- 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.
- (1995). "Quantum Hall Fluids". arXiv:cond-mat/9501022. (quote from p. 2)
- 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.
- (1996). "Law of addition in random matrix theory". arXiv:cond-mat/9602146.
- 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.”
- 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?
Quantum Field Theory in a Nutshell, 2nd edition (2010)Edit
- 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.
- p. 29
- 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.
- pp. 36–37
- 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.
- p. 77
- 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.
- p. 120
- 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.
- p. 285
- ... The quantum Hall system consists of a bunch of electrons moving in a plane in the presence of an external magnetic field 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.
- p. 322
- 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.
- p. 328
- 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.
- p. 358
- ... 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.
- 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.
Group Theory in a Nutshell for Physicists (2016)Edit
- 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.