However, gravitational perturbations from giant planets cause their orbits to change. Owing to mutual gravitational perturbations, the eccentricities of the planetary orbits vary over time. The complex motions of gravitational perturbations can be broken down. Comets are in unstable orbits that evolve over time due to perturbations and outgassing. Analogous statements are known for perturbations of more complicated orbits. Damping of acoustic perturbations or shock waves are non-stationary non-equilibrium processes. The essential spectrum is invariant under compact perturbations. Topological order is robust against any perturbations. Cranial bones can also be useful for detecting evidence of physiological perturbations. Negative feedback tends to promote a settling to equilibrium, and reduces the effects of perturbations. Solar radiation pressure is a source of orbital perturbations. It responds to local mechanical perturbations caused by morphogenetic movements. There are two fundamental types of density perturbations called "adiabatic" and "isocurvature". The explanation of this instability begins with the existence of tiny perturbations in the stream. The B-modes are not produced by standard scalar type perturbations.