More formally, in the context of QFT, the S-matrix is defined as the unitary matrix connecting sets of asymptotically free particle states (the in-states and the out-states) in the Hilbert space of physical states. A multi-particle state is said to be free (non-interacting) if it transforms under Lorentz transformations as a tensor product, or direct product in physics parlance, of one-particle states as prescribed by equation (1) below. Asymptotically free then means that the state has this appearance in either the distant past or the distant future.
It can be shown that if a quantum field theory in Minkowski space has a mass gap, the state in the asymptotic past and in the asymptotic future are both described by Fock spaces.
The S-matrix was first introduced by John Archibald Wheeler in the 1937 paper "On the Mathematical Description of Light Nuclei by the Method of Resonating Group Structure". In this paper Wheeler introduced a scattering matrix – a unitary matrix of coefficients connecting "the asymptotic behaviour of an arbitrary particular solution [of the integral equations] with that of solutions of a standard form", but did not develop it fully.
In the 1940s, Werner Heisenberg independently developed and substantiated the idea of the S-matrix. Because of the problematic divergences present in quantum field theory at that time, Heisenberg was motivated to isolate the essential features of the theory that would not be affected by future changes as the theory developed. In doing so, he was led to introduce a unitary "characteristic" S-matrix.
Today, however, exact S-matrix results are a crowning achievement of conformal field theory, integrable systems, and several further areas of quantum field theory and string theory. S-matrices are not substitutes for a field-theoretic treatment, but rather, complement the end results of such.
Making a collection of incoming particles collide (usually two particles with high energies).
Allowing the incoming particles to interact. These interactions may change the types of particles present (e.g. if an electron and a positronannihilate they may produce two photons).
Measuring the resulting outgoing particles.
The process by which the incoming particles are transformed (through their interaction) into the outgoing particles is called scattering. For particle physics, a physical theory of these processes must be able to compute the probability for different outgoing particles when different incoming particles collide with different energies.
The S-matrix in quantum field theory achieves exactly this. It is assumed that the small-energy-density approximation is valid in these cases.
A simple prototype in which the S-matrix is 2-dimensional is considered first, for the purposes of illustration. In it, particles with sharp energy E scatter from a localized potential V according to the rules of 1-dimensional quantum mechanics. Already this simple model displays some features of more general cases, but is easier to handle.
Each energy E yields a matrix S = S(E) that depends on V. Thus, the total S-matrix could, figuratively speaking, be visualized, in a suitable basis, as a "continuous matrix" with every element zero except for 2 × 2-blocks along the diagonal for a given V.
Consider a localized one dimensional potential barrierV(x), subjected to a beam of quantum particles with energy E. These particles are incident on the potential barrier from left to right.
for the region to the left of the potential barrier, and
for the region to the right to the potential barrier, where
is the wave vector. The time dependence is not needed in our overview and is hence omitted. The term with coefficient A represents the incoming wave, whereas term with coefficient C represents the outgoing wave. B stands for the reflecting wave. Since we set the incoming wave moving in the positive direction (coming from the left), D is zero and can be omitted.
The "scattering amplitude", i.e., the transition overlap of the outgoing waves with the incoming waves is a linear relation defining the S-matrix,
The above relation can be written as
The elements of S completely characterize the scattering properties of the potential barrier V(x).
Whenever V(x) is different from zero, however, there is a departure of the S-matrix from the above form, to
This departure is parameterized by two complex functions of energy, r and t.
From unitarity there also follows a relationship between these two functions,
The analogue of this identity in three dimensions is known as the optical theorem.
Definition in quantum field theory
A straightforward way to define the S-matrix begins with considering the interaction picture. Let the Hamiltonian H be split into the free part H0 and the interaction V, H = H0 + V. In this picture, the operators behave as free field operators and the state vectors have dynamics according to the interaction V. Let
denote a state that has evolved from a free initial state
The S-matrix element is then defined as the projection of this state on the final state
where S is the S-operator. The great advantage of this definition is that the time-evolution operatorU evolving a state in the interaction picture is formally known,
Being a special type of time-evolution operator, S is unitary. For any initial state and any final state one finds
This approach is somewhat naïve in that potential problems are swept under the carpet. This is intentional. The approach works in practice and some of the technical issues are addressed in the other sections.
