蝴蝶效应

 “蝴蝶效应”也可称“台球效应”,它是“混沌性系统”对初值极为敏感的形象化术语,也是非线性系统在一定条件(可称为“临界性条件”或“阈值条件”)出现混沌现象的直接原因。     

1777年11月：利希滕贝格图形的发现

 被闪电击中的人在他们的皮肤上经常会产生红色、分枝状的图案,并常会持续好几天,这很可能是皮肤下易脆的微血管因电流冲击而破裂所致,是不规则分形的自然例子;俗语称之为“闪电花”,也被叫作“利希滕贝格图形”,以纪念18 世纪的物理学家利希滕贝格（Georg Christoph Lichtenberg）。     

对称性是物理学的一把双刃剑

 对称性的概念最早源于生活。所谓“对称”,通常是指左右对称。除此之外还有轴对称、球对称等。在近代物理学中,对称性是一个很深刻的问题。在粒子物理、固体物理、原子物理等领域里,对称性问题也很重要。德国大数学家魏尔斯特拉斯(Weierstrass,1815～1897)首先提出了有关“对称性”的普遍定义,即把一个“体系”通过不同的操作达到不同的“状态”,若前后两种不同的“状态”在此操作下不变,我们可以讲这个体系对于这一操作是“对称”的,而这个操作也可以认为是这个体系的“对称操作”。常见的对称操作有空间的平移、转动以及时间的平移。     

流体力学拉氏守恒滑移线算法设计

本文在Wilkins滑移线算法的基础上, 设计了从区网格边人工黏性对主点施加算法. 结合拉氏“相容性格式”, 通过定义“接触力”和“接触力做功”, 利用局部修正内能的方法设计了总能量守恒的流体力学拉氏格式. 该修正方法可保证对称性、守恒性; 提高数值模拟分辨率.     

走向统一的自然力——电力与磁力的统一(I)

 电闪雷鸣,是人类早在远古时期就注意到的自然现象,因此,电磁相互作用,可以说是人类最先接触到的自然力.开始人们对电和磁是分开认识的,后来经过奥斯特发现“电动生磁”和法拉第发现“电磁感应”, 人们才将“电”和“磁”联系起来,最终导致麦克斯韦提出电磁理论,统一了“电力”和“磁力”.本讲座,将详细介绍“电力”和“磁力”走向统一的研究历程.     

治学之道在用心

 治学之道无他,用心而已矣:从事教学与科研工作,必须要“推陈出新”.所谓“推陈”,决不是把旧有的经验一概否定;所谓“出新”,也决不是胡思乱想.     

诠释潮汐现象

 一、潮汐及其成因潮汐是海水的一种周期性涨落运动,“潮者,据朝来也;汐者,言夕至也”（葛洪,公元281～361,东晋）,即一昼夜中两次涨起、两次跌落。白天上涨的叫做“潮”,晚上上涨的叫做“汐”,合称“潮汐”。在潮汐涨落的每一周期内,当水位上涨到最高位置时,叫做高潮;当水位下降到最低位置时,叫做低潮。相邻高潮与低潮的水位差叫做潮差。从低潮到高潮的过程中,水位逐渐上升,叫做涨潮;从高潮到低潮的过程中,水位逐渐下降,叫做落潮。我国钱塘江入海口就是世界两大观潮胜地之一（另一为亚马逊河北河口）,惊心动魄的自然界景观很早就引起人们的关注。     

超导研究大事记

 1935～1936年前苏联哈尔科夫、舒布尼可夫、乔特凯维奇、谢别列夫和里雅宾宁对第二类超导体作了重要研究,所制合金样品很接近理想第二类超导体,观察到合金存在两个临界磁场,得出较为接近理想第二类超导性的磁化曲线.30年代.物理学家对合金超导体的奇异行为提出各种解释,其中门德尔森和穆尔提出的“海绵模型”较为典型.F.伦敦在提出唯象的电动力学理论之后,提出了超导的量子解释,认为超导电性是一种“宏观尺度上的量子结构”,要求“一种平均动量的凝聚式凝固”.     

