Topics in path integration
A. Y. Shiekh
Several aspects of the application of path integrals to quantum mechanics are considered.in chapter one., the path integral is used to form gauge invariant variables [Mandelstam, 1962] and these are used in the specific calculation of the non-relativistic scattering of charged scalar particles from a cla ssica l Aharonov-Bohm solenoid.This allow s the calculation to be done without an explicit gauge choice.Chapter two continues with the application of the path integral on a multiply connected space, and forms a constructive derivation for exact scalar diffraction from an impenetrable two dimensional wedge, firs t investigated by Sornmerfeld [1896], Chapter three is more speculative; and represents a preliminary attempt to obtain a path integral, invariant under general (time dependent) canonical transformations.This would lead the way to a consistent quantization scheme, and a quantum mechanical application of the Hamilton-Jacobi philosophy [Goldstein, 1980; Schulman, 1980] Chapter four is a review of the ambiguities in the Dirac quantization scheme from a minimum set of postulates.[introduction) §1 Quantization C lassical mechanics is a description of nature using commuting variables that Is well formulated by the Hamilton least action principle.Butcla ssica l mechanics is not a complete model of nature; leaving several features unexplained, in fact c la ssica l electrodynamics predicts non-causa!behavior for an accelerating charge [Jackson, 19751 and might be considered incomplete on these grounds alone, it might even be speculated that there is only one se lf consistent theory of everything; that might then be deduced on consistency grounds without resort to observing the real thing.Some of the discrepancies in the classica l theory have been overcome in the guantum generalization.Dirac [1925, 1958] proposed how a guantum theory might be Induced from the classica l theory.He postulated that the c la ssica l commuting variables become non-commuting operators and suggested how the Quantum dvnamics be obtained from the cla ssica l Hamilton eouations of motlc)n .Since the quantum theory is the rlore f undamental theorv (the cla ss leal predl ctions follow inq in the j~\ -£ ( } imit) it might be argued that K • ; ; n should b? the start!ng point.U ! i dC s set erne, howev er has the advar tage of s tart 1 nq from a well up r j p p c f f od th sory.In this scheme to edCi : cla ssica l system corr esponds many qijantum generaliza Pons, (eact yield 1 ng the sa me classica l prediction s for 1 Dirac's metfiOd howe V c ; ■ s ambiguous In not generating one unlgue membei generalizations, some f i irf Xnr 1 Ul iCi specification (sijeh as norma'i oi uei ] ng} i e g u m e u *.o c o n i p« e « _ e * y' speeir y the quantum theory (these [edtures di reviewed in more de ^ n 1 1 i r -\ L d i ! 1 1 !chapter four). 1 i! i S i 111Q H t not seern orSadvantags; out uue lu tn is dniuiquiLV, l ids s il a i cecnniquec-soon as ine P ~i(!p j 9 {Introduction} §11 Translating between formalisms Although no formalism of quantum mechanics is more fundamental than any other, each is supposed to he equivalent and so one should he derivable from another.Assuming a knowledge of traditional quantum mechanics one may deduce the path integral formalism.Starting from the position to position amplitude for Heisenberg eigenstates: H h A -V (This may be generalized to the transition between any two states.)Inserting a set of position resolutions of unity; f dq Iq X q !leads to:t(N)iqfN-i)ft(N-i)><q(N-i.),t(N-ni ... Iq(i),tn)><q(i),t(i)iq(0},t(0i> where: qh = w U ~ uu.i and are not Intearated over.f -F L M .dq n iil,.^K ,<q(ki;t(k)iak~i);t(k-i)> Recall that for a Heisenberg eigenstate: iLRk),t(ki/ -expLiH(q,p)t/ n] iqck)/ where iqck)> Is the state at time zero and H(q,p) is the Hamiltonian operator.Pace 14 /op It ' in P a th in teg ra tio n fin iro d u ciio n ) So: < n ksl;N< quo! exp[-iH<q,p )At/ft] io(k-u> continue by inserting momentum resolutions of unity: n i=ljN^pv'.;n k=1 N < q(k)!p(k)><p(k)l exp[-iH(q,p)At/'h] !q(k-n> or [ •••/ -n , 11HdqU) n 1 , 1NdP(i) C -J jif.it.i i f . 2 nk=1 N<qfk)| exp[HH(q,p)At/ti]!pfk)><p(k)!qrk--!)>Considering the firs t alternative (the consequences of the second are discussed later) and looking at the factor: <p(k)| e x p [-iH (q p )A t/n ] !q{k-n> define a function H through: exp[-1 H(q,p)At/ h] =<p! exp[-tH (q,p)A t/h] !q>/<p|q>Even trough it is intended to take the At->0 lim it; one must work to order At, since there are N such terms, where NAt = T (the 'time of flight").In general a physical Hamiltonian hides (A t)-1 contributions (discussed later) and so agreement between all terms of the expanded exponentials is •'eauired.Agreement to lowest order yields: HsQ,p ; -U u;H \Q,p )iqx / \ u !Q/ 5o for a Hermitean Hamiltonian: < p lq X q!p> H2(q,p) = < plH (q,p )jq>< q|H (q,p )ip> integrating over q and using the resolution of unity for q: [" Ciq \ PiqX \ qip> H \*■_ !,p ) -j Qq k ujH \q,p )H *•.q,p j iq/ \ Qip/
•Jan 1, 1987•0 citations