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Galaxies 2018, 6(3), 81; https://doi.org/10.3390/galaxies6030081
Renormalizable Gravitational Action That Reduces to General Relativity on the Mass-Shell
System of Systems Analytics, Inc., 11250 Waples Mill Road, Suite 300, Fairfax, VA 22030-7400, USA
Received: 10 July 2018 / Accepted: 27 July 2018 / Published: 30 July 2018
We derive the equation that relates gravity to quantum mechanics: , where R is the scalar curvature, G is the gravitational constant, c is the speed of light and is the Standard Model Lagrangian, or its future replacement. Implications of this equation are discussed in the paper. In particular, we show (in the last section) that this equation is the transformation that relates four-dimensional physics to two-dimensional physics.
Keywords:scalar curvature; renormalizability
General Relativity (GR) is described by the following combined action1, called the Einstein–Hilbert action, , where we link the Lagrangian of the Standard Model of particle physics () to GR (using the (−+++) metric signature, with ):
In Equation (1), the constant . This action is non-renormalizable [1,2] in four-dimensional space-time. For the purposes of this paper then, we discard and replace it with a new action . In considering a replacement for , we require that the classical General Relativity equations of motion are maintained; only the off-mass-shell behavior can be modified. This requires that the high energy momentum components from the gravity Lagrangian be damped. Wilson  has shown us how to do that by introducing a smooth cutoff function in the denominator of the term in which we are interested. This program was carried through by Igarashi, Itoh and Sonoda , who showed that different choices of the smooth cutoff function merely amount to reparameterization and renormalization of the fields.
The only available is , so the ratio must be present. Finally, this ratio cannot appear in a polynomial, which would vanish when gravity is neglected, thereby giving the exponential form:where Q is a constant and (shown below). The subscript ‘NP’ designates ‘non-polynomial’. The replacement has profound implications for physics, which this paper discusses.
Donoghue  describes the present arguments that General Relativity is a perfectly acceptable effective field theory. Understanding this to mean that low energy gravity must be renormalizable, the search is then on to find the coupled-matter-gravity action that replaces . That this approach could lead to valuable results is emphasized by the work of Halpern and Huang , who showed that non-polynomial Lagrangians have surprising new physics.
2. Equations of Motion with Noether’s Theorem
The coupling of gravity and matter in prohibits the ability to add four-vector divergences to either gravity or matter, because the divergence theorem is no longer applicable. The action is stationary for fields that are solutions of the actual equations of motion; in other words, the variation is over those fields that satisfy the symmetries of the problem. This allows non-polynomial Lagrangians, if desired2. The extremum condition is:
In Equation (3), denotes the variation over the fields in . Note that and R do not involve3 the variation associated with . The individual variations are independent of each other and separately are extrema.
2.1. Gravitational Equations of Motion
The gravitational equations of motion are determined by:
In contemplating what this may reduce to, we consider the fact that , as a classical field theory, satisfies all macroscopic experimental data tests. Thus, we would expect that has the same classical behavior as , but different quantum behavior. In particular, can be a renormalizable action that will be discussed in Section 3. Equation (4) reduces to:
We do each term separately.
However, the variations must satisfy the symmetry properties of GR, which is4 . Therefore:
Working out the remaining terms:and:
In Equation (9), is the stress-energy tensor. Putting these altogether gives:
The arbitrary variation giving zero requires that the complete term in parenthesis must be zero, producing:
Now, the conservation of energy-momentum from the tensor requires:
We now can evaluate the two constants using the technique in , by requiring that the gravitational interaction reduces to the static Newtonian potential. Operating on Equation (11) by gives:where5 . Noting that all the R in Equation (13) are on the mass-shell (equation of motion), we momentarily drop the designation and have:
In order to satisfy Newton’s law of gravitation, the equation for the scalar curvature must reduce, in that particular case, to , so the constants are and . Thus, the central equations for the non-polynomial action are:with the equation of motion:
2.2. Matter Equations of Motion
The other extremum from Equation (3) is:
We work this out for a scalar field to demonstrate that the non-polynomial action is a viable action due to the symmetry properties of the variations, which gives rise to a zero four-vector divergence (Noether’s theorem). Equation (17) becomes (for a single field):
From the previous section:
The term for a single scalar field is:
We use the identity:
Equation (20) becomes:
However, the variations satisfy the symmetries of the Lagrangian and:without requiring integration by parts or the divergence theorem (Noether’s principle again). Because the integrand in Equation (18) involves a never-zero exponential7 and a factor, the terms multiplying must vanish, giving the equation of motion:
3. Renormalizability of
Before calculations are presented, we want to explain in an intuitive manner why and should differ when quantum fluctuations are taken into account. Firstly, Equation (15) shows that the scalar curvature is renormalized using the theorem in , which was derived in finite-temperature field theory for the thermodynamic potential . Secondly, the deleterious effects of having a coupling constant G with units is ameliorated by in the denominator of the exponential of . Radiative corrections will give rise to new, additional operators in the Hamiltonian: if their coefficients8 are finite, we say we are dealing with a renormalizable action; if the coefficients are infinite, the action becomes non-renormalizable. Thus, the statement that is renormalizable means that all the radiative corrections are finite after normal mass, charge and wavefunction renormalizations are performed. We now go into the details.
