We are about to hear echoes in the fabric of space for the first time

Gravitational waves can be lensed by massive galaxies so that they repeat, like an echo. Scientists are now readying to snare their first one and explore the cosmic secrets it holds

By Jonathan O'Callaghan

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Did you hear the one about the star that died twice? In 2014, astronomers saw the explosion of the Refsdal supernova. Then, 360 days later, it went bang again.

This bizarre sequence of events was down to a phenomenon called gravitational lensing, in which massive objects warp the fabric of space enough to cause light to bend. The path of the flash from the supernova was changed in this way on its journey to us, so that portions of it took different routes and arrived at different times - almost a year apart in this extreme case.

As that story shows, gravitational lensing has been around for a while, but now it is about to enter a compelling new chapter. Scientists know it isn't just light that can be lensed, but gravitational waves too. It is a mind-bending concept: ripples in space-time themselves being distorted by the curvature of space. It is also a deeply important phenomenon that could illuminate the secret interiors of neutron stars, settle a mystery about the power of dark energy and test gravity itself more keenly than ever. And here is the best part: we may be on the cusp of spotting our first lensed gravitational wave.

No one is under any illusions that this will be anything other than fiendishly difficult. Still, there is a sense it will happen sooner or later - and there are tricks we can pull to expedite the discovery. "It's exciting, and it's going to happen," says Simon Birrer at Stony Brook University in New York. "There's…