What exactly are gravitational waves? Einstein's prediction is confirmed, there are indeed gravitational waves in the universe

The universe also makes noise, and this "sound" can resonate almost throughout the universe, traveling from one end of the stars to the other.

When incredibly large objects collide violently in space, they send powerful ripples through the universe that reverberate for billions of years.

In 1916, Albert Einstein elaborated on the problem of general relativity in his scientific treatise, and predicted the existence of gravitational waves in response to this problem.

General relativity is a metric theory of gravity, the core of which is the thinking pseudo-Riemannian flow form of Einstein's equations.

It describes the geometry of the energy-donor representing spacetime, in relation to the space contained within that spacetime.

For inertial motion in the curved geometry of spacetime in general relativity, there is no gravitational force to divert the object from its natural, straight path.

Instead, gravity corresponds to a change in the properties of space and time. This, in turn, changes the straightest path that the object naturally follows.

The classic simple description is that spacetime tells matter how to move and matter tells spacetime how to bend.

Spacetime distortion under general relativity

The study of the equations of general relativity had many physical consequences, some of which came from the axioms of the theory, while others only became clear in the years of research that followed Einstein's original publication.

And in his 1916 prediction, Einstein believed that ripples in the space-time metric would propagate at the speed of light.

This is one of several analogies between weak-field gravity and electromagnetism, similar to the effect of electromagnetic waves.

As to how this effect occurs, it is that something special happens when two large objects, such as a star and a planet, orbit each other.

Einstein believed that this motion would cause ripples in space, like throwing a rock into water.

Einstein at his home in Princeton, New Jersey

The strongest gravitational waves are caused by catastrophic events, such as black hole collisions, or supernova explosions, or neutron star collisions.

Other waves can be caused by the rotation of an imperfectly spherical neutron star, or even by radiation remnants left over from the Big Bang.

Since gravitational waves are invisible, it is not possible to make direct observations. In addition they still travel at the speed of light, which makes their observation even more difficult.

Early scientists simply did not believe that such things could occur, and no substantial progress was made on this.

It was not until 1974, 20 years after Einstein's death, that the first evidence for gravitational waves appeared.

This circumstantial evidence came from the orbital decay of the Hersch-Teller binary pulsar, which scientists found to be consistent with the decay predicted by general relativity because of the loss of energy from gravitational radiation.

Gravitational wave experiments

Before we officially get into gravitational wave observations, let's learn more about gravitational waves.

Earlier we basically elaborated on the physical manifestation of general relativity, and the basic concept of gravitational waves.

In Einstein's view, gravity is a force that -could- bend space-time.

This bending is caused by the presence of mass.

Generally speaking, the more mass a given volume of space contains, the greater the curvature of spacetime at its volume boundary.

As objects with mass move through spacetime, the curvature changes to reflect the change in position of these objects.

As a gravitational wave passes through an observer, the observer will notice a distortion of spacetime due to this strain.

As the wave passes, the distance between objects increases and decreases rhythmically at the same frequency as the wave.

The effect of anodized gravitational waves on particle loops

The magnitude of this effect is inversely proportional to the distance to the gravitational source, as in the case of a large event such as a neutron star confluence, due to the extreme acceleration that their masses produce as they approach each other.

Thus, neutron star merger events can then cause them to produce powerful gravitational waves.

Gravitational waves can penetrate regions of space that electromagnetic waves cannot, and they can observe the merging of black holes and other strange objects that may be in the distant universe.

(A staff member inspects the quartz fibers of a mirror hanging inside the Virgo Gravitational Wave Observatory)

Gravitational waves cannot be observed with an optical telescope or a radio telescope, and in principle, they can exist at any frequency.

But waves at very low frequencies are impossible to detect, and there is no reliable source for detectable waves at very high frequencies.

So in the past, it was not only very difficult for scientists to observe gravitational waves, but even the corresponding technical conditions were difficult to achieve.

Just dealing with the optics was enough of a headache

In order to test Einstein's prediction, in the 1960s, American and Soviet scientists conceived a laser interferometry.

And in the late 1960s, the prototype interferometric gravitational wave detector was built by Hughes Research Laboratory.

In addition, with the support of the National Science Foundation and the California Institute of Technology, related research projects were secured with talent and funding.

During the period, the discovery of binary pulsars gave scientists hope.

Measurements of orbital periodic decay proved the existence of gravitational waves, for which Taylor and his graduate assistant were also awarded the 1993 Nobel Prize in Physics.

In 1981, orbital period decays were measured in the astronomical observing system, exhibiting magnitudes that were in perfect agreement with Einstein's theory and within a small observational uncertainty.

From this period until the late 1990s, related experimental projects and observations experienced various bumps in the road and had an on-again, off-again project course.

It was not until 2002 that the Laser Interferometric Gravitational Wave Observatory (LIGO), founded by a group of scientists and foundation executives, finally got underway in earnest.

A simplified view of the advanced LIGO detector

Laser Interference Experiment

LIGO will run two gravitational wave observatories simultaneously, the LIGO Livingston Observatory and the LIGO Hanford Observatory.

These sites are located at a linear distance of 3002 km on Earth and 3030 km on the surface.

Since gravitational waves will travel at the speed of light, this part of the distance difference will be as much as 10 milliseconds because of the time difference exhibited by the arrival of gravitational waves.

By using trilateral measurements, the difference in arrival times can help determine the source of the waves.

Each observatory supports an L-shaped ultra-high vacuum system, with each side being 4 km long, and each vacuum system can hold five interferometers.

The LIGO Livingston Observatory serves as the main configuration, where there is also a laser interferometer, which was upgraded in 2004.

It is also equipped with an active vibration isolation system based on a hydraulic actuator, which provides a 10-fold vibration isolation factor in the frequency band from 0.1 to 5 Hz.

The configuration of the LIGO Hanford Observatory is essentially the same as that of Livingston, with the half-length interferometer operating in parallel with the main interferometer during the initial and enhancement phases.

Even though this interferometer is 2 km long, the Fabry-Perot arm cavity has the same optical accuracy, so it has half the storage time of the 4 km interferometer.

LIGO Hanford Observatory Live view

As gravitational waves pass through the interferometer, the local region of spacetime changes, and depending on the wave source and its polarization, this causes the effective length of one or both cavities to change.

The effective length change between the beams causes the light currently in the cavity to become very slightly out of phase with the incident light.

As a result, there will be a periodic and slight loss of coherence within the cavity, and the beam, which is tuned for destructive interference at the detector, will have a very slight periodic change out of phase, and so a signal can be generated that can be measured.

For those less low frequencies, or other noise sources that bring interference, scientists effectively protect the detector from vibrations by means of a pendulum suspension.

After more than four decades of development and reflection, finally, near the end of the summer of 2015, the LIGO detector met the standard sensitivity criteria for detection.

As soon as the detector was turned on on Sept. 14, scientists spotted a signal strong enough to identify the source.

It took the LIGO project team several months to finalize the discovery of gravitational waves, a world first and a fulfillment of Einstein's prophecy.

The source of the signal comes from gravitational waves generated by the merger of two black holes that are about 1.23427103×10^25 meters away from Earth.

In addition, this event demonstrates that gravitational waves propagate at the same speed regardless of frequency, as described by general relativity.

Space-based observations of gravitational waves are crucial, they are astrophysically speaking, and the nature of supermassive black holes created when galaxies merge can only be determined by these waves.

This great discovery not only gives mankind new horizons and opportunities to observe the universe, but also to learn more about what is happening in the universe.