In the beginning of 20th century two new theories appeared, that radically changed our understanding of the physical world. In 1916 Einstein introduced general relativity, which was to replace Newton's theory of gravitation. General relativity was not only able to explain terrestrial gravity and the motion of astronomical bodies, as Newton's theory, but it also made exciting new predictions such as the bending of light rays by the gravitational field, the expansion of the universe, and the existence of black holes. Most of these predictions have by now been verified. The reason why Einstein's theory brought a paradigm shift in physics is that it no longer attributed gravitational attraction to a force between masses. Instead, it proposed that spacetime, a 4-dimensional continuum that combines 3-dimensional space with time, can be curved and that this is the cause for gravitational acceleration.

In the same period the scientific community was beginning to realize that matter at atomic and sub-atomic scales exhibits unexpected behaviour and that physical quantities associated to it appear to change in discrete amounts, referred to as quanta. The precise description of matter at these scales required the introduction of a new theory, quantum theory, which was pioneered by Heisenberg, Born, Bohr and Pauli. Quantum theory predicted a series of new effects, collectively termed quantum effects. Quantum effects have been detected in various experiments and nowadays the applications of quantum theory range from astrophysics to electronic engineering.

Quantum theory and general relativity set the foundations of modern physics, but at the same time they caused a divide. The former predicts and explains quantum effects but ignored spacetime curvature (and hence gravity), whereas the latter describes gravity but ignores quantum effects. There are regimes, such as the very early universe or the vicinity of black holes, in which both gravity and quantum effects are important. Therefore, the description of physical processes taking place in these regimes requires a new theory, which manages to merge quantum theory and general relativity. This theory, dubbed "quantum gravity", holds the answers to pertinent questions, such as the origin and the final fate of our universe.

One of the main reasons for which progress is quantum gravity has not been as fast as we would have hoped is precisely the fact that the regimes in which it is relevant are particularly hard to access experimentally and observationally. Thankfully, this lack of experimental guidance can be to some extend compensated by the careful analysis of "analogue systems". These are easy-to-access physical systems which can be studied in the laboratory and manage to mimic aspects of quantum gravity. For example the study of waves traveling on the surface of water draining in a bathtub sheds light into the way black holes lose mass and angular momentum. What is suggested here is a combined theoretical and experimental exploration of analogue systems. In particular, we propose to (i) investigate theoretical aspects of well-established analog systems, and attempt to identify new such systems; (ii) set up and conduct table top experiments involving some this systems; (iii) combine the output of the theoretical and experimental exploration to advance our understanding of quantum gravity effects and firm steps towards a concrete theory of quantum gravity.

## about **Quantum Gravity** LABORATORY

Black holes, accessible regions of no escape surrounded by an event horizon. From an astrophysical point of view it is essential to study rotating black holes, since any realistic gravitational collapse is not spherically symmetric, and therefore leads to the formation of a black hole with non-zero angular momentum. Rotating black holes (a) exhibit an ergosphere, that is a spacetime region where the angular velocity of the rotating black hole is high enough to “drag the surrounding space along with the velocity of light”. A bathtub vortex flow (b) can be used to mimic some of the effects predicted to arise in the vicinity of rotating black holes. (For more information on analogue rotating black holes __click here__.)