学部委员唐孝威教授主持 原子模型的建立和发展

 古代哲学家在以往漫长的岁月中对物质的结构有过许多设想.他们中具有代表性的是古希腊的留基伯(Leueippus)和他的学生德谟克利特(Dmocrituse),被称为古原子论的奠基者.他们认为物质是由简单的不可分割的基本单元--原子组成.这种朴素的思想虽属机械唯物论,却是原子论的萌芽.如今世界上通常使用的原子一词,就沿用了古希腊的atoma(原意是不可分割的).在古印度将原子称为anu(意为微小).在我国战国时期的墨家称为“端”,意思是组成物质的不可分割的最原始的东西;而名家称为“小一”,认为“小一”这东西不再有内,也就无法再分割了,即为最原始的微粒.     

ZnS晶体3C⇔2H结构转变的LAUE衍射图中“奇异点”的标定

在ZnS晶体3C⇔2H结构转变的X—射线Laue衍射分析中,为了解释一维无序造成的异常衍射现象,本文推导了Laue衍射图曲线晶带的“奇异点”计算公式。并应用此公式,计算了ZnSLaue衍射照片上数条晶带上的“奇异点”,计算值与实验值完全吻合。另外,本文还对“奇异点”的形成、特点、标定的意义以及计算公式适用范围进行了讨论。     

Number theoretic example of scale-free topology inducing self-organized criticality

In this Letter we present a general mechanism by which simple dynamics running on networks become self-organized critical for scale-free topologies. We illustrate this mechanism with a simple arithmetic model of division between integers, the division model. This is the simplest self-organized critical model advanced so far, and in this sense it may help to elucidate the mechanism of self-organization to criticality. Its simplicity allows analytical tractability, characterizing several scaling relations. Furthermore, its mathematical nature brings about interesting connections between statistical physics and number theoretical concepts. We show how this model can be understood as a self-organized stochastic process embedded on a network, where the onset of criticality is induced by the topology.     

以颗粒物理原理认识地震——地震成因、地震前兆和地震预测

本文以地壳和地幔的基本构造和己有观测事实为依据,运用颗粒物理原理,将地壳和地幔作为大尺度离散态颗粒物质体系处理,重新认识地震孕育过程,前兆产生机制及规律,探求地震预测方法和途径.主要结果是:建立了地壳与地幔构成和运动的颗粒模型;提出了引发地震的大地构造力的形成机制,以及地震前兆信息产生和传播规律;说明了地震前兆信息的主要特征及其与地震发生之间的关联,阐述了探测有效地震前兆信息的方法原理;用颗粒流动的阻塞-解阻塞转变原理解释了深源地震发生机制;对以前难以理解的若干地震学现象进行了解释,并讨论了地震的可预测性。由于地壳和地幔的离散结构特征,对于地震孕育的准静力学过程,连续介质理论不再适用.以颗粒物理原理研究地震成因、地震前兆和地震预测,所获得的新认识与传统连续介质地震学观点有本质区别。     

Do earthquakes exhibit self-organized criticality?

If earthquakes are phenomena of self-organized criticality (SOC), statistical characteristics of the earthquake time series should be invariant after the sequence of events in an earthquake catalog are randomly rearranged. In this Letter we argue that earthquakes are unlikely phenomena of SOC because our analysis of the Southern California Earthquake Catalog shows that the first-return-time probability PM(T) is apparently changed after the time series is rearranged. This suggests that the SOC theory should not be used to oppose the efforts of earthquake prediction.     

Self-organized criticality with and without conservation

The self-organized sandpile models lose criticality if dissipation is introduced. Recently Christensen et al. have shown that dissipative automata based on the Burridge-Knopoff earthquake model exhibit critical behavior. Criticality is qualitatively different for the cases with and without conservation: A new characteristic length appears for the dissipative case which diverges slower than the system size. For all dissipative models we have found a characteristic frequency in the power spectrum of the released energy, which is absent for the conservative case. The exponents describing criticality change continuously as a function of the strength of dissipation and crossover phenomena occur in the vicinity of conservation. Disorder is irrelevant if conservation is present while it destroys criticality in the dissipative case.     

