Both the Netherlands and Italy are candidates to host the Einstein Telescope. A triangular design is ready for that, but the organisation behind the world's most sensitive detector for gravitational waves is also investigating a variant with two L-shaped detectors, it was recently announced. What about it?
Work on the Einstein Telescope has been going on for years, at least preparations for the huge instrument. There is a design, a triangle with sides ten kilometres long, with two V-shaped interferometers at each vertex (see the box for an explanation of how the Einstein Telescope works).
The instrument should measure gravitational waves much more sensitively than existing detectors in the United States (LIGO), Japan (KAGRA) and Italy (Virgo). The extremely sensitive device will be located underground, at a depth of almost three hundred metres, to minimise outside disturbances. Two sites are, for years, in the race for the location: the province of Limburg and the Italian island of Sardinia. Construction of the Einstein Telescope is roughly budgeted at €2 billion.
Bottom
In both places, scientists are currently investigating the soil: is it suitable to drill into to build the detector? This will provide drill cores of the subsurface, and seismic sensors will be inserted in the holes.
'By generating a vibration further down and measuring it, you gain knowledge about the structure of the subsurface,' says Andreas Freise, professor of Gravitational Wave Physics at VU University Amsterdam and project leader for the international Einstein Telescope organisation. Freise has been researching gravitational waves for many years and was also involved in previous detectors, such as Virgo.
This soil survey should provide a map of the subsurface near the Limburg site. In addition, hydrologists are preparing a model of the underground water flows. Freise: 'All these things we want to have mapped before we start building underground, but also for the buildings above ground and for constructing the access shafts.'
This is how the Einstein Telescope works
You could call it the world's most sensitive measuring device, the Einstein Telescope. That is an instrument designed to detect gravitational waves, ripples in space-time that occasionally pass by from outer space. The waves are caused by major events in the universe, such as collisions between giant objects or the merging of two black holes.
The Einstein Telescope is so sensitive that measurements are based on displacements of mirrors the size of 10-19 metres. To give a sense: that is one millionth of the diameter of an atom. Laser beams pass back and forth between those mirrors. Normally, those two beams going in opposite directions extinguish each other. But the moment a gravitational wave passes by, the distance between the mirrors stretches (or contracts) a little and then the beams actually amplify each other, which can be measured as a light signal. Want to know more? Then check out the project's excellent website.
L-shaped detectors
Besides the triangular detector design, the organisation behind the Einstein Telescope is now investigating another option: two L-shaped detectors, according to a recent news report in Nature. Such an 'L' has arms 15 kilometres long, and in this variant there would be one underground near Maastricht, and one in Sardinia. Nearly a thousand kilometres apart, the two Ls would form one large detector for gravitational waves.
But where did the idea for the L-shaped detectors suddenly come from? 'To be honest, this option has been on the table all along, only with that Nature article is it getting wide attention for the first time,' says Freise. The US Laser Interferometer Gravitational-Wave Observatory(LIGO), the instrument that first confirmed Einstein's theory by actually measuring gravitational waves in 2016, also consists of two L-shaped detectors.
Triangle the best design
For the Einstein Telescope, the triangle emerged in 2010 as probably the best design, 'but in the background the L-shape always remained an option,' says Freise. 'Right now we are doing more detailed studies on the different variants and the question is again on the table as to which is the best. Our aim is to gather as much accurate information as possible on the two variants, which governments can then use to make a good comparison between the triangle and the two Ls. Then it is a political choice as to which one it will be. The politicians get to weigh up scientific aspects, costs, feasibility of variants and risks.'
Detail of the Einstein Telescope: in the columns, mirrors are suspended virtually vibration-free. Illustration Marco Kraan / Nikhef
There are also disadvantages
By itself, a single L detector could also measure a gravitational wave, but the desire is to build a more powerful instrument, says Freise. By putting two Ls at a great distance, it is more accurate to measure from which direction the wave came. 'But there are also disadvantages. Costs will be higher because you have to build in two locations underground. And running the eventual Einstein Telescope is more complex when there are two sites.'
CERN
Freise himself prefers the triangular variant. 'Because one site offers the chance to create the centre for gravitational wave research. One place where all scientists should be for gravitational wave research, but where there is also collaboration with companies and where citizens can visit to learn about physics and science. Think of a centre similar to what CERN is for elementary particle research.'
Currently, the triangle is preferred in the Netherlands, and the L-shape has more supporters in Italy, Freise observes. 'There is friendly competition. But the decision will be made on the basis of hard facts. That will be a political process, based on the facts and numbers we collect.'
Chance of success
Now what if the exploratory study shows that the L-shape has a better chance of success? Freise: 'Then I will be the first to say: let's do it! Whichever variant it becomes, we know it will be about a hundred times better than existing detectors, so the most important thing is THAT the Einstein Telescope will be made.'
Research into the two variants will continue until at least 2026. It will then be up to politicians to make a choice: one large Einstein Telescope in one place, or an arm of the instrument in Limburg and one in Italy. See alsohttps://deingenier.nl/ET
Opening image Impression of the Limburg hills with the Einstein Telescope below. Illustration Marco Kraan / Nikhef
