Spatio-temporal fluctuations produce "ripples"
What is gravitational wave? Hu Yiming, a member of LIGO scientific organization and a postdoctoral fellow in Tsinghua University, said that general relativity can be summarized in one sentence: "Time and space tell how matter moves, and matter guides how time and space bend." When the distribution of matter changes, time and space will change accordingly, and this change will spread around at the speed of light, just like dropping a stone on a calm lake, waves will appear on the lake. When "large-scale events" such as the merger of black holes occur in the universe, spatiotemporal fluctuations will spread like ripples, which is gravitational waves.
Gravitational waves propagate in space and will never decay. It is precisely because there is almost no attenuation in propagation that people have the illusion of using gravitational waves for communication in the future.
Zhuang, an optical expert and academician of the University of Shanghai for Science and Technology, said that the potential applications such as gravitational wave communication are still far away, but now the scientific value of gravitational wave detection is obvious. The merger of massive celestial bodies such as black holes, supernova explosions and big explosions more than 654.38 billion years ago is expected to reveal the mystery of the origin and evolution of the universe through the detected gravitational waves. Gravitational wave astronomy will be a new window for human beings to understand the universe after electromagnetic wave astronomy, cosmic ray astronomy and neutrino astronomy.
1 sec releases "3 suns".
Because gravitational waves are very weak, detecting gravitational waves has been considered as an "impossible task" for a long time. In 1970s, Lena Weiss of California Institute of Technology and others put forward the laser interferometry. This method uses Michelson interferometer principle to make the laser reflect back and forth in the mirror to form interference fringes. Gravitational waves will cause the displacement of interference fringes when they reach the earth. Although the displacement distance is extremely small, if the detection sensitivity reaches the order of magnitude below minus 2 10, gravitational waves are expected to be found.
199 1 year, with the support of the National Science Foundation of the United States, California Institute of Technology and Massachusetts Institute of Technology began to jointly build LIGO. The main part of the gravitational wave detector is two mutually perpendicular interference arms, both of which are 4 kilometers long. At the intersection of the two arms, the beam emitted by the laser light source is divided into two parts, which respectively enter the hollow cylinder perpendicular to each other and maintain the ultra-vacuum state, and then are reflected by the mirror surface of the terminal back to the original starting point, where interference occurs. If gravitational waves pass through, it will cause space-time deformation, the length of one arm will be slightly longer and the length of the other arm will be slightly shorter, which will cause the change of optical path difference and the laser interference fringe will change accordingly.
After upgrading from 20 10 to 20 15, the sensitivity of the detector is increased by 10 times, reaching the negative 23rd power of 10, which is called "advanced LIGO". The gravitational wave signal detected this time was captured in September last year 14. It comes from1.300 million light years away and is formed by the merger of two black holes. At the moment of merging, the energy of the three solar masses is released in the form of gravitational waves in less than 1 second. After1300 million years of travel, this "cosmic ripple" circle was captured by two detectors of LIGO.
I should start ground detection first.
In the field of gravitational wave detection, although the LIGO plan of the United States is ahead, many countries and regions such as Europe, Japan and India are also carrying out gravitational wave detection projects with laser interferometer.
In China, the scientific community has great differences in the field of gravitational wave detection. One school thinks that detecting gravitational waves on the ground is too noisy, and it should be detected in space first, so that China can occupy the leading position in this field. Another school thinks that, given the current technical level, it is difficult to establish a very stable jamming arm in space, and China should start the ground exploration project first.
Last year, Sun Yat-sen University launched the "Qin Tian Plan", which put forward all the key technologies needed to complete the space gravitational wave detection in four stages, and finally launched three Earth high-orbit satellites for gravitational wave detection. Zhuang believes that the urgent task for China's scientific and technological community is to start the international mainstream ground detection project and carry out gravitational wave astronomical research as soon as possible.
A series of question marks caused by gravitational waves
Are gravitational waves sound? Can it be used in science fiction? Gravitational waves that "take away" three solar masses are coming. Are you scared? So why bother to detect the gravitational waves predicted by Einstein a hundred years ago?
Gravitational Wave: Sound or Instrument?
"We like to call it sound, but gravitational waves are not sound," explained Allen Weinstein, a data analysis expert at LIGO Observatory and a physics professor at California Institute of Technology. Sound travels at the speed of sound in air, while gravitational waves travel at the speed of light and can travel in a vacuum. Both are a kind of vibration, but gravitational wave is a brand-new way of vibration. LIGO Observatory connects the detector to the loudspeaker so as to "hear" the sound of gravitational waves.
Chen Yanbei, a research member of LIGO Scientific Cooperation Organization and a professor of physics at California Institute of Technology, pointed out that gravitational waves are also a kind of mechanical vibration, just like knocking a drum with a tight skin with very great energy in the universe.
What happens when gravitational waves strike?
Chen Yanbei said that gravitational waves carry a lot of energy, but the actual effect on matter is very weak, which is why it is difficult to detect. If we calculate the energy flow of gravitational waves in unit time and area near the ground, we will find that it can't move charges, so its effect is very weak. If someone stands near the source of gravitational waves, gravitational waves will attack him. In theory, people will become short and fat, then long, then short and fat ... This is repeated, but in fact, it is difficult to detect gravitational waves on the ground.
Gravitational waves can make science fiction come true?
Can gravitational waves achieve interstellar navigation, time travel or interstellar communication?
Chen Yanbei said that gravitational waves are very weak, so it is difficult to emit gravitational waves that can be received and detected. Theoretically, superimposed gravitational waves can be emitted to the merged double black holes, which is expected to produce gravitational wave amplification effect, but it is unlikely to be realized in practice. In addition, because the curvature of spacetime caused by gravitational waves is very small, it is unrealistic to "travel through time and space and go back to the past" with the help of gravitational waves.
Weinstein said that gravitational waves are still far from the application stage. It is still too early to talk about sci-fi ideas such as "traveling in time and space with gravitational waves", and it is only a weak possibility to use gravitational waves for cosmic communication.
How does "Qin Tian Plan" exert its leverage?
What are the similarities and differences between this research and China's gravitational wave detection project "Qin Tian Project"? Chen Yanbei said that he hoped that the Qin Tian Plan would realize three visions. First of all, different "windows" can be used to measure gravitational waves. If China's space gravitational wave project observes the merger of massive or supermassive black holes, it will play a very important role in understanding the evolution of the universe.
Secondly, Qin Tian Project can carry out joint observation with LIGO Observatory. For example, the signal first enters the observation range of Qin Tian, then reaches LIGO Observatory and is observed again, which will enable scientists to know more about the same wave source. In addition, China plans to use the "Qin Tian Plan" as a platform for developing space technology and promoting cooperation among scientists in different fields, which is undoubtedly of great significance.