Putting Einstein to the test

The nearest supermassive black hole will give astronomers a great laboratory


In 2015, almost exactly a century after he concocted it, Albert Einstein’s theory of general relativity got a spectacular confirmation when scientists detected gravitational waves. These undulations, launched by the dances of distant, massive objects, minutely stretch the weft and warp of space­time itself (and thereby of experiments designed to intercept them). 

In 2018 the theory will get its most gruelling test yet, in the “high-mass limit”—where the masses in question are thousands or millions of times that of the sun. So-called supermassive black holes provide just such an environment, and conveniently (or, as astronomers think these days, simply because every galaxy does) Earthlings’ home galaxy has one: Sagittarius A*. Situated 26,000 light-years away, it is a 4m-solar-mass monster, surrounded by swirling dust and galactic detritus, and a handful of stars. The closest, called S2, makes an elliptical circuit every 16 years; in 2018, as it again makes its closest approach, astronomers will be watching. 

Most will be collecting its infra-red emissions, as it is those rays that best penetrate the maelstrom. And the best gizmo for doing that is GRAVITY, a device built for this purpose over the past decade and now bolted onto the Very Large Telescope Interferometer (VLTI) high in Chile’s Atacama Desert. It will use the VLTIs four telescopes, collecting and precisely overlapping their light such that the facility has the acuity of one giant telescope as large as the separation between the four. 

Its masters should be able to spot tiny deviations in S2’s path that show precisely how spacetime is stretched in some of the strongest gravity the universe can muster. Most scientists expect that Einstein’s relativity will again pass the test: that GRAVITY will see S2’s path deviate just a smidgen in 2018, and the whole orbit’s centre shift by 2019, just as general relativity predicts. 

But a still more exciting prospect is that it does not, thereby revealing the first shortcoming of relativity. If so it would hint that an even more all-encompassing theory awaits discovery.

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