Listening to the Universe: Building the Einstein Telescope

My earlier career was as a particle physicist in CERN. We identify four fundamental forces in physics:

• electro-magnetism
• weak interaction
• strong interaction
• gravitation

We have the quantum theory to explain three of them. In 2012 at CERN we discovered the Higgs particle using the Large Hadron Collider at CERN.

Gravity was discovered in the 17th century. It got replaced by Einstein's general relativity. This is poorly tested. Central are gravitational waves.

Einstein: gravity is not a force, but is a curve in space and time itself. If it is curved, it should be possible to create ripples in it. This is similar to electro-magnetism. Gravitational waves are generated by movements of mass: if I shake this pointer, technically it will create tiny gravitational waves. Einstein made a prediction about gravitational waves, but said they could probably never be measured.

If you have two black holes that circulate around each other, they generate gravitational waves. They lose energy because they generate those waves. They collide, become one black hole, and stop. Then the gravitational waves also stop being sent. Eventually these waves will reach the earth. Detecting them requires precision.

Compare: take the Mediterranean Sea, add one drop of water, and the wave this generates is comparable to the effect of a gravitational wave.

Observation GW150914 was the first: two black holes of 36 and 29 solar masses merge into a single black hole of 62 solar masses. 3 solar masses were converted into gravitational wave energy. The most energetic event ever measured. Since then, we measured about 400 of these events.

Current detectors observe about one signal per week. We want to build the Einstein Telescope (ET). This will detect about 100 thousand to 1 million mergers of black holes per year, is the expectation. Current detectors are until about 4 kilometers, the ET will be about 10 kilometers.

If you observe a wave, you look into the past. The oldest black hole observed, is from at most about 6 billion years ago. With the ET, we could look all the way back to the Big Bang. So it is really in this sense the ultimate telescope: there is no sense in building an even bigger one, because we will already be able to see everything.

The overal structure of science related to this in Europe is a board of governmental representatives, then the ET project organisation, site organisations, and ET collaboration. There is competition for three spots to build the ET: Italy (Sardinia), the region at the border of Germany and Poland (Saxony), and the region where you are now, in the Maastricht region, or EMR (Euregio Maas-Rhine).

We don't know yet how we want to build it. There are two promising geometries: a triangle, or two L-shapes. Nice thing about the triangle, is that you can really do this in a way that you can have three L-shapes hooking into each other, in the same tunnel. With two L-shapes, one L would need to be in Sardinia, and the other either in the Maastricht region or in Saxony.

ET project timeline: in October there will be a decision on which geometry to use. In december there is a call for tenders, open for half a year, and in third quarter next year the final decision of location (or locations) will be made.

In the ET we want to lower the frequency that we can detect from 40 Herz to maybe as low as 8 Herz. The lower you can get this, the longer the period you can hear and study an event: at 40 Herz it was half a second, at lower frequencies it can be half an hour. Here in Maastricht we are researching the low-frequency challenge, with ETpathfinder.

We have a special geological environment in this region. The top layer is quite soft, but below is hard rock. Vibrations made by man and nature are absorbed by the soft top layer. We made many drill holes to find the optimal location. The triangle will be completely in Belgium, in Wallonia. We are developing the models needed to predict how the site will affect the telescope.