Gravity wave
Gravity waves, or gravitational waves, are ripples in the fabric of space-time caused by the accelerated movement of massive objects in the universe. The concept was first proposed by physicist Albert Einstein in his general theory of relativity, published in 1915, although he could not confirm their existence due to the technological constraints of his time. It wasn't until September 2015 that scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves for the first time, resulting from the collision of two supermassive black holes located over a billion light-years away.
These waves travel at the speed of light and can pass through matter, causing small vibrations in atoms. The detection of gravitational waves confirmed a significant aspect of Einstein's theory, earning the LIGO co-founders the Nobel Prize in Physics in 2017. Since then, LIGO has observed multiple gravitational wave events, including collisions of black holes and the first known collision between a black hole and a neutron star in 2021. As of 2023, researchers have also detected low-frequency gravitational waves, believed to originate from supermassive black hole binaries in the early universe, enhancing our understanding of black hole formation and behavior over cosmic time.
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Gravity wave
A gravity wave, or gravitational wave, is a ripple in the fabric of space-time caused by the accelerated movement of massive objects in outer space. The concept of gravity waves was first theorized in 1915 by physicist Albert Einstein in his general theory of relativity. Einstein, however, could not prove the existence of gravity waves because of the technological limitations of the era. It was not until the early twenty-first century that science developed the capability to test Einstein's theory. In September of 2015, scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO)—a system of two large laser telescopes in the United States—announced that they had detected gravity waves made by a collision between two supermassive black holes more than a billion light-years away.
Background
In 1905, German-born physicist Albert Einstein published his theory on special relativity, the concept that the speed of light remains constant at 186,000 miles per second no matter how fast an observer is traveling. The theory also stated that the laws of physics are the same for observers moving at equal speeds. As a result, Einstein theorized, time and space were not constants, but that time would appear to slow down to observers traveling at speeds close to light. To observers in a stationary position, time would move at its normal pace. This led Einstein to formulate the concept of space-time—the idea that physical space is made up of three dimensions (width, length, and depth), plus a fourth dimension, time.
Einstein's idea was groundbreaking, but it was limited to movement in the absence of acceleration or a gravitational field. When he considered these factors, he noticed some inconsistencies with his calculations. For the next decade, Einstein worked to refine his ideas and in November of 1915, he published the findings as his general theory of relativity. He hypothesized that large gravitational objects produced a distortion in the fabric of space-time, like a dip or pucker in a trampoline when a heavy object is placed upon it. Massive accelerating objects that orbit around each other would cause disruptions in this fabric, producing ripples that spread away from the source, similar to the disturbances made by a stone on the surface of a pond. These waves move through the universe at the speed of light, passing through matter and causing small vibrations in atoms. Einstein called these disturbances gravitationswellen, or gravitational waves. Collisions between massive interstellar objects or other cataclysmic events would produce the largest gravity waves.
Impact
Within four years, scientists were able to confirm Einstein's contention that massive objects cause distortions in the fabric of space-time. Using measurements taken during a solar eclipse in 1919, astronomers noticed the light from distant stars seemed to "bend" slightly as it passed near the disc of the sun. This phenomenon, called gravitational lensing, showed Einstein was right on one aspect of his theory, but did little to prove the existence of gravity waves. The technology available in 1919 was not able to detect the faint signals that such waves would leave behind, leading scientists to argue for years whether Einstein was correct about their existence. Even Einstein himself began to doubt their existence, changing his mind on the subject a number of times.
In the 1950s and 1960s, physicists attempted to build devices to detect gravity waves. These instruments, which used aluminum bars covered with sensors in hopes of detecting vibrations, met with little success. In 1974, astronomers observed an orbiting system of two dense stars that fit the profile to radiate gravity waves. By recording how the stars' orbits changed over time, the astronomers noticed that the stars were getting closer to each other in exactly the same way predicted by Einstein's general theory of relativity. Although the observations seemed to mathematically confirm the existence of gravity waves, scientists had yet to detect any direct evidence of them.
