[lip-muhn; also Fr. leep-man for 1]
Lippmann, Walter, 1889-1974, American essayist and editor, b. New York City. He was associate editor of the New Republic in its early days (1914-17), but at the outbreak of World War I he left to become Assistant Secretary of War, later helping to prepare data for the peace conference. From 1921 to 1931 he was on the editorial staff of the New York World, serving as editor the last two years. In 1931 he began writing for the New York Herald Tribune a highly influential syndicated column, which moved to the Washington Post in 1962. He ceased writing a regular newspaper column in 1967. Lippmann's early books, written when he was a champion of liberalism, include A Preface to Politics (1913), Public Opinion (1922), and A Preface to Morals (1929). An early supporter of Franklin D. Roosevelt and the New Deal, Lippmann became disillusioned and condemned collectivism in The Good Society (1937). His political stance became one of moderate detachment, and he won distinction as a farsighted and incisive analyst of foreign policy. A special Pulitzer Prize citation (1958) praised his powers of news analysis, which he demonstrated in U.S. War Aims (1944), The Cold War (1947), Isolation and Alliances (1952), and Western Unity and the Common Market (1962).

See M. W. Childs and J. B. Reston, ed., Walter Lippmann and His Times (1959); E. W. Weeks, ed., Conversations with Walter Lippmann (1965); R. Steel, Walter Lippmann and the American Century (1980).

The Lippmann-Schwinger equation (named after Bernard A. Lippmann and Julian Schwinger) is of importance to scattering theory. The equation is
| psi^{(pm)} rangle = | phi rangle + frac{1}{E - H_0 pm i epsilon} V |psi^{(pm)} rangle. ,


We will assume that the Hamiltonian may be written as
H = H_0 + V ,
where H and H0 have the same eigenvalues and H0 is a free Hamiltonian. For example in nonrelativistic quantum mechanics H0 may be
H_0 = frac{p^2}{2m}. ,

Intuitively V , is the interaction energy of the system. This analogy is somewhat misleading, as interactions generically change the energy levels E of steady states of the system, but H and H0 have identical spectra Eα. This means that, for example, a bound state that is an eigenstate of the interacting Hamiltonian will also be an eigenstate of the free Hamiltonian. This is in contrast with the Hamiltonian obtained by turning off all interactions, in which case there would be no bound states. Thus one may think of H0 as the free Hamiltonian for the boundstates with effective parameters that are determined by the interactions.

Let there be an eigenstate of H_0 ,:

H_0 | phi rangle = E_0 | phi rangle ,

Now if we add the interaction V , into the mix, we need to solve

left(H_0 + V right) | psi rangle = E | psi rangle ,

Because of the continuity of the energy eigenvalues, we wish that | psi rangle to | phi rangle , as V to 0 ,.

A potential solution to this situation is

|psi rangle = | phi rangle + frac{1}{E - H_0} V | psi rangle ,

However (E-H_0) is singular since E is an eigenvalue of H_0 .

As is described below, this singularity is eliminated in two distinct ways by making the denominator slightly complex:

| psi^{(pm)} rangle = | phi rangle + frac{1}{E - H_0 pm i epsilon} V |psi^{(pm)} rangle ,

Interpretation as in and out states

The S-matrix paradigm

In the S-matrix formulation of quantum field theory, which was pioneered by John Archibald Wheeler among others, all physical processes are modeled according to the following paradigm.

One begins with a non-interacting multiparticle state in the distant past. Non-interacting does not mean that all of the forces have been turned off, in which case for example protons would fall apart, but rather that there exists an interaction-free Hamiltonian H0 for the bound states which has the same spectrum as the actual Hamiltonian H. This initial state is referred to as the in state. Intuitively it consists of boundstates that are sufficiently well separated that their interactions with each other are ignored.

The idea is that whatever physical process one is trying to study may be modeled as a scattering process of these well separated bound states. This process is described by the full Hamiltonian H, but once its over all of the new bound states separate again and one finds a new noninteracting state called the out state.

