European scientists take a major step forward towards detecting gravitational waves
Scientists operating Europe’s gravitational wave observatories have combined efforts this summer to search for gravitational waves.
Europe’s two ground-based gravitational wave detectors GEO600 (a German/UK collaboration) and Virgo (a collaboration between Italy, France, the Netherlands, Poland and Hungary) have started a joint observation programme that will continue over the summer, ending in September 2011.
These detectors work by measuring tiny changes (less than the diameter of a proton), caused by a passing gravitational wave, in the lengths (hundreds or thousands of metres) of two joined arms lying in a horizontal L-shaped configuration. Laser beams are sent down the arms and are reflected from mirrors, suspended under vacuum at the ends of the arms, to a central photodetector. The periodic stretching and shrinking of the arms is then recorded as interference patterns.
“Listening” for gravitational waves benefits enormously from simultaneously deploying two or more such laser interferometers located at different points on the Earth's surface. In this way, any extraneous, terrestrially generated noise mimicking a genuine gravitational wave signal can be eliminated, since it is unlikely to have the same characteristics at the different locations while the gravitational wave signal would remain the same. Moreover, just as our brains can work out the direction of a sound source from the difference in signals received by our two ears, detectors in separate locations can help reconstruct the position in the sky of a gravitational wave source. (With two detectors, the most likely sky position lies in a circle; in the case of three or more detectors, it can be pinned down to few spot locations).
“If you compare GEO600 and Virgo, you can see that both detectors have similar sensitivities at high frequencies, at around 600Hz and above”, says Dr Hartmut Grote, a scientist at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) and the Leibniz Universität in Hannover, Germany. “That makes it very interesting for us to search this band for possible gravitational waves associated with supernovae or gamma-ray bursts that are observed with conventional telescopes.”
Gamma-ray bursts – the most luminous transient events in the Universe – may result from the collapse of a supermassive star core into neutron star or black hole. These phenomena are expected to generate strong gravitational radiation, and so provide ideal references for gravitational wave searches. The expected frequencies depend on the mass of the objects and may extend up to the kHz band. However, given the weakness of the expected gravitational wave signal, the likelihood of detecting such an event is low. How often such events can be detected therefore depends strongly on the sensitivity of the detectors.
Thanks to its excellent sensitivity at low frequencies (below 100 Hz), Virgo will also search for signals from isolated pulsars such as Vela, the remnant of a massive supernova explosion that emits regular pulses of electromagnetic radiation, from gamma-rays to radio waves. The gravitational wave signal frequency should be at around 22Hz.
In addition, the programme will test new technology that will be used in the next (second) generation of gravitational wave observatories.
GEO600: The German-British detector is located near Hannover, Germany and is run by scientists of the AEI and the British universities of Glasgow, Cardiff and Birmingham. The GEO project is funded by the Max Planck Society, the state of Lower Saxony, the Volkswagen Foundation and the British Science and Technologies Facilities Council (STFC). GEO works in close cooperation with the cluster of excellence QUEST (Centre for Quantum Engineering and Space-Time Research) in Hannover.
Virgo: a French-Italian-Dutch project with 3 km arms at Cascina near Pisa, Italy. This project has the additional goal of measuring at the low frequency end of the scale. Virgo is funded by CNRS (Centre national de la recherche scientifique) and the INFN (Istituto Nazionale de Fisica Nucleare).
About the GEO600 experiment
GEO600 has just completed a major upgrade. Many of the new technologies that have been developed and tested are being employed in the next generation of laser-interferometric gravitational wave detectors.
- Signal recycling, a technology to enhance the gravitational wave signal.
- Monolithic suspensions (with special silicate fibres) of the optical mirrors, which greatly reduce thermal noise.
- Electrostatic actuators keep the mirrors of the laser interferometer within optimal operating parameters. Electrostatic actuators cause fewer disturbances in the system than the electromagnetic actuators used to date.
