Apr 13, 1999
Yakov Itin
1. Introduction
The grand challenge facing theoretical physics is the unification of our understanding of particle interactions within the Standard Model, with Einstein's theory of gravity. They are independently consistent with all known observable phenomena, yet come from fundamentally different conceptual frameworks. Notably, the Standard Model is based on quantum field theory within a Minkowskian flat space-time, contrasting with Einstein's gravity which is a classical field theory intrinsically linked with the geometrical properties of pseudo-Riemannian manifolds. The expected unification of these theories thus requires substantial modifications in our understanding.
More recently, significant strides have been made in the modernization of the Standard Model. This paper will focus on exploring one such alternative- teleparallelism. A concept which was first introduced by Cartan and was used by Einstein in his unification theory, providing an alternative model of gravity and describing the spin properties of matter.
2. Formulation and Variations in Field Theory
Employing a Lagrangian formulation for field theory is convenient and insightful. In classical relativistic field theory, the action functional dictates the dynamics of the field. This functional is an integral on the entire four-dimensional differential manifold and incorporates only local densities, being the squares of the first-order derivatives of field variables.
Applying the variational principle within the framework of teleparallelism presents distinctive challenges - primarily stemming from the fact that certain operators, such as the Hodge dual operator and interior product, do not commute with the variational derivative.
3. Gearing upto Teleparallel Manifold
Teleparallel space is a generalized geometric construct, a simpler alternative to the pseudo-Riemannian geometry, known to provide a potential framework for gravity theorized with quadratic Lagrangians. A teleparallel manifold is constructed from a four-dimensional differential manifold paired with a smooth field of frames which are pseudo-orthonormal.
While presenting numerous opportunities, working within a teleparallel manifold also introduces a unique set of challenges. This significantly changes the mathematical properties, such as non-commutativity of operations, of the geometric space employed in our theories.
4. Variations with Teleparallelism
We are at the infancy of our understanding of the effects and implications of variations within teleparallel theories. The study presented in this paper offers insights into this aspect, providing a variational matrix for determining necessary and sufficient conditions for commutativity and anti-commutativity of the variation derivative with the Hodge dual operator. This allows us to establish a general formula for the variation of quadratic-type expressions within teleparallelism theories.
5. In conclusion
Our exploration into the elaborate dance of variations within teleparallelism has provided new insight and understanding. We have established foundational properties and formulas, offering a different and sometimes easier approach to understanding variations within teleparallel gravity theories. This development can have a significant impact on field theories such as the electromagnetic Lagrangian on a curved manifold and the Rumpf Lagrangian of the translation invariant gravity.
However, this work has merely scratched the surface and there remains a vast territory left to explore. The formulation and understanding of such complex theories are painstaking but crucial in our quest to unify our physical laws.
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