Even in the most extreme environments in our galaxy, light behaves as predicted by Albert Einstein’s general theory of relativity, a decades-long study suggests.

Einstein’s theory says that light travelling out of a strong gravitational field loses energy and stretches to a longer, redder wavelength — an effect known as gravitational redshift. The first observations of this phenomenon around a black hole were made in 2018 by the GRAVITY collaboration during a study of the S-02 star, which orbits the black hole at the centre of the Milky Way.

Now Tuan Do at the University of California, Los Angeles, and his colleagues have independently confirmed GRAVITY’s finding with measurements that they say are more robust.

The team used the Keck telescopes in Hawaii and other instruments to measure S-02’s position and velocity over 24 years. The researchers analysed these data to precisely trace the star’s 16-year orbit around the black hole. They then combined these movement data with observations of the star’s closest approach to the black hole, in 2018, and detected a redshift that closely matches the predictions of general relativity.