Earthquakes: from chemical alteration to mechanical rupture

In the standard rebound theory of earthquakes, elastic deformation energy is progressively stored in the crust until a threshold is reached at which it is suddenly released in an earthquake. We review three important paradoxes, the strain paradox, the stress paradox and the heat flow paradox, that are difficult to account for in this picture, either individually or when taken together. Resolutions of these paradoxes usually call for additional assumptions on the nature of the rupture process (such as novel modes of deformations and ruptures) prior to and/or during an earthquake, on the nature of the fault and on the effect of trapped fluids within the crust at seismogenic depths. We review the evidence for the essential importance of water and its interaction with the modes of deformations. Water is usually seen to have mainly the mechanical effect of decreasing the normal lithostatic stress in the fault core on one hand and to weaken rock materials via hydrolytic weakening and stress corrosion on the other hand. We also review the evidences that water plays a major role in the alteration of minerals subjected to finite strains into other structures in out-of-equilibrium conditions. This suggests novel exciting routes to understand what is an earthquake, that requires to develop a truly multidisciplinary approach involving mineral chemistry, geology, rupture mechanics and statistical physics.     

One-Way Markov Process Approach to Repeat Times of Large Earthquakes in Faults

One of the uses of Markov Chains is the simulation of the seismic cycle in a fault, i.e. as a renewal model for the repetition of its characteristic earthquakes. This representation is consistent with Reid??s elastic rebound theory. We propose a general one-way Markovian model in which the waiting time distribution, its first moments, coefficient of variation, and functions of error and alarm (related to the predictability of the model) can be obtained analytically. The fact that in any one-way Markov cycle the coefficient of variation of the corresponding distribution of cycle lengths is always lower than one concurs with observations of large earthquakes in seismic faults. The waiting time distribution of one of the limits of this model is the negative binomial distribution; as an application, we use it to fit the Parkfield earthquake series in the San Andreas fault, California.     

Delayed self-organized criticality and earthquake modeling

Delayed self-organized criticality is defined. It is shown to preserve the power-law behavior of self-organized criticality with a significant change in the exponents. A delayed version of the Ito—Matsuzaki model for earthquakes is constructed and studied. This model explains some fractal features of earthquakes as well as the Gutenberg-Richter and Omori laws. Furthermore theb value obtained from the delayed model is closer to observations than theb value of the undelayed model.     

Perturbative approach to the Bak-Sneppen model

We study the Bak-Sneppen model in the probabilistic framework of the run time statistics (RTS). This model has attracted a large interest for its simplicity being a prototype for the whole class of models showing self-organized criticality. The dynamics is characterized by a self-organization of almost all the species fitnesses above a nontrivial threshold value, and by a lack of spatial and temporal characteristic scales. This results in avalanches of activity power law distributed. In this Letter we use the RTS approach to compute the value of x(c), the value of the avalanche exponent tau, and the asymptotic distribution of minimal fitnesses.     

Modeling fast and slow earthquakes at various scales

Earthquake sources represent dynamic rupture within rocky materials at depth and often can be modeled as propagating shear slip controlled by friction laws. These laws provide boundary conditions on fault planes embedded in elastic media. Recent developments in observation networks, laboratory experiments, and methods of data analysis have expanded our knowledge of the physics of earthquakes. Newly discovered slow earthquakes are qualitatively different phenomena from ordinary fast earthquakes and provide independent information on slow deformation at depth. Many numerical simulations have been carried out to model both fast and slow earthquakes, but problems remain, especially with scaling laws. Some mechanisms are required to explain the power-law nature of earthquake rupture and the lack of characteristic length. Conceptual models that include a hierarchical structure over a wide range of scales would be helpful for characterizing diverse behavior in different seismic regions and for improving probabilistic forecasts of earthquakes.     

On Origin of Power-Law Distributions in Self-Organized Criticality from Random Walk Treatment

The origin of power-law distributions in self-organized criticality is investigated by treating the variation of the number of active sites in the system as a stochastic process. An avalanche is then regarded as a first-return random walk process in a one-dimensional lattice. We assume that the variation of the number of active sites has three possibilities in each update： to increase by 1 with probability f1, to decrease by 1 with probability f2, or remain unchanged with probability 1 - f1 - f2. This mimics the dynamics in the system. Power-law distributions of the lifetime are found when the random walk is unbiased with equal probability to move in opposite directions. This shows that power-law distributions in self-organized criticality may be caused by the balance of competitive interactions.