Visual interface telescope： taking the first picture of a black hole
After studying black holes for nearly 20 years, I am often asked "What are black holes like?".
Honestly, I’m not sure.
In addition to the self-made renderings in the paper courseware, the first time I saw the image of a black hole with my own eyes was in the cinema in 2015. On the screen, the black hole "КаΥан tuya" in the Interstellar-bottomless black center and bright three-dimensional gas ring-was designed for the film by relativistic physicist Kip Thorne (see Figure 1), which was almost the same as imagined.
There is no doubt that black holes inferred from general relativity exist in the depths of the universe today in the 21st century.
Black holes exist in countless observational data, but we don’t know what they really look like in reality.
Now, humans are finally going to take the first real photo of the black hole.
Between April 5 and 14 this year, scientists from more than 30 research institutes around the world will launch an ambitious and huge observation plan, using eight radio telescope arrays distributed in different parts of the world to form a virtual telescope network, and humans will see the visual interface of black holes for the first time. This virtual telescope network is called "Event Horizon Telescope" (EHT), and its effective aperture size will reach the diameter of the earth.
In 2015, humans first heard the sound of gravitational waves generated by the merger of two black holes, and now scientists are working hard to witness the true content of black holes.
However, because of the huge amount of data to be processed by the visual interface telescope, it may take a long time to "develop photos" for the black hole. Whether the face of the black hole is really as presented by writers, artists or film directors will not be known until 2018.
No matter what kind of black hole image we finally get-whether it is as spectacular as a movie screen or only a few fuzzy pixels-the viewing interface telescope is of great significance, which is an important step in the history of black hole observation. The observation result is not just as simple as a photo. On the one hand, it echoes Einstein’s general theory of relativity, and on the other hand, it will help us to answer how spectacular jets in galaxies are produced and affect the evolution of galaxies.
We will be the first humans to "see" black holes in history, which is really good luck.
1. How do scientists know where the black hole is without a picture and truth?
Although the term "black hole" was put forward by American astrophysicist john wheeler in 1968, as early as 1783, British geographer John Michell realized that the density of a dense celestial body could be so high that even light could not escape. This is also the most basic understanding of black holes by ordinary people today: everything is inhaled, and even light cannot escape.
Since we want to see the black hole "Fang Rong", we have to know more about this distant celestial body. Almost all the mass of a black hole is concentrated at the center "singularity", which forms a powerful gravitational field around it. Within a certain range, even light cannot escape.
The critical radius from which light cannot escape is called the "visual interface"-as the name implies, it is the interface that the line of sight can reach.
You are probably curious: the reason why mountaineers climb the peak bravely is that the mountain is there. However, since astronomers can’t see the black hole at all, how can they be sure that the black hole is there?
The black hole itself does not emit light, so it is difficult to detect it directly, and telescopes of all sizes can’t catch the direct observation of distant black holes. Scientists can only "save the country by curves" and use some indirect methods to detect black holes-such as observing accretion disks and jets.
At some point, a black hole of the magnitude of a star (from 3 solar masses to 100 solar masses) will exist around a star, tearing the gas of the star to its own side, creating a gas disk rotating around the black hole, that is, an accretion disk.
When there is too much accretion gas, a part of the gas is thrown out in the rotating direction under the action of the magnetic field before falling into the black hole visual interface, forming a jet.
Both accretion disks and jets (see Figure 2) generate bright light and a lot of radiation due to gas friction, so they are easily detected by scientists, and the hiding place of black holes is exposed.
The theory is full and the reality is skinny.
Taking our Milky Way as an example, according to theoretical calculation, there should be tens of millions of black holes of the magnitude of stars in the Milky Way, but so far, we have only confirmed the existence of more than 20 black holes, in addition to 40 or 50 black hole candidates.
