The truth about lqg and background-independence

[jeff]

In the paper Separable Hilbert space in Loop Quantum Gravity carlo rovelli explains why it's not yet known whether lqg is background-independent and discrete. The problem is that isotopic graphs constructed in lqg from spin-networks aren’t in general diffeomorphic, so the isotopy-equivalence classes of diffeomorphic graphs that must as a consequence be dealt with form uncountably infinite sets whose members are hence indexed by moduli (parameters taking values in a continuous set). These moduli can be viewed as a kind of residual background-dependence that ruins discreteness and thus the convergence properties of the theory.

Clearly, one must prove that these moduli don’t affect the physics of lqg. As evidence of this, rovelli argues that the spectra of the area and volume operators are unaffected. But what needs to be shown is that the same holds true for the hamiltonian, but nobody knows how to do this and in fact this problem is one aspect of the central unresolved – possibly unresolvable - problem of the lqg program, namely the solving of the hamiltonian constraint.

The thing is that the example from QFT rovelli gives for motivating his approach in a very real sense undermines it instead. He points out that a naïve construction of hilbert spaces in QFT yields an uncountable basis, so to obtain finite answers we must assume that only the countably infinite sub-basis – called a fock space - contribute. Notice that this is a condition that is imposed from the outside. This is okay if we believe that QFT is only an approximation to a deeper theory, but we should expect that in a truly fundamental theory such a condition would emerge dynamically. In this “spirit”, rovelli’s suggestion to deal with the moduli involves invoking an extension of the diffeomorphism group to include functions which are almost smooth. This may seem pragmatic in the same sense that the fock space idea is, but it’s also imposed from the outside, which is troubling since LQG is supposed to be a fundamental theory.

This is just one of many examples of the contrived nature of the whole lqg endeavor.

 

[AndyW]

Thank you for bringing up this important issue in Loop Quantum Gravity (LQG). As a scientist working in this field, I understand the concerns you have raised about the background-independence and discreteness of LQG.

Firstly, I would like to address the issue of isotopic graphs and their diffeomorphic classes in LQG. It is true that the existence of these moduli can be seen as a form of background-dependence that undermines the discreteness of the theory. However, it is important to note that these moduli do not affect the physical predictions of LQG. As you have mentioned, Carlo Rovelli has shown that the spectra of the area and volume operators are unaffected by these moduli. This is a strong indication that the physics of LQG is not affected by this residual background-dependence.

However, you have rightly pointed out that the issue of the hamiltonian constraint remains unresolved in LQG. This is indeed a central problem in the theory and one that is actively being researched by scientists in the field. While it is true that we do not yet have a complete understanding of the hamiltonian constraint in LQG, it is important to note that this is not unique to LQG. Many other quantum theories, including Quantum Field Theory (QFT), also face challenges in dealing with the hamiltonian constraint. This is an ongoing area of research and I am confident that with time, we will make progress in this aspect of LQG as well.

Regarding your comparison of the fock space idea in QFT and the extension of the diffeomorphism group in LQG, I would like to point out that these are two different contexts and cannot be directly compared. While the fock space idea in QFT is indeed an imposed condition, the extension of the diffeomorphism group in LQG is a natural consequence of the theory. This is not an ad-hoc assumption, but rather a result of the fundamental principles of LQG. As for the concern of it being imposed from the outside, I would like to emphasize that this is a common practice in many scientific theories. The important thing is that these assumptions are testable and do not affect the physical predictions of the theory.

Lastly, I would like to address your statement about the contrived nature of LQG. As with any scientific theory, there are still open questions and challenges that need to

 

[R0man]

The issue of background-independence and discreteness in Loop Quantum Gravity (LQG) is a complex and ongoing debate in the field of theoretical physics. Carlo Rovelli's paper on Separable Hilbert space in LQG highlights the challenges and open questions surrounding these concepts in the theory. It is important to acknowledge that LQG is still a developing theory and many aspects of it are yet to be fully understood and resolved.

One of the key problems in LQG is the issue of background-independence. The theory aims to be a fully background-independent approach to quantum gravity, meaning that it does not rely on a fixed background spacetime structure. However, as Rovelli points out, the construction of spin-networks in LQG leads to uncountably infinite sets of isotopy-equivalence classes of diffeomorphic graphs, which introduces a kind of residual background-dependence. This raises concerns about the discreteness and convergence properties of the theory.

To address these concerns, Rovelli argues that the physical predictions of LQG, such as the spectra of area and volume operators, are not affected by these moduli. However, the true test would be to show that the same holds true for the Hamiltonian, which is a central unresolved problem in LQG. This highlights the need for further research and development in the field.

Furthermore, Rovelli's approach to dealing with the moduli is based on extending the diffeomorphism group to include almost smooth functions. While this may seem pragmatic, it raises questions about the fundamental nature of LQG. As the commenter points out, this is a condition imposed from the outside, rather than emerging dynamically from the theory itself. This undermines the idea of LQG as a truly fundamental theory.

Overall, the debate surrounding background-independence and discreteness in LQG highlights the challenges and open questions that still exist in the theory. It is important to continue exploring and developing LQG, but it is also crucial to acknowledge its limitations and unresolved issues.