In and out states
Here a slightly more rigorous approach is taken in order to address potential problems that were disregarded in the interaction picture approach of above. The final outcome is, of course, the same as when taking the quicker route. For this, the notions of in and out states are needed. These will be developed in two ways, from vacua, and from free particle states. Needless to say, the two approaches are equivalent, but they illuminate matters from different angles.
as a vacuum quantum state, i.e. a state without real particles. The asterisk signifies that not all vacua are necessarily equal, and certainly not equal to the Hilbert space zero state 0. All vacuum states are assumed Poincaré invariant, invariance under translations, rotations and boosts, formally,
where Pμ is the generator of translation in space and time, and Mμν is the generator of Lorentz transformations. Thus the description of the vacuum is independent of the frame of reference. Associated to the in and out states to be defined are the in and out field operators (aka fields) Φi and Φo. Attention is here focused to the simplest case, that of a scalar theory in order to exemplify with the least possible cluttering of the notation. The in and out fields satisfy
the free Klein–Gordon equation. These fields are postulated to have the same equal time commutation relations (ETCR) as the free fields,
where πi,j is the field canonically conjugate to Φi,j. Associated to the in and out fields are two sets of creation and annihilation operators, a†i(k) and a†f (k), acting in the sameHilbert space, on two distinct complete sets (Fock spaces; initial space i, final space f). These operators satisfy the usual commutation rules,
The action of the creation operators on their respective vacua and states with a finite number of particles in the in and out states is given by
where issues of normalization have been ignored. See the next section for a detailed account on how a general n-particle state is normalized. The initial and final spaces are defined by
The asymptotic states are assumed to have well defined Poincaré transformation properties, i.e. they are assumed to transform as a direct product of one-particle states. This is a characteristic of a non-interacting field. From this follows that the asymptotic states are all eigenstates of the momentum operator Pμ,
In particular, they are eigenstates of the full Hamiltonian,
The vacuum is usually postulated to be stable and unique,[nb 1]
The interaction is assumed adiabatically turned on and off.
The Heisenberg picture is employed henceforth. In this picture, the states are time-independent. A Heisenberg state vector thus represents the complete spacetime history of a system of particles. The labeling of the in and out states refers to the asymptotic appearance. A state Ψα, in is characterized by that as t → −∞ the particle content is that represented collectively by α. Likewise, a state Ψβ, out will have the particle content represented by β for t → +∞. Using the assumption that the in and out states, as well as the interacting states, inhabit the same Hilbert space and assuming completeness of the normalized in and out states (postulate of asymptotic completeness), the initial states can be expanded in a basis of final states (or vice versa). The explicit expression is given later after more notation and terminology has been introduced. The expansion coefficients are precisely the S-matrix elements to be defined below.
While the state vectors are constant in time in the Heisenberg picture, the physical states they represent are not. If a system is found to be in a state Ψ at time t = 0, then it will be found in the state U(τ)Ψ = e−iHτΨ at time t = τ. This is not (necessarily) the same Heisenberg state vector, but it is an equivalent state vector, meaning that it will, upon measurement, be found to be one of the final states from the expansion with nonzero coefficient. Letting τ vary one sees that the observed Ψ (not measured) is indeed the Schrödinger picture state vector. By repeating the measurement sufficiently many times and averaging, one may say that the same state vector is indeed found at time t = τ as at time t = 0. This reflects the expansion above of an in state into out states.
From free particle states
For this viewpoint, one should consider how the archetypical scattering experiment is performed. The initial particles are prepared in well defined states where they are so far apart that they don't interact. They are somehow made to interact, and the final particles are registered when they are so far apart that they have ceased to interact. The idea is to look for states in the Heisenberg picture that in the distant past had the appearance of free particle states. This will be the in states. Likewise, an out state will be a state that in the distant future has the appearance of a free particle state.
The notation from the general reference for this section, Weinberg (2002) will be used. A general non-interacting multi-particle state is given by
p is momentum,
σ is spin z-component or, in the massless case, helicity,
n is particle species.
