Ashtekar variables

In the ADM formulation of general relativity, spacetime is split into spatial slices and a time axis. The basic variables are taken to be the induced metric ${\displaystyle q_{ab}(x)}$ on the spatial slice and the metric's conjugate momentum ${\displaystyle K^{ab}(x)}$, which is related to the extrinsic curvature and is a measure of how the induced metric evolves in time.[1] These are the metric canonical coordinates.

Main articles: Frame fields in general relativity, spin connection, and Self-dual Palatini action

In 1986 Abhay Ashtekar introduced a new set of canonical variables, Ashtekar (new) variables to represent an unusual way of rewriting the metric canonical variables on the three-dimensional spatial slices in terms of an SU(2) gauge field and its complementary variable.[2]

Overview

Ashtekar variables provide what is called the connection representation of canonical general relativity, which led to the loop representation of quantum general relativity[3] and in turn loop quantum gravity and quantum holonomy theory.[4]

Let us introduce a set of three vector fields ${\displaystyle E_{i}^{a}}$, ${\displaystyle i=1,2,3}$ that are orthogonal, that is,

${\displaystyle \delta _{ij}=q_{ab}E_{i}^{a}E_{j}^{b}}$.

The ${\displaystyle E_{i}^{a}}$ are called a triad or drei-bein (German literal translation, "three-leg"). There are now two different types of indices, "space" indices ${\displaystyle a,b,c}$ that behave like regular indices in a curved space, and "internal" indices ${\displaystyle i,j,k}$ which behave like indices of flat-space (the corresponding "metric" which raises and lowers internal indices is simply ${\displaystyle \delta _{ij}}$). Define the dual drei-bein ${\displaystyle E_{a}^{i}}$ as

${\displaystyle E_{a}^{i}=q_{ab}E_{i}^{b}}$.

We then have the two orthogonality relationships

${\displaystyle \delta ^{ij}=q^{ab}E_{a}^{i}E_{b}^{j}}$

where ${\displaystyle q^{ab}}$ is the inverse matrix of the metric ${\displaystyle q_{ab}}$ (this comes from substituting the formula for the dual drei-bein in terms of the drei-bein into ${\displaystyle q^{ab}E_{a}^{i}E_{b}^{j}}$ and using the orthogonality of the drei-beins).

and

${\displaystyle E_{i}^{a}E_{b}^{i}=\delta _{b}^{a}}$

(this comes about from contracting ${\displaystyle \delta _{ij}=q_{ab}E_{j}^{b}E_{i}^{a}}$ with ${\displaystyle E_{c}^{i}}$ and using the linear independence of the ${\displaystyle E_{a}^{j}}$). It is then easy to verify from the first orthogonality relation (employing ${\displaystyle E_{i}^{a}E_{b}^{i}=\delta _{b}^{a}}$) that

${\displaystyle q^{ab}=\sum _{i,j=1}^{3}\delta _{ij}E_{i}^{a}E_{j}^{b}=\sum _{i=1}^{3}E_{i}^{a}E_{i}^{b},}$

we have obtained a formula for the inverse metric in terms of the drei-beins - the drei-beins may be thought of as the "square-root" of the metric (the physical meaning to this is that the metric ${\displaystyle q^{ab}}$, when written in terms of a basis ${\displaystyle E_{i}^{a}}$, is locally flat). Actually what is really considered is

${\displaystyle (\mathrm {det} (q))q^{ab}=\sum _{i=1}^{3}{\tilde {E}}_{i}^{a}{\tilde {E}}_{i}^{b},}$,

which involves the densitized drei-bein ${\displaystyle {\tilde {E}}_{i}^{a}}$ instead (densitized as ${\displaystyle {\tilde {E}}_{i}^{a}={\sqrt {\det(q)}}E_{i}^{a}}$). One recovers from ${\displaystyle {\tilde {E}}_{i}^{a}}$ the metric times a factor given by its determinant. It is clear that ${\displaystyle {\tilde {E}}_{i}^{a}}$ and ${\displaystyle E_{i}^{a}}$ contain the same information, just rearranged. Now the choice for ${\displaystyle {\tilde {E}}_{i}^{a}}$ is not unique, and in fact one can perform a local in space rotation with respect to the internal indices ${\displaystyle i}$ without changing the (inverse) metric. This is the origin of the ${\displaystyle SU(2)}$ gauge invariance. Now if one is going to operate on objects that have internal indices one needs to introduce an appropriate derivative (covariant derivative), for example the covariant derivative for the object ${\displaystyle V_{i}^{b}}$ will be

${\displaystyle D_{a}V_{i}^{b}=\partial _{a}V_{i}^{b}-\Gamma _{a\;\;i}^{\;\;j}V_{j}^{b}+\Gamma _{ac}^{b}V_{i}^{c}}$

where ${\displaystyle \Gamma _{ac}^{b}}$ is the usual Levi-Civita connection and ${\displaystyle \Gamma _{a\;\;i}^{\;\;j}}$ is the so-called spin connection. Let us take the configuration variable to be

${\displaystyle A_{a}^{i}=\Gamma _{a}^{i}+\beta K_{a}^{i}}$

where ${\displaystyle \Gamma _{a}^{i}=\Gamma _{ajk}\epsilon ^{jki}}$ and ${\displaystyle K_{a}^{i}=K_{ab}{\tilde {E}}^{bi}/{\sqrt {\det(q)}}}$. The densitized drei-bein is the conjugate momentum variable of this three-dimensional SU(2) gauge field (or connection) ${\displaystyle A_{b}^{j}}$, in that it satisfies the Poisson bracket relation

${\displaystyle \{{\tilde {E}}_{i}^{a}(x),A_{b}^{j}(y)\}=8\pi G_{\mathrm {Newton} }\beta \delta _{b}^{a}\delta _{i}^{j}\delta ^{3}(x-y)}$.

