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Where do gravitational waves come from?

Where do gravitational waves come from?
Where do gravitational waves come from?

Gravitational waves are a hot topic in the media and news sites. These “ripples on the tissue of space-time” convey much more information about the universe than electromagnetic waves can give, because their propagation is practically undisturbed. Thanks to them, we can learn more about the hot beginnings of the so-called the early universe.

When the name “gravitational waves” is mentioned, the image of a wave at sea, or at best something similar to an electromagnetic wave, usually comes to mind. However, this concept is actually something completely different

Some history

While at the turn of the nineteenth and twentieth centuries, even the most prominent physicists had a problem with accepting the concept of a wave as a propagating field disturbance (as shown, among others, by the hypothesis of ether, being the center of propagation of electromagnetic waves), here we are dealing with the disturbance of space-time itself. What does it mean? In 1915, the General Theory of Relativity (GRN) was published by Albert Einstein. Already ten years earlier, his special theory of relativity (STR), rejecting the concept of ether and adopting the postulate of the constant speed of light in all inertial systems, regardless of the speed of the observer, managed to disturb the scientific community and provoke an initial aversion to this idea. However, its generalization (or GOT), aimed at reconciling gravity with the theory of relativity, seemed to be an even more radical step.

According to the OTW, gravity is not a Newtonian force, but the result of the geometry of space-time itself. This means that, for example, the Earth’s rotation around the Sun is caused by the curvature of the “scene” on which it takes place, and the Earth moves along the so-called geodetic curve, i.e. the shortest (or longest) possible road. Of course, the source of this curvature is the distribution of masses or energy in the Universe and we get a specific coupling – matter bends space-time, which causes the movement of matter to change.

We can imagine it as a rubber, stretched, flat surface on which we have a “dimple” created by a lying heavy ball. Now let us ask – what will happen if we raise and lower this ball? Then a wave-like disturbance will appear on that rubber surface, depending on the surface tension and other factors. It is true that we cannot do this in the universe, but if we change the position of a mass or energy cluster very quickly, then we will be able to produce something similar.

This is how the concept of gravitational waves can be presented in an overview.

A bit of theory and history

The theoretical work correctly describing gravitational waves (regardless of the selected coordinate system and defined space-time) by Felix Pirani appeared only in 1956, i.e. forty years after Albert Einstein had initially predicted the existence of these waves. However, it passed unnoticed in the scientific community as all his attention was focused on whether these gravitational waves carry energy. This was one of the key issues because the concept of wave equates to energy transport without matter transport. However, this problem was resolved a year later by Richard Feynman (the so-called sticky bead argument) with the following thought experiment (which Einstein himself also often used) at a conference in Chapel Hill. Generally, when a gravitational wave passes through two fixed points, it does not affect their positions in any way – the only thing that changes is the distance between them due to the corresponding deformation of space.

Let’s imagine the simplest possible detector of such waves – a stiff rod with rough beads on it. When a gravitational wave passes, the length of the stiff rod does not change due to the atomic forces. As the beads move freely along the rod, the distance between them changes as the wave travels, causing them to rub against the rod and generate heat. This heat was created by the passage of a gravitational wave, which means that the wave itself must carry energy.

After resolving this issue, astrophysicist Joseph Weber set about building the first gravitational wave detector. Its design went down in history as a Weber cylinder. According to its originator, it picked up the first gravitational waves in 1969 and recorded regular signals until the end of that year. To eliminate local vibration and other site-dependent factors, Weber has located two detectors approximately 1,000 kilometers apart – in Chicago and Maryland, respectively.

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The frequency of the received waves then called into question the authenticity of their detection for two reasons. First, it showed energy loss through the Milky Way that was inconsistent with its age. Second, other experimental groups did not detect any signal until almost the late 1970s.

