# Gell-Mann and Low theorem

Gell-Mann and Low theorem The Gell-Mann and Low theorem is a theorem in quantum field theory that allows one to relate the ground (or vacuum) state of an interacting system to the ground state of the corresponding non-interacting theory. It was proved in 1951 by Murray Gell-Mann and Francis E. Low. The theorem is useful because, among other things, by relating the ground state of the interacting theory to its non-interacting ground state, it allows one to express Green's functions (which are defined as expectation values of Heisenberg-picture fields in the interacting vacuum) as expectation values of interaction picture fields in the non-interacting vacuum. While typically applied to the ground state, the Gell-Mann and Low theorem applies to any eigenstate of the Hamiltonian. Its proof relies on the concept of starting with a non-interacting Hamiltonian and adiabatically switching on the interactions.

Contents 1 History 2 Statement of the theorem 3 Proof 4 References History The theorem was proved first by Gell-Mann and Low in 1951, making use of the Dyson series. In 1969 Klaus Hepp provided an alternative derivation for the case where the original Hamiltonian describes free particles and the interaction is norm bounded. In 1989 Nenciu and Rasche proved it using the adiabatic theorem. A proof that does not rely on the Dyson expansion was given in 2007 by Molinari.

Statement of the theorem Let {displaystyle |Psi _{0}rangle } be an eigenstate of {displaystyle H_{0}} with energy {displaystyle E_{0}} and let the 'interacting' Hamiltonian be {displaystyle H=H_{0}+gV} , where {displaystyle g} is a coupling constant and {displaystyle V} the interaction term. We define a Hamiltonian {displaystyle H_{epsilon }=H_{0}+e^{-epsilon |t|}gV} which effectively interpolates between {displaystyle H} and {displaystyle H_{0}} in the limit {displaystyle epsilon rightarrow 0^{+}} and {displaystyle |t|rightarrow infty } . Let {displaystyle U_{epsilon I}} denote the evolution operator in the interaction picture. The Gell-Mann and Low theorem asserts that if the limit as {displaystyle epsilon rightarrow 0^{+}} of {displaystyle |Psi _{epsilon }^{(pm )}rangle ={frac {U_{epsilon I}(0,pm infty )|Psi _{0}rangle }{langle Psi _{0}|U_{epsilon I}(0,pm infty )|Psi _{0}rangle }}} exists, then {displaystyle |Psi _{epsilon }^{(pm )}rangle } are eigenstates of {displaystyle H} .

Note that when applied to, say, the ground-state, the theorem does not guarantee that the evolved state will be a ground state. In other words, level crossing is not excluded.

