Condensed concepts

Nuclear physics exhibits many characteristics associated with emergent phenomena. These include a hierarchy of scales, effective interactions and theories, and universality. The table below summarises how nuclear physics is concerned with phenomena that occur at a range of length and number scales. At each level of the hierarchy, there are effective interactions that are described by effective theories. Some of the biggest questions in the field concern how the effective theories that operate at each level are related to the levels above and below. Moving from the bottom level to the second top level, relevant length scales increase from less than a femtometre to several femtometres. The challenge in the 1950s was to reconcile the liquid drop model and the nuclear shell model. This led to the discovery of collective rotations and shape deformations. The observed small moments of inertia were explained by BCS theory.  Integration of the liquid drop and shell models  led to the award of

Recently I bought a small wire of NiTinol to have fun with and use in demonstrations to kids. This video gives a spectacular demonstration and attempts to explain how it works. I did not know about their use in stents for heart surgery. I am still struggling to understand exactly how shape-memory alloys  work. According to Wikipedia The shape memory effect occurs because a temperature-induced phase transformation reverses deformation...Typically the martensitic (low-temperature) phase is monoclinic or orthorhombic . Since t hese crystal structures do not have enough slip systems for easy dislocation motion, they deform by twinning—or rather, detwinning . Martensite is thermodynamically favored at lower temperatures, while austenite (B2 cubic) is thermodynamically favored at higher temperatures. Since these structures have different lattice sizes and symmetry, cooling austenite into martensite introduces internal strain energy in the martensitic phase. To reduce this energy,

How is the whole related to the parts? Which type of economy will produce the best outcomes: laissez-faire or regulated? Can a government end an economic recession by "stimulus" spending?   What is the relative importance of individual agency and social structures in causing social problems such as poverty and racism? These questions are all related to the first one. Let's look at it from an emergentist perspective, with reference to physics.  Consider the Ising model in two or more dimensions. The presence of nearest neighbour interactions between spins leads to emergent properties: long-range ordering of the spins, spontaneous symmetry breaking below the critical temperature, and singularities in the temperature dependence of thermodynamic properties such as the specific heat and magnetic susceptibility. Individual uncoupled spins have neither property. Even a finite number of spins do not. (Although, a large number of spins do exhibit suggestive properties such as an e

Consider a system composed of many interacting parts. I take the defining characteristic of an emergent property is novelty . That is, the whole has a property not possessed by the parts alone. I argue that there are five other characteristics of emergent properties . These characteristics are common but they are neither necessary nor sufficient for novelty. 1. Discontinuities 2. Unpredictability 3. Universality 4. Irreducibility 5. Modification of parts and their relations I now add another characteristic. 6. Diversity Although a system may be composed of only a small number of different components and interactions, the large number of possible emergent states that the system can take is amazing. Every snowflake is different. Water is found in 18 distinct solid states. All proteins are composed of linear chains of 20 different amino acids. Yet in the human body there are more than 100,000 different proteins and all perform specific biochemical functions. We encounter an