Gravitational diffraction radiation

We show that if the visible universe is a membrane embedded in a higher-dimensional space, particles in uniform motion radiate gravitational waves because of spacetime lumpiness. This phenomenon is analogous to the electromagnetic diffraction radiation of a charge moving near to a metallic grating. In the gravitational case, the role of the metallic grating is played by the inhomogeneities of the extra-dimensional space, such as a hidden brane. We derive a general formula for gravitational diffraction radiation and apply it to a higher-dimensional scenario with flat compact extra dimensions. Gravitational diffraction radiation may carry away a significant portion of the particle’s initial energy. This allows to set stringent limits on the scale of brane perturbations. Physical effects of gravitational diffraction radiation are briefly discussed.

Figure 1

Pictorial representation of the braneworld scenario considered in the text. The hidden brane at $z=0$ is identified with the brane at $z=L$. The visible brane is located at $z=h=L/2$. The particle moves on the visible brane along $x$ with constant velocity $v$. Only one transverse coordinate, $y$, is shown. The right panel shows the wrapping around the extra dimension $z$. If the two halves of the cylinder are identified, the Randall-Sundrum model I [8] is obtained.

Figure 2

Pictorial representation of brane perturbations. The longitudinal brane perturbations are periodic in $x$ with length $l$. The transverse perturbations have length $b$. Only one transverse coordinate, $y$, is shown.