Gravitational waves have yet to be directly detected, but that isn’t stopping European scientists advancing plans for a €1 billion observatory to study them. Today a European team released the results of a €3 million design study on the Einstein Telescope (ET), a facility that would be constructed in around 2025 and would represent the third generation of gravitational wave detection instruments.
“The timescale for building such observatories is long. This is exactly the right time to investigate it,” says Harald Lück of the Max-Planck-Institute for Gravitational Physics in Hannover, the study team’s deputy scientific coordinator. The team calls the project a “telescope” or “observatory” rather than simply a “detector,” because it will have a greater capacity than current generation of detectors to discern the nature of the sources producing the waves it detects.
The idea that dramatic astrophysical events, like the merger of black holes or neutron stars, produce large ripples in space-time, follows from Albert Einstein’s theory of general relativity. Detailed studies of the radio emissions from inspiralling pulsars have already shown energy is indeed carried away, apparently by gravitational waves. Yet the first generation of gravitational detectors, including the Laser Interferometer Gravitational Observatory (LIGO) in Hanford, Washington, and Livingston, Louisiana, in the US, Virgo near PISA in Italy, GEO600 near Hannover in Germany, and TAMA300 near Tokyo, Japan, have so far failed to detect the waves directly. The detectors work by comparing the length of perpendicular arms as measured by laser beams sent along them. This length is expected to be distorted when gravitational waves pass by Earth.
Physicists have been undaunted by the no-show, saying that a detectable event is only expected to occur every 10 to 50 years within the region of space that current experiments are sensitive to. They’re betting that a second generation of detectors, called Advanced LIGO at the Hanford and Livingston sites and Advanced VIRGO near PISA, will see something within a year of coming online in around 2015. Generation two is expected to be 10 times more sensitive than generation one, and so able to pick up sources in 1000 times greater volume of the sky. But even generation two won’t be sensitive enough to engage in detailed observation of the waves’ sources, argues Lück. He makes the case for a future, ultrahigh sensitive observatory that he says would be able to probe the internal composition of merging neutron stars, and measure the distance to merging black holes at the edge of the universe.
The ET would consist of three nested detectors in a triangular configuration, with an arm length of 10km, as opposed to 4km for LIGO. It would be underground, to shield it from vibrations on the surface of the Earth. The triangular shape should enable it to determine the polarization of the waves – in which direction they are oscillating – and so learn extra information about the objects that emitted them. The ET would be 10 times more sensitive again than Advanced LIGO, and should be able to pick up gravitational waves from events at the edge of the universe, enabling not only studies of the astrophysics involved in the events, but even cosmological measurements – for example of the universe’s expansion, says Lueck. The team is considering underground sites in Sardinia, Italy, Hungary, and the Spanish Pyrenees, among others.
David Reitze, a physicist at the University of Florida in Gainesville who was the spokesman for LIGO, disagrees with part of Lueck’s argument for the ET, saying that Advanced LIGO will already be able to learn about the nature of gravitational wave sources by studying the waveform, the shape of the waves, that it detects. But he agrees more advanced astrophysics to study the cores of the objects, and cosmology, will only be possible with a more sensitive observatory. He’s the principal investigator on a $15 million proposal to the US National Science Foundation to build a generation 3 observatory in the US. “European funding agencies are making a conscious decision to have the most sensitive detector in 2030,” he says, “in the US there isn’t yet a great focus on the third generation, most are focused on Advanced LIGO,” he says.
Although the Europeans are moving ahead quickly, Michele Punturo of the National Institute of Nuclear Physics in Perugia, Italy, the design study’s scientific coordinator, says the team is aware that the true value of gravitational wave observatories depends on having a network of at least three of them worldwide. Multiple detection sites are needed to triangulate the source on the sky, and attempt to correlate that with gamma-ray or X-ray emissions caused by the same events.
Lück insists it’s not too soon to start planning for generation three, as once the waves are detected, there will be a strong case for beginning construction of the ET, and the advanced planning will pay off.
Image: Einstein Telescope
Correction 3pm BST: the text was updated to reflect that the ET would consist of three nested detectors, not two