The 2020 Nobel Prize in Physics was announced : Half of the prize was awarded to Roger Penrose for “discovering that general relativity predicts the formation of black holes”; the other half was awarded to Reinhard Genzel and Andrea Ghez for “discovering the center of the galaxy” Super dense objects”. Three physicists shared this year’s Nobel Prize in Physics. They discovered one of the most bizarre phenomena in the universe-black holes .
The darkest secrets of black holes and the Milky Way
The three scientists shared this year’s Nobel Prize in Physics for their research on one of the strangest phenomena in the universe-black holes. Roger Penrose invented a clever mathematical method to explore Einstein’s general theory of relativity. His research reveals how general relativity predicts the formation of black holes. These time and space monsters will capture everything that enters them. Nothing, not even light, can escape the black hole.
Reinhard Genzel and Andrea Ghez each led a group of astronomers to study the central region of the Milky Way since the early 1990s. As the accuracy improved, they succeeded in mapping the orbits of the brightest star closest to the center of the Milky Way. Both groups of researchers discovered that there is an invisible but heavy object that causes these stars to circle around.
This invisible matter is about the mass of 4 million suns, but its size is about the same as our entire solar system. What makes the stars near the center of the Milky Way spin at such an astonishing speed? According to the current theory of gravity, there is only one possible explanation: supermassive black holes.
Breakthrough beyond Einstein
Einstein, the father of general relativity, did not think that black holes would really exist. However, ten years after Einstein’s death, British theorist Roger Penrose proved that black holes can be formed and described their characteristics. There is a singularity hidden in the center of the black hole, and all known natural laws no longer apply here.
In order to prove that the formation of black holes is a stable process, Penrose needs to expand the method used to study the theory of relativity, that is, to use new mathematical concepts to solve this theoretical problem. Penrose’s breakthrough article was published in January 1965 and is still considered to be the most important contribution to general relativity since Einstein.
Gravity firmly controls the entire universe
Black holes are probably the strangest result of general relativity. When Einstein proposed his theory in November 1915, it overturned all previous concepts of time and space. This theory provides a new foundation for understanding gravity. Gravity shapes the universe to the greatest extent. Since then, general relativity has provided the basis for all universe research, and it has also been practically used in our most commonly used navigation tool-GPS.
Einstein’s theory describes how gravity controls everything in the entire universe. Gravity makes us stand on the earth, and gravity also controls the orbit of the planets around the sun and the orbit of the sun around the Milky Way. Gravity also promotes the birth of stars from interstellar clouds, and eventually the stars die under gravitational collapse. Massive matter will bend space and slow down time; very massive matter can even cut and wrap space-forming black holes.
The first theory describing black holes appeared a few weeks after the publication of general relativity. Although the mathematical equations of the theory are extremely complicated, German astrophysicist Karl Schwarzschild still brought Einstein a solution to explain how massive matter bends space-time.
Later studies have shown that once a black hole is formed, it will be surrounded by the event horizon, which moves around the material in the center of the black hole like a veil. The black hole is always hidden in its event horizon. The greater the mass, the larger the black hole and its horizon. For matter equivalent to the mass of the sun, the diameter of the event horizon is about three kilometers; while for matter equivalent to the mass of the earth, the diameter of the event horizon is only 9 millimeters.
Beyond the perfect solution
The concept of “black hole” has found new meanings in many cultural expressions, but for physicists, black holes are the natural end of the evolution of giant stars. In the late 1930s, physicist Robert Oppenheimer (Robert Oppenheimer) first calculated the violent collapse of a massive star. Oppenheimer later led the “Manhattan Project” that produced the first atomic bomb. When giant stars with a mass many times the sun run out of fuel, they first burst into supernovae, and then collapse into extremely dense debris, so massive that gravity can pull everything inside, even light.
As early as the end of the 18th century, the British philosopher and mathematician John Michell and the famous French scientist Pierre Simon de Laplace proposed the “dark star” ( dark star) concept. Both believe that the density of celestial bodies can be so high that they are invisible, because the speed of light is not enough to escape their gravity.
More than a century later, Einstein published the general theory of relativity, in which the solutions of some equations described exactly such dark stars. Until the 1960s, these solutions were considered pure theoretical speculations, describing the ideal state of stars and their black holes in perfect circles and symmetry. However, nothing in the universe is perfect, and Roger Penrose first succeeded in finding a realistic solution for all collapsed matter.
The mystery of quasars
In 1963, with the discovery of the brightest object in the universe-quasars-the question of whether black holes existed once again surfaced. For nearly a decade, astronomers have been confused by radio rays from mysterious sources (such as 3C273 in Virgo). Visible light radiation finally reveals the true position of the quasar—3C273 is so far from the earth that these rays have been propagating towards the earth for more than 1 billion years.
