nLab
unitary representation of the Poincaré group

Idea

The Poincaré group is the group of rigid spacetime symmetries of Minkowski spacetime. It is a topological group and as such has unitary representations on infinite-dimensional Hilbert spaces. For any quantum field theory in Minkowski space its space of states therefore decomposes into irreducible representations of the Poincaré group. As was first observed by Hermann Weyl, these irreducible representations encode the particle spectrum of the QFT.

We are interested in the unitary representations of a topological group $G$, i.e., the continuous group homomorphisms

$$U:G\to U(H)$$U\colon G \to U(H)

into the unitary group of a Hilbert space $H$, especially those that are irreducible in the usual sense of representation theory. The topology on $U(H)$ here is understood to be the strong operator topology.

In this and related articles, we study such representations in the case where

$$G={\mathrm{SL}}_2(ℂ)⋉{ℝ}^4$$G = SL_2(\mathbb{C}) \ltimes \mathbb{R}^4

is the universal cover of the connected component of the identity of the Poincaré group, which is important in the study of quantum field theory. A more physical name for such a representation is “elementary particle?”, and we will often use that term in this article. (NB: “elementary particle” will always refer to the formal mathematical notion.)

A full rounded account could become large; see the blog discussion, which despite its size was left in a still-nascent state. However, in a nutshell, the basic theorem is that (elementary) particles are classified up to isomorphism according to their mass and helicity; mass is a continuous parameter and helicity is a discrete parameter.

This theorem in commonly ascribed to Eugene Wigner and often refereed to as the Wigner classification or similar. I

n the blog discussion, John Baez wrote

• I think he completely classified the ‘positive energy’ representations, which are the ones of greatest importance in physics. This leaves out exotic, unobserved representations like negative-energy particles, tachyons and other freaks too scary to mention here. But to be honest, I’m not sure exactly how far he got. What really matters to me are the positive energy reps. I hope that we can use this blog entry as a forum to find out what Wigner proved, and understand its implications for particle physics. If we all team up, we’ll all learn a lot of stuff.

I’m not sure any of these ‘freaks’ are too scary to mention here; this is mathematics after all. In any event, John later clarified what he meant by positive energy representations:

• Those for which time translation acts as

  $$\exp (-iHt)$$\exp(- i H t)

  where $H$ is a nonnegative, nonzero selfadjoint operator. (In physics, $H$ is called the ‘Hamiltonian’ or ‘energy’.)

Related entries

Relevant topics in a full account will include

• Spectral theorem

• Rigged Hilbert spaces and direct integrals?

• induced representations

• helicity, spin

• Klein-Gordon equation?

• Dirac equation?

• spin-statistics theorem

• Maxwell equation

• Stone's theorem?

• Mackey theory?

Much lies ahead…

Tentative notes, to be expanded on…

In the first place, physicists tend to be a little carefree with the mathematics, so this account is written from the point of view of a ‘stupid’ mathematician (for the moment Todd Trimble) who wants to get details straight and precise.

For example, physicists tend to talk about “eigenstates” as if they were elements of the Hilbert space, and other states as linear combinations of eigenstates, whereas really we are dealing with some more complicated technology like rigged Hilbert spaces or direct integrals instead of direct sums. Failure to mention such details places hurdles of communication between physicists and mathematicians. In addition, there are stylistic differences in presentation, where a physicist will happily deal with formulas replete with lots of subscripts and superscripts, whereas many mathematicians prefer dealing with more conceptual, less notation-heavy explanations.

There seem to be at least three ways of dealing with spectral theory of (unbounded) self-adjoint operators on a Hilbert space:

• The usual Stone theory

• Direct integrals of Hilbert spaces

• Rigged Hilbert spaces

Rigged Hilbert spaces

A rigged Hilbert space

Induced representations

Simultaneous diagonalization.

Fact that Poincaré group is a semidirect product.

Related concepts

• Wigner classification

• unitary representation of the super Poincaré group

• Doplicher-Roberts reconstruction theorem

References

The observation that the irreps of the Poincaré group correspond to fundamental particles as well as much of the classification is due to

• Eugene Wigner, On unitary representations of the inhomogeneous Lorentz group , Ann. Math. 40, 149 (1993)

A survey is for instance in section I.3.1

• Rudolf Haag, Local Quantum Physics -- Fields, Particles, Algebras