Quantum Gravity – Is There a Reliable Quantum Theory of Gravitation?

effective-field-theorygeneral-relativityqft-in-curved-spacetimequantum-gravityrenormalization

The notion that "we have no reliable quantum theory of gravitation" seems to be widely accepted (see example PSE discussion here). But is it really so?

According to the modern Effective Field Theory (EFT), at experimentally accessible energies, we DO have exceptionally reliable quantum theory of gravitation (see e.g. here, which gives a very accurate formula eq. 20 for the quantum corrections to the Newton potential between two masses at low energies). We just don't have reliable quantum theory of gravitation at Planck scale energies. However, note that the standard model (SM) is also merely an effective field theory of a more fundamental UV complete theory at Planck scale. Therefore, we don't have a reliable quantum theory of any force at Planck scale. In this regard, can we say that the status of quantum theory of gravitation (QG) is not different from e.g. QED?

One may disagree with the above comparison of QG with QED. The rationale is that QED is renormalizable and QG is not, since gravity coupling has negative mass dimension. However, in the modern EFT/Wilsonian RG point of view, coupling with negative dimension is perfectly allowable. QG is renormalizable as well: you just have to carefully absorb divergences into higher order Lagrangian terms. For more details of QG renormalization, see Section 4 of the paper referenced above. The paper says that:

The renormalization of divergences is also not that big of a deal, although it was the focus of this subject for many years. The divergences themselves come from the high energy end of the theory, which we know is not reliable. The ultimate UV completion will eventually tell us the correct way to treat this domain, and will predict the value of the coefficients. So renormalization is a necessary step, but one without much content…The real power of the effective field theory is that it shifts our attention from the UV (where we do not know the physics) to the IR (where we do). There, EFT
techniques allow one to make real predictions. This is because we know the light
degrees of freedom active there and we know their interactions.

So, is there really a reliable quantum theory of gravitation?

Best Answer

We don't expect quantum gravity effects to become observable until we approach the Planck energy, so the effective field theories for quantum gravity work at energies where they don't predict anything observable, and they don't work at energies where the effects would be observable. This is a strange way to define a reliable theory.

You are quite correct that the Standard Model is also (probably) an effective field theory, but it does make predictions in regimes where we can make experimental observations. This is the big difference from quantum gravity.

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