Gravitational waves are simply the solutions of the linearized Einstein equations, that is, the solution of the Einstein equations in weak gravitational fields or, equivalently, in spacetimes that are 'almost flat'. In this respect, they are not very different from the sound waves we hear when we listen to music or from the electromagnetic waves we receive when looking at the screen of our cellphone. However, it is not pressure or electromagnetic fields that are propagated, but rather the very same curvature we have encountered already. Hence, gravitational waves represent the propagation at the speed of light of small ripples in the curvature in spacetime. This chapter will make use of simple mechanical analogues to explain how gravitational waves are produced every time a mass or energy is set in motion, and how the amplitude of gravitational waves produced on Earth can only be extremely small.

Photon transduction is a fundamental process of any optical detector or image sensor where the basic task is to estimate an average quantity of photons versus time and/or space. We start from basic physical phenomena of the optical transduction considering photon flux as an average quantity, disregarding the quantum mechanics characteristics of a single photon. Then, we investigate the role of noise in the transduction process to better assess design rules in electronic design of interfaces. As in the other transduction chapters, we treat only a very small part of existing optical sensor implementations to serve as examples of the application of the transduction principle.