“The eternal mystery of the world is its comprehensibility … The fact that it is comprehensible is a miracle.” —Albert Einstein
In 2015, we humans recorded outer-space "gravitational waves" and captured "sounds" from the universe for the first time. This successful detection of the first gravitational waves brought a beam of light to scientists to further explore the universe. Around 2016, scientists recorded more gravitational waves, and the mysterious space figure responsible for the waves became clearer. As clues accumulated more and more, the gurgling creeks gathered into a torrent of sounds until finally, in 2019, human beings gathered enough of these sounds to piece together the first real image of the black hole, proving its existence and revealing its figure. And for the first time, the black hole - a legend hidden within the vast depths of the starry sky - finally revealed itself to human vision.
On January 19, 2021, The Affiliated High School of Peking University had the honor to invite Dr. Lijun Gou, researcher of the National Astronomical Observatory of the Chinese Academy of Sciences, the chief scientist of the permanent star black hole research group, professor of astronomy at the University of the Chinese Academy of Sciences, Ph.D. from Pennsylvania State University, and a postdoctoral fellow at the Smithsonian Center for Astrophysics at Harvard University. Dr. Gou brought a memorable lecture to the students who, under his leadership, were able to wander in the endless stars and get a glimpse of the secrets of black holes.
What are black holes?
So, what is a black hole, this seemingly empty and mysterious celestial body? To make it easier for the students to understand, Dr. Gou used Chinese mythology to draw an analogy. He compared black holes to the ancient Chinese Pi Xiu, whose peculiarity - according to folklore records - is to only eat and never excrete. The most attractive feature of black holes is, just like Pi Xiu, to absorbs everything to the point, where nothing that falls within its "grasp" can escape. Today, scientists believe that black holes are the products left by stars after they reach their decaying stage. As the stars' lifespan reaches the end, they collapse and compress inward, releasing large amounts of violent gamma-ray bursts to form a supernova until eventually, they become neutron stars or black holes. Since black holes form by the collapse and compression of dying stars, their sizes are small. The black holes carry the mass of original stars before the explosion, and the small particles that carry the masses are called "singularity points".
Black holes have become objects that swallow everything, and it is for this reason that black holes have an unparalleled gravitational force that even light cannot escape. Since light cannot escape the black hole, the most central area is invisible to the human eye, and thus the range that humans can observe is only outside the orbit where the escape velocity is equal to the speed of light. This observable area is called the visual interface.
In 1915, a German physicist Schwarzschild proposed the famous Schwarzschild radius formula to calculate the radius of the visual interface of black holes. Schwarzschild's ability to perform such a calculation when classical physics and the theory of general relativity become invalidated while being used on stars with such large masses and small volumes truly show his genius. In his time, methods of quantum gravity would need to be considered to study the properties of black holes. In the end, scientists discovered that black holes cause strong distortion in time and space.
Since light cannot escape the black hole, how did humans observe the black hole and even take pictures? To answer this, Dr. Gou introduced the "No-hair Theorem" of black holes. The "No-hair Theorem" is a theorem that uses three physical quantities to describe black holes: the mass of the black hole, the angular momentum, and the amount of charge in the plasma mass hovering around the black hole, in which the latter became a beacon to guide scientists in further discovery. Although objects on the black hole's visual interface cannot be observed, there is, however, an accretion disk composed of bright plasma on the periphery of the black hole that emits light when the black hole swallows other objects. Because the black hole can, and will, bend space-time to form a gravitational lens phenomenon, the shape of the accretion disk will change with the angle of the viewer, thus forming a unique spectacle in the universe. Moreover, the time around the black hole will also be bent. In theory, the closer an object is to the black hole, the slower time will pass for that object - this phenomenon coincides with the ancient Chinese saying "a day in the sky, a year on the ground". Black holes aren't the only objects that bend space-time, in fact, all objects with mass do. For example, the time on satellites orbiting the earth will be slower than the actual time on Earth itself. But because the mass of black holes is much larger than that of orbiting satellites, this space-time curvature is less obvious for satellites than black holes.
Dr. Gou also introduced the hard work undergone by scientists working in the field of discovering the types and sizes of newly uncovered black holes and answered the students' questions.
If you want to know more about the secret of black holes, you can copy the link below to your browser and go back to watch Dr. Gou's lecture.
Although human beings have never been to a black hole or even flown near it, they now have finally heard and seen it. And even though human beings seem to be stunned by the universe's endless unanswered questions, thoughts and knowledge have already set foot and are progressing further out to space. Black holes are the end of everything, but they are also the future of mankind. The physicist Faraday said, “Once science is plugged in the wings of fantasy, it will win.” Perhaps one-day humans will be able to fully understand the mystery of black holes and enter an era of great voyages, breakthroughs, and discoveries, heading down to the vast sea of stars.
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