Representation theory of the Galilean group

Lie algebra

We focus on the (centrally extended, Bargmann) Lie algebra here, because it is simpler to analyze and we can always extend the results to the full Lie group through the Frobenius theorem.

${\displaystyle [E,P_i]=0}$

${\displaystyle [P_i,P_j]=0}$

${\displaystyle [L_{ij},E]=0}$

${\displaystyle [C_i,C_j]=0}$

${\displaystyle [L_{ij},L_{kl}]=i\hslash [{\delta }_{ik}{L}_{jl}-{\delta }_{il}{L}_{jk}-{\delta }_{jk}{L}_{il}+{\delta }_{jl}{L}_{ik}]}$

${\displaystyle [L_{ij},P_k]=i\hslash [{\delta }_{ik}{P}_j-{\delta }_{jk}{P}_i]}$

${\displaystyle [L_{ij},C_k]=i\hslash [{\delta }_{ik}{C}_j-{\delta }_{jk}{C}_i]}$

${\displaystyle [C_i,E]=i\hslash P_i}$

${\displaystyle [C_i,P_j]=i\hslash M{\delta }_{ij}.}$

E is the generator of time translations (Hamiltonian), P_i is the generator of translations (momentum operator), C_i is the generator of Galilean boosts, and L_ij stands for a generator of rotations (angular momentum operator).

Casimir invariants

The central charge M is a Casimir invariant.

The mass-shell invariant

${\displaystyle ME-\frac{P^2}{2}}$

is an additional Casimir invariant.

In 3 + 1 dimensions, a third Casimir invariant is W², where

${\displaystyle \overrightarrow{W}\equiv M\overrightarrow{L}+\overrightarrow{P}\times \overrightarrow{C},}$

somewhat analogous to the Pauli–Lubanski pseudovector of relativistic mechanics.

More generally, in n + 1 dimensions, invariants will be a function of

${\displaystyle W_{ij}=M{L}_{ij}+P_i{C}_j-P_j{C}_i}$

and

${\displaystyle W_{ijk}=P_i{L}_{jk}+P_j{L}_{ki}+P_k{L}_{ij},}$

as well as of the above mass-shell invariant and central charge.

Schur's lemma

Using Schur's lemma, in an irreducible unitary representation, all these Casimir invariants are multiples of the identity. Call these coefficients m and mE₀ and (in the case of 3 + 1 dimensions) w, respectively. Recalling that we are considering unitary representations here, we see that these eigenvalues have to be real numbers.

Thus, m > 0, m = 0 and m < 0. (The last case is similar to the first.) In 3 + 1 dimensions, when In m > 0, we can write, w = ms for the third invariant, where s represents the spin, or intrinsic angular momentum. More generally, in n + 1 dimensions, the generators L and C will be related, respectively, to the total angular momentum and center-of-mass moment by

${\displaystyle W_{ij}=M{S}_{ij}}$

${\displaystyle L_{ij}=S_{ij}+X_i{P}_j-X_j{P}_i}$

${\displaystyle C_i=M{X}_i-P_{i}t.}$

From a purely representation-theoretic point of view, one would have to study all of the representations; but, here, we are only interested in applications to quantum mechanics. There, E represents the energy, which has to be bounded below, if thermodynamic stability is required. Consider first the case where m is nonzero.

Considering the (E, P→) space with the constraint $${\displaystyle mE=m{E}_0+\frac{P^2}{2},}$$ we see that the Galilean boosts act transitively on this hypersurface. In fact, treating the energy E as the Hamiltonian, differentiating with respect to P, and applying Hamilton's equations, we obtain the mass-velocity relation m v→ = P→.

The hypersurface is parametrized by this velocity In v→. Consider the stabilizer of a point on the orbit, (E₀, 0), where the velocity is 0. Because of transitivity, we know the unitary irrep contains a nontrivial linear subspace with these energy-momentum eigenvalues. (This subspace only exists in a rigged Hilbert space, because the momentum spectrum is continuous.)

Little group

The subspace is spanned by E, P→, M and L_ij. We already know how the subspace of the irrep transforms under all operators but the angular momentum. Note that the rotation subgroup is Spin(3). We have to look at its double cover, because we are considering projective representations. This is called the little group, a name given by Eugene Wigner. His method of induced representations specifies that the irrep is given by the direct sum of all the fibers in a vector bundle over the mE = mE₀ + P²/2 hypersurface, whose fibers are a unitary irrep of Spin(3).

Spin(3) is none other than SU(2). (See representation theory of SU(2), where it is shown that the unitary irreps of SU(2) are labeled by s, a non-negative integer multiple of one half. This is called spin, for historical reasons.)

• Consequently, for m ≠ 0, the unitary irreps are classified by m, E₀ and a spin s.
• Looking at the spectrum of E, it is evident that if m is negative, the spectrum of E is not bounded below. Hence, only the case with a positive mass is physical.
• Now, consider the case m = 0. By unitarity, $${\displaystyle mE-\frac{P^2}{2}=\frac{-P^2}{2}}$$

is nonpositive. Suppose it is zero. Here, it is also the boosts as well as the rotations that constitute the little group. Any unitary irrep of this little group also gives rise to a projective irrep of the Galilean group. As far as we can tell, only the case which transforms trivially under the little group has any physical interpretation, and it corresponds to the no-particle state, the vacuum.

The case where the invariant is negative requires additional comment. This corresponds to the representation class for m = 0 and non-zero P→. Extending the bradyon, luxon, tachyon classification from the representation theory of the Poincaré group to an analogous classification, here, one may term these states as synchrons. They represent an instantaneous transfer of non-zero momentum across a (possibly large) distance. Associated with them, by above, is a "time" operator

${\displaystyle t=-\frac{\overrightarrow{P}\cdot \overrightarrow{C}}{P^2},}$

which may be identified with the time of transfer. These states are naturally interpreted as the carriers of instantaneous action-at-a-distance forces.

N.B. In the 3 + 1-dimensional Galilei group, the boost generator may be decomposed into

${\displaystyle \overrightarrow{C}=\frac{\overrightarrow{W}\times \overrightarrow{P}}{P^2}-\overrightarrow{P}t,}$

with W→ playing a role analogous to helicity.