guia do novo coronavirus
Imprimir Republish

Physics

On the Graviton wave

Brazil joins the race to capture gravitational waves from the Cosmos

With the Graviton project, coordinated by Odylio Aguiar, of the National Space Research Institute (Inpe), a group of researchers intends, for the first time in history, to detect gravitational waves in space. Kicked off in May last year, with financing of around US$ 1 million from FAPESP, the project brings together a group of institutions and is starting out with the construction of a powerful wave detector with a spherical antenna that bears the name of the Brazilian physicist Mario Schenberg (1914-1990). Besides whatever results it may achieve, the project is important because it develops tools in this country that may also be useful in other fields.

Gravitational waves are deformations in the space-time structure produced by accelerated masses – for example, in the explosions of supernovas or in the orbit of a binary system formed by two neutron stars – and which, according to the Theory of Relativity, spread out at the speed of light. Suggested between the end of the 19th and the beginning of the 20th century by scientists such as Heaviside, Lorentz and Poincaré, they were predicted in 1916 by the equations in Albert Einstein’s General Theory of Relativity.

Since the 60s, when technological progress enabled scientists to think of detecting these waves, physicists have envisaged devices to do this and there are various detectors involved in the attempt. The Brazilian proposal involved heavy, spherical, copper and aluminum antennas operating at a temperature of between 15 and 20 mK (millikelvins) a few hundredths of a degree above absolute zero (-273.16°C). Temperature that stalls all atomic movement and, therefore, there is a total absence of heat.

The Graviton project includes various items of equipment of the sort. The first is the Schenberg detector, that will operate at between 3 and 3,4 kHz (kilohertz). In this waveband, “it will have the sensitivity of the great laser interferometers”, points out Aguiar, “with the advantage of being cheaper”. Laser interferometers are detectors costing around US$ 100 million each, while the Schenberg, of the resonant type, should cost US$ 1 million.

Revolution
Whoever detects gravitational waves will start a revolution with unpredictable consequences in Physics, forecast Aguiar, of the Inpe, and his assistant coordinators Nei Oliveira Jr., of the Solid State and Low Temperature Laboratory  (LESBT) of the Physics Institute of the University of São Paulo (USP), and Giorgio Frossati, of Leiden University (Holland). Researchers from the Bandeirantes University, the Federal Center for Technological Teaching of São Paulo and Air Force Technology Institute are also taking part. Six other institutions abroad and six from Brazil are also collaborating.

“It is no easy task”, admits Aguiar. As fleeting as the electromagnetic waves are, which travel at 300,000 kilometers a second, gravitational waves are much weaker: around 36 times less strong than electromagnetic radiation. Frossati, an Italian physicist trained in Brazil and a specialist in cryogenics, draws the analogy between electromagnetic radiation, deciphered in the 19th and 20th centuries and gravitational radiation: “Electromagnetic waves can be seen as billiard balls which move about on the flat surface of the table, representing the dimensions we know, while gravitational waves would be free to perforate the flat surface of the table, up and down”. In this pattern, they would flee to other dimensions in agreement with the theory of superstrings, which states that multiple dimensions existed in the early stage of the universe, when the four basic forces of nature would have been just one. These forces, reminds Aguiar, “are the strong force responsible for the cohesion of nuclear particles and atomic nucleuses; the weak force, which governs transmutation between particles; electromagnetic force, which make the existence of atoms and molecular and crystalline structures possible; and, finally, gravitational force, responsible for forming the galaxies, stars, planets and other bodies in the universe”.

Witchcraft
Knowledge of the nature of gravitational waves would be the answer that Isaac Newton wasn’t able to give to his Cartesian critics in the 17th century when they accused him of “witchcraft” when he announced his Theory of Universal Gravitation.

The distortion of the space-time structure by the presence of a large mass body, predicted in the General Theory of Relativity, was confirmed on May 29, 1919 in a total eclipse of the sun seen by an international team in Sobral, Ceará.

