencies a little below one hundred hertz, and those with few solar masses have frequencies above a hundred.

### Gravitational waves carry information from the Big Bang

Researchers suspect particularly diverse sources of gravitational waves in the early days of the universe: “There are all possible scenarios of how gravitational waves were created by the Big Bang. Practically every scenario has produced such waves, ”says Danzmann. The big bang was the most violent event in the history of the cosmos. The waves released at that time contain many frequencies of a broad spectrum and still pervade the universe today as omnipresent noise - this is the so-called stochastic background radiation. Gravitational waves can not only provide new knowledge about major cosmic events in the universe, but also about the formation of the universe itself.