1. Introduction
Over the past decades, great strides have been made in attempts to combine quantum description of interactions with General Relativity [
1]. There are currently many promising approaches to connecting the Quantum Mechanics and General Relativity, including perhaps the most promising ones: Loop Quantum Gravity [
2,
3,
4], String Theory [
5,
6,
7] and Noncommutative Spacetime Theory [
8,
9].
A lot of work has also been done to clear up some challenges related to General Relativity and
model [
10]. An explanation for the problem of dark energy [
11] and dark matter [
12] is still being sought, and efforts are still being made to explain the origin of the cosmological constant [
13,
14,
15].
The author also tries to bring his own contribution to the explanation of the above physics challenges, based on a recently discovered method, described in [
16]. As this article will show, this method seems very promising and may help clarify at least some of the issues mentioned above. The author’s method, similar to the approach presented in [
17,
18,
19] also points to the essential connections between electromagnetism and Gereral Relativity, however, the postulated relationship is of a different nature and can be percieved as some generalization of the direction of research proposed in [
20,
21,
22] and [
23].
According to conclusions from [
16], the description of motion in curved spacetime and its description in flat Minkowski spacetime with fields are equivalent, and the transformation between curved spacetime and Minkowski spacetime is known, because the geometry of curved spacetime depends on the field tensor. This allows for a significant simplification of research, because the results obtained in flat Minkowski spacetime can be easily transformed into curved spacetime. The last missing link seems to be the quantum description.
In this article, the author will focus on developing the method proposed in [
16] in such a way, as to obtain the convergence of the description with the description of quantum mechanics. In the first chapter, the Lagrangian density for the system will be derived, allowing to obtain the tensor described in [
16]. These conclusions will be used later in the article to propose quantum description of the system.
The author uses the Einstein summation convention, metric signature
and commonly used notations. In order to facilitate the analysis of the article, the key conclusions from [
16] are quoted in the subsection below.
1.1. Short summary of the method
According to [
16], stress-energy tensor
for a system with electromagnetic field in a given spacetime, described by a metric tensor
is equal to
where
is for rest mass density and
where
represents electromagnetic field tensor, and where the stress–energy tensor for electromagnetic filed, denoted as
may be presented as follows
Thanks to the proposed amendment to the continuum mechanics, in flat Minkowski spacetime occurs
thus denoting four-momentum density as
, total four-force density
acting in the system is
Denoting rest charge density in the system as
and
electromagnetic four-current
is equal to
The pressure p in the system is equal to
In the flat Minkowski spacetime, total four-force density
acting in the system calculated from
is the sum of electromagnetic (
), gravitational (
) and the sum of remaining (
) four-force densities
As was shown in [
16], in curved spacetime (
) presented method reproduces Einstein Field Equations with an accuracy of
constant and with cosmological constant
dependent on invariant of electromagnetic field tensor
where
appears to be metric tensor of the spacetime in which all motion occurs along geodesics and where
describes vacuum energy density.
It was also shown, that in this solution, Einstein tensor describes the spacetime curvature related to vanishing in curved spacetime four-force densities .
The proposed solution allows to add additional fields while maintaining its properties.
2. Lagrangian density for the system
Since for the considered method the transition to curved spacetime is known (based on electromagnetic field tensor), the rest of the article will focus on the calculations in the Minkowski spacetime with presence of electromagnetic field, where represents Minkowski metric tensor.
Using a simplified notation
it can be seen that the four-force densities resulting from the obtained stress-energy tensor (
12) in flat Minkowski spacetime can be written as follows
where
can also be represented in terms of electromagnetic four-potential and four-current. This means that to fully describe the system and derive the Lagrangian density, it is enough to find an explicit equation for the gravitational force or some gauge of electromagnetic four potential.
Referring to definitions from
Section 1.1 one may notice, that by proposing following electromagnetic four-potential
one obtains electromagnetic four-force density
in form of
where
is electromagnetic four-current and where Minkowski metric property was utilized
Four-force densities acting in the system may be now described by the following equality
Comparing (
15) and (
17) it is seen, that introduced electromagnetic four-potential yields
which is equivalent to imposing following condition on normalized stress-energy tensor
and what yields gravitational four-force density in Minkowski spacetime in form of
Now, one may show, that proposed electromagnetic four-potential leads to correct solutions.
At first, recalling the classical Lagrangian density [
24] for electromagnetism one may show why, in the light of the conclusions from [
16] and above, it does not seem to be correct and thus makes it difficult to create a symmetric stress-energy tensor [
25]. The classical value of the Lagrangian density for electromagnetic field, written with the notation used in the article, is
In addition to the obvious doubt that by taking the different gauge of the four-potential
one changes the value of the Lagrangian density, one may notice, that with considered electromagnetic four-potential, such Lagrangian density is equal to
As it is seen, above Lagrangian density is not invariant under gradient over four-position and
and
are dependent, what is not taken into account in classical calculation
One may decompose
and since
are constants, one may simplify (
26) to
Above yields
which leads to the conclusion that
acts as the Lagrangian density for the system
which would support conclusions from [
26] and what yields stress-energy tensor for the system in form of
In fact, the proof of correctness of the electromagnetic field tensor (noted as
) allows to see this solution
what yields following property of electromagnetic field tensor
Using the above substitution, one may note
Therefore the invariance of
with respect to
and
is both a condition on the correctness of the electromagnetic stress-energy tensor and on
in the role of Lagrangian density
what yields for (
34) that
Equations (
1), (
6) and (
31) yield
what yields second representation of the stress-energy tensor
After four-divergence, it gives additional expression for relation between forces and gives useful clues about the behavior of the system when transitioning to the description in curved spacetime.
