The Earth and the Sun will rotate, and the Milky Way will spin, so will black holes spin? This problem that has troubled physicists for more than 50 years was recently solved by an international cooperation team led by a group of Chinese scientists. Not only did they calculate that the precession period of the M87 galaxy's jets is about 11 years, breaking the previous impression that the direction of the M87 jets was unchanged, they also obtained the most powerful evidence of the black hole's spin.


Late at night on September 27, the top international academic journal "Nature" published this blockbuster discovery online. In the animation simulation, the M87 black hole spins like a top about to stop, swinging and spinning in the deep universe. Although all this seems to have nothing to do with people's current basic necessities, perhaps humans will recall this moment when they conduct interstellar travel in the future.

Several "abnormal" data trigger exploration inspiration

In the vast universe, there are supermassive black holes in the centers of active galaxies. This mysterious celestial body predicted by Einstein nearly a hundred years ago was finally photographed in silhouette for the first time in 2019.

Mass and spin are the two basic parameters of a black hole. At present, there are mature methods for estimating the mass of black holes, but whether black holes spin is still a mystery.

In 1963, astronomers theoretically proved the existence of black hole spin. The discovery of gravitational waves in 2016 provided indirect evidence of "black hole spin" - when two black holes spin and merge, they will drag the surrounding space and time, causing "space-time ripples" of gravitational waves.

So, how can we obtain a more direct proof of the spin of a black hole? In 2017, when Cui Yuzhu, who was studying for a doctoral degree at the National Astronomical Observatory of Japan/Japan Graduate University, was processing the jet data of the East Asian Very Long Baseline Interference (VLBI) observation network on the central black hole of the M87 galaxy, he discovered that the jet structure of M87 in 2017 pointed in a different direction than the previous structure.


▲M87 jet structure merged every two years from 2013 to 2020 (observation frequency band is 43GHz). The corresponding year is displayed in the upper left corner. The white arrows indicate the direction of the jet axis in each subfigure (representing different jet position angles). (YuzhuCuietal.2023)

When the material around the black hole is sucked in by the black hole, it will emit extremely bright light and look like a shiny flat disk, called an accretion disk. After matter is sucked into the black hole, the huge angular momentum they carry is eventually ejected out of the black hole in the form of a jet. This makes black holes, accretion disks, and jets look like giant cosmic tops.

"The black hole jet of M87 is very bright and is 5,000 light-years long. In the past, everyone always thought that its jet angle was constant." Cui Yuzhu said, but several data show that the jet angle is actually different from what was previously known.

"Is this an observation error, or does it mean that the jet is rotating?" With this question, Cui Yuzhu inquired about 170 observation data of the M87 black hole from the international VLBI observation network from 2000 to 2022, and found that its angle is indeed changing.

As a result, more than 70 colleagues from 45 institutions in 10 countries around the world worked with Cui Yuzhu to organize, analyze and simulate relevant data. After six years of hard work, they finally determined that the M87 jet should rotate around an "invisible axis" with a period of about 11 years. The black hole at the center of M87 should be in a spin state.

Lin Weikang, an associate researcher at the Southwest China Institute of Astronomy at Yunnan University, said that through computer data fitting, the periodic changes in the direction of the jet are very consistent with the black hole's spin axis. "This directly proves the existence of the black hole's spin."

Decades of VLBI observations have brought about accumulated knowledge

In the universe known to humans, M87 is a star galaxy. It is large, with a mass of about 6.5 billion times that of the sun; it is close to the earth, only 55 million light-years away, making it one of the best objects for astronomers to observe. As early as 1918, people discovered the jet of M87, becoming the first celestial body in the universe to discover a jet.

Since then, the M87 jet has become the object of observation by major radio telescopes on the earth. Especially after the global network of radio telescopes established the VLBI observation network, the accuracy of M87 observation data has continued to improve. The data used in this study has the largest time span and the largest amount of data in the VLBI observation network.


▲Distribution of telescopes participating in this paper in the EATING observation network composed of the East Asian VLBI Network and Italian/Russian radio telescopes (YuzhuCuietal.2023, IntouchableLab@Openverse and Zhijiang Laboratory)

"The key data discovered this time mainly benefited from the accuracy improvement of the East Asia VLBI network, and the Tianma Telescope in Sheshan, Shanghai, and the 26-meter radio telescope in Nanshan, Xinjiang contributed the main data." Cui Yuzhu told reporters that a total of 26 domestic colleagues participated in this research.

As early as 1986, the Shanghai Observatory built the Sheshan 25-meter radio telescope, became a consortium member of the European VLBI Network in 1991, and participated in the observations of the international VLBI Network in 1998. In 2017, the 65-meter Tianma Telescope was completed and joined the international VLBI network. Shen Zhiqiang, director of the Shanghai Observatory of the Chinese Academy of Sciences, said that due to the high sensitivity of the Tianma telescope, the observation capabilities of the entire network have been improved, "especially the imaging quality of the East Asian VLBI observation network has been improved by about 50%."

The Nanshan Telescope of the Xinjiang Observatory has expanded the networking diameter of the East Asia VLBI Observation Network from 3,000 kilometers to 5,000 kilometers due to its unique geographical location. Cui Lang, a researcher at the Xinjiang Observatory of the Chinese Academy of Sciences, said that this 26-meter-diameter radio telescope joined the East Asia VLBI network in 2017 and devotes 300 hours to relevant observations every year.

Just two weeks ago, the Shanghai Observatory's Xigaze 40-meter radio telescope started construction, and the Xinjiang Observatory's Qitai 110-meter radio telescope is also under construction. Shen Zhiqiang said, "In the future, the addition of these rising stars will further enhance observation capabilities and help astronomers discover more mysteries of the universe."

Opening up a new milestone in in-depth research on black hole spin

In the past, the U.S. Very Long Baseline Array (VLBA)'s year-round observations of the M87 jet helped everyone understand a lot of the physical properties of M87. They thought they knew enough about M87, so they gradually canceled the observation time and turned to other observation targets. The discovery by Chinese scientists has prompted them to resume their long-term monitoring plan for the M87 jet.

In the simulation animation, assuming that the spin direction of the M87 black hole is perpendicular to the ground, its accretion disk is like a gyro that forms a certain angle with the ground, and the shaking gyro axis is a jet that is 5,000 light-years long. However, unlike a gyroscope, the center of motion of an accretion disk is the black hole at its center.

"This is a very beautiful and clean result, and it is also a very basic and important discovery." Lai Dong, professor at Cornell University in the United States and Tsung-Dao Visiting Professor at the Tsung-Dao Lee Institute of Shanghai Jiao Tong University, mentioned that both Italy and the United States have launched satellites to specifically detect the space-time drag effect of celestial bodies, but they were unsuccessful. "This proof of the existence of black hole spin will bring substantial impetus to the research on this effect."

Today, Cui Yuzhu is a postdoctoral researcher at Zhijiang Laboratory. She said that after obtaining the most powerful evidence of black hole spin, there are still a series of questions that require more in-depth study: What is the spin speed of the M87 black hole? Is black hole spin universal? Where does the external force that drives the black hole spin come from? In addition, spin is likely to be the key to the generation of black hole jets, so will this bring a new perspective to the study of the mechanism of black hole material jets? All of these are waiting for her and many colleagues to find answers.