13 February 2011, 19:00 CET
An international team of physicists have proposed a technique that could enable the detection of spinning black holes. The large mass of these as-yet unidentified objects makes space and time swirl around. The technique, published in the February 13 issue of the renowned journal Nature Physics, uses the fact that photons from radiation sources very near the black-hole event horizon act as tiny gyroscopes that sense this swirling space-time. As a result the emitted electromagnetic radiation, from radio to X-ray and gamma, is imprinted with orbital angular momentum (OAM) that can be observed with the best telescopes if set up properly. By observing at different wavelengths, possible intervening plasma clouds may be mapped out.
OAM is one of many properties that are carried by all types of electromagnetic radiation, including radio and light, that exist in nature. It is a kind of twist that causes the beam of radiation to spiral around its axis in a vortex like a tornado. Just as there is light of different colours, there is light of different twists. It is only that these twists have gone mostly unnoticed by astronomers and space physicists until now.
In the Nature Physics article, Fabrizio Tamburini (University of Padua, Italy), Bo Thidé (Swedish Institute of Space Physics), Gabriel Molina-Terriza (Macquarie University, Australia), and Gabriele Anzolin (Institute of Photonic Sciences, Parc Mediterrani de la Tecnologia, Barcelona, Spain) show how a wavefront emanating from radiation sources in a disk around a rotating black hole will get twisted. This happens because half of the wavefront moves in the direction of advancing spacetime, and the other in the direction of receding spacetime. As a result, the phase of the radiation has a distinctive distribution in space, a unique "fingerprint" that can be observed to confirm the existence of the spinning black hole.
Dr Tamburini says: "We were looking exactly for this effect and found that rotating black holes not only induce so-called gravitational Faraday rotation, but also induce vorticity."
As has been shown in previous articles by members of the team, radio or light beams that propagate through a plasma (a gas where there are free electrons and ions present) such as the Earth's ionosphere will acquire an OAM or twist that can be measured on the ground. By measuring the electromagnetic OAM acquired, the vorticity of the ionosphere, or any other plasma in space or in the laboratory, can be imaged. How this spiral imaging technique works has been described in other articles by members of the team.
Dr. Molina-Terriza emphasises: "All of us have been involved in one way or another in measuring the OAM of light, as in the digital spiralling imaging technique developed by Lluis Torner at ICFO in Barcelona. With hard work and enthusiasm we may be able to develop new tools for astronomers, astrophysicists, and space physicists."
The detection of OAM in radiation requires that the received signals be processed in a specific way to decode them into their "twist states". In optics, this can be done with spiral lenses, holograms and interferometers. In radio, this can be done with antenna arrays such as the huge radio telescope LOFAR that is under construction in Europe. The Swedish test-bed LOIS is being developed precisely for the purpose of perfecting the OAM radio observation technique.
Prof. Bo Thidé comments: "We have recently shown experimentally how OAM and vorticity can be readily imparted onto low-frequency radio beams and received far away and analysed there. This opens the possibility to work with photon OAM at frequencies low enough to allow the use of antennas and digital signal processing, thus enabling software-controlled experimentation and space observations in manner that is not possible with other means."
Comments from experts in the field:
Dr. Martin Bojowald, Assistant professor of Physics, Penn State University, USA, writes in a News & Views article in the March 2011 issue of Nature Physics: "These results open the way to new observational tests of general relativity", and "OAM might provide a clear, distinguishing signature that could one day lead to the observation of Hawking radiation...but before one can address that, the twisting of light already opens the way to exciting new possibilities in black-hole physics."
Dr. Saul Teukolsky, Hans A. Bethe Professor of Physics and Astrophysics at Cornell University, says: "This paper describes a novel approach to measuring the spin of the black hole at the center of our Galaxy. Perhaps even more important, in combination with other measurements it might confirm that the geometry around the black hole is indeed the geometry predicted by General Relativity. This is a very exciting prospect."
Dr. Robert Williams, Distinguished Research Scholar, Space Telescope Science Institute, Baltimore, and President of the International Astronomical Union (IAU), comments: "New ideas like this are what drive fundamental discoveries in physics."
Article and authors:
The Nature Physics article is entitled "Twisting of light around rotating black holes".
The authors, representing astronomy, astrophysics, space physics and quantum optics, are:
The software used in the experiments was adapted from software that was written and made available by:
The Swedish Institute of Space Physics (IRF) is a governmental research institute which conducts research and postgraduate education in atmospheric physics, space physics and space technology. Measurements are made in the atmosphere, ionosphere, magnetosphere and around other planets with the help of ground-based equipment (including radar), stratospheric balloons and satellites. IRF was established (as Kiruna Geophysical Observatory) in 1957 and its first satellite instrument was launched in 1968. The head office is in Kiruna (geographic coordinates 67.84° N, 20.41° E) and IRF also has offices in Umeĺ, Uppsala and Lund.