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At 21 o'clock on April 10th, Beijing time, astronomers held a global press conference to announce the first direct photograph of the black hole. This photo is hard to come by. In order to get this photo, astronomers used eight millimeter/submillimeter wave radio telescopes all over the world to form a so-called "Event Horizon Telescope" (EHT).

From April 5, 2017, the eight radio telescopes have been conducting joint observations for several days, and then after two years of data analysis, let us see the true meaning of the black hole.

Black hole photos taken directly by humans for the first time

This black hole is located in the galaxies code-named M87, 53 million light-years away from the Earth, and has a mass equivalent to 6 billion suns.

When you read science news, popular science books, and science fiction movies, you can often see black holes, but they are all based on scientific theories, not direct observations. In 2014, the sci-fi movie "Interstellar Crossing" directed by Nolan was in great heat. In this film, the super-mass black hole under the aura - "Gargantua" is awe-inspiring, the image of the black hole here It was simulated using a computer. Under the guidance of the famous theoretical physicist Jeep Thorne, the simulation here is very close to reality, but after all, it is still a simulation. This time it is really fun.

Legend: Black hole image simulated by computer in the science fiction movie "Interstellar Crossing".

Why can you take pictures of black holes that don't shine?

In recent years, the term black hole has appeared frequently in media reports, and many people already know something about it. The black hole of the star-quality quality is formed by the gravitational collapse of the massive star to the end of the core. The specific way of forming medium-mass black holes and massive black holes is still inconclusive: it may be formed by the combination of small black holes, or it may be formed by black holes through phagocytic matter, or it may be formed by direct collapse of a large amount of gaseous substances.

The most impressive impression of a black hole is that it devours everything, even light. If it is a solitary black hole, we really can't use electromagnetic waves to shoot.

Black hole simulation

But usually there is material around the black hole, forming a disk-like structure called "accumulation plate." The material in the accretion disk rotates around the black hole at high speed, and emits a radiant glow due to friction, including continuous radiation from radio waves to visible light to the X-ray band. The accretion disk is outside the "hole" of the black hole, so the emitted radiation can escape to the distant location and we are detected.

Therefore, what we photographed is not the black hole itself, but the outline of the black hole outlined by the radiation from the material on its boundary, just like watching a shadow play.

What is the "event horizon" of a black hole?

In simple terms, the event horizon of a black hole refers to a space-time boundary around a black hole. Once a substance or even light crosses this boundary, it can never return. But for an object that enters the horizon, there is actually no surprise in the event horizon. In addition to the event horizon, there are also absolute horizons and visual horizons. We won't go into details here.

The size of the black hole we usually say is actually the size of the black hole interface. If you compress the sun into a black hole, its horizon radius is only 3 kilometers! If you compress the earth into a black hole, its horizon radius is only 9 mm! No mistakes, it is 9 mm.

What is the "Event Vision Telescope"?

At the beginning of the article, we mentioned that in order to observe the physical processes on the edge of the black hole horizon, astronomers used eight millimeter/millimeter-wave radio telescopes distributed around the world. These telescopes formed a virtual telescope with a caliber close to the whole earth. A virtual telescope called the "Event Vision Telescope."

Legend: Eight global millimeter-wave sub-millimeter-wave radio telescopes around the world virtualize an Earth-sized “event vision telescope”

This great observation was taken from the 30-meter millimeter-wave telescope (IRAM 30-meter telescope) in Spain, the two radio telescopes in Hawaii, and the South Pole Telescope in Antarctica. The eight millimeter/submillimeter wave radio telescopes are:

30m millimeter-wave telescope (IRAM 30m) located in the Sierra Nevada Mountains, Spain;

Heinrich Herzian Millimeter Wave Telescope (SMT) in Arizona, USA;

Large millimeter-wave telescope (LMT) at the top of an extinct volcano in Mexico;

James Clark Maxwell Telescope (JCMT) in Hawaii;

Submillimeter wave array (SMA) located in Hawaii;

Atacama large millimeter wave array (ALMA) in the Chilean desert;

Atacama Pathfinder Experimental Telescope in the Chilean Desert (APEX;

Antarctic telescope (SPT) at the Amundsen Scott Observatory in Antarctica;

Legend: The large millimeter-wave array telescope (ALMA), located in the Atacama Desert in northern Chile, is the world's most powerful telescope array in this band.

Among the eight radio telescopes, the Atacama Large Millimeter Wave Array (ALMA) is the most powerful! ALMA is located in the Atacama Desert in northern Chile at an altitude of 5,000 meters. It is a year-round drought and has created good conditions for observation. At present, ALMA is an interference array consisting of 66 movable single-lens telescopes that transmit information between optical fibers through optical fibers. With a cost of $1.4 billion, ALMA is one of the most expensive ground-based telescopes available today. If there is no ALMA to join, observing the black hole's horizon is simply an impossible task.

What is the working principle of the "Event Vision Telescope"?

This earth-sized virtual telescope utilizes a technique called Very Long Baseline Interferometry (VLBI). It allows simultaneous observation of a celestial body with multiple astronomical telescopes, simulating the observation of a giant telescope of the size equivalent to the maximum separation distance between telescopes. In order to understand this principle, we should briefly understand the historical context of this technology.

In 1962, Martin Ryle of the Cavendish Laboratory at the University of Cambridge in England used the principle of baseline interference to invent a synthetic aperture telescope that greatly improved the resolution of radio telescopes. The basic principle is that two radio telescopes separated by two places receive the radio waves of the same celestial body, and the two waves interfere with each other. The equivalent resolution can be equivalent to a single-caliber radio with a diameter equivalent to the distance between the two places. telescope. Ryle won the 1974 Nobel Prize in Physics for this invention.

