91av

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 , 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 , 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 . Using little more than vibrating aluminium bars, he picked up a signal that he claimed was the first detection of gravitational waves. Weber’s announcement created a media sensation, but his peers were more cautious. Few physicists doubted the existence of gravitational waves, which fall directly out of the equations of general relativity, but the signal was expected to be so weak that it seemed unlikely Weber had seen it with his modest equipment.

Gravitational waves

Among the critics were two physicists called Alexander Polnarev and Yakov Zeldovich. To try to prove Weber wrong, they calculated how the biggest gravitational waves possible would . They imagined a super-dense cluster of stars in the centre of our Milky Way – far bigger than any cluster that actually exists there – making waves that disturbed two particles separated by 1000 kilometres. This was the same as the distance between Weber’s bars, which he had placed in labs across the globe. Even in such an extreme case, they calculated, Weber’s equipment needed to be 100 million times more sensitive to spot gravitational waves. “The detection was impossible,” says Polnarev.

Subscriber-only newsletter

Sign up to Lost in Space-Time

Sign up to newsletter

In proving Weber wrong, however, the pair happened upon a curious effect. The calculations revealed that particles vibrated by gravitational waves don’t return to their original locations. Instead, their positions are shifted by a minuscule amount. This happens because space-time, which combines the three dimensions of space with one of time into a four-dimensional fabric, is permanently stretched in one direction and squeezed in another by the gravitational wave.

Polnarev knew that this idea of a permanent mark in space-time left by a passing gravitational wave might turn out to be useful. But back then, the idea that we might detect such waves was a moonshot, let alone this far weaker distortion of space-time. Over the next few decades, most physicists didn’t give gravitational memory a second thought. Even in 2016, when the international collaboration behind the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US announced the discovery of gravitational waves, the idea that we might someday see gravitational memory seemed a stretch to most.

Not to , though. In the months before LIGO’s announcement, he was among a select group invited to the Hilton hotel in Pasadena, California, to discuss the detection’s implications. “We were all super excited,” says Lasky, an astrophysicist at Monash University in Melbourne, Australia. Dozens of researchers darted between seminar rooms, discussing everything from how black holes merge to the nuances of LIGO’s beam splitters. But gravitational memory? “I don’t think it was on many people’s minds, to be honest,” he says.

Even so, Lasky found a small band of gravitational memory devotees. Alongside his colleagues from Monash, Eric Thrane and Yuri Levin, Lasky got chatting with Jonathan Blackman and Yanbei Chen from the California Institute of Technology. Unknown to each other, both groups had been working on a way to detect gravitational memory. The smoking gun would be a tiny, underlying shift in the ripples of the gravitational wave signal. That would be too weak for detectors like LIGO to see in individual events. But, by combining multiple events, they argued that they should be able to pick it out.

It wasn’t straightforward. And Chen immediately spotted something Lasky had missed that would make detection even more difficult. But the researchers kept pushing. “It was a rollercoaster ride,” says Lasky. “We were doing calculations in the middle of talks. Instead of going out for dinner, we were sitting in our rooms trying to solve this problem.”

A week later, at the end of the meeting, they had nailed it. Contrary to the prevailing wisdom, their calculations showed that it was possible to find gravitational memory by . Exactly how many signals they would need to pull together was hard to predict – it could be as few as 500 or as many as 4000 – but the hope was that with about 1000 they would be able to magnify this minuscule effect enough to see it.

Now, with LIGO, Virgo and the Kamioka Gravitational Wave Detector in Japan having switched on again after upgrades, the milestone is within reach. New observations are rolling in every week, pushing the current total to over 100 and counting. At this rate, experimentalists hope they will detect gravitational memory within a few years.

That would be yet another confirmation of the predictions of Einstein’s theory of gravity. Paradoxically, however, it could also help to demonstrate that it has its limits: gravitational memory could show how the black holes predicted by general relativity aren’t the black holes we see.

Such a discrepancy could reveal itself in the very final moments of a merger between two black holes, as they orbit each other in a spiral before finally becoming one. The resulting black hole starts “ringing” – another way of saying it is wobbling about because of the collision – before it settles down to being a normal, well-behaved black hole, emitting some more gravitational waves in the process. From those gravitational waves, we can detect the shape of a black hole’s “ringdown”. And this will be slightly different depending on whether black holes obey the laws of general relativity or an alternative theory of gravity, says Lasky.

