Astrophysicists seek to understand the nature of memory in the universe

When black holes collide in distant parts of the universe, energy is released in the form of gravitational waves. This can be compared to the ripples caused by throwing a pebble onto the surface of a pond. Only in this case these waves cross space and time. “In bodies of water, after the waves disappear, the water surface becomes smooth again,” says cosmologist David Garfinkle of Oakland University in Michigan. Some may think that after the gravitational waves pass, the structure of the universe will also return to normal. “But that’s not the case,” Garfinkle says. In fact, Albert Einstein’s theory of general relativity (which states that gravity results from the curvature of spacetime due to the mass of an object) predicts that gravitational waves should subtly change the structure of spacetime as they pass through. . In other words, the universe remembers everything. This “gravitational memory effect” is very weak, so it is thought to have a homeopathic effect. But in recent years, some optimistic astrophysicists have taken on the challenge of proving its existence. “Andrew Strominger, a theoretical physicist at Harvard University, says, ‘They assume when it will happen, but no one says they can’t measure it.'” And We are probably here now because gravitational waves are becoming more and more abundant. We are on the brink of a breakthrough. Such a discovery would have far-reaching and even revolutionary consequences. Gravitational memory would provide evidence of a hidden form of symmetry that is thought to pervade the universe. This, in turn, could provide important and groundbreaking clues about the quantum theory of gravity and what space-time is ultimately made of. The roots of this idea go back to his distant 1960s. At the time, physicist Joseph Weber believed that he had made an incredible discovery. He used a small vibrating aluminum rod to receive a signal that he claims is the first discovery of gravitational waves. Weber’s comments caused a sensation in the press, but his colleagues were more cautious. Few physicists doubted the existence of gravitational waves, which were “derived” directly from the equations of general relativity, but the signal was so weak that Weber’s very modest equipment might not be able to detect them. It was expected that the sex would be high. gravitational waves Critics included two physicists, Alexander Polnareff and Yakov Zeldovich. To prove Weber wrong, they calculated how Weber’s vibrating rod would be affected by the strongest possible gravitational waves. They believe that at the center of our Milky Way is a hypothetical ultra-dense star cluster, much larger than any star cluster that actually exists, which generates waves that disrupt two particles 1,000 kilometers apart. I suggested that there be. This is exactly the distance between the rods that Weber has installed in laboratories around the world. They calculate that even in this extreme case, Weber’s instrument would have to be 100 million times more sensitive than his to detect gravitational waves. “In this case, they were impossible to detect,” Polnareff says. But to prove Weber wrong, scientists discovered a strange effect. Calculations showed that particles that vibrated under the influence of gravitational waves do not return to their original position. Instead, their positions shift by a negligible amount. This happens because the spacetime, which connects space and time from the 3rd dimension to his 4th dimensional structure, is constantly stretched in one direction and compressed in another direction by the influence of gravitational waves.

But Paul Lasky is different. A few months before the LIGO results were announced, he was invited to the Hilton Hotel in Pasadena, California, to discuss the prospects for the research. “We were all very excited,” says Lasky, an Australian astrophysicist at Monash University in Melbourne. “Dozens of researchers ran from room to room, discussing everything from black hole mergers to the nuances of LIGO’s optical couplers. What about gravitational memory? I don’t know if too many people have thought about it, to be honest,” he says. Nevertheless, Lasky found a small group that supported gravitational memory. Lasky met with Caltech’s Jonathan Blackman and Yangbei Chen, along with Monash colleagues Eric Train and Yuri Levin. Both groups, previously unaware of each other’s existence, were researching ways to discover gravitational memory. The “smoking gun” could be small hidden changes in the pulsations of the gravitational wave signal. Detectors like LIGO are too weak to detect individual events. However, scientists have concluded that it can be detected by combining several events. However, everything turned out not to be so simple. Chen quickly noticed something that Lasky didn’t. And that will make discovery even more difficult. But the researchers didn’t give up. “It’s been a roller coaster,” Lasky said. “We did the math during the negotiations. Instead of going to dinner, we sat in our rooms and tried to solve this problem.” A week later, at the very end of the conference, they managed to resolve the issue. Contrary to popular belief, their calculations showed that combining data from LIGO and Italy’s Virgo detector could prove the existence of gravitational memory. It’s difficult to predict exactly how many signals you’ll need to collect, it could be 500 or even 4,000, but starting with 1,000 will ensure that this “small” effect is amplified enough to be noticeable. It was expected that With LIGO, Virgo, and Japan’s Kamioka gravitational wave detector back online after upgrades, this milestone is within reach. New observations are added every week, and the number already exceeds 100. At this rate, experimenters hope to discover gravitational memory within a few years. This would further support the predictions of Einstein’s theory of gravity. But, paradoxically, it may also serve to show that there are limits. Gravitational memory could reveal that the black holes predicted by general relativity are not the black holes we imagine. Such a discrepancy can occur in the final moments of merging, when two black holes spiral into each other, eventually merging into one black hole. The resulting black hole begins to “ring” before turning into a regular quiet black hole and sending out more gravitational waves. In other words, it vibrates due to collisions. From these gravitational waves, the ring-down structure of the black hole can be determined. And it’s likely to be a little different depending on whether the black hole follows general relativity or another theory of gravity, Lasky said. In general relativity, black holes are described by two quantities: mass and spin. Anything outside these two parameters is called “hairy,” Lasky says, and all black holes that don’t obey general relativity are “hairy.” This means that a hairy black hole sounds different than a hairless black hole. This is why scientists are trying to make “as accurate measurements as possible” of black holes. Lasky believes he can discover hidden hairs by studying gravitational waves. If you really want to test general relativity, you can test this “hairless theorem,” Lasky says. The problem, and the reason gravitational memory is useful, is that some of the signals that gravitational memory generates are expected to occur simultaneously with collapse. Therefore, to truly understand these gravitational waves, we first need to know what role gravitational memory plays. If the researcher were able to separate the two signals and eventually discover that the black hole is hairy, it would be the clearest sign yet that general relativity is about to be replaced by quantum gravity. This makes it possible to combine gravity with other natural forces described by quantum mechanics. It is not yet clear what this quantum gravity is, as experiments have not yet provided any clues. But even here, gravitational memory offers hope. It’s all thanks to a strange quirk of nature that Strominger discovered several years ago.