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International Innovation - Department of Physics - University of Florida

International Innovation - Department of Physics - University of Florida

International Innovation - Department of Physics - University of

Listening to the cosmos PROFESSOR GUIDO MUELLER Professor Guido Mueller is committed to exploiting gravitational waves to better understand the Universe around us. Here, he talks in detail about his latest research endeavour and scientific goals To start, could you briefly describe gravitational waves and where they come from? Gravitational waves are distortions in spacetime generated by accelerated masses. Everybody knows these depictions of curved spacetime around a black hole or a star that look like a bowling ball resting on a rubber sheet. Imagine two of those orbiting around each other; the dents in the rubber sheet will move with the bowling balls and send ripples or waves across the rubber sheet. These waves are Nature’s way of telling us that the location of the two balls is constantly changing. Newtonian gravity was not able to describe these waves. Laplace and others tried to solve this puzzle within Newtonian gravity but failed; it was Einstein’s theory of general relativity (GR) which provided the solution. GR allows wave solutions and was able to put a speed limit – the speed of light – on changes in gravity. The bowling ball picture also tells us that the most effective sources are binary systems where two compact objects orbit around each other. What is the general principle behind measuring these waves? Gravitational waves are quadrupole waves which stretch and squeeze spacetime, changing the distances between freefalling masses in opposite ways along orthogonal directions. Imagine a circle that a wave turns into a horizontal ellipse and then into a vertical ellipse back into a horizontal ellipse and so on. The best way to measure these waves is to use a Michelson interferometer where the beam splitter is located at the centre of the circle and the two mirrors are on the circumference cutting out a 90° segment. The resulting interference fringes strongly depend on differential changes in the distances between the beam splitter and the two mirrors, exactly what gravitational waves change. As simple as this sounds, the tricky part is that gravitational wave strain amplitudes are not expected to exceed 10 -21 . In other words, they change the distance to the Sun by the diameter of a hydrogen atom. Measuring them is non-trivial. However, we expect to make the first direct detection of gravitational waves within the next five years with Laser Interferometer Gravitational-Wave Observatory (LIGO) and VIRGO. Why are you so confident that the Advanced LIGO will be able to detect gravitational waves? I am leading one of the large subsystems – Input Optics – for Advanced LIGO, and I see the progress and the problems first hand. None of the issues will preclude us from reaching our target sensitivity between 30 Hz and a few hundred Hz; below 30 Hz is new territory for us, and we will have to see how this evolves. Because of data from NASA’s Swift satellite observatory, we now have a better handle on the neutron star merger rate; we should detect gravitational waves during the first extended science run. Once we improve by a factor of 10, in order to probe a thousand times larger volume, we will hear these mergers on a regular basis. As Chair of NASAs Gravitational Wave Science Analysis Group (GW-SAG), could you outline the main aims and activities of the group? The topical SAGs are providing a way for scientists working in a specific area to organise themselves; to provide input to NASA on how they see their field evolving and possible ways forward; and to engage in and focus the scientific discussions. The SAGs are also a way for NASA to obtain outside expertise and to release information to the scientific community about what will be possible in the future and 12 INTERNATIONAL INNOVATION

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