In the late 1960s, Rainer Weiss, a physicist at the Massachusetts Institute of Technology, had the idea of using laser beams to measure the frequencies of passing gravity waves. Weiss joined with scientists from the California Institute of Technology to develop a system of laser telescopes designed to detect such waves. In the 1990s, the Laser Interferometer Gravitational-Wave Observatory (LIGO) was built on two sparsely populated sites in Hanford, Washington, and Livingston, Louisiana. The distance between the detectors was necessary to ensure the highly sensitive equipment would not pick up a false signal from the same Earth-based sources.
LIGO was operational for a number of years before receiving an upgrade that began construction in 2008. On September 14, 2015, during a system check as the improved LIGO was preparing to go back online, scientists detected a signal from the collision of two black holes about 1.4 billion light-years away. A black hole is the remains of a supermassive star that has collapsed to become so dense that even light cannot escape from inside it. A light-year is the distance that light travels in the course of a year, meaning the interstellar collision between black holes occurred 1.4 billion years ago. Light from the event took that long just to reach Earth.
The signal—a strong "chirp" believed to have originated from the sky in the Southern Hemisphere—was the first direct evidence of the existence of gravity waves, and confirmed Einstein's theory a century after it was first published. About three months later, LIGO detected the signs of another gravity wave, this time a fainter signal from a collision between two less massive black holes.
The initial discovery of gravitational waves in 2015 earned Rainer Weiss and his colleagues Barry Barish and Kip Thorne—the three co-founders of the LIGO Project—the 2017 Nobel Prize in Physics. Since 2015, LIGO has conducted several observations of the sky and detected about ninety gravity waves. In 2021, LIGO, in conjunction with similar observatories across the globe, recorded the first-known collision between a black hole and a neutron star, the incredibly dense remains of a stellar explosion. By 2023, astronomers had detected low-frequency gravitational waves for the first time. They believed these waves were created by supermassive black hole binaries in the very early universe. The astronomers hoped that the discovery would help them better understand how black holes grow and join over long periods of time.
Bibliography
"A Brief History of LIGO." LIGO Caltech, www.ligo.caltech.edu/system/media‗files/binaries/313/original/LIGOHistory.pdf. Accessed 18 Jan. 2023.
Cole, Brendan. "LIGO Has Detected Gravitational Waves for the Second Time." Wired, 15 June 2016, www.wired.com/2016/06/ligo-announces-new-gravitational-wave-observation/. Accessed 7 Nov. 2016.
"Gravitational Waves Detected 100 Years after Einstein's Prediction." LIGO Caltech, 11 Feb. 2016, www.ligo.caltech.edu/news/ligo20160211. Accessed 7 Nov. 2016.
Gribbin, John. Einstein's Masterwork: 1915 and the General Theory of Relativity. Icon Books, 2015.
Lea, Robert. "The Universe Is Humming with Gravitational Waves. Here's Why Scientists Are So Excited About the Discovery." Space.com, 3 July 2023, www.space.com/gravitational-waves-astronomers-why-so-excited. Accessed 25 Nov. 2024.
Lea, Robert. "What Are Gravitational Waves?" Space.com, 30 Mar. 2023, www.space.com/25088-gravitational-waves.html. Accessed 25 Nov. 2024.
"LIGO-Virgo-KAGRA Finds Elusive Mergers of Black Holes With Neutron Stars." LIGO Caltech, 29 June 2021, www.ligo.caltech.edu/news/ligo20210629. Accessed 18 Jan. 2023.
Pruitt, Sarah. "Scientists Detect Gravitational Waves, Just Like Einstein Predicted." History.com, 9 Mar. 2016, www.history.com/news/scientists-detect-gravitational-waves-just-like-einstein-predicted. Accessed 7 Nov. 2016.
Redd, Nola Taylor. "Einstein's Theory of General Relativity." Space.com, 12 July 2016, www.space.com/17661-theory-general-relativity.html. Accessed 7 Nov. 2016.
ScienceNews. Einstein's Gravity: One Big Idea Forever Changed How We Understand the Universe. Diversion Books, 2016.
"2017 Nobel Prize in Physics Recognises the Observation of Gravitational Waves." United Nations Educational, Scientific and Cultural Organization, 21 Apr. 2022, www.unesco.org/en/articles/2017-nobel-prize-physics-recognises-observation-gravitational-waves. Accessed 18 Jan. 2023.
"What Are Gravitational Waves?" LIGO Caltech, www.ligo.caltech.edu/page/what-are-gw. Accessed 7 Nov. 2016.