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Echoes of gravity

The Brazilian gravitational wave detector begins to function

SERGIO TURANO DE SOUZA/USPHeart of metal: copper sphere weighing 1.1 tons is ready to record the wavesSERGIO TURANO DE SOUZA/USP

When he has time, physicist Sérgio Turano de Souza plays the guitar in a rock band. He was already not very assiduous at the rehearsals, and perhaps now he may have to miss them a bit more, because he is one of those responsible for the Mario Schenberg Gravitational Wave Detector, a unique piece of equipment in the country that began to operate on an experimental basis on September 8, with no set time for unexpected events. For the time being, there have not been any – none at least as disheartening as the overflow of the water table, six years ago, over the trench that was dug out to house the detector. In one of the sheds of the Physics Institute of the University of São Paulo (USP), the really concrete stage was then beginning of the pursuit of the waves that, if they are encountered and if they are pulsed, may show that elementary particles known as gravitons, to which the force of gravity is attributed, do in fact exist; for the time being, they are only forecast theoretically.

Weeks ago, albeit waiting for extra tasks, the team coordinated by physicists Odylio Aguiar, from the National Institute for Space Research (INPE), and Nei Oliveira Jr., from USP, accompanied the first collection of data, which went on for five days running, and all the time kept glancing at the assortment of meticulously organized electronic apparatuses, wires, vacuum pumps and thermometers that cover an aluminum cylinder 1 meter in diameter and 3 in length. It is inside this cylinder that the heart of the detector is hidden: a solid sphere of copper and aluminum, 65 centimeters in diameter and weighing 1.15 tons, suspended by a copper rod and kept in the vacuum under a layer of liquid helium at almost minus 270 degrees Celsius. Its extremely subtle pulsation – or oscillation – will indicate when the gravitational waves are finally detected; they constitute one of the most challenging objects of study for contemporary physics.

Known so far only by means of indirect evidence, such as the reduction of the orbit of binary stars, gravitational waves are defined as deformations in space resulting from the accelerated movement of large bodies like the stars themselves. They can be compared with the waves that form when we throw a stone onto the water of a lake, even though they are extremely weak, and, theoretically, they can propagate themselves at the speed of light. They fascinate the physicists because they represent the last test of the General Theory of Relativity, formulated in 1916 by Albert Einstein – all the other predictions, like the deviation of light when passing close to stars, like the Sun, have already been proved.

Exhausted, voiceless, and getting far less sleep than usual because of the detector coming into operation and the constant journeys between São Paulo and São José dos Campos, where the Inpe and his home are, Aguiar knows that it will not be possible to put himself so soon on an even footing with the teams of another 11 similar detectors – in operation for many years in the United States, Germany, Italy and Australia – that are also running after the gravitational waves. “The detector is now like a car that is moving for the first time, after we have joined together parts that had never been together before” in the comparison of the 52 year old physicist, who for at least 20 years has been working with this sort of equipment. “Its sensitiveness still falls short of what we need to be competitive.” In this initial stage, of testing and adjusting the components, the detector operates with only three of the six sensors that transform the oscillations of the sphere into electrical signals, captured by microwave antennas and amplified in a fraction of a second before arriving at the computer installed on a mezzanine close to one of the walls of the shed.

Audible vibrations
By the end of the year, though, the definitive sensors have to be installed. According to Aguiar, they will be much more refined and sensitive. Constructed in one of Inpe’s laboratories by physicist Sérgio Ricardo Furtado, the new sensors – or transducers – will have a cavity of niobium, a superconducting chemical element, and a silicon membrane with a thickness of 20 thousandths of a millimeter. The membrane of the current sensors is metallic and has a thickness of from 200 to 300 thousandths of a millimeter, which results in a far lower sensitivity to the waves that may arrive at any moment from the depths of space.

With the definitive sensors, the detector will be able to capture gravitational waves in the frequencies from 3,100 to 3,300 Hertz, which is in the range of sounds that can be captured by the human ear. Accordingly, it will be possible to hear the vibration of the bronze sphere, provided that the signals pass through a microphone and are amplified, since the sphere is covered by a vacuum.

The detector installed in USP cost roughly US$ 800 thousand, financed by FAPESP, and absorbed the work of about 30 experimental and theoretical physicists from USP itself, from the Inpe, from the Technological Institute of Aeronautics (ITA), also from São José dos Campos, from the São Paulo unit of the Federal Technological Education Center (Cefet) and from the State University of Campinas (Unicamp). Even though it is only now joining a race that has already begun, it enjoys two advantages that result from the fact that it is spherical:  it will be able to capture the waves that arrive from all over the sky and also determine the direction they came from.

On the other hand, the detectors that work with aluminum bars instead of spheres, or even by means of a laser, will only be able to register the waves, according to Aguiar. “It’s almost impossible for us to catch the first wave” he says. “But it will be the spherical detectors that are going to determine the form and the direction of the waves.” Aguiar believes that the Brazilian physicists, should they arrive at these equally strategic information, may be able to talk as equals with the representatives of the teams in charge of equipment that cost 400 times more.