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Einstein's theory of general relativity revealed a dynamic and strange cosmos

Albert Einstein’s mind reinvented space and time, foretelling a universe so strange and grandiose that it challenged the limits of the human imagination. An idea born in a Swiss patent office that evolved into a mature theory in Berlin exposed a radical new image of the cosmos, rooted in a new deeper understanding of gravity.

Outside was Newton's idea, which reigned for nearly two centuries, of masses that seemed to pull each other. Instead, Einstein presented space and time as a unified fabric distorted by mass and energy. Objects deform the fabric of space-time like a weight resting on a trampoline and the curvature of the fabric guides its movements. With this view, gravity was explained.

Einstein presented his general theory of relativity in late 1915 at a series of lectures in Berlin. But it wasn’t until a solar eclipse in 1919 that everyone realized. His theory predicted that a massive object (e.g., the sun) could distort space-time close enough to bend light from its straight course. Distant stars would thus appear not exactly where expected. Photographs taken during the eclipse proved that the change of position coincided with Einstein's prediction. “Lights haunted in the heavens; more or less exhausted men of science, ”a New York Times headline stated.

Even a decade later, a story in Science News Letter, the predecessor of Science News, he wrote about “Riots to understand Einstein theory” (SN: 30/01/30, p. 79). Apparently, additional police had to be called in to control a crowd of 4,500 who “broke iron doors and hammered” at the American Museum of Natural History in New York City to hear an explanation of general relativity.

In 1931, the physicist Albert A. Michelson, the first American to win a Nobel Prize in science, called the theory "a revolution in scientific thought unprecedented in the history of science."

But for all the powers of divination we attribute to Einstein today, he was a reluctant fortune teller. We now know that general relativity offered much more than Einstein was willing or able to see. "It was a profoundly different way of looking at the universe," says astrophysicist David Spergel of the Simiron Foundation's Flatiron Institute in New York, "and it had some wild implications that Einstein himself didn't want to accept." Moreover, says Spergel (Honorary Councilman of the Society for Science, editor of Science News), "the wildest aspects of general relativity have turned out to be true."

What was pretended to be a quiet, static, finite place is, instead, a dynamic, ever-expanding arena full of its own riot of beasts that double space. Galaxies gather in superclusters on much larger scales than experts had considered before the 20th century. Within these galaxies reside not only stars and planets, but also a zoo of exotic objects that illustrate the trend of the strangeness of general relativity, including neutron stars, which pack the value of the mass of a fat star the size of a city and black holes, which pervert space-time so strongly that no light can escape. And when these giants collide, they shake space-time, throwing out huge amounts of energy. Our cosmos is violent, evolving, and full of science fiction possibilities that really come straight out of general relativity.

“General relativity has opened up a huge stage for us to look and try and play,” says astrophysicist Saul Perlmutter of the University of California, Berkeley. He points to the idea that the universe changes drastically throughout its life – "the idea of ​​a life in a universe is a strange concept" – and the idea that the cosmos is expanding, as well as the idea that it could collapse and come to an end. , and even that there may be other universes. "You're starting to realize that the world could be a lot more interesting even than we ever imagined it could be."

General relativity has become the basis for current understanding of the cosmos. But the current picture is far from complete. There are many questions left about matter and mysterious forces, about the beginnings and the end of the universe, about how the science of large meshes with quantum mechanics, the science of the very small. Some astronomers believe that a promising route to answering some of these unknowns is another of the initially underestimated features of general relativity: the power of bent light to magnify the features of the cosmos.

Today’s scientists continue to sting and incite general relativity to find clues of what might be missing. Now general relativity is being tested to a level of accuracy previously impossible, says astrophysicist Priyamvada Natarajan of Yale University. “General relativity broadened our cosmic view, gave a sharper focus to the cosmos, and then changed its words and said,‘ Now we can test it with much more force. ’It is this proof that perhaps discovers problems with theory that may indicate the path to a more complete picture.

And so, more than a century after the debut of general relativity, much remains to be predicted. The universe may turn out to be even wilder.

Voracious beasts

Just over a century after Einstein unveiled general relativity, scientists obtained visual confirmation of one of his most impressive beasts. In 2019, a global network of telescopes revealed a mass-deformed space-time with so much fervor that nothing, not even light, could escape its trap. The Event Horizon Telescope released the first image of a black hole, in the center of the galaxy M87 (SN: 27/04/19, p. 6).