In general relativity, black holes are described by two numbers: their mass and their spin. Anything beyond these two parameters is known as “hair”, so any black holes that didn’t obey general relativity would be hairy, says Lasky. That means hairy black holes would ring differently from bald black holes. Which is why Lasky is trying to make “really precise measurements” of the black holes we can investigate via gravitational waves to see if there is any hidden hair lurking. If you want to really test general relativity, you can test this “no-hair theorem”, says Lasky.

The problem, and the reason why gravitational memory could be useful, is that some of the signal it would produce is predicted to appear at the same time as the ringdown. To truly understand these ringdown gravitational waves, then, we must first know what contribution gravitational memory is making.

If researchers can tease the two signals apart and find that black holes are hairy after all, it would be the clearest sign so far that general relativity must be replaced by a theory of quantum gravity. This would unify gravity with the other forces of nature, which are described by quantum mechanics. What this quantum gravity might look like is far from clear, and experiments are yet to yield many clues. But gravitational memory offers hope on this front too, thanks to a strange quirk of nature that Strominger uncovered a few years ago.

Quantum gravity

This starts with the idea that instead of having a structure like a rigid crystal, with its three symmetries, empty space-time has an infinite collection of symmetries linked to gravity. These persist far from gravitational influence, however, as if there is a residual effect of gravity even when there is no matter around. These supertranslation symmetries, as they are known, can be described using the same mathematics used to describe gravitational memory – in other words, they are one and the same. So, an observation of gravitational memory would be a “spectacular confirmation” that supertranslational symmetries exist, says at the Centre for Theoretical Physics in France.

What Strominger realised, and what makes this connection particularly intriguing when it comes to quantum gravity, is that gravitational memory and supertranslational symmetries can be connected to a third, seemingly disparate part of reality: quantum particles with zero energy, known as soft particles. Strominger showed that the could be described by the same that produce gravitational memory. Both sets of equations had been known about for decades, but nobody had made the connection.

Strominger had found the third corner of what is known as the infrared triangle (see above): a mathematical connection that essentially says the soft particles theorems are equivalent to the supertranslation symmetries and to gravitational memory. That is a big deal because each corner of the triangle brings something to the table that helps us understand something about the others. The symmetries are intuitive, says Strominger, while the soft particle theorems are mathematically precise. “With gravitational memory,” he says, “you connect it to observable reality.”

There are also electromagnetic and quark versions of gravitational memory (see “Electromagnetic echo”), each with their own triangles. Across the board, the memory effect is equivalent to special symmetries and to the interactions of soft particles. But these equivalent effects don’t garner as much excitement, because the gravitational version of the triangle tells us what properties a viable theory of quantum gravity should have.

In short, this triangle suggest that any quantum theory of gravity must obey supertranslation symmetries. It is hard to find a theory that does this, which should help theorists to narrow their search. “It doesn’t tell us what quantum gravity is, but it’s going to help,” says Lasky.

The infrared triangle could even help to prove that our universe is a hologram. A 25-year-old conjecture argues that the 4D space-time we experience is projected from a 2D surface governed by quantum theory, with no gravity, in much the same way a hologram projects from a surface. The idea that the universe is a hologram has been a favourite of physicists since it was first proposed, but the problem is that it only works for a strange kind of saddle-shaped space-time that doesn’t resemble our universe. Finding a 2D quantum theory that maps onto our universe’s space-time has proven tricky.

In June, Puhm presented calculations showing that if our universe’s space-time abides by supertranslation symmetries then the corresponding 2D quantum theory must too. “Experimental confirmation of these symmetries would be a very important result,” said Puhm at a recent gravitational memory conference at Queen Mary University of London. “But it would just be the start.”

As if all that weren’t enough, the infrared triangle has led to another surprising effect. When black holes obey supertranslation symmetries, they emit soft particles that . The physical process in which this happens is thought to involve some kind of – the point of no return for objects falling into a black hole. This is similar to the deformation of space-time caused by gravitational waves. “We refer to this as a ,” says Puhm.

First, though, the focus is on understanding gravitational memory and what it means for quantum gravity. For Polnarev, detecting gravitational memory will be a fitting end to 50 years of research into it, although he isn’t yet sure how he will react. This doesn’t worry him. “I am sure that if LIGO and Virgo manage to detect [the] gravitational memory effect,” he says, “I will manage to solve the problem of the best way to celebrate.”

Abigail Beall is a features editor at 91av