These radiation sources are so far away from us that their intensity is equivalent to the light emitted by hundreds of galaxies. These celestial bodies are named “quasars”. Astronomers soon discovered more distant quasars that had emitted radiation in the early universe. Where does this incredible radiation come from? There is only one way to get so much energy in the finite volume of a quasar—from the matter falling into a huge black hole.
Capture surface
Whether black holes can be formed under realistic conditions is a problem that plagues Roger Penrose. He later recalled that the answer came in the fall of 1964, when he was walking in London with a colleague. Penrose was then a professor of mathematics at Birkbeck College. When they stopped talking for a while and crossed a side street, an idea suddenly appeared in his mind. Later that afternoon, he recalled this idea, which he called the “trapped surface” concept. This is the key he has been looking for and an important mathematical tool needed to describe black holes.
A trapping surface forces all light to a center, regardless of whether the surface is curved outward or inward. Using the bound surface, Penrose proved that black holes always hide a singularity, that is, a boundary between time and space. The density of the singularity is infinite, but so far, no theory can explain this strangest phenomenon in physics.
When Penrose perfected the proof of the singularity theorem, the capture surface became a central concept. In today’s research on the curved universe, the topology method he introduced plays an important role.
One-way road to the end of time
Once the material begins to collapse and forms a trapping surface, the collapse is no longer possible to stop. As the physicist and Nobel Prize winner Subrahmanyan Chandrasekhar tells, there is no turning back. His story is about dragonflies and their larvae living under water. When the larva is ready to spread its wings, it promises its surrounding companions that it will come back and tell them about the world on the water. But once the larva really rushed out of the water and flew like a dragonfly, it would never go back. The larvae in the water can never hear the story of the world beyond the surface.
Similarly, all matter can only pass through the event horizon of a black hole in one direction. Then, time replaces space, all possible paths point to the inside, and the passage of time pushes everything to the inevitable end-the singularity. If you cross the event horizon and fall into a supermassive black hole, you won’t feel anything. But from the outside of the black hole, no one will see you fall into it, and your journey will continue. Within the laws of physics, it is impossible to peek inside a black hole; all the secrets of black holes are hidden in their event horizon.
Black holes control the path of stars
Black hole formation (top left) Cross section of a black hole. When a huge star collapses under its own gravity, it will form a massive black hole that captures everything that passes through its event horizon. Even light cannot escape the black hole. In the event horizon, time replaces space, and all paths point inward. The flow of time brings everything to the deepest singularity of the black hole—here, the density is infinite, and the time ends here. (Bottom right) The light cone represents the path of light forward and backward in time. When matter collapses and forms a black hole, the cone of light passing through the event horizon of the black hole will move inward toward the singularity. Outside observers will never actually see the light reaching the event horizon. All they saw was the light approaching the event horizon. No one can see afterwards.
Even if we can’t see the black hole, we can still determine its characteristics by observing the huge gravitational force of the black hole guiding the movement of the surrounding stars.
Reinhard Genzel and Andrea Ghez each lead an independent research team to explore the central region of our galaxy. Our Milky Way galaxy is like a circular disk with a diameter of 100,000 light-years. It contains clouds and dust, as well as hundreds of billions of stars; one of them is our sun. As we look from the earth, huge interstellar gas and dust obscures most of the visible light from the center of the Milky Way. For the first time, infrared telescopes and radio technology allowed astronomers to cross these obstacles and observe the stars in the center of the Milky Way.
Genzel and Ghez followed the orbit of the star and presented the most convincing evidence so far: an invisible supermassive object is hidden in the center of the Milky Way. Black holes are the only possible explanation.
Focus center
Figure 3: Top view of the Milky Way. Our Milky Way is like a circular disk with a diameter of 100,000 light years. The vortex arms of the Milky Way are made up of clouds and dust and hundreds of billions of stars; one of them is our sun.
For more than 50 years, physicists have suspected that a black hole may exist in the center of the Milky Way. Since the discovery of quasars in the early 1960s, physicists have speculated that most large galaxies (including the Milky Way) may have supermassive black holes inside. However, no one can explain how galaxies and their black holes are formed.
One hundred years ago, American astronomer Harlow Shapley took the lead in determining the center of the Milky Way, pointing to the constellation Sagittarius. In later observations, astronomers found a powerful radio wave source there, and they called this radio wave source “Sagittarius A*”. By the end of the 1960s, it was discovered that Sagittarius A* occupies the center of the Milky Way, and all stars in the Milky Way are orbiting around it.
But it wasn’t until the 1990s that we had larger telescopes and better equipment to conduct more systematic research on Sagittarius A*. Reinhard Genzel and Andrea Ghez started their own projects, trying to observe the center of the Milky Way through a thick cloud of dust. Together with their own research team, they develop and improve their respective technologies, build unique instruments and devote themselves to long-term research.
