从宏观量子电动力学分析色散力毕业论文外文翻译.docx

1、从宏观量子电动力学分析色散力 毕业论文外文翻译附 录附录A：英文原文Dispersion Forces within the Framework of Macroscopic QEDChristian Raabe and Dirk-Gunnar WelschAbstract. Dispersion forces, which material objects in the ground state are subject to, originate from the Lorentz force with which the fluctuating, object-assisted electr

2、omagnetic vacuum acts on the fluctuating charge and currentdensities associated with the objects. We calculate them within the frame-work of macroscopic QED, considering magnetodielectric objects described in terms of spatially varying permittivities and permeabilities which are complex functions of

3、 frequency. The result enables us to give a unified approach to dispersion forces on both macroscopic and microscopic levels.Keywords: dispersion forces, Lorentz-force approach, QED in linear causal media1. IntroductionAs known, electromagnetic fields can exert forces on electrically neutral, unpola

4、r-ized and unmagnetized material objects, provided that these are polarizable and/or magnetizable. Classically, it is the lack of precise knowledge of the state of the sources of a field what lets one resort to a probabilistic description of the field, so that, as a matter of principle, a classical

5、field can be non-fluctuating. In practice, this would be the case when the sources, and thus the field, were under strict de-terministic control. In quantum mechanics, the situation is quite different, as field fluctuations are present even if complete knowledge of the quantum state would be achieve

6、d; a strictly non-probabilistic regime simply does not exist. Similarly, polarization and magnetization of any material object are fluctuating quantities in quantum mechanics. As a result, the interaction of the fluctuating electromagnetic vacuum with the fluctuating polarization and magnetization o

7、f material objects in the ground state can give rise to non-vanishing Lorentz forces; these are commonly referred to as dispersion forces.In the following we will refer to dispersion forces acting between atoms, between atoms and bodies, and between bodies as van der Waals (vdW) forces, Casimir-Pold

8、er (CP) forces and Casimir forces, respectively. This terminology also reflects the fact that, although the three types of forces have the same physical origin, different methods to calculate them have been developed. The CP force that acts on an atom (Hamiltonian RA) in an energy eigenstate la) (RA

9、la) = nwala) at position rAin the presence of (linearly responding) macroscopic bodies is cornmonly regarded as being the negative gradient of the position-dependent part of the shift of the energy of the overall system, Ea, with the atom being in the state la) and the body-assisted electromagnetic

10、field being in the ground state. The interaction of the atorn with the field, which is responsible for the energy shift, is typically treated in the electric-dipole approximation, Le. Hint = -d.E(rA) in the multipolar coupling scheme, and the energy shift is calculated in leading-order perturbation

11、theory. In this way, one finds 1,2 (1) (P, principal value; Wba=Wb-Wa), where G(r,r,w) is the classical (retarded) Greentensor (in the frequency domain) for the electric field, which takes the presence ofthe macroscopic bodies into account. It can then be argued that, in order to obtainthe CP potent

12、ial Ua(rA) as the position-dependent part of the energy shift, onemay replace G(rA,rA,w) in Eq. (1) with G(S)(rA,rA,w), where G(S)(r,r,w) is thescattering part of the Green tensor. Hence, (2) (3) (4)where Ua(rA) has been decomposed into an off-resonant part Uf(rA) and a resonant part U(rA), by takin

13、g into account the analytic properties of the Green tensor as a function of complex w, and considering explicitly the singularities excluded by the principal-val ne integration in Eq. (1). Let us restrict our attention to ground-state at0111S. (Forces on excited atoms lead to dynamical problems in g

14、eneral 2). In this case, there are of course no resonant contributions, as only upward transitions are possible Wab 0 in Eq. (4). Thus, on identifying the (isotropic) ground-state polarizability of an atom aswe may write the CP potential of a ground-state atom in the form of (see, e.g.Refs. 1-6)from

15、 which the force acting on the at0111 follows as (7)Now consider, instead of the force on a single ground-state atom, the force on a collection of ground-state at0111S distributed with a (coarse-grained) nUInber density 7(r) inside a space region of volume Vr-iI. When the mutual interaction of the a

16、toms can be disregarded, it is permissible to simply add up the CP forces on the individual atoms to obtain the force acting on the collection of atoms due to their interaction with the bodies outside the volume Vm, Le.Since the collection of atoms can be regarded as constituting a weakly dielectric

