In November 1783, a British natural philosopher and pastor John Michell wrote to the physicist Henry Cavendish (Henry Cavendish) in a letter that first proposed that there may be a dark body or dark star. (Dark star) , its density is extremely high, and it has a huge gravitational force that even light cannot escape.
Dark Celestial Body,
Since John Michell first proposed the existence of a “dark celestial body”, humans have never stopped exploring this mysterious celestial body.
Fortunately, although it is difficult for us living in the 21st century to know the whole picture, we have also seen precious black hole photos.
In April 2019, the Event Horizon Telescope (EHT) team composed of more than 30 research units around the world released the first black hole photo, which is from the M87 galaxy black hole.
Then in April 2020, astronomers released the world’s second black hole photo, a picture of the central core of the quasar 3C 279 and its jet origin taken in April 2017, 5.5 billion light-years away.
The latest news about black hole research is: On March 24, 2021, the EHT team once again announced a black hole photo- this is the image of the M87 galaxy black hole photo released in April 2019 under polarized light.
Some people say that the first black hole photo in human history is now clearer. In fact, the lines in the photo mark the polarization direction, which is related to the magnetic field around the shadow of the black hole.
In the words of scientists: The EHT team provided a new perspective for revealing this supermassive black hole.
What is a black hole?
Before understanding the EHT team’s research, let’s first understand the black hole.
Approximately 132 years after John Michell’s letter mentioned the existence of “dark celestial bodies”, a physicist who is no stranger to you and I once again predicted its existence.
In 1915, Albert Einstein completed the foundation of general relativity and officially published it the following year.
Einstein’s general theory of relativity predicted that there is a kind of celestial body in the universe, which is produced by the gravitational collapse of a star with a sufficiently large mass after the nuclear fusion reaction fuel exhaustion and death.
This kind of celestial body is extremely dense, small in size, and extremely strong in gravity, so strong that even light is attracted and cannot escape.
Soon after, German physicist Karl Schwarzschild made a precise solution for this prediction.
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Schwarzschild obtained a vacuum solution of Einstein’s field equation by calculation. This solution shows that if the actual radius of a static spherically symmetric star is smaller than a fixed value (here refers to the famous Schwarzschild radius), there will be singularities around it. Phenomenon:
Once entering an interface called “Horizon”, even light cannot escape (here refers to the event horizon, which is a boundary of time and space.
In the event horizon, the gravitational force is huge, the escape speed near the black hole is greater than the speed of light, and no light can escape; outside the event horizon, objects will not be affected by the black hole).
Until 1969, American astrophysicist John Archibald Wheeler first proposed the concept of “black holes”.
Since then, the definition of a black hole is: a celestial body whose space-time curvature is so great that light cannot escape from its event horizon.
How difficult it is to explore black holes
Regardless of predictions or calculations, it is impossible to truly perceive the existence of black holes, let alone further exploration.
But it is worth mentioning that in 1964, the American sounding rocket discovered a binary star system “Cygnus X-1” located in the direction of Cygnus.
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As an X-ray binary star system, Cygnus X-1 contains a compact star capable of generating X-ray sources and a blue giant star.
At that time, British theoretical physicist Stephen Hawking and American theoretical physicist Kip Stephen Thorne had bet on Cygnus X-1.
Hawking bet that Cygnus X-1 is not a black hole, but then more and more observational evidence is in front of him. Hawking also admitted that Cygnus X-1 is a black hole in 1990, and it is also the first black hole discovered by human beings in actual exploration. .
Through this example, it is not difficult to see that human exploration of black holes is quite difficult. A large part of the reason is that black holes cannot be directly observed. Scientists can only indirectly know its existence, mass, and influence on other things.
Specifically, before the object is sucked into the black hole, the acceleration caused by its gravity will cause friction, and then release some information, and this is the key evidence for scientists to judge the existence of the black hole (by indirectly observing the trajectory of stars or interstellar cloud gas clusters) , Can also find some clues).
