Physicists From China

"Selection rules governing the disintegration of a particle into two photons are derived from the general principle of invariance under rotation and inversion. The polarization state of the photons is completely fixed by the selection rules for initial particles with spin less than 2. These results which are independent of any specific assumption about the interactions may possibly offer a method of deciding the symmetry nature of mesons which decay into two photons."

"Many physicists recall October 1957 as a time of excitement and legend. In that year, at the age of 35, Yang won the Nobel Prize in Physics. Yang and Lee thereby became the first Chinese laureates. The significance of the award lay not only in the academic achievement, but also in the boost it provided to the self-belief of a nation. Before that, the scientific talent of the Chinese had been questioned. Ching-Wu Chu, a distinguished physicist specialized in superconductivity and a member of the US National Academy of Sciences, was in high school at the time. He spent his spare time reading every news report he could find about Yang, and talked earnestly to his classmates about “parity non-conservation” – a subject on which they could understand nothing. Tsu-Teh Chou, a professor of physics at the University of Georgia, was dining at a tiny Chinese restaurant in Liverpool, England, 12 years later, and overheard both the chef and the owner talking proudly about Yang’s achievements."

"In a letter to Ampère dated 3 September 1822, Faraday lamented, "I am unfortunate in a want of mathematical knowledge and the power of entering with facility into abstract reasoning, I am obliged to feel my way by facts closely placed together." ... Faraday's "facts" were his experiments, both published and unpublished. During a period of 23 years, 1831–54, he compiled the results of those experiments into three volumes, called Experimental Researches in Electricity ... A most remarkable thing is that there was not a single formula in this monumental compilation, which showed that Faraday was feeling his way, guided only by geometric intuition without any precise algebraic formulation."

"The repulsive δ interaction problem in one dimension for N particles is reduced, through the use of Bethe's hypothesis, to an eigenvalue problem of matrices of the same sizes as the irreducible representations R of the permutation group S'N. For some Rs this eigenvalue problem itself is solved by a second use of Bethe's hypothesis, in a generalized form. In particular, the ground-state problem of spin-½ fermions is reduced to a generalized Fredholm equation."

"With the advent of special and general relativity, the symmetry laws gained new importance. Their connection with the dynamic laws of physics takes on a much more integrated and interdependent relationship than in classical mechanics, where logically the symmetry laws were only conse- quences of the dynamical laws that by chance possess the symmetries. Also in the relativity theories the realm of the symmetry laws was greatly enriched to include invariances that were by no means apparent from daily experience. Their validity rather was deduced from, or was later confirmed by complicated experimentation. Let me emphasize that the conceptual simplicity and intrinsic beauty of the symmetries that so evolve from complex experiments are for the physicists great sources of encouragement. One learns to hope that Nature possesses an order that one may aspire to comprehend. It was, however, not until the development of quantum mechanics that the use of the symmetry principles began to permeate into the very language of physics. The quantum numbers that designate the states of a system are often identical with those that represent the symmetries of the system. It in- deed is scarcely possible to overemphasize the role played by the symmetry principles in quantum mechanics."

"The spontaneous magnetization of a two-dimensional Ising model is calculated exactly. The result also gives the long-range order in the lattice."

"Although group theory is certainly relevant for nineteenth-century physics, it really started to play an important role with the work of Lorentz and Poincaré, and became essential with quantum mechanics. Heisenberg opened up an entirely new world with his vision of an internal symmetry, the exploration of which continues to this day in one form or another. Beginning in the 1950s, group theory has come to play a central role in several areas of physics, perhaps none more so than in what I call fundamental physics ..."

"Rotations in 3-dimensional Euclidean space ... form the poster child of group theory and are almost indispensable in physics. Think of rotating a rigid object, such as a bust of Newton. After two rotations in succession, the bust, being rigid, has not been deformed in any way; it merely has a different orientation. Thus, the composition of two rotations is another rotation."

"Normally, the entropy of a system is extensive, that is, proportional to its volume. Somehow, a black hole has an entropy proportional to its surface rather than to its volume. This fact has led to the so-called holographic principle. Many fundamental physicists believe that this mysterious property of black holes holds the key to quantum gravity."

"Ah, group theory! The entire subject is amazing and amusing. Who would have expected that three Platonic solids—the cube, tetrahedron, and icosahedron—would pop up in constructing the Dynkin diagrams of the exceptional Lie algebras? Or that finite group theory could determine the remainder when 1010 is divided by 11?"

