Boson

In physics, a boson is an elementary particle with integral spin. According to the Pauli spin statistics postulate, systems of identical bosons are described by totally symmetric (under permutations of the bosons) wave functions.

A composite system of an even number of fermions may behave as a boson when the coupling between the constituting fermions is strong. For instance, atomic nuclei are composed of protons and neutrons; both types of nucleons are fermions. Atomic nuclei whose mass number A (the total number of nucleons) is even are bosons. Nuclei with odd A are fermions.

A thermodynamical system of N bosons satisfies Bose-Einstein statistics.

The boson is called after the Indian physicist Satyendra Nath Bose (1894–1974), who was the first to note that photons satisfying Planck's law for blackbody radiation obey a special kind of statistics, now called the Bose-Einstein statistics. (Photons have integral spin 1.)

Mathematical description

Let an elementary boson have coordinates

$\mathbf {r} =(x,\;y,\;z),\quad {\hbox{with}}\quad -\infty <x,y,z<\infty ,\qquad \sigma =-I,-I+1,\ldots ,I-1,I.$

The space coordinates x, y, and z are continuous and take on infinitely many values. The spin coordinate σ is discrete and can have 2I+1 different values. For bosons I is integral.

A one-particle wave function and an N-particle wave function are written as

$\phi _{a}(\mathbf {r} _{1},\sigma _{1})\equiv \phi _{a}(1)\quad {\hbox{and}}\quad \Phi (1,2,\ldots ,N)=\phi _{1}(1)\phi _{2}(2)\cdots \phi _{N}(N),$

where (k) stands for (r_k, σ_k), k=1,...,N. Here Φ(1,2,...,N) is the simplest possible N-particle function (a product, which can be exact only if the bosons do not interact). The N-boson function Φ, written here, has as a major defect: it does not satisfy Pauli's spin statistics postulate. This postulate states that the function must be symmetric under interchange of any two boson coordinates (a "transposition").

Example

As a first example we take N = 2. The following function is not symmetric under interchange of 1 and 2 (given by the permutation operator P₁₂), unless φ_a = φ_b,

$\phi _{a}(1)\phi _{b}(2)\neq P_{12}\left[\phi _{a}(1)\phi _{b}(2)\right]=\phi _{a}(2)\phi _{b}(1)\quad {\hbox{if}}\quad \phi _{a}\neq \phi _{b}.$

The symmetrized form is symmetric, even when a ≠ b, and thus is in accordance with the Pauli postulate:

$\phi _{a}(1)\phi _{b}(2)+\phi _{a}(2)\phi _{b}(1)=P_{12}\left[\phi _{a}(1)\phi _{b}(2)+\phi _{a}(2)\phi _{b}(1)\right]=\phi _{a}(2)\phi _{b}(1)+\phi _{a}(1)\phi _{b}(2).$

For the second example we take N = 3 and write the following non-symmetric function with a ≠ b:

$\phi _{a}(1)\phi _{b}(2)\phi _{b}(3)\neq \,P_{12}\left[\phi _{a}(1)\phi _{b}(2)\phi _{b}(3)\right]\equiv \phi _{a}(2)\phi _{b}(1)\phi _{b}(3)$

but

$\phi _{a}(1)\phi _{b}(2)\phi _{b}(3)+\phi _{a}(2)\phi _{b}(1)\phi _{b}(3)=P_{12}\left[\phi _{a}(1)\phi _{b}(2)\phi _{b}(3)+\phi _{a}(2)\phi _{b}(1)\phi _{b}(3)\right]=\phi _{a}(2)\phi _{b}(1)\phi _{b}(3)+\phi _{a}(1)\phi _{b}(2)\phi _{b}(3).$

It is easily verified that the latter function is symmetric under all 3! = 6 permutations of the three space-spin coordinates.

The last example shows that two bosons may occupy the same one-particle function (two bosons occupy φ_b). This is in contrast to fermions: as soon as two fermions occupy the same one-particle function, the total N fermion function vanishes.

Composite systems of fermions

One may define bosons by their collective behavior, circumventing spin. That is, a system of N identical (not necessarily elementary) particles consists of bosons if the N particle wave function of the system is symmetric under transpositions of the particle (space plus spin) coordinates.

In order to show that systems consisting of fermions may behave as bosons, we must first recall that the Pauli statistics postulate requires fermionic wave functions to be antisymmetric (to change sign) under interchange of space-spin coordinates of any two fermions. Consider two systems A and B each consisting of two fermions. The space-spin coordinates of the four identical elementary fermions are labeled 1,..., 4. The total wave function is

$\Phi _{A}(1,2)\Phi _{B}(3,4)\;.$

If the fermions within A and B are strongly coupled (for instance by nuclear forces), permutations that effectively interchange 1 and 2 and/or 3 and 4 do not have to be considered, so that the only permutation is

$P_{AB}=P_{12}P_{34}\quad {\hbox{with}}\quad P_{AB}\Phi _{A}(1,2)\Phi _{B}(3,4)=\Phi _{B}(1,2)\Phi _{A}(3,4)=\Phi _{A}(3,4)\Phi _{B}(1,2).$

Since this permutation consists of two transpositions of fermions, it gives the sign (-1)×(-1) = 1 and hence the identical systems A and B are bosons (i.e., their wave function is symmetric under transposition).

In this way one can explain why an ideal gas, which by definition consists of non-interacting particles, is sometimes bosonic and sometimes fermionic. For instance, an ideal gas of H-atoms is bosonic, while that consisting of D (= ²H) atoms is fermionic. Deuterium consists of three fermions: a proton, a neutron, and an electron. A simultaneous permutation of three fermions that is equivalent to the permutation of two D-atoms gives a minus sign.