In 2019, the Event Horizon Telescope Collaboration released this first image of a black hole, in the heart of the galaxy M87. The image shows the shadow of the monster surrounded by a bright disk of gas.Collaboration in Telescope Horizon of Events

“The power of an image is strong,” says Kazunori Akiyama, an astrophysicist at the MIT Haystack Observatory in Westford, Massachusetts, who led one of the teams that created the image. "Something hoped we could see something exotic," says Akiyama. But after looking at the first picture, "Oh, my God," he remembers thinking, "it perfectly coincides with our expectation of general relativity."

For a long time, black holes were mere mathematical curiosities. Evidence that they actually reside in space did not begin to arrive until the second half of the twentieth century. It is a common history in the annals of physics. A rarity in the equation of some theorist points to a previously unknown phenomenon, which initiates a search for evidence. Once the data is reachable and if physicists are a little lucky, the search gives way to discovery.

In the case of black holes, the German physicist Karl Schwarzschild found a solution to Einstein's equations near a single spherical mass, such as a planet or a star, in 1916, shortly after Einstein proposed general relativity. Schwarzschild’s mathematics revealed how the curvature of space-time would differ around stars of the same mass but smaller and smaller sizes, in other words, stars that were increasingly compact. Outside of math came out a limit to the little one that could squeeze a dough. In the 1930s, J. Robert Oppenheimer and Hartland Snyder described what would happen if a massive star falling under the weight of its own gravity shrank beyond that critical size – now known as the "Schwarzschild ray" – to reach a point. from which its light could never reach us. Still, Einstein — and most others — doubted that what we now call black holes was plausible in reality.

The term “black hole” first appeared in Science News Letter. It was in a 1964 account by Ann Ewing, which covered a meeting in Cleveland of the American Association for the Advancement of Science (SN: 18/01/64, p. 39). It is also about the time he gave to understand the reality of black holes.

A few months later, Ewing reported on the discovery of quasars, describing them in the Science News Letter as "the most distant, bright, violent, heavy, and baffling light sources and radio waves" (SN: 8/15/64, p. 106) . Although they were not attached to black holes at the time, quasars hinted at some cosmic powers needed to provide that energy. The use of X-ray astronomy in the 1960s revealed new features of the cosmos, including bright beacons that could have come from a black hole suffocating a companion star. And the movements of stars and gas clouds near the centers of galaxies pointed to something extremely dense lurking inside them.

Quasar illustrationQuasars (an illustrated one) are so bright that they can surpass their galaxies. Although baffling when first discovered, these explosions are fueled by massive black holes that feed.Mark Garlick / Science Source

Black holes stand out among other cosmic beasts for how extreme they are. The largest are billions of times the mass of the sun, and when they pull out a star, they can spit out particles with 200 trillion volts of energy. This is about 30 times the energy of the protons that run around the world's largest and most powerful particle accelerator, the Large Hadron Collider.

As evidence incorporated in the 1990s and to this day, scientists have realized that these great beasts not only exist, but also help form the cosmos. “These objects that general relativity predicted, which were mathematical curiosities, became real, then they were marginal. Now they have become power plants, ”says Natarajan.

We now know that supermassive black holes reside in the centers of most, if not all, galaxies, where they generate energy outputs that affect how and where stars form. “At the center of the galaxy, they define everything,” he says.

Although the visual confirmation is recent, it appears that the black holes have been familiar for a long time. They are a fundamental metaphor for any unknowable space, any deep abyss, any effort that consumes all our efforts while giving little in return.

The real black holes, of course, returned a lot of answers: answers about our cosmos and new questions to ponder, wonder and entertainment for space fans, a lost Weezer album, numerous episodes of Doctor Who, the Hollywood blockbuster Interstellar

For physicist Nicolas Yunes of the University of Illinois at Urbana-Champaign, black holes and other cosmic giants continue to amaze. "Just thinking about the dimensions of these objects, how big they are, how heavy they are, how dense they are," he says, "is really impressive."

In 2019, scientists gave us the first real image of the supermassive black hole in the center of the galaxy M87. How? We explain.

Space-time waves

When the monsters of general relativity collide, they disturb the cosmic tissue. Waves in space-time called gravitational waves emanate outward, a business card of a tumultuous and more energetic tango.

Einstein's mathematics predicted that such waves could be created, not only by gigantic collisions but also by explosions and other accelerated bodies. But for a long time, seeing any kind of ripple in space-time was an out-of-measure dream. Only the most dramatic cosmic facts would create signals large enough for direct detection. Einstein, who called the gravitationswellen waves, was unaware that such large events existed in the cosmos.

