A unifying concept in physics: Reciprocity

In physics, we have many different branches. For example, the theory of elasticity, thermodynamics, quantum mechanics and general relativity are all constructed on different mathematical and physical frameworks. They apply to different scales and they refer to different mechanisms that explain our physical world. In this talk, I will refer to a concept which applies to all of the aforementioned branches of physics. This common property is called “reciprocity”. Loosely speaking, it refers to the fact that the measurement outcome of a system does not change when the input and the output variables are exchanged. Although it might seem trivial at a first glance, this property signals an important feature of linear systems. Being a relativist, I will mostly focus on the reciprocity theorem of distances in the context of cosmology. We will explore what this means for the current data analysis for observations on the sky.