Does space-time remember? The search for gravitational memory

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WHEN black holes collide in the distant reaches of the universe, they release energy in the form of gravitational waves. You can picture these passing through space-time like the ripples a dropped pebble creates on the surface of a pond.

“In a pond, after the ripples pass, the water returns to its old level,” says David Garfinkle, a cosmologist at Oakland University in Michigan. You might imagine that after the gravitational wave has passed, the fabric of the universe returns to normal too. “But it doesn’t,” says Garfinkle. In fact, Albert Einstein’s general theory of relativity, which says that gravity results from mass warping space-time, predicts that gravitational waves should ever-so-subtly shift the structure of space-time in their wake. In other words, the universe remembers.

This “gravitational memory” effect is so weak that it might as well be homeopathic. But, in recent years, a few optimistic astrophysicists have taken up the challenge of trying to demonstrate its existence. “They hedge their bets about when,” says Andrew Strominger, a theoretical physicist at Harvard University, “but nobody’s saying we can’t measure it.” And now, as more gravitational waves roll in, we might be on the cusp of a breakthrough.

The implications of such a discovery would be far-reaching. Gravitational memory would be evidence of a hidden form of symmetry that is thought to saturate the whole universe. This, in turn, would provide vital and potentially decisive clues about a quantum theory of gravity – and what space-time is ultimately made of.

The roots of this idea stretch back to the late 1960s, when physicist Joseph Weber thought he had made a startling discovery …

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