APC Science Union · Online Library

How Flip-Flops Work and What They Do Author: 浮槎 Reviewed by: 时光   Any discussion of how computers work must include one kind of electronic device: the flip-flop. A flip-flop consists mainly of two vacuum tubes, with current flowing through one of them at a time. It has four terminals: two receive external pulses, while the other two supply response pulses. The instant an external pulse arrives, the flip-flop “flips.” The tube that was conducting switches off, and the current moves to the other tube. When one tube switches off and the other begins conducting, the flip-flop produces a response pulse.   Now suppose we feed a continuous series of pulses into a flip-flop and define its state by the condition of one of the two tubes in Figure 1—the tube on the right. When the right-hand tube ...

Entangled States—from Theory to Technology Alain Aspect, John Clauser, and Anton Zeilinger conducted groundbreaking experiments using entangled quantum states. In such states, two particles behave as a single unit even when separated. Their results cleared the way for new technologies based on quantum information. The once-ineffable effects of quantum mechanics are beginning to find practical applications. They are now the subject of a major research field that includes quantum computers, quantum networks, and secure quantum-encrypted communication. A key to this progress is the way quantum mechanics allows two or more particles to exist in what is known as an entangled state. What happens to one particle in an entangled pair determines what happens to the other, even when the two are far ...

Introduction Humanity has always been curious about its origins: Where do we come from? How are we, Homo sapiens, related to the other humans who came before us? What makes Homo sapiens different from other humans? Through his pioneering research, Professor Svante Pääbo accomplished something seemingly impossible: sequencing the genome of an extinct human relative, the Neanderthal. He also made the sensational discovery of a previously unknown human group, the Denisovans. Crucially, Pääbo found that genes from these now-extinct groups were transferred to Homo sapiens after our ancestors left Africa about 70,000 years ago. This ancient genetic legacy remains physiologically significant in people today, affecting, for example, how our immune systems respond to infection. Pääbo’s groundbreak ...

Author: I Am Zhang Yixian Author’s Zhihu profile: Thinker Variational Inference The basic idea is this: in probabilistic models, we often need to approximate probability distributions that are difficult to calculate. Inference about any unknown quantity can be regarded as posterior-probability inference, because Bayes’ theorem can be used to construct it: p(x)=∑p(x∣z)p(z)p(x) = \sum p(x|z)p(z) p(x)=∑p(x∣z)p(z) For large datasets, Markov chain Monte Carlo methods are too slow, so we need variational inference. We frequently encounter two kinds of variables here: real data, xxx, and latent data, zzz​. The resulting inference problem is to determine the posterior conditional probability distribution p(z∣x)p(z|x)p(z∣x)​​ of the input data. Using the ELBO method, we hope to find a concrete d ...

Author: Enthusiastic A-Jiang Reviewer: Saito Shin   Everyone has played a game by “Paradox,” right? (Figure 1 shows Paradox’s official logo.) Anyone who enjoys historical strategy or simulation games will know the famous company. Headquartered in Stockholm, Sweden, Paradox has created many classic game series. Its games go on sale every year from April to May, so if you have not played one yet, now is the time. You may find something you like. (Figure 1: Paradox logo)   Why is Paradox so famous? Its players are said to be the most inhuman people in gaming history. Search for “Paradox players,” and you will see descriptions such as “demons” and “perverts.” But why are they considered inhuman? Take Stellaris as an example. Ordinary players fly spaceships through space, make friends with a ...

Author: Silicon-Based Reviewer: Xian’e   I wrote this lecture while my school was under a pandemic lockdown. I had already reached the library’s borrowing limit, and the books in which I had taken notes were far away at my home in southern China. Almost everything here was written from my rather unreliable memory, so this account of traditional Chinese philosophy will inevitably contain major omissions and errors. Please read it with appropriate caution and a critical eye.   Traditional Chinese philosophy generally refers to the Chinese philosophical systems—or “Eastern philosophy”—that occupied a dominant ideological position in East Asia from the Hundred Schools of Thought in pre-Qin China until Marxism entered China. Compared with its counterpart, “Western philosophy,” traditional Chi ...

Author: 丛雨 Reviewer: 白烟   If you have excellent eyesight, you may see more than 2,500 stars on a clear, dark night, while nearly 6,000 stars are visible to the naked eye across the entire celestial sphere. Ancient observers noticed that, unlike the five planets Mercury, Venus, Mars, Jupiter, and Saturn, the relative positions of most celestial objects seemed eternally unchanged. This gave rise to the Chinese term for a star, literally a “constant star.” We now know that stars only appear fixed because their great distances make their motion across the celestial sphere difficult to perceive. This article briefly introduces three ways in which stars move relative to the celestial background: Parallax Proper motion Aberration of light Parallax   Hold one finger upright in front of your e ...

Author: 未名 Reviewer: 未名 Introduction   In 1886, Karl Benz built the world’s first three-wheeled automobile powered by an internal-combustion engine. Its main difference from a horse-drawn carriage was the powertrain, while the basic form of the modern four-wheeled car came from Daimler’s ideas. Over the following century and more, car bodies changed continually as people demanded greater mobility and aesthetic tastes evolved. From the Beetle through the ponton body to the wedge shape, designers consistently sought forms that combined lower drag with visual appeal. This article offers a brief look at the relationship between automotive styling and aerodynamics. Car Design and Aerodynamic Drag   At constant speed on a level road under normal atmospheric pressure, a car's aerody ...

The Philosophy and Theology of Saint Augustine Author: Михaїл-А-Булгáкoв☦️ Reviewed by: 观复·钧天   Saint Augustine was an extraordinarily prolific writer whose works dealt mainly with theological questions. His polemical writings addressed issues of the day and, once they had served their purpose, generally lost all significance. Some of them, however—especially those concerning the Pelagians—have continued to exert a real influence into modern times. I do not intend to cover everything he wrote. Instead, I will examine the works and arguments that seem to me intrinsically or historically important: First, his pure philosophy, especially his theory of time Second, the philosophy of history presented in The City of God Third, the theory of salvation developed in opposition to Pel ...

Author: Shiguang Reviewed by: Dongda   Many readers probably first encountered “procedural art” in an elementary-school information technology class, where the textbook introduced a fascinating program called Turtle Graphics. Think back: how did we make the little turtle move? With code. In 1996, Seymour Papert and Wally Feurzig invented Logo, a language designed to teach children programming, and we used it to direct a turtle as it drew on the screen. As technology has evolved, however, Logo has gradually faded from view, and mainstream approaches to drawing have changed as well.   This article explains how to use a shader language—referred to below simply as Shader—to draw a 3*3 checkerboard. I use UE4 for the demonstration. If you are interested, you can try the al ...