Researchers have measured the gravitational redshift of the Sun with the highest precision yet, confirming a theoretical prediction made by Einstein in 1920.

Photons are particles of light that are affected by gravitational fields. The path of photons can be bent by dense bodies and their wavelength can stretch as they climb out of a gravitational potential well. This wavelength of light escaping a gravitational well is shifted slightly towards the red and is referred to as a gravitational redshift. As reported in Astronomy & Astrophysics, the effect is small but measurable even for the Sun.

Einstein wrote a century ago: "For the Sun, the theoretical redshift predicted is approximately two-millionths of the wavelength. Whether this effect really exists is an open question, and astronomers are currently working hard to resolve it. For the Sun, its existence is difficult to judge because the effect is so small."

The measurement of the solar gravitational redshift required an ingenious approach. The first step was to look at the spectrum of the Sun. In simple terms, this is like using a prism and revealing all the colors that make up the light of the Sun. In the spectrum, there are dark lines created by elements in the Sun.

The gravitational redshift moves these lines to longer wavelengths than they would appear in the lab. The team focused on some of the iron lines and used the light bouncing off the Moon to make the measurement. The instrument they used was the High Accuracy Radial-velocity Planet Searcher (HARPS), calibrated with a state-of-the-art laser frequency comb, allowing it to make extremely precise measurements.

The measurements are very much consistent with theoretical predictions of the solar gravitational redshift.

"Combining the precision of the HARPS instrument with the laser frequency comb, we have been able to measure with high accuracy the position of the iron lines in the solar spectrum," lead author Dr Jonay González Hernández, from the Instituto de Astrofísica de Canarias, said in a statement. "This has enabled us to verify one of the predictions of Einstein's Theory of General Relativity, the gravitational redshift, to a precision of just a few meters per second."

The gravitational redshift phenomenon has also been confirmed for the stars that orbit the supermassive black hole at the center of the Milky Way, which was part of the work of the 2020 Nobel Prize winners Reinhard Genzel and Andrea Ghez.