How to measure waves in space-time

When a gravitational wave passes through the Earth, it causes space itself to stretch in one direction and compress in another, so that the two “arms” of the detector actually grow and shrink by small amounts. This means that each beam of light travels a slightly different distance, which shows up in the recombined laser light pattern as a jump in frequency called a “cosmic chirp” – this is a 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 it absorbs only 1 in 3 million photons that hit it. It is polished to an atomic level, leaving it so smooth that there is virtually no light scattering. And it’s coated with a thin layer of material that’s so reflective that less than 0.0001 percent of laser light is lost on contact.

Inside one of the 3-kilometer-long Virgo, with a 1.2-meter-diameter main vacuum tube in which the laser light travels.\

Photo: EGO/Devica

Each mirror hangs below the super-attenuator to protect it from seismic vibrations. They consist of a chain of seismic filters that act like pendulums, tucked into a vacuum chamber inside a 10-meter-high tower. The setup is designed to counter the Earth’s motions, which can be nine orders of magnitude stronger than the gravitational waves that Virgo is trying to detect. The superattenuators are so efficient that, at least in the horizontal direction, the mirrors behave as if they are floating in space.

A more recent innovation is Virgo’s “squeezing” system, which combats the effects of Heisenberg’s uncertainty principle, a strange feature of the subatomic world that states that certain pairs of properties of a quantum particle cannot be precisely measured simultaneously. For example, you cannot measure both the position and momentum of a photon with absolute precision. The more precisely you know its position, the less you know about its momentum and vice versa.

Within Virgo, the uncertainty principle manifests as quantum noise, obscuring the gravitational wave signal. But by injecting a special state of light into a tube that runs parallel to the main vacuum tubes and then overlaps the main laser field at the beam splitter, the researchers can “squeeze” or reduce the uncertainty in the properties of the laser light, reducing quantum noise and improving Virgo’s sensitivity to gravitational wave signals.

Since 2015, nearly 100 gravitational wave events have been recorded during three observations by Virgo and its American counterpart LIGO. With both facilities upgraded and joining KAGRA, the next observation—starting in March 2023—promises much more. Researchers hope to gain a deeper understanding of black holes and neutron stars, and the sheer scale of the expected events offers tantalizing prospects for building a picture of the evolution of the cosmos through gravitational waves. “This is just the beginning of a new way of understanding the universe,” says Losurdo. “A lot is going to happen in the next few years.”

This article was originally published in the January/February 2023 issue of WIRED UK.

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