Einstein’s general relativity stubbornly passes yet another test

Time and time again, Einstein's theory of general relativity passes cosmic tests near and far. It's a testament to the physicist's ingenuity and scientific and technological progress. But physicists also want to see general relativity fail a test.

Driving the news: Andrea Ghez of the University of California, Los Angeles, and her colleagues report today in Science that light from a star (dubbed S0-2) passing close to the supermassive black hole at the center of the Milky Way is affected as Einstein's theory predicts.

The big picture: Scientists want to find a test where Einstein's theory doesn't hold. The hope is to discover physics that can explain what Einstein's and Newton's equations cannot — the creation of the universe or what happens inside a black hole.

• The new work complements that of the GRAVITY collaboration at the European Southern Observatory.
• Both teams relied on detailed measurements of the star's 16-year orbit — Ghez helped to take the first measurement in 1995 — to detect the effects of gravity.
• Test time came in 2018 when the star passed within 120 astronomical units of the black hole — about 3 times the distance between our Sun and Pluto.

Using the historical data for the star's orbit as a reference, the teams took measurements of the light emitted from the star as it moved by the black hole. They observed it was redder because, as predicted, it lost energy due to the effects of gravity from the black hole.

• In the new study, the researchers report general relativity is 43,000 times more likely than Newtonian physics to describe the observations.

Researchers applaud the new study — the precision of the measurements, the patience of the endeavor, and the avenues of investigation it opens.

But Einstein's continued success is a challenge for the field.

• "One of the problems we have is that Einstein’s theory just keeps passing the test over and over again. And we know that it can’t be the complete story," says Geraint Lewis, a professor of astrophysics at the University of Sydney.
• "It is exciting and disappointing at the same time."
• Einstein's and Newton's equations are correct — under certain conditions.
• The search is for an all-encompassing construct that Newton and Einstein would be part of, says Zoltan Haiman, a theoretical physicist at Columbia University.

What's next: Knowing details of S0-2's path, researchers want to track the star to see if its orbit will rotate like general relativity predicts.

• They also want to use results from recent experiments to home in on even powerful tests where different aspects of Einstein's theory might not hold.
• "It has been tested in so many different ways that any deviation would be instantaneously surprising," says Tuan Do, a research scientist at UCLA who helped to lead the study with Ghez.