Illustration of gravitational wavesGravitational waves move away from two black holes that orbit each other before merging (shown in this simulation). The black holes that merged created a new black hole much larger than those found in previous collisions.Deborah Ferguson, Karan Jani, Deirdre Shoemaker and Pablo Laguna / Georgia Tech, Maya Collaboration

Beginning in the 1950s, when others were still debating whether gravitational waves actually existed, physicist Joseph Weber sank his career in an attempt to detect them. After more than a decade of effort, it claimed to have been detected in 1969, identifying an apparent signal perhaps from a supernova or a newly discovered type of rapidly rotating star called a pulsar. In the few years after reporting the initial discovery, Science News published more than a dozen reports on what it came to call the “Weber problem” (SN: 21/06/69, p. 593). Study after study could not confirm the results. Moreover, no sources of the waves were found. A 1973 headline read: "The deep doubt about Weber's waves" (SN: 26/05/73, p. 338).

Weber was trapped by his claim until his death in 2000, but his waves were never verified. However, scientists increasingly believed that gravitational waves would be found. In 1974, radio astronomers Russell Hulse and Joseph Taylor saw a neutron star orbiting a dense companion. In the following years, the neutron star and its companion seemed to be closer and closer together by the distance that would be expected if they were losing energy by gravitational waves. Scientists soon spoke not of Weber's problem, but of what equipment could catch the waves. “Now, even though they haven’t seen it yet, physicists believe it,” Dietrick E. Thomsen wrote in Science News in 1984 (SN: 8/4/84, p. 76).

It was a different detection strategy, decades in progress, that would provide the necessary sensitivity. The Advanced Laser Interferometry Gravitational-wave Observatory, or LIGO, which reported the first confirmed gravitational waves in 2016, relies on two detectors, one in Hanford, Washington and one in Livingston, La. Each detector divides the beam of a powerful laser. in two, with each beam traversing one of the two arms of the detector. In the absence of gravitational waves, the two beams recombine and cancel each other out. But if the gravitational waves stretch one arm of the detector while squeezing the other, the laser light no longer matches.

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Machines are an incredible engineering feat. Even space-time waves detected by colliding black holes can stretch an arm of the LIGO detector up to one-tenth of the width of a proton.

When the first detection was announced, coming from two colliding black holes, the discovery was heralded as the beginning of a new era of astronomy. It was the story of the year for Science News in 2016 and such a huge success that the pioneers of the LIGO detector won the Nobel Prize in Physics the following year.

Scientists with LIGO and another gravitational wave detector, Virgo, based in Italy, recorded dozens more detections (SN: 30/01/21, p. 30). Most waves emanated from fusions of black holes, although some events featured neutron stars. Smashups have so far revealed previously unknown birthplaces of some heavy elements and pointed to a bright jet of charged subatomic particles that could offer clues to mysterious high-energy flashes of light known as gamma-ray bursts. The waves also revealed that there really are medium-sized black holes, between 100 and 100,000 times the mass of the sun, along with confirmation that Einstein was right, at least so far.

Virgo installation in ItalyResearchers at two gravitational wave observatories, LIGO in the United States and Virgo in Italy (shown), have reported in the past five years dozens of detections of black hole destruction as well as neutron star fusions.The Virgo Collaboration

Just five years later, some scientists are already anxious for something even more exotic. In a Science News article on the detection of black holes orbiting wormholes through gravitational waves, physicist Vítor Cardoso of the Instituto Superior Técnico de Lisboa, Portugal, suggested an upcoming shift to more unusual phenomena: "We need to look for strange signals but exciting, ”he said. (SN: 29/08/20, p. 12).

The astronomy of gravitational waves is only in its infancy. Improved sensitivity in existing Earth-based detectors will increase the volume of gravitational waves, allowing detections from less energetic and more distant sources. Future detectors, including space-based LISA, scheduled for launch in the 2030s, will prevent the problematic noise that interferes when the Earth’s surface shakes.

“Perhaps the most exciting thing would be to watch a small black hole fall into a large black hole, an inspiring extreme mass relationship,” says Yunes. In such a case, the small black hole would approach back and forth, rotating in different directions as it followed eccentric orbits, perhaps for years. That could provide the definitive proof of Einstein’s equations, revealing whether we really understand how space-time is deformed at the extreme.

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