Self-dual Palatini action
Ashtekar variables, which were a new canonical formalism of general relativity, raised new hopes for the canonical quantization of general relativity and eventually led to loop quantum gravity. 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. These proofs were given in terms of spinors. A purely tensorial proof of the new variables in terms of triads was given by Goldberg and in terms of tetrads by Henneaux et al. Here we in particular fill in details of the proof of results for self-dual variables not given in text books.
- 1 The Palatini action
- 2 Self-dual variables
- 3 The Self-dual Palatini action
- 4 Derivation of main results for self-dual variables
- 5 Derivation of Ashtekar's Formalism from the Self-dual Action
- 6 Reality conditions
- 7 See also
- 8 References
The Palatini action
The Palatini action for general relativity has as its independent variables the tetrad and a spin connection . Much more details and derivations can be found in the article tetradic Palatini action. The spin connection defines a covariant derivative . The space-time metric is recovered from the tetrad by the formula We define the `curvature' by
The Ricci scalar of this curvature is given by . The Palatini action for general relativity reads
where . Variation with respect to the spin connection implies that the spin connection is determined by the compatibility condition and hence becomes the usual covariant derivative . Hence the connection becomes a function of the tetrads and the curvature is replaced by the curvature of . Then is the actual Ricci scalar . Variation with respect to the tetrad gives Einsteins equation .
(Anti-)self-dual parts of a tensor
We will need what is called the totally antisymmetry tensor or Levi-Civita symbol, . This is equal to either +1 or -1 depending on whether is either an even or odd permutation of , respectively, and zero if any two indices take the same value. The internal indices of are raised with the Minkowski metric .
Now, given any anti-symmetric tensor , we define its dual as
The self-dual part of any tensor is defined as
with the anti-self-dual part defined as
(the appearance of the imaginary unit is related to the Minkowski signature as we will see below).
Now given any anti-symmetric tensor , we can decompose it as
where and are the self-dual and anti-self-dual parts of respectively. Define the projector onto (anti-)self-dual part of any tensor as
The meaning of these projectors can be made explicit. Let us concentrate of ,
The Lie bracket
An important object is the Lie bracket defined by
it appears in the curvature tensor (see the last two terms of ), it also defines the algebraic structure. We have the results (proved below):
That is the Lie bracket, which defines an algebra, decomposes into two separate independent parts. We write
where contains only the self-dual (anti-self-dual) elements of .
The Self-dual Palatini action
We define the self-dual part, , of the connection as
which can be more compactly written
Define as the curvature of the self-dual connection
Using it is easy to see that the curvature of the self-dual connection is the self-dual part of the curvature of the connection,
The self-dual action is
As the connection is complex we are dealing with complex general relativity and appropriate conditions must be specified to recover the real theory. One can repeat the same calculations done for the Palatini action but now with respect to the self-dual connection . Varying the tetrad field, one obtains a self-dual analog of Einstein's equation:
That the curvature of the self-dual connection is the self-dual part of the curvature of the connection helps to simplify the 3+1 formalism (details of the decomposition into the 3+1 formalism are to be given below). The resulting Hamiltonian formalism resembles that of a Yang-Mills gauge theory (this does not happen with the 3+1 Palatini formalism which basically collapses down to the usual ADM formalism).
Derivation of main results for self-dual variables
The results of calculations done here can be found in chapter 3 of notes Ashtekar Variables in Classical Relativity. The method of proof follows that given in section II of The Ashtekar Hamiltonian for General Relativity. We need to establish some results for (anti-)self-dual Lorentzian tensors.
Identities for the totally anti-symmetric tensor
Since has signature , it follows that
to see this consider,
With this definition one can obtain the following identities,
(the square brackets denote anti-symmetrizing over the indices).
Definition of self-dual tensor
It follows from that the square of the duality operator is minus the identity,
The minus sign here is due to the minus sign in , which is in turn due to the Minkowski signature. Had we used Euclidean signature, i.e. , instead there would have been a positive sign. We define to be self-dual if and only if
(with Euclidean signature the self-duality condition would have been ). Say is self-dual, write it as a real and imaginary part,
Write the self-dual condition in terms of and ,
Equating real parts we read off
where is the real part of .