Legend: Very large array of antennas (VLA) in the United States. Each antenna weighs 230 tons and is mounted on rails and can be moved.

The radio telescope based on synthetic aperture technology is represented by the Very Large Array (VLA) in the United States. It is a radio telescope array consisting of 27 25-meter antennas. It is located in San Agustin, New Mexico, USA. On the plain, at an altitude of 2,124 meters, it is the world's largest comprehensive aperture radio telescope. Very large antenna arrays each weighing 230 tons, are mounted on rails and can be moved. All antennas are arranged in a Y-shape with a length of 21 kilometers per arm, and the combined longest baseline can reach 36 kilometers. The Very Large Antenna Array is part of the National Radio Astronomy Observatory (NRAO). It was built in 1981 and operates in six bands with a maximum resolution of 0.05 arc seconds, which is comparable to the resolution of a large optical telescope on the ground. This radio telescope array is also often seen in film and television dramas. For example, in 1997, the famous science fiction film "Contact" has VLA.

Why not use optical telescopes for observation?

We know that the light that the human eye can see is called visible light and is part of the electromagnetic spectrum. The frequency ranges from 430 terahertz to 750 terahertz, and the corresponding wavelength ranges from 400 nm to 700 nm.

Radio telescopes are telescopes that use radio waves to observe. Radio waves are also part of the electromagnetic spectrum. The frequency ranges from 300 GHz at high frequencies to 30 Hz at low frequencies, with wavelengths ranging from 1 mm to 10,000 km. In nature, radio waves are emitted from lightning to cosmic objects.

Legend: Black holes are usually surrounded by thick gas and dust.

Since the black hole in the center of the galaxy is blocked by thick interstellar dust and gas, the optical band telescope is powerless and can only use the radio band. The millimeter wave is already the lower limit of the wavelength used by the radio telescope, and it has been bordered by infrared rays on the electromagnetic spectrum.

The resolution of the telescope mainly depends on two parameters, one is the wavelength used, and the other is the size of the aperture: the aperture is fixed, the shorter the wavelength, the higher the resolution; the longer the wavelength, the larger the aperture, the higher the resolution.

In order to be able to observe the behavior of matter in the black hole horizon, the Event Vision Telescope has increased the resolution of the radio telescope to an unprecedented height, to the extent of 10 to 20 micro-angle seconds! This is equivalent to seeing the date of issue on coins 4,000 kilometers away. In contrast, the resolution of the human eye is about 1 arc second, and the resolution of the Hubble telescope is 0.05 arc seconds, which means that the resolution of the event vision telescope is thousands of times that of the Hubble telescope. Of course, although this virtual telescope has an amazing resolution, it is not completely satisfactory because it is composed of a widely dispersed telescope.

Why choose the black hole in the center of the Milky Way and the center of the M87 galaxy as the research object?

This time, the first photo of the galaxy M87 is published. The black hole photo in the center of the Milky Way is still in data processing. It is reported that in the Milky Way, humans have discovered more than 20 stellar-quality black holes, more than 3,400 light-years away from us, but why not choose these relatively close black holes for observation, rather than staying close to 26,000 light years away. The black hole in the center of the Milky Way and the black hole in the center of the M87 galaxy beyond 53 million light years? This is because the quality of these star-rated black holes is too small and the diameter is relatively small. Therefore, from the perspective of the earth, the opening angle is not as large as the super-mass black hole at a long distance.

Legend: This is the central area of the Milky Way captured by the Chandra X-ray. The place marked "SgrA Star" in the figure is where the big black hole is located.

The two black holes observed by the incident vision telescope are supermassive black holes. The mass of the black hole in the center of the Milky Way is equivalent to 4 million times the mass of the sun. The diameter of the horizon is about 24 million kilometers, which is equivalent to the 17 suns connected together. The quality of the black hole in the center of the M87 galaxy It is equivalent to 6 billion times the mass of the sun, and the diameter of the horizon is about 36 billion kilometers, which is equivalent to the distance between six Pluto and the Sun! Why are two such huge cosmic monsters still seem so small? Although the black holes are huge, they are as far away as the Earth. The black hole in the center of the Milky Way is about 26,000 light years from Earth, and the black hole in the center of M87 is about 53 million light years away from Earth. At such a long distance, the huge black hole is also a point, so the telescope is required to have an abnormal resolution.

 Legend: This is the two possible appearances of the M87 galaxy center black hole previously generated by computer simulation.

The calculations show that it takes 53 micro-seconds of angular resolution to see the black hole in the center of the Milky Way. If you look at the black hole in the center of the M87 galaxies, you need 22 micro-seconds of angular resolution, all of which fall within the scope of the observational telescope. Inside. Therefore, the apparent diameter of the black hole in the center of the Milky Way is larger than the apparent diameter of the black hole in the center of the M87 galaxy.

Legend: The jet from the center of the M87 galaxy can be as long as 5,000 light years. Scientific research has shown that the jet is driven by a massive black hole that rotates in the center.

The black hole in the center of the M87 galaxies is in a very active state. A very typical feature is that a jet of near-light velocity motion is ejected from the center, and the length of the jet can reach 5,000 light years. Scientific research has shown that the jet is driven by a massive black hole that rotates in the center.

What is the purpose of taking pictures of black holes?

Through direct observation of black holes, scientists hope to test the general theory of relativity in a more intense gravitational field, directly verify the existence of the event horizon, study the accretion and spray prevalence on the edge of the black hole, and the basic black hole physics.