Proof As in the original paper, the theorem is typically proved making use of Dyson's expansion of the evolution operator. Its validity however extends beyond the scope of perturbation theory as has been demonstrated by Molinari. We follow Molinari's method here. Focus on {displaystyle H_{epsilon }} and let {displaystyle g=e^{epsilon theta }} . From Schrödinger's equation for the time-evolution operator {displaystyle ihbar partial _{t_{1}}U_{epsilon }(t_{1},t_{2})=H_{epsilon }(t_{1})U_{epsilon }(t_{1},t_{2})} and the boundary condition {displaystyle U_{epsilon }(t_{2},t_{2})=1} we can formally write {displaystyle U_{epsilon }(t_{1},t_{2})=1+{frac {1}{ihbar }}int _{t_{2}}^{t_{1}}dt'(H_{0}+e^{epsilon (theta -|t'|)}V)U_{epsilon }(t',t_{2}).} Focus for the moment on the case {displaystyle 0geq t_{1}geq t_{2}} . Through a change of variables {displaystyle tau =t'+theta } we can write {displaystyle U_{epsilon }(t_{1},t_{2})=1+{frac {1}{ihbar }}int _{theta +t_{2}}^{theta +t_{1}}dtau (H_{0}+e^{epsilon tau }V)U_{epsilon }(tau -theta ,t_{2}).} We therefore have that {displaystyle partial _{theta }U_{epsilon }(t_{1},t_{2})=epsilon gpartial _{g}U_{epsilon }(t_{1},t_{2})=partial _{t_{1}}U_{epsilon }(t_{1},t_{2})+partial _{t_{2}}U_{epsilon }(t_{1},t_{2}).} This result can be combined with the Schrödinger equation and its adjoint {displaystyle -ihbar partial _{t_{1}}U_{epsilon }(t_{2},t_{1})=U_{epsilon }(t_{2},t_{1})H_{epsilon }(t_{1})} to obtain {displaystyle ihbar epsilon gpartial _{g}U_{epsilon }(t_{1},t_{2})=H_{epsilon }(t_{1})U_{epsilon }(t_{1},t_{2})-U_{epsilon }(t_{1},t_{2})H_{epsilon }(t_{2}).} The corresponding equation between {displaystyle H_{epsilon I},U_{epsilon I}} is the same. It can be obtained by pre-multiplying both sides with {displaystyle e^{iH_{0}t_{1}/hbar }} , post-multiplying with {displaystyle e^{iH_{0}t_{2}/hbar }} and making use of {displaystyle U_{epsilon I}(t_{1},t_{2})=e^{iH_{0}t_{1}/hbar }U_{epsilon }(t_{1},t_{2})e^{-iH_{0}t_{2}/hbar }.} The other case we are interested in, namely {displaystyle t_{2}geq t_{1}geq 0} can be treated in an analogous fashion and yields an additional minus sign in front of the commutator (we are not concerned here with the case where {displaystyle t_{1,2}} have mixed signs). In summary, we obtain {displaystyle left(H_{epsilon ,t=0}-E_{0}pm ihbar epsilon gpartial _{g}right)U_{epsilon I}(0,pm infty )|Psi _{0}rangle =0.} We proceed for the negative-times case. Abbreviating the various operators for clarity {displaystyle ihbar epsilon gpartial _{g}left(U|Psi _{0}rangle right)=(H_{epsilon }-E_{0})U|Psi _{0}rangle .} Now using the definition of {displaystyle Psi _{epsilon }} we differentiate and eliminate derivatives {displaystyle partial _{g}(U|Psi _{0}rangle )} using the above expression, finding {displaystyle {begin{aligned}ihbar epsilon gpartial _{g}|Psi _{epsilon }rangle &={frac {1}{langle Psi _{0}|U|Psi _{0}rangle }}(H_{epsilon }-E_{0})U|Psi _{0}rangle -{frac {U|Psi _{0}rangle }{{langle Psi _{0}|U|Psi _{0}rangle }^{2}}}langle Psi _{0}|(H_{epsilon }-E_{0})U|Psi _{0}rangle \&=(H_{epsilon }-E_{0})|Psi _{epsilon }rangle -|Psi _{epsilon }rangle langle Psi _{0}|H_{epsilon }-E_{0}|Psi _{epsilon }rangle \&=left[H_{epsilon }-Eright]|Psi _{epsilon }rangle .end{aligned}}} where {displaystyle E=E_{0}+langle Psi _{0}|H_{epsilon }-H_{0}|Psi _{epsilon }rangle } . We can now let {displaystyle epsilon rightarrow 0^{+}} as by assumption the {displaystyle gpartial _{g}|Psi _{epsilon }rangle } in left hand side is finite. We then clearly see that {displaystyle |Psi _{epsilon }rangle } is an eigenstate of {displaystyle H} and the proof is complete.

References 1. Gell-Mann, Murray; Low, Francis (1951-10-15). "Bound States in Quantum Field Theory" (PDF). Physical Review. American Physical Society (APS). 84 (2): 350–354. doi:10.1103/physrev.84.350. ISSN 0031-899X.

2. K. Hepp: Lecture Notes in Physics (Springer-Verlag, New York, 1969), Vol. 2.

3. G. Nenciu and G. Rasche: "Adiabatic theorem and Gell-Mann-Low formula", Helv. Phys. Acta 62, 372 (1989).

4. Molinari, Luca Guido (2007). "Another proof of Gell-Mann and Low's theorem". Journal of Mathematical Physics. AIP Publishing. 48 (5): 052113. CiteSeerX 10.1.1.340.5866. doi:10.1063/1.2740469. ISSN 0022-2488. S2CID 119665963.

5. A.L. Fetter and J.D. Walecka: "Quantum Theory of Many-Particle Systems", McGraw–Hill (1971) Categories: Quantum field theoryTheorems in quantum mechanics

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