We present a simple two-dimensional model of a “cat”—a body with zero angular momentum that can rotate itself with no external forces. The model is used to explain the nature of a gauge theory and to illustrate the importance of noncommutative operators. We compare the free-space cat in Newtonian mechanics and the same problem in Aristotelian mechanics at low Reynolds numbers (with the velocity proportional to the force rather than to the acceleration). This example shows the analogy between (angular) momentum in Newtonian mechanics and (torque) force in Aristotelian mechanics. We discuss a topological invariant common to the model in free space and at low Reynolds number.
We present a simple two-dimensional model of a “cat”—a body with zero angular momentum that can rotate itself with no external forces. The model is used to explain the nature of a gauge theory and to illustrate the importance of noncommutative operators. We compare the free-space cat in Newtonian mechanics and the same problem in Aristotelian mechanics at low Reynolds numbers (with the velocity proportional to the force rather than to the acceleration). This example shows the analogy between (angular) momentum in Newtonian mechanics and (torque) force in Aristotelian mechanics. We discuss a topological invariant common to the model in free space and at low Reynolds number.
Abstract This paper is concerned chiefly with the introduction of a co-ordinate operator in the quantum mechanics of the electron. It differs from other work on the same subject in the derivation, of the form of the operator by an analogy with the quantum expression for the angular momentum of the electron. Expressions for velocity and momentum operators are derived, and the relation between them is formally that of the classical theory in the absence of a field of force. By the introduction of reciprocal relations for the co-ordinate and momentum operators a condition is obtained which is applicable when the finite extension of the particle cannot be ignored. This is similar to the condition introduced by Yukawa in the theory of non-local fields. In the course of the development of the work, an explanation is offered of the significance of the fifth co-ordinate introduced in Kaluza’s theory of the gravitational and electromagnetic field and in Klein’s adaptation of it. Further, a new aspect of the question of the quantization of space and time is revealed by the use of co-ordinate operators (Xk), for it appears that the interest lies, not in a property of space-time, but in the quantization of localization of a particle. It should, however, be emphasized that the co-ordinates (xk) are retained with their usual meaning, but it is suggested that they are insufficient as a basis of a mechanical description of the electron. It is by operations upon (Xk) and not upon (xk) that mechanical quantities are obtained. The momentum (uk), like (xk), keeps its usual meaning, but it also is part of an operator (Mk) of wider significance. This is analogous to the case of the angular momentum of the electron where Dirac’s introduction of ‘spin’ shows that the orbital angular momentum is a part of a complete angular momentum operator. The application has been limited to the electron, because the derivation of the form of the operator (Xk) rests upon this analogy. The intention has not been to develop a calculus in which ordinary geometrical co-ordinates are replaced by operators. Finally, in proceeding to a force operator an additional term arises proportional to the derivative of the acceleration with respect to time. This term, which is usually ascribed to the reaction of the field of the electron, occurs here without reference to the self field, and suggests that, in addition to charge and current, a non-Maxwellian quantity must be considered as interacting with the external field.
Spin-s systems Angular momentum is one of the fundamental quantities of Newtonian physics, and in quantum physics its importance is at least as great. In quantum mechanics we often distinguish between two types of angular momentum: orbital angular momentum , which a system of particles possesses due to particle motion through space; and spin angular momentum , which is an intrinsic property of a particle. The distinction will be important later, but for now we will ignore it. We will here refer to angular momentum of any sort as “spin” and develop general-purpose mathematical tools for its description. We have already dealt with spin systems, particularly the example of a spin-½ particle. Our approach began with the empirical observation that a measurement of any spin component of a spin-½ particle could yield only the results +ħ/2 or −ħ/2. We introduced the basis states | z ±〉 for the two-dimensional Hilbert space ℋ. We also gave other basis states such as {| x ±〉} and {| y ±〉} in terms of the | z ±〉 states. From basis states and measurement values we constructed operators for the spin components S x , S y , and S z . With the operators in hand, we could then examine the algebraic relations between them (such as the commutation relation in Exercise 3.56). Our job here is to generalize our analysis to systems of arbitrary spin. To do this, we will reverse our chain of logic.
In the separation of rotations from internal motions in the n -body problem, there appear some gauge fields which physically represent Coriolis effects. These fields are also present in the "falling cat" problem: at the kinematical level they map changes in the cat's shape to changes in its orientation whereas at the dynamical level they show up as gauge potentials in the Hamiltonian. Classically, the vanishing angular momentum condition allows for the orientation degrees of freedom to decouple from the internal ones and the cat's re-orientation can be accounted for at the kinematical level, partially. In the quantum case the cat's reorientation description requires to be done on dynamical grounds. In this paper we explore the quantum version of the falling cat modelled as a three body problem.
For pedagogical purposes it is demonstrated that with little effort the customary discussion of rotations in introductory courses can be extended to naturally produce a number of angular momentum operator properties, if coordinate-free rotation operators are used in their exponential form. This form gives rise to a particularly useful coordinate-free representation of the generators of rotations in three-space, from which the student can derive the usual angular momentum formalism for the case j=1 by elementary vector algebra alone. Since in this approach the j=1 formalism appears as a set of relations among simple geometric objects in ordinary space, it admits of a geometric interpretation and will leave the student better prepared for a more profound appreciation of the abstract concepts of the full theory.
It is shown by direct computation that angular momentum cannot be reduced to two dimensions by assuming lack of dependence on the z coordinate. Formally, this is because the independence of z breaks rotational invariance. As a physical example, it is shown that a gauge- and rotation-invariant description of planar electrons in a perpendicular magnetic field must have a three-dimensional form, even though all the terms refer to planar coordinates alone. The physical concepts underlying common formal manipulations with the quantum angular momentum operator are treated at a level directly useful to students encountering them thoroughly for the first time. The letter may also serve as a contextual introduction to advanced quantization issues, usually studied at graduate level.
A particle moving along a straight-line has a linear momentum mv. A revolving particle has angular momentum due to its mass and angular velocity. Let us imagine the electron as a small ball, revolving about some axis. It possesses angular momentum. Precisely, the angular momentum vector is a vector pointing along the axis of rotation with magnitude proportional to the speed of rotation. In classical mechanics the symmetry of a physical system leads to conservation laws. If the space is isotropic, it possesses rotational symmetry. The number of elements in a group is called its order. A group with finite number of elements is called a finite group whereas an infinite group contains infinite number of elements. An infinite group may further be either discrete or continuous. In fact, it is impossible to tell whether an electron is spinning. The word spin is a convenient way of representing the intrinsic angular momentum of a particle.