Gupta–Bleuler formalism
In
Overview
Firstly, consider a single
- ,
where the factor is to implement Lorentz covariance. The metric signature used here is +−−−. However, this sesquilinear form gives positive norms for spatial polarizations but negative norms for time-like polarizations. Negative probabilities are unphysical, not to mention a physical photon only has two transverse polarizations, not four.
If one includes gauge covariance, one realizes a photon can have three possible polarizations (two transverse and one longitudinal (i.e. parallel to the 4-momentum)). This is given by the restriction . However, the longitudinal component is merely an unphysical gauge. While it would be nice to define a stricter restriction than the one given above which only leaves the two transverse components, it is easy to check that this can't be defined in a
To resolve this difficulty, first look at the subspace with three polarizations. The sesquilinear form restricted to it is merely
This technique can be similarly extended to the bosonic
with the condition
for physical states and in the Fock space (it is understood that physical states are really equivalence classes of states that differ by a state of zero norm).
This is not the same thing as
- .
Note that if O is any gauge invariant operator,
does not depend upon the choice of the representatives of the equivalence classes, and so, this quantity is well-defined.
This is not true for non-gauge-invariant operators in general because the
In an interacting theory of quantum electrodynamics, the Lorenz gauge condition still applies, but no longer satisfies the free wave equation.
See also
- BRST formalism
- Quantum gauge theory
- Quantum electrodynamics
- ξ gauge
Notes
References
- Bleuler, K. (1950), "Eine neue Methode zur Behandlung der longitudinalen und skalaren Photonen", Helv. Phys. Acta (in German), 23 (5): 567–586, doi:10.5169/seals-112124(pdf download available))
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: CS1 maint: postscript (link - Gupta, S. (1950), "Theory of Longitudinal Photons in Quantum Electrodynamics", Proc. Phys. Soc., 63A (7): 681–691,