We need to make more measurements and calculations to finally confirm whether a celestial body is a black hole. It is even more challenging to detect a supermassive black hole with hundreds of thousands of solar masses to billions or even billions of solar masses. Scientists have really spent a lot of effort to confirm the existence of the black hole Sgr A* in the center of the Milky Way.
Second, looking at the visual interface of the black hole in the center of the Milky Way is like looking at oranges on the moon on the earth.
It’s so hard to find a black hole, isn’t it even harder to take pictures of it?
Since the invention of the telescope in the early 17th century, the aperture of astronomical telescope has become larger and larger, from the earliest 2.5cm aperture to the largest 10m aperture optical telescope at present, as well as the 500m aperture radio telescope in Guizhou, China, and the next generation of larger aperture telescopes are also being planned or built. All these telescopes have condensed the wisdom of human beings and even represented the highest scientific and technological level of human society.
However, to observe distant black holes, it is not enough to rely on any single telescope at present.
Therefore, in the past 10 years, scientists from Massachusetts Institute of Technology (MIT) have joined hands with researchers from other research institutions to launch an exciting "Visual Interface Telescope" project, in which eight submillimeter radio telescopes around the world will simultaneously observe black holes (see Figure 3).
These eight brothers go to Spain in the north and to the South Pole in the south. They will cast a big net to the selected target, retrieve massive data and outline the appearance of a black hole for us.
These telescopes are:
(1) the South Pole Telescope.
(2) Atacama Large Millimeter Array (ALMA) located in Chile.
(3) Atacama Pathfinder Experiment in Chile.
(4) Large Millimeter Telescope in Mexico.
(5) Submillimeter Telescope located in Arizona, USA.
(6) James Clerk Maxwell Telescope (JCMT) located in Hawaii.
(7) Submillimeter Array in Hawaii
(8) The 30m millimeter-wave telescope of the Millimeter-wave Radio Astronomy Institute in Spain.
Most of them are single telescopes, such as JCMT and Antarctic telescope in Hawaii; There are also telescope arrays. For example, the ALMA telescope is composed of more than 70 small telescopes.
The visual interface telescope has two main observation targets, one is the black hole Sgr A* in the center of the Milky Way, and the other is the black hole in the galaxy M87.
These two black holes are chosen as observation targets because their visual interfaces look the largest on the earth. Other black holes are more difficult to observe because they are farther away from the earth or have limited mass.
The mass of Sgr A* black hole is about 4 million suns, and the corresponding apparent interface size is about 24 million kilometers, which is equivalent to the size of 17 suns.
Wow, it’s huge! ! However … the Earth is 25,000 light years away from Sgr A*, which means that its huge visual interface is as small as the tip of a needle, just like we stand on the Earth and watch an orange on the surface of the moon.
The mass of the black hole in the center of M87 has reached 6 billion solar masses. Although it is farther away from the earth than Sgr A*, its horizon may be about the size of Sgr A* or even a little bigger for us because of its huge mass.
Three or eight telescopes can see two black holes at the same time, and there is only a window period of 10 days every year.
In order to see the details of the visual interface of two black holes clearly, the spatial resolution of the visual interface telescope must be high enough.
How high is it? It is more than 1000 times higher than the resolution of Hubble telescope.
Scientists used to measure the position of stars around black holes with a single telescope, but compared with the distance between stars and black holes (1 trillion kilometers), the scale of the viewing interface is too small (at least one time smaller than 100,000), so it is difficult to complete it with a single mirror.
At this time, in order to enhance the spatial resolution, we need to use the "interference" technology, that is, using multiple telescopes located in different places to carry out joint observation at the same time, and finally combining the data after correlation analysis. This technology is quite mature in the radio band.
In this case, the resolution of the telescope depends on the distance between the telescopes, rather than the size of the aperture of a single telescope. Therefore, the resolution of the viewing interface telescope is equivalent to that of a radio telescope with the aperture of the earth.