"We show that the introduction of a charged scalar particle into the standard theory leads to numerous phenomenological consequences. In particular, muon-neutrino scattering on electron can resonate in the s-channel, a fact which is potentially important in high-energy neutrino experiments and conceivably relevant in explaining the recently reported underground muon events from Cygnus X-3. We focus on the violation of various quantum numbers, including electron, muon, and tauon numbers."

"In the last few years, an interesting new subject, the study of topological quantum fluids, has emerged. Examples of topological quantum fluids include the Hall fluid, the chiral spin fluid, and the anyon superfluid."

"We discuss the problem of adding random matrices, which enable us to study Hamiltonians consisting of a deterministic term plus a random term. Using a diagrammatic approach and introducing the concept of "gluon connectedness," we calculate the density of energy levels for a wide class of probability distributions governing the random term, thus generalizing a result obtained recently by Brézin, Hikami, and Zee. The method used here may be applied to a broad class of problems involving random matrices."

"One reason I went to Princeton University as an undergraduate was that I had read about a Professor John Wheeler suggesting that the atomic nucleus might take on the form of a doughnut. When I got there, I learned that Wheeler was going to give a novel type of course for freshmen. A group of us were asked a few physics questions by Wheeler, and those who answered correctly were allowed into the course. The first homework assignment consisted of standing for 15 minutes in front of the house that Albert Einstein had lived in. It turned out that we were to learn physics from the top down: For example, we were taught “F = ma” as a limiting case of special relativity. If I remember correctly, the department did not allow Wheeler to teach the course again. But I learned a lot; in particular, I learned to “never calculate without first knowing the answer.”"

"That the exchange of a particle can produce a force was one of the most profound conceptual advances in physics. We now associate a particle with each of the known forces: for example, the photon with the electromagnetic force and the graviton with the gravitational force; the former is experimentally well established and while the latter has not yet been detected experimentally hardly anyone doubts its existence."

"It is difficult to overstate the importance (not to speak of the beauty) of what we have learned: The exchange of a spin 0 particle produces an attractive force, of a spin 1 particle a repulsive force, and of a spin 2 particle an attractive force, realized in the hadronic strong interaction, the electromagnetic interaction, and the gravitational interaction, respectively. The universal attraction of gravity produces an instability that drives the formation of structure in the early universe. ... Denser regions become denser yet. The attractive nuclear force mediated by the spin 0 particle eventually ignites the stars. Furthermore, the attractive force between protons and neutrons mediated by the spin 0 particle is able to overcome the repulsive electric force between protons mediated by the spin 1 particle to form a variety of nuclei without which the world would certainly be rather boring. The repulsion between likes and hence attraction between opposites generated by the spin 1 particle allow electrically neutral atoms to form. The world results from a subtle interplay among spin 0, 1, and 2. In this lightning tour of the universe, we did not mention the weak interaction. In fact, the weak interaction plays a crucial role in keeping stars such as our sun burning at a steady rate."

"It is almost an article of faith among theoretical physicists, enunciated forcefully by Einstein among others, that the fundamental laws should be orderly and simple, rather than arbitrarily and complicated."

"In a course on nonrelativistic quantum mechanics you learned about the Pauli exclusion principle2 and its later generalization stating that particles with half integer spins, such as electrons, obey Fermi-Dirac statistics and want to stay apart, while in contrast particles with integer spins, such as photons or pairs of electrons, obey Bose-Einstein statistics and love to stick together. From the microscopic structure of atoms to the macroscopic structure of neutron stars, a dazzling wealth of physical phenomena would be incomprehensible without this spin-statistics rule. Many elements of condensed matter physics, for instance, band structure, Fermi liquid theory, superfluidity, superconductivity, quantum Hall effect, and so on and so forth, are consequences of this rule."

"As was emphasized by Feynman ... among others, the physics of superfluidity lies not in the presence of gapless excitations, but in the paucity of gapless excitations. (After all, the Fermi liquid has a continuum of gapless modes.) There are too few modes that the superfluid can lose energy and momentum to."

"... The quantum Hall system consists of a bunch of electrons moving in a plane in the presence of an external magnetic field B perpendicular to the plane. The magnetic field is assumed to be sufficiently strong so that the electrons all have spin up, say, so they may be treated as spinless fermions. As is well known, this seemingly innocuous and simple physical situation contains a wealth of physics, the elucidation of which has led to two Nobel prizes."

"The goal of condensed matter physics is to understand the various states of matter. States of matter are characterized by the presence (or absence) of order: a ferromagnet becomes ordered below the transition temperature. In the Landau-Ginzburg theory ... , order is associated with spontaneous symmetry breaking, described naturally with group theory. Girvin and MacDonald first noted that the order in Hall fluids does not really fit into the Landau-Ginzburg scheme: We have not broken any obvious symmetry. The topological property of the Hall fluids provides a clue to what is going on. As explained in the preceding chapter, the ground state degeneracy of a Hall fluid depends on the topology of the manifold it lives on, a dependence group theory is incapable of accounting for. Wen has forcefully emphasized that the study of topological order, or more generally quantum order, may open up a vast new vista on the possible states of matter."

