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CRC Press eBooks

Permutation Symmetry

A. Abbas

Published Aug 17, 2016• 0 citations
Publisher

Abstract

Two particles are said to be identical if all their intrinsic properties, e.g., mass, electrical charge, spin, colour are exactly the same. Similarly, all protons are identical to one another, and so are all quarks. They possess the same defining properties under similar interactions with each other. The exchange in position of identical particles has a so-called permutation symmetry. Clearly, this permutation symmetry is extremely important for quantum mechanical calculations of the multi-indistinguishable particles. So knowing how a collection of fermions (or bosons) behave is kinematically determined by their permutation symmetry. If we have different cycles in a permutation and there is no element common to any two different cycles, then all these cycles are termed disjoint. In fact any permutation of a finite set is the product of disjoint cycles. A cycle of length two is called a transposition.

DOI
10.1201/9781315371702-4

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Two particles are said to be identical if all their intrinsic properties, e.g., mass, electrical charge, spin, colour are exactly the same. Similarly, all protons are identical to one another, and so are all quarks. They possess the same defining properties under similar interactions with each other. The exchange in position of identical particles has a so-called permutation symmetry. Clearly, this permutation symmetry is extremely important for quantum mechanical calculations of the multi-indistinguishable particles. So knowing how a collection of fermions (or bosons) behave is kinematically determined by their permutation symmetry. If we have different cycles in a permutation and there is no element common to any two different cycles, then all these cycles are termed disjoint. In fact any permutation of a finite set is the product of disjoint cycles. A cycle of length two is called a transposition.
Particles that must have symmetric states are called bosons; pions, photons, and He 4+ nuclei are such particles. Particles whose states must be ant symmetric under interchange of the two particles are called fermions; electrons, protons, and neutrons are examples of fermions. Particles with integer spin are always bosons while particles with half-integer spin are always fermions. It is not at all obvious from the point of view of non-relativistic quantum mechanics why this relation, called the connection between spin and statistics, is true, and one must look within the framework of relativistic quantum field theory to understand it fully theoretically. Effects of identity can also show up in the scattering of seemingly non-identical particles. The elastic scattering of a He 4+ ion from a He 4+ atom, both are in their ground states. These are certainly distinguishable particles, since they have different charges. Particle identity also plays an important role in determining the properties of molecules made of identical atoms.
Abstract If we imagine a Hamiltonian, H^(r1,r2), describing two identical particles at positions r1 and r2 and then we interchange the particles, the Hamiltonian will be unaffected, i.e. H^(r1,r2)=H^(r2,r1). If we introduce an exchange operator P^r1,r2 such that P^r1,r2H^(r1,r2)=H^(r2,r1)P^r1,r2=H^(r1,r2)P^r1,r2, we see that they commute, or [P^r1,r2,H^(r1,r2)]=0. We know then that P^r1,r2andH^(r1,r2) have common eigenfunctions. We can then easily show that the eigenfunctions of the exchange operator must be either even or odd. Experiments show that odd exchange symmetry corresponds to half-integer spin particles called fermions, while even exchange symmetry corresponds to integer spin particles called bosons. The notes then discuss the implications of the new postulate and then presents the Heitler–London theory and the Heisenberg exchange Hamiltonian which has been so successful in predicting molecular structure.
Elementary particles appear in different ways.Particles of the same kind (similar particles) share all state-independent properties like mass, charge, and spin quantum number; there are many similar particles, but they are indistinguishable with respect to a certain class of properties.However, the case might even be stronger: similar particles apparently share all state-dependent properties, like spatial location and spin projection.This threatens the validity of Leibniz's famous Principle of the Identity of Indiscernibles (PII; see Rodriguez-Pereyra (2014) for a historical reconstruction).As it is usually understood, quantum mechanics requires for similar particles to be in permutation invariant states.Take the following state, for example:
The permutation symmetry of Newtonian stationary-state functions – such as the total momentum and the Hamiltonian – is crucial for correct state counting in classical statistical mechanics, but plays virtually no role for classical many-body systems. In contrast, it yields novel effects for quantum many-body systems of equal/identical particles. In order to avoid the misunderstandings Jaynes (1992) has rightly stressed, basic notions, such as equal and identical particles, (in)distinguishability and exchangeability (the really relevant notion) are discussed first within classical mechanics. Then, they are applied to the weight (limiting) functions. The permutation symmetry of the wave functions follow largely from that of the weight functions. The different effects for bosons and fermions are mentioned and illustrated. For the generalization towards anyons and the fractional quantum Hall effect, some ideas for further exploration are proposed. Entanglement is treated as a consequence of the conservation laws, in particular, for the angular momentum.
In non-relativistic quantum mechanics, each particle is characterized by a set of parameters. These are intrinsic to a given particle, and they are the same for a given particle species. Think, for instance, of the mass, the charge, or the spin of an electron. All electrons have the same mass, charge and spin and, from these properties alone, we cannot distinguish two different electrons. Based on these properties, we can divide particles into classes depending on their intrinsic parameters. Particles belonging to the same class are said to be indistinguishable .