Quantum Mechanics – Importance of Canonical Commutation Relations

commutatoroperatorsphase-spacequantum mechanics

Suppose $\hat{x}$ and $\hat{p}$ are the position and momentum operators, it can be shown that
$$[\hat{x}, \hat{p}] = i\hbar\mathbb{I}.$$
The Stone-von Neumann theorem tells us that that the above is unique up to unitary equivalence.

I am unclear on the significance of the canonical commutation relations shown above. My current interpretation of commutators is, informally speaking, that they measure the extent to which two operators commute. What further information does the canonical commutation relation give us, and why is its uniqueness up to unitary equivalence such a big deal?

Best Answer

There's more to it, and the deeper content it encodes is related to symmetries and their associated conserved quantities. Let us start with the classical theory, to see that this is indeed already present there. In Classical Mechanics we may formulate our theory in the Hamiltonian Formalism. In that case we have a phase space $(\Gamma,\Omega)$ where $\Gamma$ is a space, which in basic mechanics courses is usually described as the space of pairs $(q^i,p_i)$ of position and momenta, and where $\Omega$ is an object called sympletic form.

The sympletic form gives rise to an operation among functions on $\Gamma$ called the Poisson bracket $\{,\}$. The Poisson bracket between position and momenta obey $$\{q^i,p_j\}=\delta^i_{\phantom i j}\tag{1}\label{ccr}.$$

Now, you might be aware of a result known as Noether's theorem which puts in correspondence symmetries and conservation laws. In the Hamiltonian Formalism it can be phrased as follows. For a given symmetry we have a function in $\Gamma$, called its Hamiltonian charge $Q$, which has the property that $$\{Q,f\}=-\delta_Q f\tag{2}$$

where $\delta_Q$ is the variation of the observable according to the symmetry corresponding to $Q$.

Now let us consider translations. Consider a translation by $\epsilon^i$ so that the coordinates get transformed as $q^i\to q^i+\epsilon^i$. We will have $\delta q^i = \epsilon^i$. In that regard, observe that if we define $Q = \epsilon^i p_i$ we have $$\{ Q,q^i\}=\epsilon^j\{p_j,q^i\}=-\epsilon^i = -\delta_Q q^i\tag{3}.$$

Observe that (1) has been used in the second equality. What this tells is that (1) is the statement that momentum is the generator of translations, or else that momentum is the Hamiltonian charge associated to translations. In particular, momentum in the $i$-th direction generates translations in the $i$-th direction, that is the content of (1).

This then naturally generalizes to Quantum Mechanics. And it is not so surprising that it happens, since we know that the correspondence principle gives the quantization rule $[] \leftrightarrow i\{\}$. In that setting, the Canonical Commutation Relations are just saying that momentum should be the generator of translations.

Obviously, the whole analysis of symmetries that I have outlined above in Classical Mechanics can be made in a self-contained manner in Quantum Mechanics. I only did it in Classical Mechanics to show you that there is a classical version of the story, which may be easier to understand first.

In summary, commutation relations often encode symmetry statements and their associated charges, and the CCR is just one example of that.

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