So far, the evidence for gravitational waves is indirect. In the 70s, American scientists observed, with the help of the 305-meter radio telescope at Arecibo, in Porto Rico, a decrease of 76 microseconds a year in the orbit of the PSR1913+16 binary system, made up of a pulsar and a neutron star, both collapsed stars. The system’s rotational variation was explained by the emission of gravitational waves.

According to Frossati, any detection of the undulation of space-time “will lead to a profound rethink ranging from the micro to the macro cosmos and the field of particle physics, consolidating everything from approaches such as the superstring theory to new mapping and knowledge of the universe”. The superstring theory maintains that the basic constituents of matter – the quarks –  derive from the vibration of infinitely small strings – the building bricks of the world, produced as notes are produced by the vibration of a piano’s strings. From the combination of various types of quarks, protons and neutrons are constituted – particles forming the nucleus of atoms and, wrapped in layers of electrons, form the known world.

Frossati lived in Brazil from 8 to 30 years of age and studied at USP’s Physics Institute. Today, while he observes the tree-lined landscape through the glass windows of the Mario Schenberg building at the USP campus , he warns that the project “may seem pure science fiction” and speaks of the possible ramifications of detecting gravitational waves, such as the possibility that these waves “break away to other dimensions”.

Bell alloy
“ Not only the first signs that will be important”, says Aguiar. “The regular observation of these waves will be as or more important, since they carry information about the cosmos that cannot be obtained by detecting electromagnetic waves (microwaves, infrared, light, gamma rays, etc.). Therefore, detecting these gravitational waves will open a new window through which to see the universe.

The development of this new generation of gravitational wave detectors is based on spherical resonant antennas built with a copper (94%) and aluminum (6%) alloy. “In recent work, the Dutch group showed that this alloy combines a high resonant capacity (like the alloy of a bell) with high thermal conductivity. The latter property will enable the antenna to be cooled to very low temperatures. Cooling the antenna is very important in minimizing thermal noise. One of the great trump cards of this new generation of antennas is that they can be cooled down to temperatures almost an order of magnitude below existing antennas – the coldest of them, the Nautilus at the National Nuclear Physics Institute (INFN), at Frascati, Italy, works at around 100 mK”, emphasizes Oliveira.

Project Omega
Aguiar’s team’s most ambitious plan is to produce three spherical detectors of different sizes in Brazil and, consequently, different wavebands. The Schenberg is the first; then the Newton and the Einstein will come, and they will make up the Brazilian Gravitational Wave Observatory.

There will be three units of the size of the Schenberg detector, with a diameter of 65 centimeters and weighing 1,150 kilograms. The first is already taking shape in the Italbronze foundry, in Arujá. The Newton will have twice the diameter of the Schenberg and the Einstein will be bigger still: 3 meters in diameter and weighing 100 tons. Each version represents a challenge, both in terms of construction and cooling.

The three units of the first detector size, all produced by Italbronze, will take part of the Omega project, an international network of spherical detectors. The Schenberg will be located in the LESBT of USP’s Physics Institute in the building named after the Brazilian physicist. The others, Mini-Grail and Sfera, will be installed at Leiden University, Holland, and at the National Nuclear Physics Institute (INFN), in Frascati, Italy. The work with three similar instruments, operating on the same waveband, is required to ensure the reliability of the measurements, justify Frossati and Aguiar.

Thermos bottle
The challenges of building and cooling begin with the casting of the copper-aluminum alloy, the composition Frossati claims is the one that offers the best resonance for gravitational waves. But casting a body of this sort, he recognizes, “involves difficulties such as the appearance of faults capable of altering the desired mechanical standards”. The solution Italbronze employs is to cast blocks in cylindrical form so that any structural faults tend to shift to the upper region of the part, which “are then cut off as a means of eliminating the faults and ensuring the desired performance”, says Aguiar.

Cooling a part weighing more than a ton down to a few hundredths of a degree above the absolute zero demands considerable cryogenic infrastructure. For this reason, the antenna will assembled in USP’s LESBT under the command of Nei Oliveira. “It will be the biggest mass ever cooled to this temperature in the world”, he says. And, he adds that with projects of this sort “Brazil takes part in the international circuit of high scientific technology”.