Although GEO600 has relatively short arm lengths compared with other gravitational wave detectors, the new technology allows the observatory to specialise in the high-frequency range.
“We are actually very sensitive at high frequencies,” says Dr Grote. “In part, this is due to our having increased the laser power by 50%. The key technology, however, is what we call squeezed light, which allows us to reduce fluctuations in the signal at the quantum level. We installed it in GEO600 last year and are now testing it for the first time in an official science run.”
After completing the joint GEO600-Virgo Science Run
GEO600 will be the only laser interferometer gravitational wave observatory operating world-wide, along with the Italian low temperature and bar detectors. In this period, GEO600 will be able to observe gravitational waves from cosmic events which can also be seen by other means, such as gamma-ray or x-ray satellites.
GEO600 can already measure length changes as small as 10-18m – about one-thousandth the diameter of a proton. The next generation of detectors will be even more sensitive. They should be able to search for signals from a wider range of more distant sources.
Building larger detectors with increased sensitivity is extremely challenging. Many new technologies, including the most stable lasers in the world had to be specially developed. Even though systems have become ever more complex, successive data runs have repeatedly showed that they were absolutely reliable.
In recent years, GEO600 has become an international think-tank and research centre for gravitational wave technology, spawning new developments that are now used in gravitational wave detectors worldwide.
The new key technologies include highly stable laser light, triple pendulum mirror suspensions, as well as an optical method for recycling laser light. The technologies were developed at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) Hannover and Leibniz Universität Hannover in close cooperation with the Laser Zentrum Hannover eV (LZH, Laser Centre Hannover) and the University of Glasgow in the UK.
- A highly stable laser
The German scientists’ long experience with laser development has paid off at an international level: in close cooperation of AEI and LZH they have developed a new type of high-performance laser for use in the next generation of gravitational wave detectors. The first of these new lasers is just being installed in the LIGO project. A special feature is that it provides 200 W of power at a wavelength of 1064 nm. Its unsurpassed stability in both output power and frequency is what will make the high sensitivity of the new generation of gravitational wave detectors possible.
- Quiet light
GEO600 sensitivity has now reached the limits set by the laws of nature. At frequencies around 1000Hz – exactly where signals from supernovae and the birth of neutron stars are expected – the quantum nature of light comes into play. Just like pellets fired from a shotgun, individual photons will hit the detector at an uneven rate. Because of quantum fluctuations resulting form the Uncertainty Principle, this ‘shot-noise’ will show up as a fluctuating background signal that could completely obscure the expected short gravitational wave signal from the event itself. GEO scientists have managed to tame this unwanted signal noise by manipulating the fluctuations so as to produce what is called “squeezed light”. GEO600 was upgraded with a source of squeezed light in mid-2010 and has since been testing it under operating conditions.
- Monolithic suspensions
The central elements in all gravitational wave detectors are mirrors weighing up to 10 kg, which are used to direct the laser beams. These mirrors are suspended as pendulums, so that they are isolated from various disturbances. The mirror suspensions must meet several special requirements: they have to hold the heavy mirrors securely and must not cause disturbances of their own.
The Institute for Gravitational Research (IGR) of the University of Glasgow has developed suspensions meeting these requirements: thin threads made of quartz glass – fused silica fibres. Such fibres have far less internal losses than equivalent steel wires, for instance. They are bonded directly onto the mirrors and a second pendulum mass, which means there is no friction at the point of contact. This increases the overall sensitivity of GEO600 through reduced mechanical loss.
Many of the disturbing influences in a gravitational wave detector are seismic in origin. They are especially noticeable when making measurements in the low-frequency range below 100Hz. To reduce their influence, the Glasgow scientists have developed a multiple pendulum system which works to dampen these disturbances. They can reduce external vibration by nine orders of magnitude. In addition, the individual components of the pendulum are driven by electromagnetic or electrostatic actuators to reduce the residual noise. The result is that only an earthquake with the strength of 6 and above on the Richter scale occurring anywhere in the world can jolt the gravitational wave detector out of alignment. In all, 260 control loops are needed to move the mirror back into alignment, hold it there and dampen external vibrations on the system.