"Electromagnetism becomes stronger as we go to higher energies, or equivalently shorter distances. Physically, the origin of this phenomenon is closely related to the physics of dielectrics. Consider a photon interacting with an electron, which we will call the test electron to avoid confusion in what follows. Due to quantum fluctuations ... , spacetime is full of electron-positron pairs, popping in and out of existence. Near the test electron, the electrons in these virtual pairs are repelled by the test electron and thus tend to move away from the test electron while the positrons tend to move toward the test electron. Thus, at long distances, the charge of the test electron is shielded to some extent by the cloud of positrons, causing a weaker coupling to the photon, while at short distances the coupling to the photon becomes stronger. The quantum vacuum is just as much a dielectric as a lump of actual material."

"... the fight for credit goes on in every field, but in theoretical physics, it is almost a way of life, since ideas are by nature ethereal. And the stakes are high: the victor gets to go to Stockholm, while the loser is consigned to the dustbin of history; a history largely written by the victor with the help of an army of idolaters and science writers."

"I do not plan to come back. I have no reason to come back.. I plan to do my best to help the Chinese people build up the nation to where they can live with dignity and happiness."

"The celebrated physicist and mathematician A.M. Ampere coined the word cybernetique to mean the science of civil government (Part II of "Essai sur la philosophic des sciences", 1845, Paris). Ampere's grandiose scheme of political sciences has not, and perhaps never will, come to fruition. In the meantime, conflict between governments with the use of force greatly accelerated the development of another branch of science, the science of control and guidance of mechanical and electrical systems. It is thus perhaps ironical that Ampere's word should be borrowed by N. Wiener to name this new science, so important to modern warfare. The "cybernetics" of Wiener ("Cybernetics, or Control and Communication in the animal and the Machine," John Wiley & Sons, Inc., New York, 1948) is the science of organization of mechanical and electrical components for stability and purposeful actions. A distinguishing feature of this new science is the total absence of considerations of energy, heat, and efficiency, which are so important in other natural sciences. In fact, the primary concern of cybernetics is on the qualitative aspects of the interrelations among the various components of a system and the synthetic behavior of the complete mechanism."

"An engineering science aims to organize the design principles used in engineering practice into a discipline and thus to exhibit the similarities between different areas of engineering practice and to emphasize the power of fundamental concepts. In short, an engineering science is predominated by theoretical analysis and very often uses the tool of advanced mathematics."

"The purpose of "Engineering Cybernetics" is then to study those parts of the broad science of cybernetics which have direct engineering applications in designing controlled or guided systems. It certainly includes such topics usually treated in books on servomechanisms. But a wider range of topics is only one difference between engineering cybernetics and servomechanisms engineering. A deeper - and thus more important - difference lies in the fact that engineering cybernetics is an engineering science, while servomechanisms engineering is an engineering practice."

"Qian Xuesen... didn't like being called the father of China's guided-missile program: he felt that the title didn't give credit to his fellow researchers. Indeed, while the Chinese-born, U.S.-educated rocket scientist was technically brilliant, he also realized that legions of bright thinkers can do far more than one genius ever could."

"It was the stupidest thing this country ever did. He was no more a Communist than I was, and we forced him to go."

"That the government permitted this genius, this scientific genius, to be sent to Communist China to pick his brains is one of the tragedies of this century."

"In 1948, the MIT mathematician Norbert Wiener gave a widely read, albeit completely nonmathematical, account of cybernetics. A more mathematical treatment of the elements of engineering cybernetics was presented by H.S. Tsien in 1954, driven by problems related to control of missiles. Together, these works and others of that time form much of the intellectual basis for modern work in robotics and control."

"All of us have direct experience of the Supreme."

"If every one of those good words — liberty, equality, fraternity, democracy, human rights — has been called "bourgeois", what on earth does that leave for us?"

"[Marxism-Leninism is] a worn-out dress that should be thrown away."

"A rising economic power that violates human rights is a threat to peace."

"If you really look at it, I was trying to sell a dream … There was very little I could put in concrete to tell these people it was really real."

"… it is shameful that there are so few women in science... In China there are many, many women in physics. There is a misconception in America that women scientists are all dowdy spinsters. This is the fault of men. In Chinese society, a woman is valued for what she is, and men encourage her to accomplishments yet she remains eternally feminine."