Sensors

Not only the sea has a tidal effect, but also the Earth follows the tidal waves generated by the Moon. There are constantly very small earthquakes that can disturb the measurements. Even distant mining activities, passing trains, or windmills can have an influence on the Einstein Telescope.

The ET mirrors that will detect the gravitational waves must be protected from all these external disturbances and noise. The measurement and control systems that must achieve the isolation from external gravitational influences use extremely precise lasers and sensors.

A first group of sensors will measure the laser light reflected by the mirrors. The lasers operate at the uncommon wavelengths of 1550 nanometers and 2090 nm (frequencies from 286 THz to 143 THz ). The sensors must be able to detect changes in laser light in the order of 10-15 m. Therefore, new laser sensors and cameras must be developed with extremely high accuracy.

The Einstein Telescope is constantly impacted by seismic noise and gravity gradients. The Earth itself moves due to tidal waves, but even the tidal waves of the ocean can have a microseismic effect at great distances. Other seismic influences are caused by local effects or have an anthropomorphic (man-made) source.

An important source of seismic noise is the so-called Newtonian noise (also called Gravity (called gradients ). This noise is produced by small fluctuations in the Earth’s gravitational field. The Einstein Telescope is so sensitive that it can measure these small changes.

For example, if the ET were on the surface, even the movement of the air above the ET would affect the detector. Even if the detector were underground, there are small variations in the ground due to seismic waves. The influence of Newtonian noise affects the Einstein Telescope at very low frequencies, down to 10 Hz. The effect can result in a ‘seismic wall’ that hinders the sensitivity of the ET at low frequencies.

Geologists identify four types of waves that could affect the Einstein Telescope.

• S-waves move up and down and can best be compared to ripples on the surface of water.
• P-waves travel forward and backward, and are therefore local compressions that travel through the Earth, much like a sound wave travels through air.
• Love waves are snake-like sideways vibrations.
• Rayleigh waves are a more complex combination of motions, as if a spinning wheel of vibrations were moving through the Earth.

The Einstein Telescope must be protected from all these seismic influences, and this is done on two levels.

The vacuum towers holding the mirrors are placed on heavy concrete floors that can move freely in six degrees of freedom (X, Y, Z, tilt, roll and yaw). Any change in the movement of the concrete floors must be measured and controlled to a level of 10-15

Inside the towers, a series of inverted pendulum and multiple damping systems further ensure that the mirrors remain stable to the desired level of 10-21 m.

In the Einstein Telescope , laser beams pass through 10 km long arms and the light is captured using optical mirrors. These optical elements are suspended as still as possible to reduce the earth’s vibrations and to measure only gravitational waves.

Despite the advanced suspension systems keeping the mirrors as stable as possible, seismic waves still pass through the tunnels. These vibrations can be up to a billion times stronger than gravitational waves. Therefore, it cannot be ruled out that the vibrations still have an impact on the mirrors.

This is where differential sensors come in. Inertial sensors accurately measure the earth’s vibrations so that these vibrations can be digitally subtracted from the mirror’s readings. The sensor achieves these signals by measuring its movement with a laser relative to a decoupled mass.

The Einstein Telescope (ET) is not the only sensor in the tunnels. Thousands of environmental sensors will also be needed, both on the surface and in the underground caverns and tunnels.

The following list provides some examples of sensors that monitor the ET’s environment, and measure external disturbances that could lead to erroneous measurements.

• In the larger area around ET, very sensitive seismometers and accelerometers will be placed for precise measurement of seismic vibrations. Some seismometers are placed on the Earth’s surface, others are located in boreholes 250 to 400 meters deep or are placed inside the caverns and tunnels of the Einstein Telescope itself.
• Sensitive microphones pick up all sounds in the caves. In the operational phase, the tunnels must be completely silent. That is also why no one is allowed in the tunnels in the first place when the ET is in operation. Nevertheless, sounds from the surface can still enter the tunnels. Acoustic sensors must measure the sound from the surface that propagates in the caves and tunnels. These sounds can generate so-called standing waves in the caverns and tunnels, which are picked up by the ET as erroneous measurements. This includes infrasound generated by, for example, trains, road traffic, compressors, … or from natural sources such as thunderstorms or earthquakes.
• Barometers and pressure sensors will measure variations in air pressure. These variations in air pressure can also cause unwanted air movements. The barometers are supported by thermo-hygrometers, thermometers and weather stations that can be part of a worldwide network of weather sensors.
• Lightning detectors at the vertices could be part of a similar global network of detectors. It is expected that the ET will be influenced by lightning strikes from all over the world, which create so-called Schumann resonances in the atmosphere. The electromagnetic waves can be picked up by the coils of the sensitive vibration dampers in the vacuum towers and thus cause vibrations on the mirrors.
• Flow meters will measure the water discharged from the tunnels.
• Gauss sensors and magnetometers will measure so-called eddy currents and electromagnetic fields in the caves. These fields can also be picked up by the coils controlling the dampers, so they should be avoided. The sensors can be supported by gamma-ray detectors.
• The large number of sensors in the caves requires a separate (SCADA) instrumentation network and additional computer equipment. The platform will become an integral part of the control center that monitors and controls the Einstein Telescope .

The unique environment of the Einstein Telescope makes new types of measurements possible. Fiber optic cables can be installed in the tunnels to measure small displacements of the Earth over the lifetime of the telescope (50 years). The results of this scientific research will be valuable for other large civil constructions such as railway tunnels and bridges.

The ET would also be the ideal candidate to demonstrate the use of new ultra-precise quantum sensors. In the field of quantum technology, new sensors are being developed for environmental measurements (e.g. temperature and pressure, but also magnetic fields). Research into quantum detectors for laser light is also in full swing. The detectors should meet the high precision requirements of the Einstein Telescope , which makes them useful for a wide range of industries.

The development of sensors, photodiodes and micro-electromechanical systems (MEMS) is done in close cooperation between universities, knowledge institutions and industry. The sensors developed for the ET will not be usable for other gravity detectors, but can also be used in other industries.

The sensors are expected to find their way into aerospace and space applications, the automotive industry, in the energy sectors (e.g. batteries) and telecommunications industries, and by extension to all applications requiring highly sensitive electronics.