3.1. Renormalization of the Scalar Curvature R
The inputs to a gravitational calculation are the stress-energy tensor, , with boundary conditions. The Lagrangian in Equation (15) is used to calculate the components of , and Equation (16) gives the solution for the Einstein tensor. R in finite-temperature quantum field theory is given by vacuum diagrams. The thermodynamic potential in finite-temperature quantum field theory is renormalized by the theorem9 of :where B and R refer to bare and renormalized quantities, , refer to the collection of coupling constants and masses and T, refer to temperature and the various chemical potentials. When quantum corrections are derived for R, the renormalized10 scalar curvature is:
From Equation (26), we see immediately that the various condensates (Higgs, chiral, etc.) associated with phase transitions do not contribute to 11.
The finite and tiny cosmological constant has an unknown origin at the present time. To illustrate Equation (15), we give a toy model for : a condensation of a sterile neutrino of mass that fills up positive energy levels in the Universe to Fermi momentum . The Fermi momentum replaces the chemical potential in Equation (26). From Equation (15):where is the mass density. The mass density of such a neutrino is given by Equation (10) in .where .
The relationship between and is:
Then, for this toy model:
A value of /m is an approximation, . If we arbitrarily assign a value of = 10 meV (0.01 eV), then x =
3.2. Finite Radiative Corrections to
We do background gauge field quantization: by expanding locally, the short distance behavior is always flat space for a smooth manifold. Following , leads to and in Equation (2), letting :
The term in Equation (31) acts like a smooth momentum cutoff function, as long as itself is renormalizable12. Thus, the matter Lagrangian plays a critical role in the renormalization of gravity. This cutoff behavior results in an effective re-scaling of the metric:where is a finite quantity, after the standard mass, coupling constant and wavefunction renormalizations are performed.
4. General Implications of
4.1. Comment on the Black Hole Information Paradox
The black hole information paradox refers to the Hawking radiation being emitted by a black hole13. If the photon radiation is truly thermal, it cannot carry away information14. The radiation need not be photons. A positron-electron pair can be created outside a black hole, and one Fermion traverses the event horizon, while the other goes to infinity. The black hole has then “eaten up” information because ostensibly the scalar curvature R just records an energy density/angular momentum change, which could have been any particles. However, Equation (15) changes this situation. Instead of an energy density change, the Lagrangian density changes. This quantity carries with it all the symmetries of matter, including the Fermion number. Equation (15) is a ledger inside the black hole, even if no experiment can do an audit. A detailed calculation has to be implemented on the eventual end process of black hole evaporation, but the back-reaction is now just an elementary particle recoil, with all symmetries of having to be conserved.
4.2. Implications for the Matter Lagrangian
The QCD (Quantum Chromodynamics) Lagrangian piece of has a possible effective Lagrangian , which is:where and are the familiar gluon color matrix tensor and color dual-field matrix tensor, respectively, and means the trace in color space. The extra piece is the four-divergence of a vector. If is being used, the Divergence Theorem can be invoked, and then, non-zero surface terms from instantons can lead to this term contributing to quantum processes. If instead, is used, then the extra -piece stands on its own depending on experimental data determining the value of the CP (charge conjugation x Parity) -violating parameter . Using means just another parameter , but no -vacuum, since instantons are no longer relevant in the presence of this gravity-matter coupling.
An interesting possibility is the following: starting with , compute the radiative corrections for the confining QCD ground state to see if is the minimum. This would be the normal QCD ground state that conserves CP. However, the excited ground state of freed quarks may have a different minimum . The latter calculation can be done in a perturbative manner. This is the suggestion put forward by  to explain CP violations in the early Universe.
4.3. Reduction of Quantum Mechanical Probability Amplitudes
Feynman  shows that the quantum mechanical amplitude is the sum over all paths, where each path has a phase proportional to the action S:where is a constant. Using Equation (15), the dominant contribution to can be evaluated directly. With , , and the definition of the Planck length :
Equation (34) reduces to:
Now, is the scalar curvature per unit area, and we conclude that the probability amplitude is the integration over space-time of the scalar curvature per unit area. The inescapable conclusion is that quantum mechanical information resides on two-dimensional surfaces. This is a concrete realization of the ‘holographic principle’ .
This paper discards the historical Einstein–Hilbert action and replaces it with a coupled gravity-matter non-polynomial interaction. The renormalizability of the scalar curvature follows the same mathematics as the renormalizability of the thermodynamic potential in finite-temperature quantum field theory. Radiative corrections to are finite due to the high momentum cutoff from the renormalized matter fields in . This leads to an effective finite metric tensor. Many details still remain to be worked out, including the calculation of the Hawking radiation, with the constraint now imposed by Equation (15).
This research received no external funding.
Conflicts of Interest
The author declares no conflict of interest.
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We discuss the cosmological constant in Section 3.1 below.
No integration by parts is required.
This is true even though the gravitational field has matter fields as sources.
This is a four-vector divergence (see for example ), which is zero by Noether’s principle.
In the static limit.
The scalar curvature can be positive or negative, consistent with Equation (15).
These being after having done the mass, charge and wavefunction renormalizations.
Here, this is written for on-mass-shell renormalization, so no Euclidean subtraction momentum squares appear.
The set of coupling constants includes Newton’s constant G.
This is because these condensates are present under the conditions and so cancel from the two terms on the left-hand side of Equation (26).
As shown in , appears in the numerator of the renormalization group flow for changes of renormalization scales, where t is a renormalization ‘time’ and as decreases.
For a review, see .
For example, it cannot radiate polarized light.
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