# Quantum Gravity - Nihon â€؛ ~fffujita â€؛ 170 Chapter 9. Quantum Gravity Before...

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Chapter 9

Quantum Gravity

The quantum field theory of gravitation is constructed in terms of Lagrangian density of Dirac fields which couple to the electromagnetic fieldAµ as well as the gravitational field G. The gravity appears in the mass term asm(1 + gG)ψ̄ψ with the coupling constant of g. In addition to the gravitational force between fermions, the electromagnetic fieldAµ interacts with the gravity as the fourth order effects and its strength amounts toα times the gravitational force. Therefore, the interaction of photon with gravity is not originated from Einstein’s general relativity. Further, we present a renormalization scheme for the gravity and show that the graviton stays massless.

9.1 Problems of General Relativity

The motion of the earth is governed by the gravitational force between the earth and the sun, and the Newton equation is written as

mr̈ = −G0mM r r3

, (9.1)

whereG0, m andM denote the gravitational constant, the mass of the earth and the mass of the sun, respectively. This is the classical mechanics which works quite well.

The gravitational potential that appears in eq.(9.1) is experimentally determined. How- ever, the theoretical derivation of the gravity cannot be achieved in any of the equations such as Newton equation or Maxwell equations. Einstein presented the equation of gen- eral relativity which should be some analogous equations to the Maxwell equations in the sense that the gravitational field should be determined by the equation of general relativity. However, since he employed the principle of equivalence which has nothing to do with real experiments, the general relativity became an equation that determines the metric tensor. This does not mean that one can determine the gravitational interaction, and indeed, we have completely lost the correct direction in physics. Therefore, we should find a theoreti- cal frame work to determine the gravitational interaction with fermions in some way or the other.

169

170 Chapter 9. Quantum Gravity

Before constructing a theory that can describe the field equation under the gravity, we discuss the fundamental problems in the theory of general relativity. Basically, there are two serious problems in the general relativity, the lack of field equation under the gravity and the assumption of the principle of equivalence.

9.1.1 Field Equation of Gravity

When one wishes to write the Dirac equation for a particle under the gravitational interac- tion, then one faces to the difficulty. Since the Dirac equation for a hydrogen-like atom can be written as (

−i∇ ·α + mβ − Ze 2

r

) Ψ = EΨ (9.2a)

one may write the Dirac equation for the gravitational potentialV (r) = −G0mMr as ( −i∇ ·α + mβ − G0mM

r

) Ψ = EΨ. (9.2b)

But there is no foundation for this equation. At least, one cannot write the Lagrangian density which can describe the Dirac equation for the gravitational interaction. This is clear since one does not know whether the interaction can be put into the zero-th component of a vector type or a simple scalar type in the Dirac equation. That is, it may be of the following type [

−i∇ ·α + (

m− G0mM r

) β

] Ψ = EΨ. (9.2c)

In fact, this is a right Dirac equation for a particle in the gravitational potential.

9.1.2 Principle of Equivalence

The theory of general relativity is entirely based on the principle of equivalence. Namely, Einstein started from the Gedanken experiment that physics of the two systems (a system under the uniform external gravity and a system that moves with a constant acceleration) must be the same. This looks plausible from the experience on the earth. However, one can easily convince oneself that the system that moves with a constant acceleration cannot be defined properly since there is no such an isolated system in a physical world. The basic problem is that the assumption of the principle of equivalence is concerned with the two systems which specify space and time, not just the numbers in connection with the acceleration of a particle. Note that the acceleration of a particle is indeed connected to the gravitational acceleration,̈z = −g, but this is, of course, just the Newton equation. Therefore, the principle of equivalence inevitably leads Einstein to the space deformation. It is clear that physics must be the same between two inertia systems, and any assumption which contradicts this basic principle cannot be justified at all.

