By NASA/CXC/M.Weiss – http://chandra.harvard.edu/photo/2001/blackholes/ (image link), Public Domain, https://commons.wikimedia.org/w/index.php?curid=40060179

The black hole at the center of the Milky Way has proven one of Albert Einstein’s key predictions is correct: light climbing out of a black hole is ‘stretched’ so that it turns red.

The theory of general relativity addressed the issue of how photons would get out of a gravitational well such as a black hole. To do so it would have to expand energy. In most circumstances you’d expect something that expands energy to slow down. However, by definition photons travel at the speed of light, which is a constant.

Einstein’s theories suggested the result would be gravitational redshift, in which the loss of energy comes in the form of the light’s wavelength being drawn out, moving it towards the light spectrum’s red end (as shown in the artist’s conception above).

That’s never been confirmed in reality until now with the results of the wonderfully-named Very Large Telescope in Chile being able to observe the star S2 moving past the black hole Sagittarius A.

Gravitational redshift itself has been observed before and is already factored in to the operations of GPS satellites. However, this is the first time it’s been observed in the context of a black hole and is effectively the first time tenets of the theory of general relativity have been proven in that setting.

An ordinary telescope would not have been able to make the observation as it would be distorted by both the contents of the Earth’s atmosphere and the sheer amount of light from stars around the black hole. The Very Large Telescope is actually made up of multiple telescopes, with information from four combined for this observation.