APC Science Union · Online Library

Author: 阿白特尔 Reviewed by: 一毫秒的永恒   When we think of states of matter, the first three that probably come to mind are solids, liquids, and gases. In a solid, particles are confined near fixed positions, where they vibrate randomly. The material therefore has a fixed volume and shape as a whole. As the kinetic energy of the particles’ random thermal motion increases, a point eventually comes when they break free from those fixed positions and begin to flow. The particles no longer occupy fixed locations, but attraction from their neighbors still prevents them from escaping the group. At the macroscopic scale, the material has a relatively fixed volume but no fixed shape: the familiar liquid state. Raise the temperature further and the particles’ thermal motion grows so intense that they al ...

  The immune system keeps pathogens from invading the body. Having previously examined how our defenses work, we will now look at how pathogens attack. Today’s subject is one of history’s most notorious diseases: plague. Its immense harm to humanity deserves our attention. Plague is also known as the “Black Death” because many people who died from the disease developed dark purple or black discoloration. Author: 神州 Reviewers: 未名 and Thinker Introduction   Where there is attack, there must be defense. The body’s immune system exists to block invading pathogens. Here, we will take the opposite perspective and examine how those pathogens mount an attack.   We begin with the agent behind one of history’s best-known diseases: plague. The devastation it has caused is reason enough to take it ...

Author: Kon Reviewed by: Xiao   The Solar System that humanity inhabits contains planets, moons, comets, and other celestial bodies of every size. The Sun itself is a thoroughly ordinary member of the Milky Way’s “family of stars.” This raises two questions: Is our existence unique? In more specific astronomical terms: are planetary systems like the Solar System common throughout the universe? How did the Solar System on which we depend come into being?   Before humanity could search for exoplanets on a large scale, it was difficult to answer either question satisfactorily. Without a substantial set of comparison samples, the Solar System itself was our only model for understanding planetary systems. The two questions are also linked to some extent: once the mechanism of planetary-sys ...

Author: 绿水青山 Reviewers: 食野; 未名   Throughout our lives, we are inevitably affected by things in the surrounding environment. The body naturally responds by first trying to adapt. If a stimulus is too strong, however, the body may be injured. Today we will look at how the body repairs that damage.   An injury is the destruction or loss of some of the body’s cells and tissues; repair is the process of mending and restoring what is missing. Repair generally takes one of two forms: regeneration or fibrous repair. In regeneration, cells of the same type divide at the injured site and restore the tissue’s original function. This form of repair is usually more complete and can restore all function. Fibrous repair uses fibrous connective tissue. It produces a scar and does not completely restore ...

Author: 白書 Reviewers: 蜗牛; 一毫秒的永恒 Note: “Mars One” was later officially renamed “Tianwen-1.” It successfully completed the three major stages of Mars exploration: orbiting, landing, and roving.   On October 11, 019, the China Aerospace Science and Technology Corporation published photographs of the physical spacecraft for China’s second Mars probe on its official Weibo account. It was the first time the public had seen the probe in its finished form; the mission was scheduled for launch the following year. “Mars One” | Credit: China National Space Administration   According to Ye Peijian, chief scientist for space science and deep-space exploration at the China Academy of Space Technology, a subsidiary of the China Aerospace Science and Technology Corporation, preparations for China’s Ma ...

Author: 绿水青山 Reviewer: 未名   Previously, we looked at several mechanisms of specific immunity. This time, we will turn to nonspecific immunity.   Nonspecific immunity is provided by the body’s physical barriers, immune cells such as phagocytes, and antimicrobial substances in body fluids. Here we will examine how phagocytes engulf and kill pathogens.   The process begins with chemotaxis: a phagocyte moves in a particular direction from an area with a low concentration of chemoattractants toward one with a higher concentration. These chemical signals include certain components of the complement system and substances secreted by other immune cells. Phagocytosis Opsonization   Once a phagocyte reaches a pathogen, it begins to engulf it. A pathogen coated with antibodies or complement is mu ...

Author: 丛雨 Reviewed by: 毫秒   When we think about the lengths of a year, month, or day, the first figures that probably come to mind are the familiar 365 days, 30 days, and 24 hours. In astronomy, however, years, months, and days come in several different kinds and lengths. Here is a brief introduction to a few of them. Sidereal and Solar Days   The Sun, Moon, and stars appear to rise in the east and set in the west because Earth rotates. Earth’s rotation period is 23 h 56 m 4 s, known as one sidereal day. Clearly, this is not the 24-hour day of everyday life, which is one mean solar day. Why are they different? Because Earth rotates on its axis while also orbiting the Sun. Imagine that Earth stopped rotating: during the time it took to complete one orbit around the Sun, people on Earth ...

Author: 赖渊   An Android app with the package name com.sgzh.dt, called “For the Best One,” recently spread wildly online. Universities across the country were said to have been hit, leaving victims distraught and miserable in what became known as the “September 27 app” incident. Online rumors even claimed that simply installing the app would leave harmful files behind after it was uninstalled, causing endless trouble.   Could it really be that powerful? Did it have privileges even higher than TWRP??   In a spirit of scientific rigor, I tracked down this so-called “Sounds of Nature” APK to find out exactly what it did.   To understand how an APK works, we first have to remove its “outer clothing.” Only by examining its code can we make sense of its operating principles.   Developers of so ...

Author: 丛雨   Across the ages, the Sun, Moon, and stars have stirred people’s imagination and inspired poets and writers. Let us explore both the poetry found in the night sky and the night sky found in poetry. “The Fisherman's Pride” Li Qingzhao The sky meets waves of cloud joined with the dawn mist; the River of Stars begins to turn, and a thousand sails dance. My dreaming soul seems to return to Heaven's court. I hear Heaven speak, asking kindly where I am bound. I answer that the road is long and daylight is fading; though I have studied poetry, my startling lines are of no use. The great roc now rises on a wind of ninety thousand li. Wind, do not cease—carry my reed boat to the three immortal isles.   When a poet known for restrained and delicate verse writes with grandeur and aband ...

Author: 神州 Reviewer: 一毫秒的永恒   In “Immunity in Five Minutes: How Do Antibodies Adapt?”, we caught a glimpse of just a few among the immune system’s many mechanisms. Today, let us continue exploring that system.   Last time, we gained a general sense of the importance of antibodies in immunity. Antibodies cannot always do everything, however. Their usual role is to bind to an antigen and change its structure, removing its original toxicity or preventing it from binding to a cell, or to cause agglutination so that phagocytes can engulf it. But when an antibody encounters an intact pathogen such as a bacterium, binding to antigens on the bacterial membrane cannot kill the whole bacterium directly. Another system is needed to help: the complement system, our subject today.   Unlike an antibod ...