Important lengthy calculation
The following lengthy calculation is important as all the other important formula can easily be derived from it. From the definition of the Lie bracket and with the use of we have
That gives the formula
which is the starting point for everything else.
Derivation of important results
where in the first step we have used the anti-symmetry of the Lie bracket to swap and , in the second step we used and in the last step we used the anti-symmetry of the Lie bracket again. Now using this we obtain
where we used in the third step. So we have then . Similarly we have
Now if we took and simply replaced with we would get . Combining () and we obtain
Summarising, we have
where we used going from the first line to the second line. Similarly we have . Now consider ,
where we have used and in going from the second line to the third line. Similarly
Starting with we have
where we have used that any can be written as a sum of its self-dual and anti-sef-dual parts, i.e. , and .
Summary of main results
Altogether we have,
which is our main result, already stated above as . We also have that any bracket splits as
into a part that depends only on self-dual Lorentzian tensors and is itself the self-dual part of , and a part that depends only on anti-self-dual Lorentzian tensors and is the anit-self-dual part of .
Derivation of Ashtekar's Formalism from the Self-dual Action
The Palatini action
where the Ricci tensor, , is thought of as constructed purely from the connection , not using the frame field. Variation with espect to the tetrad gives Einstein's equations written in terms of the tetrads, but for a Ricci tensor constructed from the connection that has no a priori relationship with the tetrad. Variation with respect to the connection tells us the connection satisfies the usual compatibility condition
This determines the connection in terms of the teterad and we recover the usual Ricci tensor.
The self-dual action for general relativity is given above.
where is the curvature of the , the self-dual part of ,
It has been shown that is the self-dual part of .
Define vector fields
(where is the projector onto the three surface), which are orthogonal to .
Writing then we can write
where we used and .
So the action can be written
We have . We now define
An internal tensor is self-dual if and only if
and given the curvature is self-dual we have
Substituting this into the action (EQ 12) we have,
where we denoted . We pick the gauge and (this means ). Writing , which in this gauge . Therefore,
The indices range over and we denote them with lower case letters in a moment. By the self-duality of ,
where we used . This implies
We replace in the second term in the action by . We need
The action becomes
where we swapped the dummy variables and in the second term of the first line. Integrating by parts on the second term,
where we have thrown away the boundary term and where we used the formula for the covariant derivative on a vector density :
The final form of the action we require is
There is a term of the form `` thus the quantity is the conjugate momentum to . Hence, we can immediately write
Variation of action with respect to the non-dynamical quantities , that is the time component of the four-connection, the shift fucntion , and lapse function give the constraints
Varying with respect to actually gives the last constraint in Eq divided by , it has been rescaled to make the constraint polynomial in the fundamental variables. The connection can be written
where we used , therefore . So the connection reads
This is the so-called chiral spin connection.
Because Ashtekar's variables are complex it results in complex general relativity. To recover the real theory one has to impose what are known as the reality conditions. These require that the densitized triad be real and that the real part of the Ashtekar connection equals the compatible spin connection.
More to be said on this, later.
- Ashtekar variables
- Einstein–Hilbert action
- General relativity
- Lie algebra
- Loop quantum gravity
- Spin connection
- J. Samuel. A Lagrangian basis for Ashtekar's formulation of canonical gravity. Pramana J. Phys. 28 (1987) L429-32
- T. Jacobson and L. Smolin. The left-handed spin connection as a variable for canonical gravity. Phys. Lett. B196 (1987) 39-42.
- T. Jacobson and L. Smolin. Covariant action for Ashtekar's form of canonical gravity. Class. Quant. Grav. 5 (1987) 583.
- Triad approach to the Hamiltonian of general relativity. Phys. Rev. D37 (1987) 2116-20.
- M. Henneaux, J.E. Nelson and C. Schomblond. Derivation of Ashtekar variables from tetrad gravity. Phys. Rev. D39 (1989) 434-7.
- Ashtekar Variables in Classical General Relativity, Domenico Giulini, Springer Lecture Notes in Physics 434 (1994), 81-112, arXiv:gr-qc/9312032
- The Ashtekar Hamiltonian for General Relativity by Ceddric Beny
- Knot theory and quantum gravity in loop space: a primer by Jorge Pullin; AIP Conf.Proc.317:141-190,1994, arXiv:hep-th/9301028