17、 body of susceptibility XNI(r, i), Eq. (8) gives the Casimir force acting on such a body. Note that special cases of this formula were already used by Lifshitz 7 in the study of Casimir forces between dielectric plates. The question is how Eq. (8) can be generalized to an arbitrary ground-state body

18、 whose susceptibility XrvI (r,i) is not necessarily small. An answer to this and related questions can be given by means of the Lorentz-force approach to dispersion forces, as developed in Refs. 8,9.2. Lorentz ForceLet us consider macroscopic QED in a linearly, locally and causally responding medium

19、 with given (complex) permittivity c( r, w) and perrneability p( r, w). Then, if the current density that enters the macroscopic Maxwell equations isthe source-quantity representations of the electric and induction fieldsread aswhere the retarded Green tensor G(r,r,w) corresponds to the prescribed m

20、edium. In Eqs. (12) and (13), it is assumed that the medium covers the entire space so that solutions of the homogeneous Maxwell equations do not appear. Free-space regions can be introduced by performing the limits E 1 and J-L 1, but not before the end of the actual calculations.Because of the pola

21、rization and/or magnetization currents attributed to the medium, the total charge and current densities are given by whereAs we have not yet specified the current density IN(r) in any way, the above formu-las are generally valid so far, and they are valid both in classical and in quantum electrodyna

22、mics, In any case, it is clear that knowledge of the correlation func-tion (IN(r,w)l(rw), where the angle brackets denote classical and/or quan-turn averaging, is sufficient to C0111pute the correlation functions (e(r,w)Et(r,w),(t(r,w),E(r,w), (l(r,w)Bt(r,w) and (t(r,w)B(r,w), from which the(slowly

23、varying part of the) Lorentz force density follows asWhere the limit r r must be understood in such a way that divergent self-forces, which would be formally present even in a uniform (bulk) medium, are omitted. The force on the matter in a volurne Vf,l is then given by the volume integralwhich can

24、be rewritten as the surface integralwhere T(r) is (the expectation value of) Maxwells stress tensor (as opposed to Minkowskis stress tensor), which is (formally) identical with the stress tensor in microscopic electrodynamics. Note that in going from Eq. (18) to Eq. (19), a term resulting from the (

25、slowly varying part of the) Poynting vector has been omitted, which is valid under stationary conditions. If IN(r) can be regarded as being a classical current density producing classical radiation, IN(r) jclass (r, t), then the Lorentz force computed in this way gives the classical radiation force

26、that acts on the material inside the chosen space region of volume Vm (see also Ref. 10).3. Dispersion ForceAs already mentioned in Sec. 1, the dispersion force is obtained if IN(r) is identified with the noise current density attributed to the polarization and magnetization of the material. Let us

27、restrict our attention to the zero-temperature limit, Le. let us assume that the overall system is in its ground state. (The generalization to thermal states is straightforward.) From macroscopic QED in dispersing and absorbing linear media 11,12 it can be shown that the relevant current correlation

28、 function reads as (I, unit tensor). Combining Eqs. (12), (13), (15), (16) and (20), and making use ofstandard properties of the Green tensor, one can then show thatand Let us consider, for instance, an isolated dielectric body of volume VlrI andsusceptibility XrvI(r,w) in the presence of arbitrary

29、rnagnetodielectric bodies, whichare well separated from the dielectric body. In this case, further evaluation ofEq. (18) leads to the following formula for the dispersion force on the dielectricbody:where GrvI(r, r,i) is the Green tensor of the system that includes the dielectricbody. When the diele

30、ctric body is not an isolated body but a part of some largerbody (again in the presence of arbitrary magnetodielectric bodies), Eq. (23) mustbe supplernented with a surface integral,which may be regarded as reflecting the screening effect due to the residual part ofthe body. At this point it should

31、be mentioned that if Minkowskis stress tensor were usedto calculate the force on a dielectric body, Eq. (24) would be replaced withAlthough both Eq. (24) and (25) properly reduce to Eq. (23) when the dielectric body is an isolated one, they differ by a surface integral in the case where the body is

32、S0111e part of a larger body. In the latter case, Minkowskis tensor is hence expected to lead to incorrect and even self-contradictory results 9,13. It should be pointed out that the differences between the Lorentz-force approach to dispersion forces and approaches based on Minkowskis tensor or related quantities are not necessarily small, For instance, the ground-state Lorentz force (per unit area) that acts on an almost perfectly reflecting planar plate