As a gravitational wave travels through Earth, it causes space itself to stretch in one direction and compress in another, so the detector’s two “arms” actually grow and contract by tiny amounts. This means that each beam of light travels a slightly different distance, which shows up in the recombined laser patterns as frequency spikes known as “cosmic chirps” — the gravitational wave signal.
To measure it, Virgo relies on state-of-the-art equipment. The mirrors at the end of each tunnel are made of synthetic quartz so pure that one in every three million photons is absorbed. It is atomically polished and extremely smooth with little light scattering. It is coated with a thin layer of material that is so reflective that it loses less than 0.0001% of the laser light on contact.
Each mirror is suspended below a superattenuator to protect it from seismic vibrations. They consist of a series of seismic filters that work like pendulums, encased in vacuum chambers inside the 10-meter tower. The device is designed to counteract Earth’s motion, which could be nine orders of magnitude stronger than the gravitational waves Virgo is trying to detect. The super attenuator is so effective that the mirrors behave as if they are floating in space, at least horizontally.
A recent innovation is Virgo’s “squeeze” system, which combats the effects of Heisenberg’s uncertainty principle, a curious feature of the subatomic world in which pairs of certain properties of quantum particles cannot be simultaneously suppressed. Measure precisely. For example, you cannot measure a photon’s position and momentum with absolute precision. The more precisely you know its position, the less you know about its momentum, and vice versa.
Inside Virgo, the uncertainty principle manifests itself as quantum noise, obscuring the gravitational wave signal. But by injecting a special state of light in a tube parallel to the main vacuum tube, which then overlaps the main laser field at a beam splitter, the researchers can “squeeze,” or reduce, the uncertainty in the properties of the laser, reducing quantum noise and Increased Virgo’s sensitivity to gravitational wave signals.
Since 2015, nearly 100 gravitational wave events have been recorded during the three observing sessions of Virgo and its US counterpart LIGO. With the upgrade of these two facilities and the addition of KAGRA, the next observing run (to begin in March 2023) promises to achieve even more. Researchers hope to gain a deeper understanding of black holes and neutron stars, and the abundance of expected events offers the tantalizing prospect of building a picture of the universe’s evolution through gravitational waves. “This is just the beginning of a new way to understand the universe,” Losurdo said. “A lot will happen in the next few years.”
This article originally appeared in the January/February 2023 issue of WIRED UK magazine.