This paradigm allows one to calculate the probabilities of all of the processes that we have observed in 70 years of particle collider experiments with remarkable accuracy. That said, many interesting physical phenomena do not fit into the above paradigm. For example, if one wishes to consider the dynamics inside of a neutron star sometimes one wants to know more than what it will finally decay into. In other words, one may be interested in measurements that are not in the asymptotic future. Sometimes an asymptotic past or future is not even available. For example, it is very possible that there is no past before the big bang, and in general relativity the future of system falling into a Schwarzschild black hole ends at a singularity and not in a well-separated asymptotic future.

The connection to Lippmann-Schwinger

Intuitively, the slightly deformed eigenfunctions psi^{(pm)} of the full Hamiltonian H are the in and out states. The phi are noninteracting states that resemble the in and out states in the infinite past and infinite future.

Creating wavepackets

This intuitive picture is not quite right, because psi^{(pm)} is an eigenfunction of the Hamiltonian and so at different times only differs by a phase, and so in particular the physical state does not evolve and so it cannot become noninteracting. This problem is easily circumvented by assembling psi^{(pm)} and phi into wavepackets with some distribution g(E) of energies E over a characteristic scale Delta E. The uncertainty principle now allows the interactions of the asymptotic states to occur over a timescale hbar/Delta E and in particular it is no longer inconceivable that the interactions may turn off outside of this interval. The following argument suggests that this is indeed the case.

Plugging the Lippmann-Schwinger equations into the definitions

psi^{(pm)}_g(t)=int dE e^{-iEt} g(E)psi^{(pm)}


phi_g(t)=int dE e^{-iEt} g(E)phi

of the wavepackets we see that, at a given time, the difference between the psi_g(t) and phi_g(t) wavepackets is given by an integral over the energy E.

A contour integral

This integral may be evaluated by defining the wave function over the complex E plane and closing the E contour using a semicircle on which the wavefunctions vanish. The integral over the closed contour may then be evaluated, using the Cauchy integral theorem, as a sum of the residues at the various poles. We will now argue that the residues of psi^{(pm)} approach those of phi at time trightarrowmpinfty and so the corresponding wavepackets are equal at temporal infinity.

In fact, for very positive times t the e^{-iEt} factor in a Schrödinger picture state forces one to close the contour on the lower half-plane. The pole in the V from the Lippmann-Schwinger equation reflects the time-uncertainty of the interaction, while that in the wavepackets weight function reflects the duration of the interaction. Both of these varieties of poles occur at finite imaginary energies and so are suppressed at very large times. The pole in the energy difference in the denominator is on the upper half-plane in the case of psi^{-}, and so does not lie inside the integral contour and does not contribute to the psi^{-} integral. The remainder is equal to the phi wavepacket. Thus, at very late times psi^{-}=phi, identifying psi^{-} as the asymptotic noninteracting out state.

Similarly one may integrate the wavepacket corresponding to psi^{+} at very negative times. In this case the contour needs to be closed over the upper half-plane, which therefore misses the energy pole of psi^{+}, which is in the lower half-plane. One then finds that the psi^{+} and phi wavepackets are equal in the asymptotic past, identifying psi^{+} as the asymptotic noninteracting in state.

The complex denominator of Lippmann-Schwinger

This identification of the psi's as asymptotic states is the justification for the pmepsilon in the denominator of the Lippmann-Schwinger equations.

A formula for the S-matrix

The S-matrix S is defined to be the inner product


of the ath and bth Heisenberg picture asymptotic states. One may obtain a formula relating the S-matrix to the potential V using the above contour integral strategy, but this time switching the roles of psi^+ and psi^-. As a result, the contour now does pick up the energy pole. This can be related to the phi's if one uses the S-matrix to swap the two psi's. Identifying the coefficients of the phi's on both sides of the equation one finds the desired formula relating S to the potential


In the Born approximation, corresponding to first order perturbation theory, one replaces this last psi^+ with the corresponding eigenfunction phi of the free Hamiltonian H0, yielding

which expresses the S-matrix entirely in terms of V and free Hamiltonian eigenfunctions.

These formulas may in turn be used to calculate the reaction rate of the process brightarrow a, which is equal to |S_{ab}-delta_{ab}|^2.,


  • Sakurai, J. J. (1994). Modern Quantum Mechanics. Addison Wesley. ISBN 0-201-53929-2.
  • Weinberg, S. (1995). The Quantum Theory of Fields. Cambridge University Press. ISBN 0-521-67053-5.

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