3. Hamiltonian density and quantum picture
Noting Hamiltonian density as
, from (
31) one gets
As it is seen, above Hamiltonian density agrees with the classical Hamiltonian density for electromagnetic field [
27] except that this Hamiltonian density was currently mainly considered for sourceless regions. According to the result above, this Hamiltonian density describes also gravitational interaction and other forces resulting from electromagnetism. Above, therefore, significantly simplifies quantum field theory equations [
28,
29,
30].
At first one may notice, that in transformed (
31)
first row of electromagnetic stress-energy tensor
is a four-vector, representing energy density of electromagnetic field and Poynting vector [
31] - the Poynting four-vector. Therefore vanishing four-divergence of the
must represent Poynting theorem. Indeed, properties (
33) and (
35) provide such equality
Next, one may introduce auxiliary variable
with the energy density dimension, defined as follows
and comparing the result
between the two tensor definitions (
31), (
38) one may notice, that it must hold
because the second component of above vanishes contracted with
, due to the property of the Minkowski metric (
18). Therefore (
31) also yields
where
is electric vacuum permittivity. Since
thus from (
44) one easy gets
and thanks to (
44) substituted to (
38) one also obtains
Comparing zero-components of (
45) and (
47) one may notice, that
is a valid solution of the system, what yields
From the analysis of the equation (
45) it may be concluded, that after integration of the
with respect to the volume, the total energy transported in the isolated system should be the sum of the four-momentum and four-vectors describing energy transport related to fields. This would be consistent with the conclusion from [
32] that "equations of motion for matter do not need to be introduced separately, but follow from the field equations". In considered approach it would mean, that the canonical four-momentum density is a part of the stress-energy tensor.
Therefore, by analogy with the Poyting four-vector
, one may introduce a four-vector
understood as its equivalent for the remaining interactions and rewrite (
45) as
where
The above result should not be surprising, because considering the system with electromagnetic field and canonical four-momentum density as part of the stress-energy tensor, actually one of components of canonical four-momentum density should be related to electromagnetic four-potential and charge density. It is also worth to notice, that , due to its properties, may be associated with some description of the spin.
All above also drives to conclusion, that (
50) may act as Lagrangian density, used in the classic relativistic description based on four-vectors.
Finally, one may define another gauge
of electromagnetic four-potential
in following way
and note, that
Four-divergence of
vanishes, therefore (
49) indicates that
what yields
Above brings two more important insights:
may play a role of the density of Hamilton’s principal function,
Hamilton’s principal function may be expressed based on the electromagnetic field only, so in the absence of the electromagnetic field it disappears.
One may now summarize all of the above findings and propose a method for quantizing the system and for the description of the system with the use of classical field theory for point-like particles.
At first, it should be noted, that the above reasoning changes the interpretation of what the relativistic principle of least action means. As one may conclude from above, there is no inertial system in which no fields act, and in the absence of fields, the Lagrangian, the Hamiltonian and Hamilton’s principal function vanish. Since the metric tensor (
5) for description in curved spacetime depends on the electromagnetic field tensor only, it seems clear, that in the considered system, the absence of the electromagnetic field means actually the disappearance of spacetime and the absence of any action.
One may then introduce generalized, canonical four-momentum
as four-gradient on Hamilton’s principal function S
where
One may also conclude from previous findings, that canonical four-momentum should be in form of
where
and where four-momentum
may be now considered as just "other gauge" of
Since in the limit of the inertial system one gets
, therefore, to ensure vanishing Hamilton’s principal function in the inertial system, one may expect that
what also yields vanishing in the inertial system Lagrangian
L in form of
where
is four-force. Eq. (
47) yields
where
and where
vanishes, what comes from
In above picture, the Hamilton’s principal function, generalized canonical four-momentum and Lagrangian vanish for inertial system as expected.
Since
therefore, to ensure compatibility with the equations of quantum mechanics it suffices to assume that
Thanks to above, by introducing quantum wave function
in form of
where
is wave four-vector related to canonical four-momentum
from (
67) one obtains Klein-Gordon equation
which allows for further analysis of the system in the quantum approach, eliminating the problem of negative energy appearing in solutions [
33].
The above representation allows the analysis of the system in the quantum approach, classical approach based on (
40) and the introduction of a field-dependent metric in (
5) for curved spacetime, which connects previously divergent descriptions of physical systems.
4. Conclusions and Discussion
As shown above, the proposed method summarized in
Section 1.1 seems to be very promising area of farther research. In addition to the earlier agreement with Einstein Field Equations in curved spacetime, by imposing condition (
20) on normalized stress-energy tensor in flat Minkowski spacetime, one obtains consistent results, developing the knowledge of the physical system with electromagnetic field:
Lagrangian density for the systems appears to be equal to
Stress-energy tensor may be simplified to familiar form:
acts as canonical four-momentum for the point-like particle
The vanishing four-divergence of turns out to be the consequence of Poynting theorem
Some gauge of electromagnetic four-potential may be expressed as
Gravitational, electromagnetic and sum of other forces acting in the system may be expressed as shown in (
15) where gravitational four-force is dependent on the pressure
p in the system as shown in (
22)
Obtained canonical four-momentum
is equal to
where
is four-momentum, L is for Lagrangian,
due to its properties, seems to be some description of the spin, and where
describes the transport of energy due to the field. It may be calculated as
where
is electromagnetic four-potential and where
is the volume integral of the Poyinting four-vector.
It also seems, that this approach allows to combine previously divergent methods of curvilinear and classic description with the quantum description. The proposed method should facilitate further research on the quantum picture of individual fields, significantly simplifying equations of the Quantum Field Theory and leading in natural way to Klein-Gordon equation (
71).
Further analysis using the variational method may also lead to next discoveries regarding both the theoretical description of quantum fields and elementary particles associated with them, and the possibility of experimental verification of the obtained results.