- Tuned signal recycling
GEO600 is the only detector amplifying the signal of the laser beam. A special signal- recycling mirror at the signal exit reflects the interference beam back into the interferometer so that the part of the laser light containing the expected gravitational wave signal is amplified. This process is repeated until the signal is 10 times stronger. GEO600s latest signal recycling mirror especially can amplify the signal in a broad frequency band. Tuned signal recycling is one of the main reasons why GEO600 has a similar sensitivity to Virgo at higher frequencies, despite its shorter arm lengths. The advanced LIGO- and Virgo detectors will also use this technology for increasing their sensitivity.
Science runs: taking data and looking for a needle in a haystack
The experimental strategy behind gravitational wave observations is to operate detectors across the world as a network. True gravitational wave events can then be identified because they should pass through each detector at the same time.
To pick out gravitational wave events in the flood of data requires complex computer simulations that can predict possible sources and patterns for gravitational waves. Theorists working in the field of “numerical relativity” provide fingerprints of the signals for their experimental colleagues.
Since GEO600 and the international gravitational wave observatory network were launched, there have been six science runs. Currently, the data from the last science run (S6), involving GEO600, LIGO and Virgo, are being evaluated at the AEI using a dedicated computer cluster ATLAS, which is one of the fastest supercomputers in the world.
In the course of LIGO Scientific Collaboration data analysis, the project Einstein@Home was born. This allows interested amateurs to contribute to the search of gravitational wave data. Since 2009 Einstein@Home has also been used to analyse data from the Arecibo Radio Telescope: in the summer of 2010 a German and two American amateur scientists discovered a new radio pulsar, and in spring 2011 a binary system of a neutron star and a white dwarf was found. In the past weeks, Einstein@Home has also discovered five new radio pulsars in data from the Australian Parkes observatory.
Expectations: gravitational-wave astronomy and collaboration with other research fields
The search for Einstein’s gravitational waves is still one of the most important questions in modern science. Scientists hope that the direct observation of gravitational waves will not only confirm the Theory of General Relativity, but also launch a new era of gravitational wave astronomy, opening up a completely new view of our Universe: For the first time, it will be possible to look back to the earliest times, when the Universe was in its infancy.
Until now, we have been able to observe the Universe only in the electromagnetic spectrum, which means that the earliest information that can reach us comes from about 380,000 years after the Big Bang. Earlier epochs remain hidden to observation, because photons of light were then constantly interacting with matter rendering the Universe opaque to electromagnetic radiation. Consequently, none of the various theories concerning the early Universe have been experimentally tested. Direct measurements of gravitational waves should allow scientists to observe as far back as the first billionth of a second after the Big Bang.
Gravitational wave astronomy will also be important for other fields in astrophysics, such as the study of very compact and massive objects. Thus, close collaboration with astronomers observing in the electromagnetic spectrum, for example, in the gamma or X-ray region would be very fruitful. For example, the nature of gamma-ray bursts has not been wholly explained, and detecting a source of gravitational waves in the area where gamma-ray bursts are observed would very likely show whether they really come from the supernova explosions of supermassive stars.
Gravitational waves would also give a clearer insight into the internal structures of single and double neutron-star systems, which to date have been observable only in radio or X-rays. While electromagnetic radiation comes mostly from the surfaces of astrophysical objects, permitting only indirect observation of their inner workings, gravitational wave radiation will give direct information of what is going on inside these objects. More significantly, gravitational waves from astrophysical events can most likely be observed before there is an eruption of visible, radio or X-ray light. If this happens, gravitational wave astronomers can alert their colleagues to the event, enabling them to turn their telescopes in the appropriate direction.