Scattered light noise due to dust particles contamination in the vacuum pipes of the Einstein Telescope

The search for gravitational waves is a quest to hear the faintest whispers of the universe, ripples in space-time caused by cataclysmic events like colliding black holes. To catch these signals, scientists build instruments of extreme sensitivity, using lasers that travel back and forth over distances of ten kilometers inside vacuum pipes. The goal is to measure changes in distance smaller than a fraction of an atom's width. However, these delicate machines are easily disturbed by stray light. If a single photon of the laser beam hits a speck of dust or a rough spot on a mirror, it can scatter, bounce off a vibrating wall, and re-enter the main beam. This re-entering light carries a distorted version of the vibration it encountered, creating a noise that can mask the cosmic signals scientists are trying to detect. For the next generation of these detectors, known as the Einstein Telescope, researchers must ensure that every surface inside the vacuum pipes is perfectly clean, because even the tiniest imperfection could drown out the universe's voice.

A team of physicists has now turned their attention to a specific source of this trouble: dust particles settling on the conical shields, or baffles, that line the inside of the vacuum pipes. These baffles are designed to catch stray light from the main mirrors before it hits the rough walls of the pipe, but the baffles themselves can become a problem if they are dirty. The researchers set out to understand how light scatters when it hits a dust particle sitting on a reflective surface, and to determine just how clean the installation environment must be to keep the noise low enough. They focused on particles ranging from one-tenth of a micrometer to three hundred micrometers in diameter, covering the size of fine smoke to that of a grain of sand.

Using computer simulations based on the physics of light scattering, the team modeled how light behaves when it strikes these particles. They found that the amount of noise generated depends heavily on the size of the particle and what it is made of. Small particles, those much smaller than the wavelength of the laser light, scatter very little. However, larger particles can scatter light much more efficiently, especially if they are made of materials that reflect light well and absorb very little of it. The researchers discovered that the most dangerous dust would likely be composed of materials like glass or mineral dust, which are common in construction and laboratory environments. They also found that the shape of the scattering pattern changes depending on whether the light is polarized in one direction or another, but for the purpose of setting safety limits, they focused on the worst-case scenarios where the scattering is strongest.

The study then applied these findings to the specific design of the Einstein Telescope. The team calculated the maximum number of dust particles allowed on each baffle to ensure that the noise they create does not exceed the noise produced by the baffle material itself. They determined that if the baffles are made of a material that reflects light strongly, the allowable density of dust is extremely low. For instance, if the surface is highly reflective, the presence of even a few large particles could create significant noise. The researchers concluded that to meet these strict limits, the assembly of the vacuum pipes must take place in an environment with a very high level of cleanliness, likely requiring a controlled clean room. They provided specific guidelines for the density of particles allowed on the surfaces, suggesting that for the most sensitive parts of the detector, the air must be virtually free of dust larger than a few micrometers.

The paper also addressed the uncertainty regarding the exact materials that might end up as dust inside the telescope. Since the final location of the detector has not been chosen, and the dust could come from the tunnel excavation, the construction process, or the handling of components, the researchers tested a wide range of possible materials. They simulated everything from typical environmental dust to human skin cells and aluminum. Their results showed that while the exact composition matters, the general rule remains the same: larger particles are the primary concern, and their presence must be minimized regardless of what they are made of. The study did not find a single magic solution to the problem, but rather provided a clear set of rules for how clean the environment needs to be. By establishing these limits, the team has given engineers a concrete target for the construction and assembly of the Einstein Telescope, ensuring that the machine will be quiet enough to listen to the universe without being drowned out by its own dust.