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Testing General Relativity

Testing General Relativity

This is the third of a three-part series on general relativity.

The Theory of General Relativity—Einstein's century-old description of gravity—presented physicists with some pretty bizarre predictions. To test them, scientists from the University of Texas at Austin have traveled to the Sahara Desert to observe a rare eclipse, launched into Earth orbit the densest known object orbiting anywhere in the Solar System, and used computers to model ripples in space and time unleashed by the mergers of black holes.

A team from the University of Texas at Austin constructed a temporary telescope house from plywood and styrofoam in the Sahara Desert to observe the bending of starlight by the sun during a total solar eclipse in June 1973. Photo: Richard Matzner.

Deflected Starlight

One prediction of general relativity is that the path of starlight passing near the sun should be bent by a subtle but specific amount, an effect most easily seen during a total solar eclipse.

Although UT Austin scientists were not the first to confirm this effect, the late Bryce DeWitt, Cécile DeWitt-Morette, Richard Matzner and others did make the most accurate measurement up to that point during a total solar eclipse in 1973. Their results, based on photographs taken from a temporary observatory in Mauritania, West Africa, confirmed the prediction with an accuracy of about 10 percent. The first successful test of the gravitational deflection of light, by a British expedition in 1919, and the one that made Einstein a global celebrity, had only a 30 percent accuracy.

The UT Austin team, led by DeWitt and DeWitt-Morette, overcame numerous technical challenges with their equipment and the unforgiving climate of the Sahara Desert. On the morning of the eclipse, a sandstorm blotted out the sun. Just in time for the brief window of eclipse totality, the wind and dust settled down and "operations proceeded flawlessly during the shortest six minutes of our lives," as team members later recalled in Sky & Telescope magazine. 

Frame Dragging

LARES satellite

A second prediction is known as frame dragging, in which a massive rotating object like the Earth actually drags space around it like an electric mixer dragging cake batter around in a mixing bowl. 

Matzner is part of a team using the LARES satellite, launched in February 2012, to measure the effects of Earth's frame dragging on orbiting satellites. The LARES satellite is the densest known object orbiting anywhere in the Solar System.

Results of an earlier satellite experiment, called Gravity Probe B, confirmed the effects of frame dragging on Earth-orbiting satellites, but with an error of about 19 percent. The LARES team will have to accumulate data for at least five years to be able to extract the science from the orbital behavior, but plan to measure frame dragging with an error of about 1 percent.

The project is headed by Ignazio Ciufolini, whose Ph.D. dissertation at UT Austin described the LARES experiment. Ciufolini, who is now a professor at the University of Rome and the University of Salento, was co-advised in his graduate work by Matzner and the late John Archibald Wheeler.

Gravity Waves

A third prediction is that when extremely massive objects interact at close distance, they emit gravity waves, ripples in space-time like those from a rock dropped in a pond. In 2011, astronomer Don Winget and his team discovered indirect evidence for gravity waves emanating from two white dwarf stars slowly spiraling toward each other 3,000 light-years away.

Now scientists with the Advanced LIGO project are searching for the first direct observations of gravity waves with two Earth based detectors. Matzner has made predictions about the kinds of gravity waves that might be unleashed when two black holes merge, work that could help the Advanced LIGO team interpret their data.

The original LIGO project searched for signs of gravity waves for a decade without luck, but physicists concede that it was probably not sensitive enough. After being offline for five years for upgrades, the new improved system began collecting data last September and might possibly confirm the existence of gravity waves in the next four or five years.

Binary white dwarfs spiral together, creating gravity waves, in this illustration from NASA. Credit: D. Berry/NASA GSF

This is the third of a three-part series on general relativity. In part 1 (featuring physicists Steven Weinberg, Willy Fischler and Raphael Flauger), we examine how general relativity breaks down inside black holes and at the origin of our universe. In part 2 (featuring astronomers Karl Gebhardt and Gary Hill), we explore how the search for dark energy might require a revamp of the equations of gravity.

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Wednesday, 28 October 2020

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