The world of quantum physics is usually associated with the microscopic cosmos of atoms and photons. In principle, but so far without any demonstration, even heavy objects can exhibit the distinguished properties of quantum world particles. In 1935, Einstein, Podolsky and Rosen (EPR) challenged a particular prediction of quantum theory saying that two particles can exist in a so-called entangled state in which the two particles do not have individually defined (‘local’) positions and momenta. Most interestingly, the existence of entangled states was subsequently fully confirmed in experiments with photons and atoms.The new project MassQ aims to test and to confirm quantum theory in the macroscopic world of massive, human-world sized objects by realizing an EPR entanglement experiment with heavy mirrors. Two kg-sized mirrors will be cooled to low temperature and their centre of mass motion driven by radiation pressure of intense laser light in such a way that the mirrors will lose their individually defined positions and momenta. As a result, their joint motion will form a unified massive quantum object.This project will realize a fundamental test of quantum theory in the so far unexplored regime of human-world sized objects. Recent advances in gravitational wave detector research and in opto-mechanics make this project feasible. The vision of this project points even further into the future. This project aims to lay the basis for a completely new class of physics experiments. Mirrors with kilogram masses have a proper gravitational field and cause a space-time curvature in their vicinity. This way, in principle, the dynamics of two heavy entangled mirrors need to be described not only by quantum theory but also by general relativity. Today it is completely unclear what the results of such a new class of physics experiments will be. Undoubtedly, they are important to illuminate the deep connection between the two most successful theories in physics.