What is gravitational wave? Does it have anything to do with gravity?

In physics, gravitational waves refer to ripples in curvature of spacetime, which propagate outward from radiation sources in the form of waves and transmit energy in the form of gravitational radiation. 19 16, Einstein predicted the existence of gravitational waves based on general relativity. The existence of gravitational waves is the result of Lorentz invariance of general relativity, because it introduces the concept that the propagation speed of interaction is limited. In contrast, gravitational waves cannot exist in Newton's classical theory of gravity, because Newton's classical theory assumes that the interaction and propagation of matter are infinite.

Various gravitational wave detectors are under construction or operation. For example, LIGO (Advanced LIGO) has been in operation since September 2065438+2005.

Possible gravitational wave detection sources include compact binary systems (white dwarfs, neutron stars and black holes). On February, 2065438 1 1, LIGO Scientific Cooperation Organization and Virgo Cooperation Team announced that they had detected the gravitational wave signal from the merger of double black holes for the first time with the advanced LIGO detector.

In the early morning of June 26th, 20 16, LIGO Cooperation Group announced that at 03:38:53 (UTC) on February 26th, 20 15, two gravitational wave detectors located in Hanford, USA and Livingston, Louisiana detected a gravitational wave signal at the same time. This is the second gravitational wave signal detected by human beings after LIGO 20 15 detected the first gravitational wave signal on September/4.

On 201710 June 16, scientists from many countries around the world held a press conference at the same time, announcing that human beings had directly detected the gravitational wave generated by the merger of two neutron stars for the first time, and at the same time "saw" the electromagnetic signal from this spectacular cosmic event. 20 17 In February, 17 was selected as one of the five candidate international words in China Inventory 20 17.

In Einstein's general theory of relativity, gravity is considered to be the effect of space-time bending. This bending is caused by the existence of mass. Generally speaking, the greater the mass contained in a given volume, the greater the curvature of spacetime at the boundary of this volume. When a mass object moves in time and space, the change of curvature reflects the position change of these objects. In some cases, an accelerating object can change this curvature and can spread outward in the form of waves at the speed of light. This propagation phenomenon is called gravitational wave.

When gravitational waves pass through the observer, the observer will find that space-time is distorted by strain effect. When gravitational waves pass by, the distance between objects will increase and decrease rhythmically, which is different from the frequency of gravitational waves. The intensity of this effect is inversely proportional to the distance between the sources that produce gravitational waves. The orbiting double neutron star system is predicted to be a very strong gravitational wave source. When they merge, because of their huge acceleration when they orbit near each other. Because it is usually far away from these sources, it has little influence when observed on the earth, and the deformation influence is less than 1.0E-2 1. Scientists have confirmed the existence of gravitational waves with more sensitive detectors. ALIGO is the most sensitive one at present, and its detection accuracy can reach 1.0E-22. More space observatories are being planned (eLISA project of European Space Agency, Taiji project of China Academy of Sciences and Qin Tian project of Sun Yat-sen University).

Gravitational waves should be able to penetrate where electromagnetic waves cannot. Therefore, it is speculated that gravitational waves can provide observers on earth with information about black holes and other strange objects in the distant universe. These celestial bodies cannot be observed by traditional methods, such as optical telescopes and radio telescopes, so the gravitational wave astronomy society gives us a new understanding of the operation of the universe. Gravitational waves, in particular, are more interesting because they can provide a way to observe the very early universe, which is impossible in traditional astronomy because the universe is opaque to electromagnetic radiation before reunion. Therefore, the accurate measurement of gravitational waves can enable scientists to verify the general theory of relativity more comprehensively.

By studying gravitational waves, scientists can tell what happened at the initial singularity of the universe. In principle, gravitational waves exist at all frequencies. It is impossible to detect extremely low frequency gravitational waves, and there is no reliable gravitational wave source in extremely high frequency region. According to Stephen Hawking and Werner israel, the frequency of gravitational waves that may be detected should be between 1.0E-7 Hz and 1E 1 1Hz.

Gravitational waves constantly pass through the earth; However, even the strongest gravitational wave effect is very small, and these sources are far away from us. For example, the gravitational wave of GW 1509 14 in the final stage of fierce merger reached the earth after crossing1300 million light years. The most time-space ripple only changed the proton diameter of LIGO 4 km arm, which is equivalent to bringing the solar system to.

The distance between our nearest stars changes the width of a thin line. This extremely small change, if we don't borrow an unusually sophisticated detector, we can't detect it at all.