Before the observation with this visual interface telescope, astronomers have used some millimeter telescopes to jointly observe the central black holes in Sgr A* and M87 galaxies, and obtained some exciting results: although the visual interface of the black hole cannot be seen clearly, the radiation in the central region of the black hole has been detected.
In order to increase the spatial resolution and see the smaller area clearly, scientists added telescopes in Chile and South Pole to the telescope array for this observation. To ensure that all eight telescopes can see these two black holes, so as to achieve the highest sensitivity and spatial resolution, the observation window for scientists is very short, only about 10 days a year (for 2017, it is between April 5 and April 14).
Among all the telescopes involved in observation, the ALMA millimeter telescope (see Figure 4) located in Chile and costing 14 billion US dollars is the most important one, because its sensitivity is the highest among single arrays at present, but its observation time is also the most precious.
Limited by a series of crowded observation plans on the full schedule of ALMA telescope, the observation of the black hole interface is only planned for 4-5 days at present, including two nights to observe the black hole Sgr A* in the center of the Milky Way, and the rest of the time will be to observe the black hole M87 in the galaxy.
Fourth, in addition to the geometry of black holes, this observation will also answer many questions for us.
It is not easy to take a photo of a black hole, and it takes a long time to "develop the photo". Radio telescopes can’t "see" black holes directly, but they will collect a lot of data about black holes and describe them to scientists with data.
For previous interferometers, because the distance between different telescopes is not too far, the observation data of different positions can usually be compared and combined in real time to get images, so it is possible for scientists to see images on the screen in real time. However, for the observation of the visual interface telescope across the northern and southern hemispheres, because the site area involved is very broad, the amount of data generated will be very huge. The visual interface telescope can generate 2pb (2PB (1PB=1000TB=1000000 GB) of data every night, which is similar to the data generated by the large proton collider in Europe in one year. Considering that the data transmission speed in some areas (such as the South Pole) is relatively slow, scientists will not make real-time correlation analysis on the data of each site during observation, so it is even more impossible to see the real-time image of the black hole on the screen.
At each observation center, scientists will use the atomic clock time calibrated in advance to calibrate and save the arrival time of each electromagnetic wave, and then summarize and compare it after the observation.
After the observation, the data collected by each station will be collected in two data centers (Haystack Observatory in Massachusetts, USA and Max Planck Radio Station in Bonn, Germany). There, a large computer cluster will merge and analyze the data time to produce an image of a black hole. The time required for this analysis ranges from half a year to one year.
Even in the most optimistic case, we won’t be able to "see" the black hole until early 2018.
Everything is available, except for the east wind.
After the equipment is ready, there is a very important factor left, which is the weather and observation time. Because the water in the atmosphere has a great influence on this observation band, in order to observe the interface telescope smoothly, it is necessary that the weather conditions of all eight telescope locations (from Hawaii to Chile, from Mexico to Antarctica) are very good.
At present, these telescopes are usually located at high altitudes, and the rainfall is very small, so the probability of all sunny days is actually very high.
When all the data are merged and finally an image is obtained, astronomers, including me, hope to see such an image: a black disk surrounded by a bright photon ring very close to the visual interface of the black hole; Because of the Doppler effect of black hole rotation, one side of the photon ring is bright and the other side is dark (see Figure 5).
The observation of visual interface telescope is of great significance to scientific research.
Astronomers hope to make the most stringent restrictions on Einstein’s general theory of relativity through this observation. At the same time, the black hole image will help us to answer how the spectacular jet in the galaxy is produced and affects the evolution of the galaxy.
Of course, this is an ideal picture in the minds of scientists, and the actual black hole image may be much worse. But no matter what the final image is, even if only a few pixels can be seen, the observation of the interface telescope will be an important step in the history of human black hole observation.
How lucky we are to be the first carbon-based creatures in the universe to see black holes with their own eyes.
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Production: Black Hole Visitor Team Gou Lijun Huang Yue
Producer: computer network information center