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The black hole holds its secret tightly in its arms. They locked up everything they had in their arms. Even light cannot surpass the gravitational cage of a black hole.

In this way, a black hole is supposed to be invisible-so it is impossible to take a picture of it. In 2019, however, astronomers released the first image of a black hole and advertised it with great fanfare. Then, in the spring of 2022, astronomers released another picture of a black hole-this time at the center of our Milky way galaxy.

This photo shows an orange doughnut-like spot that looks very similar to the black hole captured earlier in the center of the Messier 87 galaxy. But in fact, the black hole in Sagittarius A * in the Milky way is much smaller and harder to see than the black hole in Messier 87 because it needs to be observed through the hazy disk of the Milky way. So although astronomers' observations of the black hole in Sagittarius A * began at the same time as the black hole in Messier 87, it took us three more years to produce this image. The project requires the international cooperation of hundreds of astronomers, engineers and computer scientists and the development of complex computer algorithms to spell out images from raw data.

The new image of Black Cave Sagittarius A * confirms and refines astronomers' predictions of its size and orientation. The mass of a black hole determines its size, or what scientists call the gravitational diameter. The critical range in which no light can escape from a black hole is called the event horizon, which is determined by the mass and spin of the black hole. The thermal plasma rotates at a high speed around the accretion disk, emitting radio waves. These radio waves are distorted by gravity (the "gravitational lens effect") to form the appearance of an orange outer ring. The black hole shadow and emission ring shown here are the projection of the event horizon of the black hole and the distal end of the accretion disk under the gravitational lens, respectively. | Source: Knowable Magazine. The black hole is provided by the author K.McCormick and is defined as an area of space called the event horizon. These "photos" obviously do not directly show the appearance of the black hole. These photos actually record the flat hot plasma rotating at high speed around the black hole, also known as the accretion disk. Plasma consists of high-energy charged particles. As the plasma rotates around the black hole, accelerating particles emit radio waves. The blurred orange ring in the image is carefully reconstructed from radio waves captured by eight telescopes scattered around the globe (collectively known as event horizon telescopes (Event Horizon Telescope, EHT).

These black hole images tell the epic journey of radio waves from the center of the Milky way, providing unprecedented details of the black hole in Sagittarius A *. The image also constitutes "one of the most important visual proofs of general relativity," which is our best theory of gravity so far, said Sera Markoff, an astrophysicist at the University of Amsterdam and a member of the EHT collaborative project.

The study of supermassive black holes such as Sagittarius A * will help scientists gain a fuller understanding of how galaxies evolve over time and how these galaxies gather into giant clusters in the universe.

The Sagittarius A * black hole viewed from the eyes of the Milky way is 1600 times smaller and about 2100 times closer to Earth than the Messier 87 black hole photographed in 2019. This means that the two black holes look similar in size in the sky. Geoffrey Bauer, a scientist for the EHT project at the Institute of Astronomy and Astrophysics at Academia Sinica in Taipei, said that the resolution required to observe a black hole in Sagittarius A * from Earth is the same as that required to photograph oranges on the surface of the moon.

The center of our galaxy is 26000 light-years away, so the radio waves collected to take pictures of black holes were emitted during one of the earliest known permanent settlements. Their journey begins when radio waves are first emitted from particles in the black hole's accretion disk. Among the various electromagnetic waves emitted to Earth, radiation with a wavelength of about 1 mm is relatively undisturbed by galactic gas and dust. For electromagnetic waves with much shorter wavelengths, such as visible light, electromagnetic waves are scattered by dust. If the wavelength increases further, these electromagnetic waves will be bent by the charged plasma cloud, distorting the image.

Finally, after a 26, 000-year trek, this electromagnetic radiation was captured by EHT and recorded by radio observation stations scattered across our planet. The huge geographical distance between observatories is critical to collecting and analyzing these data-allowing researchers to detect extremely subtle differences in the radio waves collected at each location through a process called interferometry. These small differences are used to infer a more subtle difference in the distance of each radio wave from the source to the observer. Using computer algorithms, scientists have successfully decoded the path length differences of radio waves to reconstruct the shape of the objects that emit them.

The researchers then colored all the collected data to form a visual picture, with orange representing high-intensity electromagnetic radiation signals and black representing low-intensity electromagnetic radiation signals. Fulvio Melia, an astrophysicist who studies supermassive black holes in the Milky way, who teaches at the University of Arizona, explained: "each telescope can only receive a fraction of the electromagnetic signals emitted by the black hole. Since we cannot collect all the electromagnetic signals emitted by the black hole, what we see in this photo is not a clear picture, but some somewhat blurred images. "

Astronomers took pictures of the black hole of Sagittarius A * using a technique called interferometry. Interferometry uses eight telescopes located around the world to collect electromagnetic waves emitted by black holes and compare the collected signals. If the waves collected at the two locations are "in phase", meaning that the peaks of the waves are aligned with each other, then the two waves will be added together to form a bright spot in the image. On the other hand, if the phase of the wave is inconsistent, that is, the peak of one wave is aligned with the trough of another, then the waves cancel each other out, creating a black dot in the image. Working together, these telescopes can collect more detailed data than any other telescope. | Source: Knowable Magazine, this photo provided by the author K.McCormick will help reveal more information about the event horizon of the black hole. The event horizon is the closest point where any object approaches a black hole without being inhaled. Within the event horizon, even light cannot escape the black hole.

From the black hole image of Sagittarius A *, scientists have been able to estimate the event horizon more accurately and infer that the accretion disk of the black hole is tilted by more than 40 degrees compared with the Milky way. So, in the photo, what we see is the projection of the tilt angle of the black hole accretion disk, not its edge.

But even if the accretion disk of the black hole is parallel to the Milky way, that is, we can observe the edge of the accretion disk directly, the gravity of the black hole will distort the space around it, so that the light emitted from the back of the black hole will bend toward Earth. Therefore, regardless of the orientation of the accretion disk relative to the earth, it will form a circular image in the photo. So how do scientists judge the orientation of black hole accretion disks? Because in fact, most of the black hole emission rings (emission ring) in the photos we get are round. If we were looking at the edge of the black hole, the picture of the emission ring would look flatter and longer.

Markov believes that this new ability to observe the center of the Milky way will help fill the gaps in our understanding of the evolution of galaxies and the large-scale structure of the universe. A dense, massive object, such as a black hole at the center of a galaxy, affects the motion of nearby stars and dust, which in turn affects how the galaxy changes over time. The characteristics of a black hole, such as the direction in which it rotates, depend on its history of colliding with stars or other black holes. "A lot of people look at the sky and think the sky is still, right? but that's not the case. this is a huge ecosystem that is evolving."

So far, this image is precisely consistent with scientists' expectations, which makes it an important confirmation of the current theory of physics. "our predictions over the past 20 years tell us that we will see a ring picture of this size," Bauer said. but seeing is believing. "

Author: Katie McCormick

Translation: * 0

Revision: September 1st

Original link: What the image of the Milky Way's black hole really shows

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: Katie McCormick

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