
New edges V
T
A (and V
T
B, V
T
C, V
T
D by symmetry). Place the regular tetra-
hedron at A = (1, 1, 1), B = (1, −1, −1), C = (−1, 1, −1), D = (−1, −1, 1) (edge
L
0
= 2
√
2, centroid V
T
= (0, 0, 0), r
0
=
√
3). Exactly three sub-tetrahedra share edge
V
T
A: {V
T
, A, B, C}, {V
T
, A, B, D}, and {V
T
, A, C, D}. For sub-tet {V
T
, A, B, C}, the
faces at edge V
T
A are (V
T
, A, B) and (V
T
, A, C). Their outward normals, projected per-
pendicular to the edge direction (A−V
T
)/|A−V
T
| = (1, 1, 1)/
√
3, are found to be parallel
to (0, 1, −1)/
√
2 and (−1, 0, 1)/
√
2 respectively; their inner product is −1/2, giving dihe-
dral angle arccos(−1/2) = 120
◦
. By the three-fold symmetry of {A, B, C} about the axis
V
T
A, all three sub-tetrahedra contribute 120
◦
. Total: 3 × 120
◦
= 2π, deficit δ
V
T
A
= 0.
Old edges AB (and all six original edges by symmetry). Two sub-tetrahedra
share edge AB: {V
T
, A, B, C} and {V
T
, A, B, D}. For {V
T
, A, B, C}, the faces at AB are
(V
T
, A, B) with normal proportional to (0, 1, −1) and (A, B, C) with normal proportional
to (1, 1, −1). Both normals are already perpendicular to AB (verified: each has zero
dot product with B − A = (0, −2, −2)). Their inner product is 2/
√
6, giving dihedral
arccos(2/
√
6). The same result holds for {V
T
, A, B, D} by symmetry. The sum from
both sub-tetrahedra is 2 arccos(2/
√
6). Algebraically, cos
2 arccos(2/
√
6)
= 2(2/
√
6)
2
−
1 = 4/3 − 1 = 1/3, so 2 arccos(2/
√
6) = arccos(1/3) exactly—the original tetrahedron’s
contribution, unchanged. Together with the two unchanged octahedral contributions
arccos(−1/3) each, the total around edge AB is still 2π: δ
AB
= 0.
Elastic relaxation. After insertion, the surrounding FCC sites relax outward. Each
original FCC bond (not a new Tvoid bond) is enclosed by exactly four cells—two tetra-
hedra and two octahedra—forming a topologically closed ring homeomorphic to S
1
. The
total dihedral angle around a closed S
1
ring is topologically fixed at 2π under any defor-
mation that preserves the cell connectivity. Elastic relaxation preserves connectivity, so
δ
e
= 0 at every original FCC edge after relaxation. For the new Tvoid edges, the 120
◦
result above used only the position of V
T
at the centroid of ABCD; elastic relaxation
of the surrounding lattice does not move V
T
(it is constrained by the four Tvoid bonds
symmetrically), so the new-edge dihedrals remain 120
◦
each.
The theorem has a direct physical consequence: SSM matter does not curve the dis-
crete FCC lattice. Curvature appears only in the continuum limit.
3.4 Continuum limit: the SSM Einstein equation
In the continuum limit, the Tvoid bond prestress distorts the local metric by h
µν
∼ Gm/r
(elastic Green’s function of a point source with coupling G). Varying the total action
S = S
Regge
+ S
matter
with respect to FCC vertex positions, and using the Schl¨afli identity
∂S
Regge
/∂ℓ
e
= δ
e
, gives the discrete Einstein equation at each interior vertex:
X
e
∂ℓ
e
∂x
i
δ
e
16πG
=
X
b
k(ℓ
b
− L
0
)
∂ℓ
b
∂x
i
, (13)
whose left-hand side is the discrete Ricci tensor and whose right-hand side is the Tvoid
stress-energy. In the continuum limit this becomes
G
µν
= 8πG T
µν
, (14)
with the same G = a
2
/(8 ln 2) throughout, and T
00
= m δ
3
(x − x
0
) for a static Tvoid at
x
0
.
6