AbstractWe consider the case of scattering by several obstacles in $${\mathbb {R}}^d$$
R
d
, $$d \ge 2$$
d
≥
2
for the Laplace operator $$\Delta $$
Δ
with Dirichlet boundary conditions imposed on the obstacles. In the case of two obstacles, we have the Laplace operators $$\Delta _1$$
Δ
1
and $$\Delta _2$$
Δ
2
obtained by imposing Dirichlet boundary conditions only on one of the objects. The relative operator $$g(\Delta ) - g(\Delta _1) - g(\Delta _2) + g(\Delta _0)$$
g
(
Δ
)
-
g
(
Δ
1
)
-
g
(
Δ
2
)
+
g
(
Δ
0
)
was introduced in Hanisch, Waters and one of the authors in (A relative trace formula for obstacle scattering. arXiv:2002.07291, 2020) and shown to be trace-class for a large class of functions g, including certain functions of polynomial growth. When g is sufficiently regular at zero and fast decaying at infinity then, by the Birman–Krein formula, this trace can be computed from the relative spectral shift function $$\xi _\mathrm {rel}(\lambda ) = -\frac{1}{\pi } {\text {Im}}(\Xi (\lambda ))$$
ξ
rel
(
λ
)
=
-
1
π
Im
(
Ξ
(
λ
)
)
, where $$\Xi (\lambda )$$
Ξ
(
λ
)
is holomorphic in the upper half-plane and fast decaying. In this paper we study the wave-trace contributions to the singularities of the Fourier transform of $$\xi _\mathrm {rel}$$
ξ
rel
. In particular we prove that $${\hat{\xi }}_\mathrm {rel}$$
ξ
^
rel
is real-analytic near zero and we relate the decay of $$\Xi (\lambda )$$
Ξ
(
λ
)
along the imaginary axis to the first wave-trace invariant of the shortest bouncing ball orbit between the obstacles. The function $$\Xi (\lambda )$$
Ξ
(
λ
)
is important in the physics of quantum fields as it determines the Casimir interactions between the objects.