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This closes our short introduction to TG. Like all classi-
cal theories, whose form is established, we can put TG into a
Tonti type diagram [56] in order to clearly display its struc-
ture. This will be done for the first time in Sect. 2. Then
we turn to a closer examination of the constitutive tensor of
TG. In Sect. 3, we decompose it into smaller pieces, in par-
ticular into the irreducible pieces with respect to the linear
group GL(4, R). In Sect. 4, metric dependent constitutive
tensors will be addressed, in particular those which relate
TG to general relativity. In Sect. 5, we will study the prop-
agation of gravitational waves in TG within the geometric
optics approximation. We will follow the procedure that we
developed for electrodynamics [2,22,29]. Since in TG we
have four generators of the gauge group, things become a
bit more complicated than in electrodynamics. In Sect. 6,we
will specialize these considerations on gravitational waves to
metric dependent models.
2 Tonti diagram of the premetric teleparallel theory of
gravity
Over the past decades, Tonti [56] developed a general clas-
sification program for classical and relativistic theories in
physics, such as, e.g., for particle dynamics, electromag-
netism, the mechanics of deformable media, fluid mechan-
ics, thermodynamics, and gravitation. Here we will display in
Fig. 1 for the first time an appropriate and consistent diagram
of the teleparallel theory of gravity (TG).
If a theory is well-understood, its configuration and its
source variables can be clearly identified and their interrela-
tionships displayed in the form of a Tonti diagram. Such a
diagram defines what one may call the skeleton of a theory.
In Tonti’s book [56], for all classical theories, including the
relativistic ones, a corresponding framework was established
– and this step by step, based on an operational definition of
the quantities involved.
Tonti [56, p. 402] has also displayed a diagram for rela-
tivistic gravitation. In Tonti’s own words, it was an “attempt”
of a diagram based on an ansatz for a tetrad theory of gravity
by Kreisel and Treder, see [57, pp. 60–67, pp. 71–91]. Due
to our enhanced knowledge of TG, see [8, Chapters 5 and
6], we can now improve on Tonti’s attempt, see [23] and our
Fig. 1. The notation in Fig. 1 is based on our recent paper
[31] and on the present one.
Let us have a look at our new diagram. The configuration
variables of TG are the coordinates x
i
of the 4-dimensional
spacetime (four 0-forms) and the coframes ϑ
α
(four 1-forms).
By differentiation, we find the torsion F
β
(four 2-forms) and
by further differentiation the homogeneous field equation of
gravity dF
β
= 0 (four 3-forms), the right hand side of which
vanishes.
The round boxes on the left column depict geometrical
objects and the square boxes interrelate these geometrical
objects. The formula dF
β
= 0, for example, corresponds
to the first Bianchi identity of a teleparallel spacetime. In
a Riemann–Cartan space, we have DF
β
= ϑ
α
∧ R
α
β
.In
teleparallelism, the curvature vanishes, R
α
β
= 0, and, in the
teleparallel gauge, dF
β
= 0.
For global variables, Tonti distinguishes spatial domains,
such as volumes V, surfaces S, lines L, and points P, with
respect to time he introduces instants I and intervals T. Fur-
thermore, for the respective domains, he has inner (interior)
and outer (exterior) orientation.
Hence [
I ×
S], for example, refers to a time domain
I
with exterior orientation and a space domain
S with interior
orientation. And this domain [
I ×
S] supports the torsion
two-form. The holonomic coordinates x
j
, to take another
example, depend on an instant of time I and a spatial point
P. i.e. [I × P]. Then we have to add the orientation. All
exterior forms on the left columns are forms without twist,
see [22], those on the right columns all carry twist. This can
be read off from the corresponding orientations. This comes
about as follows.
For configuration variables the associated space elements
are endowed always with an interior orientation, whereas
in the case of source variables it is the exterior orientation
which plays a role. This behavior is found by phenomenolog-
ically examining the different theories. According to Tonti,
the underlying theoretical reason for this correspondence is
not clear, but phenomenology does not allow any other attri-
butions. The configuration variables are related to the theory
of chains of algebraic topology, whereas the source variables
are associated to co-chains. For more details, we refer to the
exhaustive monograph of Tonti [56].
Since the gravitational field itself carries energy–momen-
tum, it is also the source of a new gravitational field, which
likewise carries energy–momentum, etc. Thus, like general
relativity, TG is an intrinsically nonlinear theory, even it car-
ries a linear constitutive law. Within the field equation of TG,
dH
α
−
(ϑ)
Σ
α
=
(m)
Σ
α
, the gravitational energy-momentum
3-form
(ϑ)
Σ
α
:=
1
2
[F
β
∧ (e
α
H
β
) − H
β
∧ (e
α
F
β
)] (25)
shows up explicitly and is a manifestation of this nonlinearity.
By differentiation of the field equation, we find
dd H
α
= 0 = d
(ϑ)
Σ
α
+
(m)
Σ
α
. (26)
Thus, d
(m)
Σ
α
=−d
(ϑ)
Σ
α
is nonvanishing in general, which
clearly shows up in our Tonti diagram.
The Tonti diagram displayed in Fig. 1 was constructed for
the premetric version of teleparallelism, no metric is involved
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