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  • Listening to Black Holes
Jun 2008
LONDON, June 13, 2008 -- Can you hear black holes collide? That question, among others, will be explored at the Royal Society Summer Science Exhibition, to be held June 30 to July 3 at the Royal Society facilities in London.

Researchers from the Universities of Glasgow, Birmingham, Cardiff and Southampton are joining forces with colleagues from the Albert Einstein Institute in Potsdam, Germany, and designers from Milde Science Communication to showcase the science associated with Einstein's general theory of relativity, black holes and gravitational waves.
The results of a computer simulation of the gravitational waves produced when two black holes merge to form a larger black hole. The gravitational-wave intensity is represented by different colors.  (Photo courtesy University of Glasgow)
The exhibit will introduce the main ideas behind Einstein’s relativistic theory of gravity. Through a number of hands-on exhibits, visitors can gain an understanding of how space and time are flexible and why this leads to gravity. Black holes will be explained, and a tabletop laser-interferometer will be used to demonstrate the technology used to search for gravitational waves -- the tiny ripples in space and time that bathe the earth.

Supercomputer simulations of colliding black holes will be demonstrated, and the challenging task of digging weak gravitational-wave signals out of noisy detector data will be introduced via a game in which visitors can listen to actual black-hole signals.

Gravitational-wave astronomy will open a new window through which we will probe that dark side of our universe, said Jim Hough, a physics professor at the University of Glasgow.
"Einstein's special theory of relativity is based on the idea that the speed of light is constant regardless of the motion of the observer. It tells us that measurements involving space and time lead to surprising results if one travels near the speed of light. Time appears to slow down and objects seem to contract," Hough said. "This shows that space and time are not the rigid concepts that we are used to in everyday life. In the general theory of relativity, which reconciles the principles of relativity with gravitation, space and time become even more flexible, changing as the world changes around them."
An overlay of the results of a supercomputer simulation of colliding black holes and a piece of gravitational-wave detector equipment. (Photo courtesy University of Glasgow)
In Einstein’s theory, gravity is no longer simply a force that pulls falling apples to the ground. Instead, gravity is geometry.

"The presence of matter alters the geometry of space and time, and the geometry in turn determines how matter moves," Hough said. "One of the most fascinating predictions of general relativity is the existence of black holes. Black holes are made from matter, but they are not matter. They are formed when massive stars run out of nuclear fuel and collapse under their own weight. The collapse leads to a region of extreme space-time curvature. Objects can fall into this region -- the black hole -- but nothing can escape. Not even light."

There is strong astronomical evidence that black holes are common in the universe, he said. "Their presence may be central to the formation of galaxies. In fact, we believe that virtually all galaxies harbour a gigantic black hole in their centers. Yet we do not know how these black holes work," the societ said. "Apart from having relatively good estimates of their masses and some evidence that they may rotate very fast, we know very little."

Black holes are dynamical objects that interact with the environment. They may form binary systems where two black holes orbit each other. According to general relativity, such systems will evolve toward a final black-hole collision -- an event involving extreme space-time deformations.

Einstein's geometric theory of gravity predicts that changes in gravity propagate through the universe in the form of waves. These gravitational waves, often thought of as “ripples” in space and time, are created whenever masses accelerate. "They have not yet been detected directly, but we have strong indirect evidence that they exist," Hough said. "The best evidence comes from a double neutron-star system called PSR1913+16. As the stars orbit each other the system radiates gravitational waves and loses energy. Observations now spanning more than three decades show that the orbit of the double neutron star system shrinks at exactly the rate predicted by general relativity."
The German-British gravitational wave detector GEO600 in Ruthe near Hannover, Germany. The GEO600 project aims at the direct detection of gravitational waves by means of a laser interferometer of 600-m armlength. Front: The central building for the laser and the vacuum tanks. The 600-m tubes run in covered trenches at the edge of the field upward and to the right. Buildings for the mirrors are at the end of each tube. (Photo: Albert Einstein Institute Hannover)

Gravitational waves convey less a picture than a sound. Just as sound waves contain information about the musical instrument that created them, the gravitational waves carry an imprint of the event in which they were generated. The strongest gravitational-wave signals come from the most violent events in the universe, involving the acceleration of large masses in small regions of space. Because of their small size to mass ratio, black holes are particularly promising sources.

"If we could detect these signals, we would be able to find black holes and study them in detail," Hough said. "However, even though the events that generate the waves may be extremely powerful, the waves wane with distance. Since most cosmological events occur far from the Earth, the gravitational waves that bathe our planet are very weak."

In order to catch these waves, we must develop very sensitive detectors, it said. "One must be able to detect changes of about a thousandth of the diameter of the proton in a kilometer-sized detector. This is like comparing the width of a human hair to the distance to the nearest star."

Cutting-edge laser interferometers have been developed to detect these tiny stretches in space-time, it said, and an international network of extremely sensitive detectors is now tracking the changes in the space-time geometry, listening for tiny changes in gravity.

"Sophisticated tools are needed to dig the very weak signals out of the detector noise. This poses further challenges," Hough said. "The development of such analysis tools requires a good theoretical understanding of gravitational-wave sources. Unfortunately, even though the mathematical equations of general relativity, encoding the interaction between matter and space-time geometry, are easy to write down they are notoriously difficult to solve. In most situations, one has to resort to simplifications or costly supercomputer simulations.

"After several decades of effort, there has recently been great progress on simulating the inspiral and final merger of two black holes," Hough said. "The results of these simulations provide insights into the detailed interaction of two black holes. They also provide researchers analyzing the detector output with reliable signal templates."

We study gravity, but gravity is also the messenger, Hough said. "By detecting gravitational waves from black holes, we hope to measure black-hole parameters, provide accurate maps of the shape of space-time and test Einstein’s theory with high precision.

"At the end of the day, the weak interaction between gravitational waves and matter may be a blessing. The gravitational waves that reach our detectors are virtually unaltered since their generation. This means that we will be able to study regions of space that cannot be investigated with the traditional tools of astronomy."

The Royal Society Summer Science exhibition is a premier annual showcase for scientific excellence in the UK . Research teams are invited to bid to provide an exhibit of their work, and the best are selected for display to scientists, the media and the public. The research is funded by the Science and Technology Facilities Council in the UK and the Max Planck Gesellschaft in Germany .

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