Hawking Radiation in Dielectric Horizon Analogues

Lead Research Organisation: Heriot-Watt University
Department Name: Sch of Engineering and Physical Science


Black holes are incredibly fascinating objects. They largely populate the Universe we live in, attracting whole galaxies around them. They also attract the imagination of novel writers and scientists alike: they represent the ultimate frontier at which our knowledge and intellect can be put to the test.
In 1974 Stephen Hawking, building upon suggestions that black holes have a finite temperature, predicted that the event horizon surrounding a black hole separates regions characterized by such an intense space-time distortion that photons and particles are literally ripped out of vacuum state. These photons are then seen from outside the black hole to be emitted as a continuous flux of radiation. Black holes glow, just as if they were light bulbs. Unfortunately, this truly amazing prediction has little hope of being verified directly from astrophysical black holes. The "glow" has an extremely low temperature, of the order of tens of nano-Kelvins and cannot be distinguished amongst the much higher cosmic background temperature.
Fortunately, exactly 30 years ago, William Unruh noted that the same arguments that lead to black hole evaporation also predict that a thermal spectrum of sound waves should be given out from a flowing fluid whose velocity is made to vary from sub-sonic to super-sonic velocities. Sound waves will remain blocked at the transition between the sub- and super-sonic regions at what, to all effects, is the analogue of an horizon. It now turns out that horizons are apparently far more common than one may imagine. They appear in flowing tap water as it hits the sink and in a number of water or liquid based scenarios; they appear in flowing Bose-Einstein-Condensates, in polariton condensates and, most importantly for what concerns this project, in moving dielectric media. We may imagine moving a transparent glass sample at velocities close to that of light. We would then have a situation analogous to that of sound waves in a moving fluid: in the presence of a transition from sub-luminal to super-luminal speeds, light waves will not be able to move beyond the horizon point at which the medium velocity is exactly equal to the phase velocity of light. One of the PIs (U. Leonhardt) recently proposed an ingenious method to achieve such horizons in a very simple manner. An intense laser pulse propagating in glass will create a local perturbation in the refractive index that travels together with the pulse, i.e. it naturally travels at light speeds. Any light wave approaching the perturbation will be slowed down by the local increase in refractive index and will eventually be blocked at the horizon beyond which it will be never be able to propagate. Using this very simple proposal, the other project PI (D. Faccio) obtained the first evidence of spontaneous photon emission induced by the dielectric horizon. The perturbation is glowing and evaporating by shedding photons excited from the vacuum state, just as Hawking predicted black holes should do. This project aims at taking forth these results and taking studies on Hawking emission and horizon related effects to the next level. We are now able to plan real experiments that can give us for the first time real data describing how horizons interact with the quantum vacuum. Moreover, at the heart of Hawking emission lies a novel amplification mechanism that, due to the lack of any previous experimental possibilities, has never been truly investigated before. This new amplification channel will be studied and used to amplify light. The goal in mind is to create the first black hole laser in which light is trapped in between two separate horizons. Bouncing back and forth it is amplified at each rebound and finally exponentially explodes in laser-like amplification process.
The impact of this project therefore goes well beyond investigation of Hawking effects and invests a number of fields, ranging from quantum field theories to nonlinear optics and photonic technologies.

Planned Impact

The impact of this project extends well beyond the standard academic impact and interest for researchers working in related areas. Most importantly, we note a clear impact on society. Indeed, black holes are a source of continued fascination for the general public. For instance, many school children have not heard about gravitational fields, but most have heard of black holes. Black holes are perceived as awe-inspiring and sinister, as dark stars that may swallow everything in their paths. Our experience is that whenever we say that we create something like the horizon of a black hole in the laboratory, we immediately get the attention of many people. Moreover, names like Einstein and Hawking are familiar to countless people; we can show that their work is not as remote from everyday life as they may think and that they surely can understand some of it. In demystifying black holes we may give people clarity and trust in common sense, even in areas of science far way from our everyday life such as astrophysics. Trust in science and rationality is more important than ever, we can definitely make a contribution towards a more rational understanding of science for the general public.
We have already had many interactions with the media, from news bulletins on the internet to the international press and radio and television. We expect that the decisive experiments demonstrating Hawking radiation will create significant media interest. BBC Horizon may fly to Scotland for filming experiments on artificial black holes with light, and to Nice and Vancouver for experiments with water waves. We plan to apply for a contribution to the Royal Society Summer Exhibition that will illustrates the inner workings of black holes. Ulf Leonhardt's exhibit on Geometry and Light: the Science of Invisibility has been accepted for 2011, an exhibit on artificial black holes may follow.
Finally, the education of highly qualified people is a very important form of the impact of ambitious research. The research of HORIZONS is strongly interdisciplinary, ranging from general relativity to laser physics and back to quantum electrodynamics. The project will provide opportunities for all researchers involved at the PhD and postdoc level to interact with the very best researchers and scientists of the world in their areas. This is a unique learning and training opportunity that we believe will give the young researchers an open-minded attitude that will, in turn, benefit them in their future careers. The delivery of such high expectations will of course require additional dedication on behalf of the principal investigators in order to ensure that the people involved actually learn and gain from the project rather than being "drowned" by it.
It will be our endeavour to ensure that all team members are involved in regular email discussions regarding the details of on-going work in the various areas. These will be backed up by periodic meetings where also the external partners are present, if possible. We strongly believe that the added value of this project derives precisely from the ability to bring together theoreticians and experimentalists alike and, from our experience we have seen that this is possible only through the continuous, even day-by-day, interaction of all partners. What we can achieve in this project is for our young researchers to get a can-do mind-set - if you can build a black hole, you can build anything. People of this calibre are going to make invaluable contribution to the economy and the society as a whole.