These states are normalized as
Permutations work as such; if s ∈ Sk is a permutation of k objects (for a k-particle state) such that
then a nonzero term results. The sign is plus unless s involves an odd number of fermion transpositions, in which case it is minus. The notation is usually abbreviated letting one Greek letter stand for the whole collection describing the state. In abbreviated form the normalization becomes
When integrating over free-particle states one writes in this notation
where the sum includes only terms such that no two terms are equal modulo a permutation of the particle type indices. The sets of states sought for are supposed to be complete. This is expressed as
which could be paraphrased as
where for each fixed α, the right hand side is a projection operator onto the state α. Under an inhomogeneous Lorentz transformation (Λ, a), the field transforms according to the rule
where W(Λ, p) is the Wigner rotation and D(j) is the (2j + 1)-dimensional representation of SO(3). By putting Λ = 1, a = (τ, 0, 0, 0), for which U is exp(iHτ), in (1), it immediately follows that
so the in and out states sough after are eigenstates of the full Hamiltonian that are necessarily non-interacting due to the absence of mixed particle energy terms. The discussion in the section above suggests that the in states Ψ+ and the out states Ψ− should be such that
for large positive and negative τ has the appearance of the corresponding package, represented by g, of free-particle states, g assumed smooth and suitably localized in momentum. Wave packages are necessary, else the time evolution will yield only a phase factor indicating free particles, which cannot be the case. The right hand side follows from that the in and out states are eigenstates of the Hamiltonian per above. To formalize this requirement, assume that the full HamiltonianH can be divided into two terms, a free-particle Hamiltonian H0 and an interaction V, H = H0 + V such that the eigenstates Φγ of H0 have the same appearance as the in- and out-states with respect to normalization and Lorentz transformation properties,
The in and out states are defined as eigenstates of the full Hamiltonian,
for τ → −∞ or τ → +∞ respectively. Define
This last expression will work only using wave packages.From these definitions follow that the in and out states are normalized in the same way as the free-particle states,
and the three sets are unitarily equivalent. Now rewrite the eigenvalue equation,
where the ±iε terms has been added to make the operator on the LHS invertible. Since the in and out states reduce to the free-particle states for V → 0, put
on the RHS to obtain
Then use the completeness of the free-particle states,
Here α and β are shorthands that represent the particle content but suppresses the individual labels. Associated to the S-matrix there is the S-operatorS defined by
where the Φγ are free particle states.[nb 2] This definition conforms with the direct approach used in the interaction picture. Also, due to unitary equivalence,
As a physical requirement, S must be a unitary operator. This is a statement of conservation of probability in quantum field theory. But
By completeness then,
so S is the unitary transformation from in-states to out states.
Lorentz invariance is another crucial requirement on the S-matrix.[nb 3] The S-operator represents the quantum canonical transformation of the initial in states to the final out states. Moreover, S leaves the vacuum state invariant and transforms in-space fields to out-space fields,[nb 4]
In terms of creation and annihilation operators, this becomes
A similar expression holds when S operates to the left on an out state. This means that the S-matrix can be expressed as
If S describes an interaction correctly, these properties must be also true:
If the system is made up with a single particle in momentum eigenstate |k⟩, then S|k⟩ = |k⟩. This follows from the calculation above as a special case.
The S-matrix element may be nonzero only where the output state has the same total momentum as the input state. This follows from the required Lorentz invariance of the S-matrix.
Evolution operator U
Define a time-dependent creation and annihilation operator as follows,
so, for the fields,
We allow for a phase difference, given by
because for S,
Substituting the explicit expression for U, one has
where is the interaction part of the hamiltonian and is the time ordering.
By inspection, it can be seen that this formula is not explicitly covariant.
Since the transformation of particles from black hole to Hawking radiation could not be described with an S-matrix, Stephen Hawking proposed a "not-S-matrix", for which he used the dollar sign ($), and which therefore was also called "dollar matrix".
^This is not true if an open system is studied. Under an influence of an external field the in and out vacua can differ since the external field can produce particles.
^Here it is assumed that the full HamiltonianH can be divided into two terms, a free-particle Hamiltonian H0 and an interaction V, H = H0 + V such that the eigenstates Φγ of H0 have the same appearance as the in- and out-states with respect to normalization and Lorentz transformation properties. See Weinberg (2002), page 110.
^If Λ is a (inhomogeneous) proper orthochronous Lorentz transformation, then Wigner's theorem guarantees the existence of a unitary operator U(Λ) acting either on HiorHf. A theory is said to be Lorentz invariant if the same U(Λ) acts on HiandHf. Using the unitarity of U(Λ), Sβα = ⟨i, β|f, α⟩ = ⟨i, β|U(Λ)†U(Λ)|f, α⟩. The right-hand side can be expanded using knowledge about how the non-interacting states transform to obtain an expression, and that expression is to be taken as a definition of what it means for the S-matrix to be Lorentz invariant. See Weinberg (2002), equation 3.3.1 gives an explicit form.
^Here the postulate of asymptotic completeness is employed. The in and out states span the same Hilbert space, which is assumed to agree with the Hilbert space of the interacting theory. This is not a trivial postulate. If particles can be permanently combined into bound states, the structure of the Hilbert space changes. See Greiner & Reinhardt 1996, section 9.2.