What is the use of gravitational waves?

At present, if you want to select the most cutting-edge and hottest research fields in the scientific community, most people will vote for gravitational waves, because scientists in this field have captured gravitational waves from black holes 4 times in two years and won the Nobel Prize once, and This field has had an impact on the research programs of all technologically developed countries.

See also gravitational waves

Fortunately, in the past October 2017, scientists announced that on August 17th, the American Laser Interferometer Gravitational-Wave Observatory (LIGO) and the European Virgo Gravitational Wave Observatory (LIGO) VIRGO) For the first time, the gravitational waves emitted by the collision of neutron stars were captured at the same time from two places thousands of miles apart. This is the fifth time that scientists have detected gravitational waves in the past year. The first four were gravitational waves caused by black hole mergers, and this time it was gravitational waves caused by neutron star collisions.

A neutron star is one of the products that may be formed after a supernova explosion at the end of a star’s evolution. In the process of its formation, the star will be violently compressed, and the electrons in the internal matter merge into the protons to form neutrons, eventually becoming a compact star with a diameter of only a dozen kilometers and a mass several times the sun. Many astronomical phenomena of neutron stars are of great observational value. For example, the density of neutron stars is extremely high, weighing billions of tons per cubic centimeter. When two dense neutron stars further collide and merge, they release huge energy instantly. The intensity of gamma rays in some directions will suddenly increase in a short period of time, resulting in so-called “gamma-ray bursts.”

The discovery process this time is like this:

First of all, LIGO and VIRGO simultaneously captured a new gravitational wave signal with a duration of about 100 seconds. Through the analysis of the signal characteristics, scientists believe that this is caused by the merger of two neutron stars. About 1.7 seconds after the arrival of the gravitational wave signal, the Fermi satellite of the National Aeronautics and Space Administration (NASA) detected a gamma-ray burst. Since the gravitational wave signal and the gamma-ray burst come from the same area of ​​the sky at the same time, scientists believe that the two must be produced by the same astronomical event.

Subsequently, astronomers all over the world were notified by LIGO to use some of the most advanced telescopes, such as the Chandra X-ray Space Telescope, the Hubble Space Telescope, the Very Large Telescope, and the Atacama Large Millimeter Wave Antenna Array. Expand the observation area.

Subsequent astronomical observations lasted for several weeks. Combined with the gravitational wave data of about 100 seconds, scientists can make a comprehensive description of this astronomical event. About 130 million years ago, in the “NGC4993” galaxy at the tail of Hydra, two neutron stars that were slightly heavier than the sun met unexpectedly. They were about 400 kilometers apart at first, and they revolved around each other at a speed of 12 revolutions per second. . The huge mass stirs the universe, sending out ripples in time and space-gravitational waves.

As the neutron star gets closer and closer, the speed of the two gradually increases to 2000 revolutions per second, and the “whistle” of the gravitational wave becomes more and more rapid. Finally, the two neutron stars collided, and high-temperature matter of 1 billion ℃ spewed out from the collision. The huge shock wave also emits strong gamma rays as it passes through the spewing material. Together, these lights, cosmic rays and gravitational waves traveled at the speed of light for 130 million years, and finally came to the earth and were noticed by humans.

“Multi-messenger astronomy”

The capture of gravitational waves emitted by the collision of neutron stars this time has resulted in a milestone development in astronomy research.

We often say that astronomy research is “blind people touching the elephant” because the universe is too big to understand it, and one method of observation can often only understand one-sided information. From the ancients looking up at the stars with the naked eye, to Galileo using an astronomical telescope to the night sky for the first time, the only way for humans to observe the universe was to see with their eyes, but this observation is restricted by weather conditions, and many stars are invisible to the naked eye. of.

With the development of science, people gradually realized that in addition to visible light, there are invisible rays such as X-rays and radio waves in the universe. By detecting them, you can touch some other aspects of the “elephant” of the universe, such as the gravitational force of a black hole so that light cannot escape. People cannot see the black hole, but it emits strong X-rays, which allows astronomers to analyze several properties of the black hole. Therefore, what modern scientists are studying is “electromagnetic wave astronomy”-using visible light, X-rays, radio waves and other electromagnetic waves of different bands to “see” astronomical phenomena.

However, gravitational waves are essentially different physical phenomena from electromagnetic waves. Einstein’s general theory of relativity pointed out a hundred years ago that gravitational waves record the “ripples” of time and space changes, and their interaction with matter is very weak (unlike electromagnetic waves). The information carried from the wave source is always the same. Through this brand-new physical phenomenon, scientists have a way to “listen” to astronomy, so that “electromagnetic wave astronomy” will evolve into “multi-messenger astronomy”, which can use electromagnetic waves to “see” and gravitational waves to “listen”. Celestial bodies, and can also use electromagnetic waves to “see” those celestial bodies that are “heared” by gravitational waves. This “neutron star collision” was studied using this method. Scientists only rely on gravitational wave data to understand the process of neutron star collision, determine the origin of the gamma-ray burst, and then use electromagnetic waves to “see” this time. collision.