In the past sixty years, many physicists and astronomers have made countless efforts to prove the existence of gravitational waves. The most famous is the indirect experimental evidence of the existence of gravitational waves-pulse binary stars PSR1913+16. From 65438 to 0974, Professor joseph taylor, a physicist at the University of Massachusetts, and his student, Hall, used the American 308-meter radio telescope to discover a binary star system consisting of two neutron stars with roughly the same mass as the sun. Because one of the two neutron stars is a pulsar, using its precise periodic radio pulse signal, we can know the semi-long axis and period of the orbits of the two dense stars when they revolve around their center of mass very accurately. According to the general theory of relativity, when two dense stars orbit each other at close range, the system will produce gravitational radiation. The radiated gravitational waves take away the energy, so the total energy of the system will be less and less, and the orbital radius and period will be shorter.

Taylor and his colleagues have continuously observed PSR1913+16 for the next 30 years. The observation results are exactly the same as predicted by general relativity: the periodic change rate decreases by 76.5 microseconds every year, and the semi-major axis shortens by 3.5 meters every year. General relativity can even predict that this binary system will merge after 300 million years. This is the first time that human beings have obtained indirect evidence of the existence of gravitational waves, and it is an important verification of the theory of gravity in general relativity. Taylor and Hall won the 1993 Nobel Prize in Physics. So far, only about 10 similar double neutron star systems have been discovered. But this is the first time that the double black hole system in this meeting has never been discovered.

In the experiment, the first person who made a great attempt to directly detect gravitational waves was Joseph Weber. As early as 1950s, he was the first visionary to realize that it was not impossible to detect gravitational waves. From 1957 to 1959, Weber devoted himself to the design of gravitational wave detection scheme. Finally, Weber chose a cylindrical aluminum rod with a length of 2 meters, a diameter of 0.5 meters and a weight of about 1 ton, and its side pointed in the direction of gravitational waves. This kind of detector is called resonant rod detector in the industry: when the gravitational wave comes, both ends of the aluminum rod will be squeezed and stretched alternately, and when the frequency of the gravitational wave is consistent with the design frequency of the aluminum rod, the aluminum rod will resonate. The wafer attached to the surface of aluminum rod will generate corresponding voltage signal. The resonant rod detector has obvious limitations, such as its resonant frequency is certain, although we can adjust the resonant frequency by changing the length of the resonant rod. But for the same detector, only the gravitational wave signal of its corresponding frequency can be detected. If the frequencies of gravitational wave signals are inconsistent, then the detector can do nothing. In addition, the resonant rod detector has a serious limitation: gravitational waves will produce space-time distortion, and the longer the detector is made, the greater the change of gravitational waves in this length. Weber's resonance detector is only 2m, and the strain (2e-2 1m) of gravitational waves with the intensity of1is too small, so it is almost impossible for physicists in 1950s and 1960s to detect such a small change in length. Although the resonant rod detector failed to find gravitational waves, Weber pioneered the experimental science of gravitational waves. After him, many young and talented physicists devoted themselves to the experimental science of gravitational waves.

While Weber designed and built the resonant rod, some physicists realized the limitations of the resonant rod, so they came up with the gravitational wave laser interferometer detection scheme based on Michelson interferometer principle mentioned above. It was built by Rainer Weiss of Massachusetts Institute of Technology and Robert Forward of Malibu Hughes Laboratory in 1970s. By the end of 1970s, these interferometers had become an important substitute for resonant rod detectors. The advantages of laser interferometer for resonant rod are obvious: first, the laser interferometer can detect gravitational wave signals in a certain frequency range; Secondly, the arm length of the laser interferometer can be made very long. For example, the arm length of the ground gravitational wave interferometer is generally in the order of kilometers, far exceeding the resonant rod.

In addition to the aLIGO we just mentioned, there are many other gravitational wave observatories. Virgo; , located near Pisa, Italy, with an arm length of 3 kilometers; Geography; In Hanover, Germany, it has an arm length of 600 meters; The arm length of Tokyo National Astronomical Observatory in Japan is 300 meters. These detectors were observed together from 2002 to 20 1 1 year, but no gravitational waves were detected. So these detectors have been greatly upgraded. Two high-tech LIGO detectors began to observe in 20 15 as pioneers in the high-tech detector network with greatly improved sensitivity, and the high-tech Virgo (upgraded Virgo) will also start to operate at the end of 20 16. The project TAMA300 in Japan has been upgraded in an all-round way, and its arm length has been increased to 3 kilometers, and it has been renamed KAGRA. It is expected to run 20 18.

Physicists are also marching into space because they are easily disturbed on the ground. European space gravitational wave project eLISA (evolved laser interference space antenna). ELISA will consist of three identical detectors to form an equilateral triangle with a side length of 5 million kilometers, and also use laser interferometry to detect gravitational waves. This project has been approved by the European Space Agency and formally established. Currently in the design stage, it is planned to be launched in 2034. As a pilot project, two experimental satellites were successfully launched on February 3, 20 15, and are currently being debugged. China's researchers, in addition to actively participating in the current international cooperation, are also preparing their own gravitational wave detection projects.