Weighing Cosmic Dust with Gravity express
In absolute silence, you need to hear a feather drop. But someone keeps rustling a plastic bag. For the space-based LISA observatory, that 'rustling' will come from charged particles in space.
LISA consists of three satellites that use laser beams to measure the distance between freely floating gold-platinum cubes inside them. When a gravitational wave passes, space stretches slightly, and the distance changes. But the solar wind and galactic cosmic rays bombard the cubes. Each incoming particle leaves its charge on the cube, like dust on glass. Gradually, charge accumulates, and the cube begins to feel the electric fields of the walls of its container. A force arises that pushes the cube, and this push could be mistaken for a gravitational wave.
The main suspect was low-energy secondary electrons. When a proton hits the cube, it knocks out several electrons from its surface. If these electrons fly away to the walls, the cube's charge grows. But if some of them return, the balance changes. To catch this effect, physicists set up an experiment at an accelerator. They bombarded a mock-up of the LISA measurement system with protons and, for the first time, directly measured how charging depends on the electric potential of the cube itself. It turned out that at a small positive voltage, the cube charges less — some of the knocked-out electrons return home.
Now engineers will be able to calculate more precisely what electric 'climate' needs to be maintained inside the satellites to clean the signal from noise. And then, in the cosmic silence, we will be able to hear the Universe itself breathe.
Original abstract
as written by the authors arXiv:2606.22979
The Laser Interferometer Space Antenna (LISA) is a space-based gravitational-wave observatory that uses free-falling test masses as inertial references to detect milliHertz frequency signals. Interactions between test masses and galactic or solar energetic particles cause charge buildup and Coulomb forces, source of acceleration noise that must be accurately modeled. LISA Pathfinder measurements showed that the Poissonian test mass charging noise was considerably larger than that in pre launch simulations, indicating missing physical processes in early models. Emission of low energy secondary electrons (LEE) from test mass and housing surfaces has been proposed as a key mechanism affecting charging and the sensor electrostatic response. We report a particle accelerator based experiment that directly tests the LEE hypothesis by measuring proton induced test mass charging in a LISA like Gravitational Reference Sensor geometry as a function of the test mass electrostatic potential.
The charge that accumulates on LISA's test cube in a month is comparable to the charge from touching a doorknob after walking on a carpet, but in space, this tiny charge creates a force capable of shifting the cube by fractions of an atom.
In Arthur C. Clarke's novel 'Rendezvous with Rama', heroes explore the interior of an alien spacecraft where all objects float freely — much like the test masses inside LISA will float, except their motion must be tracked with unimaginable precision.