How to measure ripples in spacetime

When a gravitational wave passes through the Earth, it causes space to stretch itself in one direction and compress in the other, so the detector’s two “arms” actually grow and contract by a very small amount. This means that each beam of light travels a slightly different distance, appearing in the recombinant laser light pattern as a spike in frequency known as a “cosmic cry”—this is a wave signal. attractive.

To measure, 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 at the atomic level, making it so smooth that there is almost no scattering of light. And it’s coated with a thin material that’s so reflective that less than 0.0001 percent of laser light is lost on exposure.

Inside one of Virgo’s 3 km long arms, there is a 1.2 meter diameter main vacuum tube through which the laser light travels.\

Photo: EGO/Virgo

Each mirror is suspended below a super-attenuator to protect it from seismic shocks. They consist of a series of seismic filters that act like a pendulum, encased in a vacuum chamber inside a 10-meter-tall tower. The setup is designed to resist Earth’s movements, which can be 9 orders of magnitude stronger than the gravitational waves Virgo is trying to detect. The superattenuators are so efficient that, at least horizontally, the mirrors behave as if they were suspended in space.

A more recent innovation is Virgo’s “squeezing” system, which counteracts the effects of the Heisenberg uncertainty principle, a curious feature of the subatomic world that states that certain pairs of properties of a quantum particle death cannot be measured exactly at the same time. For example, you cannot measure both the position and momentum of a photon with absolute precision. The more you know exactly where it is, 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 particular state of light into a tube that runs parallel to the main vacuum tubes and then overlaying the main laser field at the beamsplitter, the researchers were able to “squeeze” or reduce the brightness. Uncertainty in the properties of laser light, quantum reduction. noise and improves Virgo’s sensitivity to gravitational wave signals.

Since 2015, nearly 100 gravitational wave events have been recorded during three observations by Virgo and US partner LIGO. With the upgrade of both facilities and KAGRA joining the team, the next round of observations—commencing in March 2023—promises even more. The researchers hope to deepen their understanding of black holes and neutron stars, and the high volume of events is expected to offer the intriguing prospect of building a picture of the evolution of the universe through through gravitational waves. “This is just the beginning of a new way of understanding the universe,” says Losurdo. “A lot of things will happen in the next few years.”

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


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