To observe distant stars, you need to use the world’s largest telescope-in astronomy, the bigger the better is an absolute truth. German astronomer Reinhard Genzel and his team initially used the New Technology Telescope (NTT) at the La Sila Observatory in Chile. Later, they transferred the observations to the Very Large Telescope (VLT) located in Paranal Mountain (also in Chile). The Very Large Telescope has four telescopes with a diameter of 8.2 meters, which is more than twice the size of the new technology telescope (3.58 meters), and the combined equivalent diameter of these telescopes can reach 16 meters.
In the United States, Andrea Ghez and her research team used the Keck Observatory in Mauna Kea, Hawaii. The observatory has two telescopes with an aperture of about 10 meters and is currently one of the largest telescopes in the world. Each lens is like a honeycomb, composed of 36 hexagonal parts, which can be individually controlled to better focus the stars.
Stars guide the way
The orbits of these stars indicate that in the center of the Milky Way, something invisible and heavy controls their orbits.
The star closest to the center of the Milky Way
The orbits of these two stars are by far the most convincing evidence that a supermassive black hole is hidden in Sagittarius A*. It is estimated that the mass of this black hole is about 4 million times the mass of the sun, and all of this mass is squeezed in an area not much larger than the solar system.
Top left: Astronomers have measured the orbits of some stars near Sagittarius A* in the center of the Milky Way;
Top right: For one of the stars, S2 (or S-02), astronomers successfully mapped its complete orbit and found that its cycle around the center of the Milky Way is less than 16 years. When the star is closest to Sagittarius A*, the distance is only about 17 light-hours (over 10 billion kilometers).
Lower left: The radial velocity of S2 increases as it approaches Sagittarius A*, and gradually decreases as it moves on an elliptical orbit. The radial velocity is the component of the star’s velocity in our line of sight.
Bottom right: At the closest approach to Sagittarius A* (2002 and 2018), the speed of the star S2 reached its highest speed of 7,000 kilometers per second
No matter how big the telescopes are, the details they can distinguish are always limited, because above us, there is an atmosphere nearly 100 kilometers thick. The large bubbles above the telescope tend to be hotter or colder than the surrounding environment. They are like lenses that refract light when it reaches the mirror surface of the telescope, distorting the light waves. This is the reason why the stars flicker and the reason why the starry sky image is blurred.
The emergence of adaptive optics technology is crucial to the improvement of astronomical observations. Now, an extra thin lens is installed on the telescope to compensate for air turbulence and correct distorted images.
For nearly thirty years, Reinhard Genzel and Andrea Ghez have been tracking certain stars in the cluster of stars in the center of the Milky Way. They continue to develop and advance this technology, using more sensitive digital light sensors and better adaptive optics to increase the image resolution by more than 1,000 times. Now they can more accurately determine the positions of stars and track them at night.
Researchers tracked the 30 brightest stars in this group of stars. These stars move fastest within a radius of a “light moon” from the center. On the other hand, stars outside this area follow their elliptical orbits more orderly (Figure 4).
A star called S2 (or S-O2) orbited the center of the Milky Way galaxy in less than 16 years. This is a very short time, so astronomers can map its entire orbit. We can compare it with the sun. It takes more than 200 million years for the sun to revolve around the center of the Milky Way; in other words, when our current circle just started, dinosaurs were still walking on the earth.
Theory and observation complement each other
The measurement results of the two groups are very consistent. They concluded that the mass of the black hole in the center of the Milky Way should be equivalent to 4 million times the mass of the sun and be squeezed into an area the size of the solar system.
We may soon see the true face of Sagittarius A*. Just a year ago, the astronomical network of the Event Horizon Telescope has successfully captured an image of a supermassive black hole-in fact, what we see is the closest environment around it. In the Virgo A galaxy (also known as the M87 galaxy), 55 million light-years away, there is a core composed of a supermassive black hole.
The core black hole of the M87 galaxy is very huge, with a mass more than 1,000 times that of Sagittarius A*. In contrast, the collision black holes behind many gravitational wave events in recent years are much lighter. Like black holes, before the first capture of gravitational wave signals by the American LIGO detector in the fall of 2015, this space-time ripple was only a theoretical prediction of Einstein’s general theory of relativity (the scientist who made the discovery won the 2017 Nobel Prize in Physics) ).
Unsolved puzzle
Roger Penrose’s work revealed that black holes are a direct inference of general relativity, but this theory is no longer applicable under the infinitely strong gravity of the singularity. A lot of work is going on in the field of theoretical physics to create a new theory of quantum gravity. This must combine the two pillars of physics-relativity and quantum mechanics-and meet under the extreme conditions inside the black hole.
At the same time, astronomers are getting closer and closer to the black hole, trying to observe more closely. The pioneering work of Reinhard Genzel and Andrea Ghez paved the way for a new generation of astronomers, enabling them to accurately verify general relativity and its most bizarre predictions. These measurements and verifications are likely to provide clues for new theoretical insights and reveal more secrets and surprises in the universe.
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