9.1: Introduction

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Quantum mechanics allows us to predict the results of experiments. If we conduct an experiment with indistinguishable particles a correct quantum description cannot allow anything which distinguishes between them. For example, if the wavefunctions of two particles overlap, and we detect a particle, which one is it? The answer to this is not only that we don’t know, but that we can’t know. Quantum mechanics can only tell us the probability of finding a particle in a given region. The wavefunction must therefore describe both particles. The Schrödinger equation is then:

\[\left[ − \frac{\hbar^2}{2m} (\nabla^2_1 + \nabla^2_2 ) + V ( {\bf r_1}) + V ( {\bf r_2}) \right] \Phi ( {\bf r_1}, {\bf r_2}) = E\Phi ( {\bf r_1}, {\bf r_2}) \nonumber\]

where the subscripts label each particle, and there are six coordinates, three for each particle. \(\Phi\) is a wave in six dimensions which contains the information we can measure: the probability of finding particles at \( {\bf r_1}\) and \( {\bf r_2}\), but not what we can’t measure: which particle is which.

What basis states would be appropriate for \(\Phi\)? An approximation is to use a product such as \(\Phi ( {\bf r_1}, {\bf r_2}) = |a( {\bf r_1})b( {\bf r_2}) \rangle\) where \(a( {\bf r_1})\) and \( b( {\bf r_2})\) are one-particle wavefunctions of atoms 1 and 2. This allows us to separate the two particle equation into two one particle equations:

\[[ \frac{−\hbar^2}{2m} \nabla^2_1 + V ( {\bf r_1})]|a( {\bf r_1}) \rangle = E_1|a( {\bf r_1}) \rangle; \quad [\frac{−\hbar^2}{2m} \nabla^2_2 + V ( {\bf r_2})]|b( {\bf r_2}) \rangle = E_2|b( {\bf r_2}) \rangle \nonumber\]

provided that the particles do not interact (n.b \(\nabla^2_1\) does not act on \(b( {\bf r_2})\)).

Unfortunately, by doing this we have introduced unphysical labels to the indistinguishable particles. And this is wrong: the effect of it is that the particles do not interfere with each other because they are in different dimensions (six dimensional space - remember?). When we construct a twoparticle wavefunction out of two one-particle wavefunctions we must be ensure that the probability density (the measurable quantity \(|\Phi |^2\)) is independent of the artificial labels.