9.1. Problems of General Relativity 171

Frame or Coordinate Transformation

Besides, this problem can be viewed differently in terms of Lagrangian. For the system under the uniform external gravity, one can write the corresponding Lagrangian. On the other hand, there is no way to construct any Lagrangian for the system that moves with a constant acceleration. One can define a Lagrangian for a particle that moves with a constant acceleration, but one cannot write the system (or space and time) that moves with a constant acceleration.

Physics in one inertia frame must be equivalent to that of another inertia frame, and this requirement is very severe. It is not only a coordinate change of space and time with Lorentz transformation, but also physical observables must be the same between two systems. In this respect, the principle of equivalence violates this important condition, and therefore, it is very hard to accept the assumption of the principle of equivalence even with the most modest physical intuition.

9.1.3 General Relativity

Einstein generalized the Poisson type equation for gravity

∇2φg = 4πG0ρ

to the tensor equations which should have some similarities with the Maxwell equations. Therefore, he had to find some tensor quantity like the field strengthFµν of the electro- magnetic field, and the metric tensorgµν is chosen as the basic tensor field since he started from the principle of equivalence. Thus, the general relativity is the equation for the metric tensorgµν which, he believed, should be connected to the gravitational fieldφg. By noting that

g00 ' 1 + 2φg c2

together with T00 ' ρ

with the energy momentum tensor ofTµν , he arrived at the equation of the general relativity

Rµν − 1 2

gµνR = 8πG0Tµν ,

whereRµν denotes the Ricci tensor which can be described in terms of the metric tensor gµν . However, the physical meaning of thegµν is unclear, and that is the basic problem of the general relativity.

Mathematics vs Physics

It should be important to note that Einstein’s equation is mathematically complicated, but physically it is just simple. First, we should understand the physics of the Poisson type equation, and the equation is to determine the gravitational fieldφg when there is a matter

172 Chapter 9. Quantum Gravity

field densityρ. Just in the same way, Einstein’s equation can describe the behavior of the metric tensorgµν when there is the energy-momentum tensorTµν which should be generated by the matter field densityρ. However, the Poisson type equation is unfortunately insufficient to determine the gravitational interaction with the matter field since one has to know in which way the matter field should be influenced by the gravitational field, and this should be determined by the equation for the matter field like the Dirac equation. In the same way, Einstein’s equation is insufficient to find the interaction with the matter field. In addition, the energy-momentum tensorTµν cannot be defined well unless one has a good field theoretical picture of fermions. In this respect, it is most important to find the Lagrangian density which includes the gravitational field interacting with the fermions, and this will be explained in the next section.

Before going to the discussion of the Lagrangian density of the gravity, it should be important to clarify the origin of the coordinatexµ in the metric tensorgµν(x) from where it is measured. From Einstein’s equation, it is clear that the origin of the coordinate should be found in the matter field center. Therefore, one can see that the metric tensorgµν should be in contradiction with the principle of special relativity since its space and time become different from the space and time of the other inertia system. This peculiar behavior of the metric tensorgµν is just the result of the principle of equivalence which is not consistent with the principle of special relativity as discussed above.

9.2 Lagrangian Density for Gravity

We should start from constructing the quantum mechanics of the gravitation. In other words, we should find the Dirac equation for electron when it moves in the gravitational potential. In this chapter, we present a model Lagrangian density which can describe electrons inter- acting with the electromagnetic fieldAµ as well as the gravitational fieldG.

9.2.1 Lagrangian Density for QED

We first write the Lagrangian density for electrons interacting with the electromagnetic field Aµ as given in eq.(5.1)

Lel = iψ̄γµ∂µψ − eψ̄γµAµψ −mψ̄ψ − 14 FµνF µν , (9.3)

where Fµν = ∂µAν − ∂νAµ.

This Lagrangian density of QED is best studied and is most reliable in many respects. In particular, the renormalization scheme of QED is theoretically well understood and is experimentally well examined, and there is no problem at all in the perturbative treatment of QED. All the physical observables can be described in terms of the free Fock space terminology after the renormalization, and therefore one can compare any predi