What are gravitational waves? The | Astronomy Essentials

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Gravitational waves are waves in the structure of space-time. Just like a ship traveling across the surface of a calm sea leaves a wake behind it, so moving objects in the universe create gravitational waves. The “ships” in the case of gravitational waves are extremely violent and cataclysmic events far in the cosmos: black hole mergers, neutron star collisions, supernovae. All of this generates waves in the structure of space-time, stretching and squeezing it as the waves travel through the universe.

Because gravitational waves are extremely weak as observed from our earthly point of view, the technology to detect them has been available only in recent years. Like all waves, gravitational waves decrease in size with distance, reducing to faint echoes of those distant “shipwrecks”, those distant violent events in the cosmos, the moment they reach us. From our location, many light years from a black hole merger or neutron star collision, the waves compress and stretch space, and everything in it, to a thousandth of the diameter of a custom proton that pass through the Earth. That’s a trillionth of a trillionth of a meter. We need very advanced technology to see that change. It’s like seeing the distance between the sun and its closest neighbor among the stars: https://en.wikipedia.org/wiki/Alpha_Centauri ”target =” _ blank ”rel =” noopener noreferrer ”> Alpha Centauri, 4.3 light years away – change by the thickness of a human hair.

It was Albert Einstein who, in his 1915 General Theory of Relativity, first postulated the existence of gravitational waves. His suggestion that gravity travels in waves seemed logical: all kinds of light in the electromagnetic spectrum, from ultraviolet to visible and radio, travels in waves. Sound travels in waves. Why should gravity not propagate in the same way? Einstein calculated that extremely violent events in the cosmos would cause room for it sounds like a bell. This was different from the idea of ​​static and immutable gravitational fields that are generated by any object that has mass, such as a star or a planet.

However, for decades after 1915, Einstein himself was not convinced of the existence of gravitational waves. In 1936, he and his colleague Nathan Rosen published an article titled Are there gravitational waves? which was initially rejected by a magazine due to a mathematical error.

It was the error that caused the authors to conclude that gravitational waves do not exist. When Einstein corrected the error, the conclusion of the article became the exact opposite! Although the evidence now pointed to their existence, Einstein was not convinced and believed that even if gravitational waves existed, they would be so weak that humans could never develop the technology to detect them.

It should be noted that Einstein was not the only theorist who worked on gravitational waves. Other famous scientists made important contributions, including Robert Oppenheimer, Roger Penrose, Karl Schwarzschild, Arthur Eddington, Kip Thorne, and Richard Feynman. But it was Feynman who, in January 1957, finally convinced those who doubted that not only gravitational waves exist, but that they can also carry energy, explaining this using something he called his Sticky Bead argument.

Feynman’s work directly paved the way for today’s gravitational wave detectors.

However, it would be another 50 years before the first gravitational waves were detected. Developing the concepts and technology to do so required decades of hard work by many scientists. Finally, LIGO, the laser interferometry gravitational wave observatory located at two sites in the United States, began observing in 2002. It took several updates to LIGO, between 2002 and 2015, to give it the necessary sensitivity to make its first historical detection .

The first detection, of two black holes merging about 1.3 billion light-years away, was made in September 2015 and announced to the world in February 2016 after months of work verifying that the signal, which had lasted Just a tenth of a second, in perfect agreement with Einstein’s predictions, was real. Unbelievably, LIGO had not yet started its official observing career when detection arrived: after its latest series of updates to improve its range and sensitivity, LIGO had been activated for engineering testing. The black hole fusion was detected almost immediately when the detector was operational.

Another key Einstein prediction was that gravitational waves would travel at the speed of light. By measuring the time difference between the time the gravitational wave signal reached the two LIGO observatories, in Hanford, Washington and Livingston, Louisiana, separated by nearly 2,000 miles (3,000 km), the scientists were able to determine that the prediction of Einstein was completely correct. Gravitational waves propagate at the speed of light.

LIGO was joined in 2018 by the European Virgo detector in Italy, which has vastly improved scientists’ ability to determine the location in the sky where gravitational waves originated. Since then, LIGO / Virgo have detected some 50 black hole mergers, but also eight neutron star collisions and six black hole neutron star collisions. Some of these may end due to so-called “ground interference”: vibrations from passing traffic and even distant ocean waves can cause false positives.

On January 14, 2020, LIGO also detected an event of completely unknown origin, which does not conform to any model or prediction, perhaps, excitingly, that points to the existence of a hitherto unknown cosmic phenomenon.

Very soon, the Japanese KAGRA observatory will join Virgo and LIGO in detecting gravitational waves. In the 2030s, the European Space Agency will launch LISA, a space-based gravitational wave detector, which should allow detection of low-frequency gravitational waves emanating from supermassive black holes and supernova explosions. China has begun work on the construction of three gravitational wave observatories, its stated intention to become the world leader in Earth and space based gravitational wave detection.

All the gravitational wave events detected so far are perfectly in line with Einstein’s predictions and with the computer simulations derived from his calculations. Einstein would surely have been surprised that he was wrong, that human intellect and ingenuity have really triumphed and created the technology that he believed impossible. You probably would have also regretted doubting your own work in predicting the existence of gravitational waves. But also surely he would have been happy if the detection of gravitational waves is also a confirmation of his theory of relativity. Now there are few places left for those who doubt Einstein’s greatest triumph.

Gravitational wave astronomy is a whole new science and promises to unlock many of the mysteries of the universe. It is not an exaggeration to say that a revolution in our vision of the universe is underway. In the future, it might even be possible to detect gravitational waves from the Big Bang itself, hear the sound of Creation ringing for billions of years.

If you want to keep up to date with the latest gravitational wave events, the University of Birmingham in the UK has created this page which is a database of LIGO and Virgo detections during your current observation run. The database is also available as a free application for Android / Apple phones, downloadable from their respective stores.

Bottom line: First postulated by Albert Einstein in 1916, but not directly observed until September 2015, gravitational waves are waves in space-time.
Read more and watch an explanatory video: What are gravitational waves?