Non-critical string theory: Lorentz invariance

Usually non-critical string theory is considered in frames of the approach proposed by Polyakov . The other approach has been developed in . It represents a universal method to maintain explicit Lorentz invariance in any quantum relativistic theory. On an example of Nambu-Goto string theory in 4-dimensional Minkowski space-time the idea can be demonstrated as follows:

Geometrically the world sheet of string is sliced by a system of parallel planes to fix a specific parametrization, or gauge on it. The planes are defined by a normal vector n_μ, the gauge axis. If this vector belongs to light cone, the parametrization corresponds to light cone gauge, if it is directed along world sheet's period P_μ, it is time-like Rohrlich's gauge. The problem of the standard light cone gauge is that the vector n_μ is constant, e.g. n_μ = (1, 1, 0, 0), and the system of planes is "frozen" in Minkowski space-time. Lorentz transformations change the position of the world sheet with respect to these fixed planes, and they are followed by reparametrizations of the world sheet. On the quantum level the reparametrization group has anomaly, which appears also in Lorentz group and violates Lorentz invariance of the theory. On the other hand, the Rohrlich's gauge relates n_μ with the world sheet itself. As a result, the Lorentz generators transform n_μ and the world sheet simultaneously, without reparametrizations. The same property holds if one relates light-like axis n_μ with the world sheet, using in addition to P_μ other dynamical vectors available in string theory. In this way one constructs Lorentz-invariant parametrization of the world sheet, where the Lorentz group acts trivially and does not have quantum anomalies.

Algebraically this corresponds to a canonical transformation a_i -> b_i in the classical mechanics to a new set of variables, explicitly containing all necessary generators of symmetries. For the standard light cone gauge the Lorentz generators M_μν are cubic in terms of oscillator variables a_i, and their quantization acquires well known anomaly. Let's consider a set b_i = (M_μν,ξ_i) which contains the Lorentz group generators and internal variables ξ_i, complementing M_μν to the full phase space. In selection of such a set, one needs to take care that ξ_i will have simple Poisson brackets with M_μν and among themselves. Local existence of such variables is provided by Darboux's theorem. Quantization in the new set of variables eliminates anomaly from the Lorentz group. It is well known that canonically equivalent classical theories do not necessarily correspond to unitary equivalent quantum theories, that's why quantum anomalies could be present in one approach and absent in the other one.

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