Rebecca Priestley: The "sound" of black holes banging together

by Rebecca Priestley / 13 May, 2016
Billions of years after the event, scientists are getting excited.
Photo/Getty Images
Photo/Getty Images

The Big Bang, black holes, gravitational waves: my science columns based on interviews with University of Auckland cosmologist Professor Richard Easther have been among the most popular, always prompting reader questions. Why are people so interested in cosmology?

“I think it’s an innate human drive,” says Pro­fessor Janna Levin, whom Easther will introduce when she appears at the Auckland Writers Festival. “There’s never been a civilisation that didn’t look up at the sky and ask what it was all about and where we are in the scheme of things.”

Today, this drive is behind the construction and operation of some of the world’s biggest and most expensive scientific instruments. In a talk called “Gravitational Sensations”, Levin will tell the story of the Laser ­Interferometer Gravitational-Wave Observatory, or Ligo, which is the subject of her recently published book, Black Hole Blues.

What is Ligo? Put simply, it consists of two 4km-long L-shaped detectors, one in Louisiana and one in Washington, designed to measure what Levin calls “Lilliputian modulations” in the shape of space-time. Any gravitational waves that reach the Earth would stretch one arm of each detector tunnel more than the other, by an amount measured using a precisely calibrated system of lasers and mirrors. Ligo scientists could then amplify the “ripple” into a sound, calculate the size of the cosmic disturbance that caused the ripple, and determine the distance and direction of the instigating event.

The occurrences Ligo is trying to measure are massive: colliding black holes and neutron stars, pulsars and exploding stars. In Black Hole Blues, Levin uses metaphors to describe the relationship between the cosmic event and Ligo. “Astrophysical calamities are the mallets, space-time is the skin of a three-dimensional drum and the apparatus records the modulations in the shape of the drum to play the silent score back to us as sound.”

Janna Levin. Photo/Sonya Levin
Janna Levin. Photo/Sonya Georgevich

But how can the “sound” of two black holes colliding be differentiated from any other event in the universe? “It’s like a cocktail-party effect,” says Levin. “If you’re at a cocktail party and it’s very loud, you can still pick out your friend’s voice.” Theoreticians such as Levin predict and model the sound that cosmic collisions make, giving experimentalists the chance to search for the distinctive signals.

On September 14, 2015, soon after the Ligo detectors had been upgraded and switched on, the instrument detected gravitational waves: it was 100 years after Einstein predicted their existence. By ­February this year, scientists had worked out that the waves were from the collision of two black holes about 1.3 billion years ago.

The black holes had probably been ­spiralling together over billions of years, says Levin, creating ripples in space-time. “And the ripples have been washing over the planet ever since the Earth started forming. But it wasn’t until the final milliseconds, when they were just a few hundred kilometres apart and travelling near the speed of light, that the banging was loud enough” for Ligo to detect.

Once Ligo comes back online later this year, “they fully expect to detect another black hole system”, says Levin. In time, the instrument should be detecting several collisions a year.

Levin expects it to also find evidence of neutron-star collisions and more. “But what would be really exciting is if space-time is ringing as a result of something we hadn’t thought of yet. That would be a great day.” As the Ligo website puts it, “the possibilities for discovery are as rich and boundless as they have been with light-based astronomy.”

Janna Levin’s “Gravitational Sensations” talk is on May 15 at the Auckland Writers Festival. She also takes part in “The State of America” talk on May 13.

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