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Title Edinburgh Science Festival 
Description In collaboration with Lily Hibberd (Australian artist) we developed a museum exhibit for the Edinburgh Science Festival. This exhibit featured four pieces that investigated the relationship between human perception and the flow of time by relating this to a series of experiments performed and recorded in our lab. 
Type Of Art Artistic/Creative Exhibition 
Year Produced 2017 
Impact The exhibit was open for 1 month and attracted significant attendance. 
URL https://youtu.be/V1KIvtsC7Zg
Description There are several key findings:
1) We developed a new platform based on laser pulse propagating in crystals with which one can create black hole analogues and artificial horizons
2) We discovered a new form of optical radiation that we call "negative frequency resonant radiation". This led to a series of works that modified the standard equations used to model optical propagation
3) We initiated a study in so-called photon fluids that allowed us to show how light can, in certain situations, behave exactly like a fluid. This work then led to a new EPSRC grant proposal that will carry forward this work in collaboration with experts in hydrodynamics
4) We developed a new camera technology that allows us to film light in flight. This was then applied to novel applications such as imaging behind walls. This activity is now currently a major research line in our group and has attracted further funding for a number of sources (EPSRC, DSTL, industry)
Exploitation Route Our findings are currently being developed by many other groups.
Negative frequency resonant radiation is currently being investigated by groups interested in expanding the wavelength range of lasers.
Our imaging technology is being investigated for novel LIDAR applications.
Sectors Aerospace, Defence and Marine,Education,Manufacturing, including Industrial Biotechology,Other

Description The findings from this project have a lead to a number of outcomes. Our research on artificial spacetimes and horizons has essentially opened a new area in the field of nonlinear optics. The use of horizons and related terminology is now relatively common. Optical horizons are being studied and proposed fro novel optical components. Another major finding of this proposal was the development of a new camera technology that allows to video light in flight. This was first used to create videos of light pulses that are now part of museum exhibitions. But it has also lead to further funding opportunities (EPSRC, CDE, DSTL, Thales) and to the creation of a camera that can see behind corners. This research has now become a whole activity of its own but was initially seeded from this blue skies project.
First Year Of Impact 2014
Sector Education,Other
Impact Types Cultural

Description Leverhulme artist in residence
Amount £12,500 (GBP)
Organisation The Leverhulme Trust 
Sector Charity/Non Profit
Country United Kingdom
Start 04/2016 
End 11/2016
Description BBC News interview 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Media (as a channel to the public)
Results and Impact BBC News sent a camera crew to interview me and talk about my research in black hole physics

We received many expressions of interest, also from the general public
Year(s) Of Engagement Activity 2013
URL http://www.bbc.co.uk/news/uk-scotland-19822295
Description BBC interview - energy extraction from rotating black holes 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Media (as a channel to the public)
Results and Impact BBC interview on our recent work (and grant award) to study the physics of energy extraction from rotating black holes with applications in fluid dynamics.
The BBC came to our labs in Edinburgh and filmed activity in the lab and an interview in my office.
An online newspiece (on the BBC website) accompanied the TC broadcast.
Year(s) Of Engagement Activity 2016
URL http://www.bbc.co.uk/news/uk-scotland-38403628
Description Discovery Channel documentary 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Windfall films visited our labs at Heriot-Watt University to film part of a documentary on experimental Black hole physics. This documentary will be aired by the Discovery Channel in November 2014.

This will have a huge outreach impact.
Year(s) Of Engagement Activity 2014
Description TED conference event 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact 50+ attendees to the Edinburgh TEDGlobal conference were given a "Behind the scenes" lab tour at Heriot-Watt University as part of a tour organised together with TED on black hole physics and experimental physics.

New contacts were made with influential thinkers. This event initiated a collaboration with Ramesh Raskar at MIT media labs, USA.
Year(s) Of Engagement Activity 2013