Linear simplicity constraint in Loop Quantum Gravity

In summary, the conversation discusses the linear simplicity constraint in covariant LQG and the interpretation of the components of the vector K as canonical generators of Lorentz boosts. This interpretation is not fully understood and attempts are being made to make sense of it. The conversation also mentions the use of tetrad notation and the verification of the algebra commutation relations for (\star(e\wedge e)+\frac{1}{\gamma}e\wedge e)|_{\Sigma}.
  • #1
Georges Simenon
1
0

Homework Statement


Hi,
I am studying covariant LQG from the recent book by Rovelli & Vidotto, and i am struggling with the linear simplicity constraint. My problems are not with its proof, that i understand, but rather with the physical interpretation in terms of boost generators.
I will try to make my question as much self-consistent as possible. I refer to ch. 3 of the Book.
They start from the Holst action
[tex]S=\int B\wedge F\quad\text{where}\quad B=\star(e\wedge e)+\frac{1}{\gamma}e\wedge e[/tex]
Then, in sec. 3.3.1, they define the two 2-forms
[tex]K^I=n_J B^{IJ}|_{\Sigma}\quad\text{and}\quad L^I=n_J(\star B^{IJ})|_{\Sigma}[/tex]
restricted on a spatial hypersurface [itex]\Sigma[/itex] with unit timelike normal [itex]n_I[/itex].
Using these definitions, they prove that the following equation
[tex]\vec{K}=\gamma\vec{L}\qquad\text{linear simplicity constraint}[/tex]
holds in the "time gauge" [itex]n_Ie^I|_{\Sigma}[/itex].
In sec. 3.4.3, they interpret the components of the vector [itex]\vec{K}[/itex] as the canonical generators of Lorentz boosts. It is this interpretation that i don't understand, and I'm trying to give sense to it.
The same claim you can find in the Zakopane lectures by C. Rovelli. In the notation used there, he identifies
[tex](\star(e\wedge e)+\frac{1}{\gamma}e\wedge e)|_{\Sigma}[/tex]
with the [itex]SL(2,\mathbb{C})[/itex] generator.

Homework Equations



The Attempt at a Solution


If i am asked to write the Lorentz generators in tetrad notation, i write
[tex]G^{IJ}=e^I\wedge e^J[/tex]
up to an overall proportionality factor.
This makes sense. Consider, for definiteness, the 2-form
[tex] e^0\wedge e^1 = d\xi^0\otimes d\xi^1-d\xi^1\otimes d\xi^0 [/tex]
It generates Lorentz boosts along the local inertial axis 1. Note: the [itex]\xi[/itex]'s are local inertial coordinates.[/B]
 
Last edited:
  • #3
Have you checked if

[tex](\star(e\wedge e)+\frac{1}{\gamma}e\wedge e)|_{\Sigma}[/tex]

obeys the so(1,3) algebra commutation relations
 

1. What is a linear simplicity constraint in Loop Quantum Gravity?

A linear simplicity constraint is a mathematical equation that is used in Loop Quantum Gravity (LQG) to impose the principle of simplicity, also known as Occam's Razor, on the quantum states of the gravitational field. It is a key component of the LQG approach to quantizing gravity.

2. Why is the linear simplicity constraint important in Loop Quantum Gravity?

The linear simplicity constraint is important because it helps to simplify the mathematical description of the gravitational field in LQG. By imposing the principle of simplicity, it allows for a more elegant and efficient representation of the quantum states of gravity, making the theory more manageable and easier to work with.

3. How is the linear simplicity constraint applied in Loop Quantum Gravity?

The linear simplicity constraint is applied by adding it to the Hamiltonian constraint of the LQG formalism. This constraint, along with other mathematical equations, then determines the quantum states of the gravitational field. The simplicity constraint is also used to define the physical observables in LQG, such as area and volume, which are crucial for the theory.

4. What are the implications of the linear simplicity constraint in Loop Quantum Gravity?

The linear simplicity constraint has several important implications in LQG. It allows for the quantization of the gravitational field, which is a key goal of the theory. It also leads to discrete, or quantized, values for physical observables, which is a defining feature of LQG. Additionally, the simplicity constraint helps to reconcile the principles of general relativity and quantum mechanics, making it a promising approach to a theory of quantum gravity.

5. How does the linear simplicity constraint differ from other constraints in Loop Quantum Gravity?

The linear simplicity constraint is unique to LQG and is not found in other approaches to quantum gravity, such as string theory. Other constraints in LQG, such as the Hamiltonian constraint and the diffeomorphism constraint, are also important, but the simplicity constraint is crucial for the formulation and success of the theory. It is considered to be one of the defining characteristics of LQG.

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