In 1979, a scientific article was published containing measurements of the extension of the cycle time of a system of two pulsars designated as PSR1913 + 16. Pulsars are rotating neutron stars that emit electromagnetic waves periodically. Due to the fact that such a system also radiates gravitational waves, the energy for their emission is derived from the energy of orbital motion, which means that with time their velocities start to decrease and thus the circulation time slowly increases. This is a discovery for which Russell Alan Hulse and Joseph Hooton Taylor Jr. were awarded the Nobel Prize in Physics in 1993, it is considered the first, indirect evidence of the existence of gravitational waves.

What can emit a gravitational wave?

Any body with a non-zero mass can become a source of a gravitational wave. It is enough that the motion of these bodies is variable and that it is not spherically symmetrical or axially symmetrical, i.e. the same in all possible directions. For this reason, the equally expanding sphere does not emit gravitational waves.

Take a dumbbell as an example – if the weight rotates around its axis of symmetry, it will not generate gravitational waves. On the other hand, if it moves around its end, its motion will have none of the above-mentioned symmetries and such a system will radiate gravitational waves.

The same is true of two orbiting stars. If they are of different masses, they emit waves because they orbit their center of mass. The greater the masses of these stars and the faster they orbit, the greater the power of gravitational waves they will emit. In the case of very massive stars, such as neutron stars or black holes, the gravitational radiation emitted can be very strong. In addition, supernova explosions are an important source of these waves, as a significant proportion of the star’s material is ejected into the surrounding space at enormous speeds, up to 10% of the speed of light. The emission of gravitational waves will occur only in the case of an asymmetrical explosion. Similarly, if we have a rotating neutron star, the chance of gravitational radiation will only arise if there are irregularities on its surface. It turns out that there may be so-called hills up to 10 cm high (they are so low due to the very high density of neutron matter). Then such a star would emit gravitational waves until its surface is even.

How to receive them?

A pulsar binary system could be used to detect gravitational waves. Unfortunately, one of its key drawbacks is a very large distance from Earth and the relatively rare occurrence of such systems. It would be much more convenient to pick up gravitational waves on or near the Earth, because we would then have some temporal reference and the ease of locating their source in space. The main problem, however, is that it is very difficult to observe them – when they reach Earth; they have a very small amplitude, on the order of 10-21. This means that you need a very sensitive detector to detect them. Moreover, the sought signal with such a small amplitude may be hidden by noise from other sources. The frequencies of gravitational waves should be in the range. 10-16-104 Hz Another issue is the enormous distance of the strongest sources of gravitational waves (such as the merging of two black holes), because the amplitude of the waves decreases inversely with the distance (i.e. twice as far from the source is twice the amplitude). We will now briefly describe the basic detectors that could detect gravitational waves directly.

The stresses in space caused by a passing gravitational wave can induce resonant vibrations of a large metal rod, protected against external vibrations. The currently used antennas have a very high sensitivity due to cryogenic cooling. The lower the temperature, the less random vibrations of molecules and atoms are. In addition, they use superconducting systems that use quantum interference, the so-called SQUID – Superconducting Quantum Interference Device. An example is the MiniGRAIL spherical antenna, which consists of a sphere weighing 1150 kg, cooled to a temperature of 20 mK. Its spherical design allows for equal sensitivity in all directions. This is important due to the unpredictability of the direction of wave emission from astrophysical sources, moreover, such an antenna is much simpler than huge linear systems. The described detector has a maximum sensitivity in the range of 2-4 kHz, which would correspond to the instability of neutron stars or the merging of black holes.