The constant ${\displaystyle \beta }$ is the Immirzi parameter, a factor that renormalizes Newton's constant ${\displaystyle G_{\mathrm {Newton} }}$. The densitized drei-bein can be used to re construct the metric as discussed above and the connection can be used to reconstruct the extrinsic curvature. Ashtekar variables correspond to the choice ${\displaystyle \beta =-i}$ (the negative of the imaginary number), ${\displaystyle A_{a}^{i}}$ is then called the chiral spin connection. The reason for this choice of spin connection was that Ashtekar could much simplify the most troublesome equation of canonical general relativity, namely the Hamiltonian constraint of LQG; this choice made its second, formidable, term vanish and the remaining term became polynomial in his new variables. This raised new hopes for the canonical quantum gravity programme.[5] However it did present certain difficulties. Although Ashtekar variables had the virtue of simplifying the Hamiltonian, it has the problem that the variables become complex.[6] When one quantizes the theory it is a difficult task to ensure that one recovers real general relativity as opposed to complex general relativity. Also the Hamiltonian constraint Ashtekar worked with was the densitized version instead of the original Hamiltonian, that is, he worked with ${\displaystyle {\tilde {H}}={\sqrt {\det(q)}}H}$. There were serious difficulties in promoting this quantity to a quantum operator. It was Thomas Thiemann who was able to use the generalization of Ashtekar's formalism to real connections (${\displaystyle \beta }$ takes real values) and in particular devised a way of simplifying the original Hamiltonian, together with the second term, in 1996. He was also able to promote this Hamiltonian constraint to a well defined quantum operator within the loop representation.[7] For an account of these developments see John Baez's homepage entry, The Hamiltonian Constraint in the Loop Representation of Quantum Gravity.[8]

Smolin and others independently discovered that there exists in fact a Lagrangian formulation of the theory by considering the self-dual formulation of the tetradic Palatini action principle of general relativity.[9][10][11] These proofs were given in terms of spinors. A purely tensorial proof of the new variables in terms of triads was given by Goldberg[12] and in terms of tetrads by Henneaux et al.[13]

References

1. Gravitation by Charles W. Misner, Kip S. Thorne, John Archibald Wheeler, published by W. H. Freeman and company. New York.
2. Ashtekar, A (1986). "New variables for classical and quantum gravity". Physical Review Letters. 57 (18): 2244–2247. Bibcode:1986PhRvL..57.2244A. doi:10.1103/physrevlett.57.2244. PMID 10033673.
3. Rovelli, C.; Smolin, L. (1988). "Knot Theory and Quantum Gravity". Physical Review Letters. 61 (10): 1155–1158. Bibcode:1988PhRvL..61.1155R. doi:10.1103/physrevlett.61.1155. PMID 10038716.
4. J. Aastrup; J. M. Grimstrup (2015). "Quantum Holonomy Theory". Fortschritte der Physik. 64 (10): 783. arXiv:1504.07100. Bibcode:2016ForPh..64..783A. doi:10.1002/prop.201600073.
5. See the book Lectures on Non-Perturbative Canonical Gravity for more details on this and the subsequent development. First published in 1991. World Scientific Publishing Co. Pte. LtD.
6. See part III chapter 5 of Gauge Fields, Knots and Gravity, John Baez, Javier P. Muniain. First published 1994. World scientific Publishing Co. Pte. LtD.
7. Thiemann, T. (1996). "Anomaly-free formulation of non-perturbative, four-dimensional Lorentzian quantum gravity". Physics Letters B. Elsevier BV. 380 (3–4): 257–264. arXiv:gr-qc/9606088. doi:10.1016/0370-2693(96)00532-1. ISSN 0370-2693.
8. The Hamiltonian Constraint in the Loop Representation of Quantum Gravity, http://math.ucr.edu/home/baez/hamiltonian/hamiltonian.html
9. Samuel, J. (April 1987). "A Lagrangian basis for Ashtekar's formulation of canonical gravity". Pramana - Journal of Physics. Indian National Science Academy. 28 (4): L429-L432.
10. Jacobson, Ted; Smolin, Lee (1987). "The left-handed spin connection as a variable for canonical gravity". Physics Letters B. Elsevier BV. 196 (1): 39–42. doi:10.1016/0370-2693(87)91672-8. ISSN 0370-2693.
11. Jacobson, T; Smolin, L (1988-04-01). "Covariant action for Ashtekar's form of canonical gravity". Classical and Quantum Gravity. IOP Publishing. 5 (4): 583–594. doi:10.1088/0264-9381/5/4/006. ISSN 0264-9381.
12. Goldberg, J. N. (1988-04-15). "Triad approach to the Hamiltonian of general relativity". Physical Review D. American Physical Society (APS). 37 (8): 2116–2120. doi:10.1103/physrevd.37.2116. ISSN 0556-2821.
13. Henneaux, M.; Nelson, J. E.; Schomblond, C. (1989-01-15). "Derivation of Ashtekar variables from tetrad gravity". Physical Review D. American Physical Society (APS). 39 (2): 434–437. doi:10.1103/physrevd.39.434. ISSN 0556-2821.

Further reading

• Ashtekar, Abhay (1986). "New Variables for Classical and Quantum Gravity". Physical Review Letters. 57 (18): 2244–2247. Bibcode:1986PhRvL..57.2244A. doi:10.1103/PhysRevLett.57.2244. PMID 10033673.