Next, scientists will use the same technical means (LIGO, VIRGO) to observe more gravitational waves generated by the merger of black holes and neutron stars, and new discoveries may be made every day in the future. At the same time, there are two more important directions for multi-messenger astronomy to explore. One is to explore weaker gravitational waves. According to Einsta’s theory, the strength of the gravitational wave signal is related to the quality and distance of the emitting source. At present, the gravitational wave signals captured by scientists are relatively easy to find, either from black holes or from close neutron stars. And more gravitational waves in the universe originate from a large number of small stars, such as planets and white dwarfs, which are more active, but the emitted gravitational wave signals are much weaker. But at present, the technology of LIGO and VIRGO has not reached the accuracy that can detect them.

Another greater goal is to try to collect the initial gravitational waves generated by the Big Bang. Because gravitational waves will not decay, the initial gravitational waves are likely to still reverberate in the universe. Finding them may help mankind begin to understand the secrets of the origin of the universe and the creation of matter, and may even begin to probe the primitive universe before light is produced.

Scientific theory speculates that during the period after the Big Bang 13.8 billion years ago, the universe was filled with plasma-like substances composed of very hot photons, electrons, and protons, which formed high-temperature, high-density plasma clouds. Photons continue to scatter with electrons and protons in this cloud of slurry, and they can’t get out of this hot particle porridge at all. Therefore, we cannot see the first 380,000-year universe. Until 380,000 years after the Big Bang, as the universe expanded and cooled, atoms began to form, and the plasma cloud gradually dispersed, and there was light (moving photons) that could propagate in the universe—this is also what “electromagnetic wave astronomy” can do. The “time starting point” of all astronomical phenomena studied, if you want to study the previous events, you can only hope for the initial gravitational waves. But the frequency of the initial gravitational wave is lower, and the wavelength is similar to the scale of the entire universe. It has higher technical requirements. Although we don’t know when it will be realized, it still brings us hope and research direction.

What else can gravitational waves do?

Finally, we can’t avoid vulgarity. We still have to discuss the value of gravitational waves to ordinary people. After all, things like “cosmic time and space” are too far away from us. In fact, many of the technological advances brought by scientists in the process of exploring gravitational waves have been transformed into civilian use.

Take the world’s most important gravitational wave detection observatory, LIGO in the United States, for example. It cost hundreds of millions of dollars and was built by thousands of scientists over 40 years, but it still needs to be “upgraded.” The reason is that gravitational waves are very weak. The tremors of crustal movement, the sound of waves hitting rocks thousands of kilometers away, and the slight rise in temperature may all affect the detection. In order to ensure the anti-interference ability, LIGO must improve the precision technology to the extreme.

For example, the lens used by LIGO is made of high-purity silica, which can achieve every 3 million photons emitted, only one photon will be absorbed by the mirror, that is, only one photon is blocked. It can be said that the lens is even more transparent than the air. This technology can be used in medical treatment, mobile phones, and cameras. When detecting gravitational waves, the LIGO laser is reflected 400 times in the observatory, and the total length of the optical path reaches 1,600 kilometers, but it can still do it. Divergence and no attenuation, in which superb laser power amplification technology must be used, then it may be able to provide some reference for lidar in unmanned vehicles; the pressure in the LIGO vacuum system can reach one trillion at sea level. One part, such a high degree of vacuum technology should be equally useful for semiconductor processing industries that need to be dust-proof; and LIGO’s shock-absorbing and anti-seismic system can be copied and applied in military missile storage.

So, what can gravitational waves do? It can be said that in the foreseeable future, gravitational waves are almost useless for daily life, and can only provide directors or writers with some creative inspiration, such as the movies “Star Trek”, “Interstellar Crossing” and the novel “Three-Body” There are bridges about gravitational waves. However, when humans first realized the existence of electromagnetic waves, they did not feel the use of electromagnetic waves. Today, electromagnetic waves are indispensable in microwave ovens, mobile phones, and aviation. It is speculated that gravitational waves may be able to repeat this story.


When the mass of an old star is greater than about 30 times the mass of the sun, the neutrons and protons in the nucleus are crushed due to the strong gravity, and become quarks. Such a star composed of quarks is called a quark star; it is more massive than a quark star. Large old stars will form black holes, and old stars with a mass smaller than quark stars will form neutron stars when their mass is 8-30 times the mass of the sun. Stars with mass less than 8 suns can often only be transformed into a white dwarf star.

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