For many people, black holes may be like Nolan’s movies. Don’t try to understand, just feel it (manually).
In order to further understand black holes, scientists used a tool-a kind of “Event Horizon Telescope” (EHT) consisting of 8 radio telescopes (radio telescope refers to the basic observation and study of radio waves from celestial bodies). It mainly tries to observe the event horizon.
In 2006, scientists from more than 30 research institutes around the world joined forces to initiate a project to take pictures of black holes.
Lei Feng.com has previously reported that under this plan, eight radio telescopes distributed in various places jointly observe the same target source and record data,
thereby forming a virtual telescope with an aperture equivalent to the diameter of the earth, increasing the telescope’s angular resolution It is sufficient to observe the scale structure of the event horizon.
Regarding the resolution ability of EHT, the Chinese Academy of Sciences has a vivid analogy: it is equivalent to the resolution required to see a credit card on the moon on the earth.
In April 2017, the shooting of the black hole photo was completed, and the scientists began the data processing process.
Finally, at 9 o’clock on April 10, 2019, U.S. Eastern Time, Washington, U.S., Shanghai and Taipei, China, Santiago, Chile, Brussels, Denmark, Lyngby, and Tokyo held a press conference at the same time, revealing the first black hole photo obtained by mankind. .
The photo shows a bright ring structure. The black area in the center of the ring structure is the shadow of the black hole.
For this reason, the EHT team won the Basic Physics Award of the Science Breakthrough Award in September 2019.
The first black hole polarization image released
It has been nearly two years since the first black hole photo was published.
On March 24, 2021 local time, 3 papers from the EHT team were published in the Astrophysical Journal Letters, with hundreds of co-authors.
The thesis mainly deals with the polarization characteristics of the black hole edge, which is related to the organized polar magnetic field.
In response, the Chinese Academy of Sciences stated:
This is the first time that astronomers have measured polarization information that characterizes the magnetic field so close to the edge of a black hole. This result is of key significance for explaining how the M87 galaxy, which is 55 million light-years away from the Earth, propagates a huge jet of energy from its core.
The black hole of the galaxy M87, which is the research object, is a huge black hole located in the galaxy M87 (that is, Virgo). Its mass is about 3-6 billion suns, and the distance from the solar system is about 50 million light-years-currently, scientists have selected The main observation targets are the M87 galaxy black hole and the Sgr A* (ie Sagittarius A*) black hole located in the center of the Milky Way galaxy.
The EHT team discovered through in-depth research on the relevant data of the black hole of the M87 galaxy: A considerable part of the light around the black hole of M87 is polarized.
Polarization, also known as polarization, refers to a phenomenon in which the transverse wave vibration vector (that is, perpendicular to the direction of wave propagation) deviates from certain directions. This asymmetry of the vibration direction to the propagation direction is the most obvious sign that transverse waves are different from other longitudinal waves.
According to the Shanghai Astronomical Observatory:
In the field of radio astronomy, most of the celestial signals we receive are polarized light. However, since the polarization characteristic will be weakened due to the superposition effect, and in the dense area near the black hole, the light depth usually also affects the degree of polarization.
The Shanghai Astronomical Observatory believes that EHT can capture high-resolution polarization images around the shadow of the M87 black hole. The main reasons are:
EHT has high resolving power and can decompose dense areas;
The observation waveband is in the short millimeter waveband, which greatly weakens the influence of the degree of polarization.
Leifeng.com noticed that Jiang Wu, a member of the EHT team and an associate researcher at the Shanghai Astronomical Observatory, said:
Conventional VLBI (Very Long Baseline Interferometry) polarization measurement is difficult, and obtaining this polarization image by EHT is even more challenging. So after the first black hole image was released, it took nearly two years for the polarization image to be available.
Undoubtedly, this scientific research has once again deepened human’s understanding of black holes.