05 August 2018

ESO: “First Successful Test of Einstein’s General Relativity Near Supermassive Black Hole”

The team compared the position and velocity measurements from GRAVITY and SINFONI respectively, along with previous observations of S2 using other instruments, with the predictions of Newtonian gravity, general relativity and other theories of gravity. The new results are inconsistent with Newtonian predictions and in excellent agreement with the predictions of general relativity.

The new measurements clearly reveal an effect called gravitational redshift. Light from the star is stretched to longer wavelengths by the very strong gravitational field of the black hole. And the change in the wavelength of light from S2 agrees precisely with that predicted by Einstein’s theory of general relativity. This is the first time that this deviation from the predictions of the simpler Newtonian theory of gravity has been observed in the motion of a star around a supermassive black hole.

eso1825 — Science Release

It’s remarkable how well Einstein’s theory of general relativity has withstood every test and experiment it has been subjected to for the past century. But I find it somewhat odd that none of the articles I’ve read about this research mention how predictions from alternative theories of gravity fit with these measurements. No serious physicist would expect Newtonian gravity to pass this test, but what about string theory, MOND or loop quantum gravity? It makes me think that none of these theories can match the predictive success of general relativity, let alone improve upon it. And that suggests physics is very far from surpassing Einstein and unlocking the elusive final unification of forces.

Artist impression of S2 passing supermassive black hole at centre of Milky Way - annotated
This artist’s impression shows the path of the star S2 as it passes very close to the supermassive black hole at the centre of the Milky Way. As it gets close to the black hole the very strong gravitational field causes the colour of the star to shift slightly to the red, an effect of Einstein’s general theory of relativity

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