After the Schenberg, the difficulties with the larger antennas will increase, both in terms of casting and cooling. Aguiar refers to the task of cooling not one but 100 tons in a short length of time. To do this, the Schenberg will be contained inside a sort of giant thermos bottle, in the cryogenics laboratory. Bathed in helium gas, in a technique known as forced convection, in three and a half days it will have reached its operating temperature. This short cooling period is one of the main advantages of this detector, arguesOliveira. In the event of any change to the configuration, he adds, “there will be no need to wait the months that the bigger detectors would require”.

In the Guinness Book of Records
It is in the cooling that Frossati’s skills come in. In the 80s he beat the record of continuous low temperatures, achieving less than two millikelvins above absolute zero, and thus gained a place in the Guinness Book of Records.

To develop the Einstein, however, the cooling time will have to rise to a month and the “thermos bottle” in which it will be contained will be significantly bigger. This, according to Aguiar, “will demand a much bigger building that the USP laboratory, equipped with special cranes to move it”. The Einstein, he says, should increase the detection energy sensitivity of the Schenberg by a factor of 100 and its cost will rise to US$ 7 million.

The Graviton project’s plan is to begin with smaller, cheaper detectors, to progress subsequently to the bigger, more sophisticated and efficient ones. Even the Schenberg will go through two stages: the Granted, formed by the initials of “gravitation radiation antenna for technological demonstration”; and the Detector, when the instrument begins operating, which should take place by May 2004.

“In the 3.0 a 3.4 kHz band, the Brazilian Schenberg, the Dutch Mini-Grail and the Italian Sfera have everything going for them to become the most sensitive in the world, since all the long range optical laser interferometers beginning operation will have a lot of background noise in this waveband”, says Aguiar. “Besides this, we will learn how to make still more sensitive detectors in other wavebands”, adds Aguiar.

The researcher compares detecting gravitational waves to the emergence of radio astronomy. “If looking at the universe in terms of radio wave lengths drew a new map of the cosmos, the observation of gravitational waves should expand these horizons”, claims Frossati. He believes that, as radio astronomy did by using microwaves, it will also be possible to record the gravitational waves of the Big Bang, the initial explosion creating the universe.

“The difference”, says Oliveira, “is that, whereas electromagnetic waves give a “snapshot” of when the universe was only 300,000 years old, gravitational waves will provide  a “snapshot” of the precise moment of creation. Something that cannot be left out from the science photo album”.

The detectors in action

There are two main types of gravitational wave detector in operation: the bar resonance and the laser types. Currently, there are five bar resonance types and one laser type among the most sensitive operating. Four laser types (interferometers) are set to begin operating and there are three spherical types under construction, among which is the Brazilian Schenberg.

In January 1960, the American physicist Joseph Weber proposed the bar resonance instruments and the first began operating five years later. The shape of a bar detector is changed by a gravitational wave: transducers, or accelerators, coupled to its surface, transform this deformation into an electrical signal that can be interpreted.

An analogy for a resonance are inflatable balloons. Anyone putting his hands on the surface can feel the vibrations caused by sound – which are distortions in the air – produced by sources such as music played loudly. In this case, the transducer would be the hands, conveying a mechanical signal to the brain.

Spherical detectors work on the same principle as bar detectors, with the difference that, in principle, they can record the waves coming from any direction, whereas bar detectors are limited to observations in directions close to the plane perpendicular to the bar. Laser detectors can record the space-time distortions directly, but they are too just expensive for Brazil. Aguiar adds: “Besides this, spherical detectors are able to determine, each of them alone, the direction a wave is coming from and its polarization (its manner of distorting space). Laser detectors, for example, need to be working in a group (of at least four of them) , in order to pinpoint the direction and polarization of the wave. The great advantage of the laser interferometers, justifying their high cost, is the simultaneous observation of a wide waveband with a single detector”.

The Project
The Schenberg Detector: Proposal for the Project, Construction, and Operation of a 0.6-Meter Diameter Spherical Wave Detector (nº 98/13468-9); Type Support for research project; Coordinator Odylio Denys de Aguiar – National Space Research Institute; Investment R$ 820,551.75 and US$ 563,000

Republish