Black Hole Superradiance in Rotating Fluids (SURF)

Some of the most fundamental and perhaps bizarre processes expected to occur in the vicinity of black holes are out of observational reach. To address this issue we utilise analogue systems where we study fluctuations on a background flow that in the experiment reproduces an effective black hole. In the literature this line of research is referred to as analogue models for gravity, or simply analogue gravity. Analogue models provide not only a theoretical but also an experimental framework in which to verify predictions of classical and quantum fields exposed to 'extreme' spacetime geometries, such as rapidly rotating black holes. This project brings together two world-wide recognised experts in the field of analogue gravity with the aim of pushing the field in a new direction: we propose ground-breaking studies to mimic some of the bizarre processes occurring in the vicinity of rotating black holes from general relativity and rotating fluids in both water and optical systems.

In particular, we will investigate both theoretically and experimentally the interaction between an input wave and a rotating black hole spacetime geometry, here recreated by the rotating fluid. This allows us to mimic a scattering process associated to rotating black hoes called superradiant scattering. From a historical viewpoint this kind of radiation is the precursor to Hawking radiation. More precisely, black hole superradiance is the scattering of waves from a rotating black hole: if the incoming wave also possesses a small amount of angular momentum, it will be reflected with an increased amplitude, i.e. it is amplified at the expense of the black hole that thus loses some of its rotational energy. It has also been pointed out that the same physics may take place in very different systems, for example light incident on a rotating metallic (or absorbing) cylinder may also be amplified upon reflection. Yet, no-one has ever attempted to experimentally investigate the underlying physics that extend beyond general relativity and are relevant to a variety of hydrodynamical and rotating systems.

We aim to provide the first ever experimental evidence of this intriguing and fundamental amplification mechanism in two different hydrodynamical systems. The first is a water spout, controlled so that the correct boundary conditions are obtained and optimised for observing BH-SS. The second is a less conventional fluid that is made out of light. Light propagating in a special medium can behave as a fluid or even a superfluid. By building upon highly developed photonic technologies e.g. for the control and measurements of laser beam wavefronts, we will implement very precisely tailored and characterised experiments. One of the unique aspects of this project is the marriage between two very different lab-based systems, one using water the other using light, to tackle an outstanding problem in physics that is of relevance to astrophysics, hydrodynamic and optical systems.

Science:
SURF is a research project at the cutting edge of modern physics that will have a profound impact on our understanding of the universality and robustness of the processes that allow rotating black holes to lose their angular momentum.
The two principal investigators of this project pioneered the first experiments in analogue gravity and analogue Hawking radiation starting in 2010 with their works on one-dimensional horizons in optics and hydrodynamics. These first experiments stimulated a widespread interest and there are now several very important results reported in literature from other research groups.
The PIs are aiming with SURF to now extend these results to two-dimensional geometries, and consequently observe series of new effects that are related to angular momentum. This will give analogue gravity a much wider remit well beyond the Hawking radiation effects studied so far.
Moreover, by combining studies in water and optics in the same project we will build upon the recent, growing interest in the physics community working at the boundary between these two fields.

Technology:
Although our research is primarily aimed at fundamental studies, past projects with a similar flavour and in related fields, lead to some remarkable technological achievements. The drive to achieve more precision and higher reproducibility in water-based experiments led to development of a new "ripple detector" that is now commercialised by a company and will be used and further improved upon during this project. Similarly, the attempt to visualise propagating light pulses used in our previous Hawking radiation experiments led to the development of new imaging technology that can freeze light in motion and has had a huge success in a variety of fields. We will build upon this track record of exploiting the technological successes of our blue-sky research, to develop any new instruments or methodologies that will emerge from our research.

Society:
The project is also an excellent opportunity for the training of highly qualified people. On the one hand the young researchers involved in the project will tackle problems that require a remarkably broad range of knowledge and expertise. On the other other, they will also be exposed to research that will require the development of bespoke methodologies or methodologies used in other fields (e.g. oceanographic techniques for measuring wave dispersion relations in optics or optical spectral methods applied to water waves).
Last but not least, SURF will provide an excellent platform for public outreach. The PIs have a strong track record of interaction with the public media, newspapers, videos etc. Resources have been allocated to further promote our research to the general public and use the strong appeal of black holes to build upon the current excitement and interest for physics.