Skip to main content

Effect predicted by Albert Einstein spotted in a double-star system

Gravitational redshift, an effect predicted by Albert Einstein that is crucial for maintaining the Global Positioning System (GPS) on Earth, has been observed in a star system in our galaxy.

Within Einstein's general theory of relativity there is an effect known as "gravitational redshift," in which light becomes redder because of the influence of gravity; the wavelength of a photon, or light particle, gets longer and appears redder as the wavelength climbs farther away from a gravitational well. 

A gravitational well is the pull of gravity exerted by large bodies in space, like Earth. Under this effect, clocks on Earth's surface actually run slower than clocks far away and experiencing less gravity, so clocks on orbiting satellites run slower. Because of this, redshift has to be factored into calculating positions on Earth with GPS.

Now, while scientists have found absolute evidence of this effect in our solar system, they have found less evidence farther away because the observations are more difficult to make. But now, in a new study, researchers have spotted gravitational redshift in a two-star system a whopping 29,000 light-years (200,000 trillion miles or 321868800000000 kilometers) away called 4U 1916-053. 

Related: Einstein's theory of relativity explained (infographic)

Scientists have observed gravitational redshift, an effect predicted by Albert Einstein in the double star system 4U 1916-053. (Image credit: NASA/CXC/M.Weiss)

This far-off system consists of a dense core remaining from a star that's had its outer layers stripped away, and a neutron star, which is even denser and is created when a massive star collapses in a supernova. The pair are orbiting just 215,000 miles (346,000 km) from one another, or about the distance between Earth and the moon. 

In this new study, researchers using NASA's Chandra X-ray Observatory not only discovered this system but also were able to show the effects of gravitational redshift. The scientists did this by analyzing X-ray spectra, or the amount of X-rays absorbed at different wavelengths, of the stars in this system. 

Not only is this an important discovery because it is outside of our solar system, but it is the first strong evidence of this effect in a two-star system that has had either a neutron star or black hole, according to a statement from the Chandra X-ray Observatory. The scientists have won more time with Chandra in the upcoming year to study the system in more detail, according to the statement. 

This work was accepted for publication to the Astrophysical Journal Letter; a version of the paper was uploaded to the preprint server on Aug. 3. 

Email Chelsea Gohd at or follow her on Twitter @chelsea_gohd. Follow us on Twitter @Spacedotcom and on Facebook.

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at:

Chelsea Gohd

Chelsea Gohd joined as an intern in the summer of 2018 and returned as a Staff Writer in 2019. After receiving a B.S. in Public Health, she worked as a science communicator at the American Museum of Natural History and even wrote an installation for the museum's permanent Hall of Meteorites. Chelsea has written for publications including Scientific American, Discover Magazine Blog, Astronomy Magazine, Live Science, All That is Interesting, AMNH Microbe Mondays blog, The Daily Targum and Roaring Earth. When not writing, reading or following the latest space and science discoveries, Chelsea is writing music and performing as her alter ego Foxanne (@foxannemusic). You can follow her on Twitter @chelsea_gohd.