Earth laser interferometers

These detectors use the phenomenon of interference, i.e. wave overlapping. If two waves interfere destructively with each other, the resultant wave disappears completely, and if constructively – the wave is amplified the most. It follows that an appropriate system should be built that will ensure the movement of waves over long distances, because the elongation or contraction caused by the gravitational wave is directly proportional to the so-called the base length of the interferometer. Normally, the circuit is tuned to destructive interference, which gives a signal of zero. However, if any gravitational wave passes through the interferometer, there will be a non-zero output signal. The most sensitive interferometric system is LIGO (Laser Interferometer Gravitational Observatory), ie laser gravitational interferometric observatory. There are three detectors in this system – one in Livingston (USA, Louisiana), one in Richland (Washington), and the third is to be launched in India. Each detector has a 4 km long arm in which the photons emitted from the laser move. This allows the detection of gravitational waves with a very small amplitude of 5·10-22, which corresponds to a change in arm length by 10-18m. For comparison, it is almost the upper limit of the size of electrons and quarks. It clearly follows that LIGO is currently the largest and most sensitive observatory ever built on the surface of the Earth.

Cosmic interferometers

Since the Earth itself is the source of the seismic vibrations and contributes a great deal to the noise, there was an idea to take the interferometer into space. Such an arrangement would be similar to a LIGO, but could be much larger. If we could master the technology of building such observatories, we could keep the disruption to a minimum. The main projects of space detectors are LISA and DECIGO. The aim of the LISA (Laser Interferometer Space Antenna Project) project is to carry out three test masses with lasers and interferometers. They would form an equilateral triangle in space with a side length of 5 million km, which is a million times longer than in the case of LIGO. Unfortunately, the construction of such a detector requires solving many problems, such as: heat, shot noise (i.e. fluctuations occurring in systems with a small number of energy-carrying particles and related to the quantization of the electric charge, i.e. its granular nature), as well as accidental excitations caused by radiation space and solar wind.

Latest discoveries

On February 11, 2016, the LIGO team announced the direct discovery of gravitational waves based on the signal received at 09:50:45 Greenwich Mean Time September 14, 2015 This signal was produced when two black holes with a mass of 29 and 36 solar masses merged, respectively, at a distance of approximately 1.3 billion light-years from Earth. During the last fractions of a second, the power released was more than 50 times the power of all the stars in the observable universe. The signal frequency increased from 35 to 250 Hz in five circuits of the system and its intensity increased for 0.2 seconds. The mass of the newly formed black hole was 62 solar masses. Energy equivalent to three solar masses was then emitted in the form of gravitational waves. The signal was recorded by both LIGO detectors within the time interval of 7 ms, caused by the difference in the position of both detectors in relation to the wave source. The detected signal came from the southern part of the sky, towards the Magellanic Cloud, but the source was much further behind it. The probability that this signal was not caused by gravitational waves is 0.00006%.

The second direct detection was announced on June 15, 2016. It was observed at 03:38:53 GMT December 23, 2015 Analysis of the received signal indicates that it was another merger of black holes. In this case, the emitted energy was equal to one mass of the sun. Since the frequencies of these waves are in the range of audible waves, we can hear them, e.g. by accessing a clip from YouTube.

Why do we need these waves?

Due to their very weak interaction with matter, gravitational waves pass without loss through areas of the cosmos that are inaccessible to electromagnetic waves. Only they are capable of observing the merging of black holes and other possible exotic phenomena in distant parts of the universe. Such cases cannot be investigated with conventional observational instruments such as optical or radio telescopes. Thanks to this, the astronomy of gravitational waves has a great potential to expand our knowledge about the universe. It allows, among others cosmologists look at the history of the formation of the universe as far as possible. This earliest period is not visible with “conventional” astronomy because before the era of radiation (which began a second after the Big Bang), the cosmos was transparent to electromagnetic waves. In addition, thanks to gravitational waves, it is possible to check the general theory of relativity even more accurately and test its predictions. Here you can refer to an article from December 2016, published in the prestigious journal “Nature”, which described the occurrence of “echoes” of gravitational waves at the fringes of black holes. Perhaps this is a new effect, but scientists are still cautious and stipulate that the detected signal changes may be within the statistical error of the observation. Let us hope for a quick resolution of these doubts and for further, sensational discoveries.