CRACKS in Einstein鈥檚 theory of relativity could be evident at the heart of our very own galaxy. If new calculations are correct, the enormous black hole that sits at the core of the Milky Way should have Saturn-like rings that offer a safe haven from its irresistible gravitational pull.
Einstein came out with his theory of general relativity in 1916. It describes how the effects of gravity emerge from the way space-time is shaped. It has passed every test thrown at it 鈥� including the latest, this week 鈥� but many physicists still believe general relativity has a chink in its armour. That鈥檚 because the theory is incompatible with quantum mechanics, the other pillar of modern physics.
Where relativity describes the universe at large scales, quantum describes the universe of the tiny, random particles that make up matter. Efforts to combine them lead to mathematical nonsense. 鈥淕eneral relativity must fail at some level,鈥� says .
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One of the best places to look for flaws should be where gravity is most intense, in the vicinity of neutron stars, say. These dense stellar corpses are left over when a massive star explodes as a supernova, and can pack the mass of the sun into a sphere just 20 kilometres across.
of the Max Planck Institute for Radioastronomy in Bonn, Germany, and colleagues have now put relativity to the test around the most massive neutron star yet discovered. PSR J0348+0432 is 2.01 times the mass of the sun. It orbits a white dwarf 鈥� the burnt-out husk of a sun-like star that has reached the end of its life. General relativity predicts that as the pair circle each other, they should lose energy in the form of gravitational waves, and their 2.46-hour orbit should shrink by about 8 millionths of a second every year.
And that is exactly what Antoniadis and colleagues saw (). 鈥淭hat鈥檚 astonishing,鈥� he says. 鈥淭his mathematically elegant theory, general relativity, was devised 100 years ago but still manages to survive every test we perform.鈥�
聯This mathematically elegant theory devised 100 years ago still survives every test we perform聰
To find general relativity鈥檚 weak spots, then, physicists may have to look to stronger gravitational fields. Bambi suggests starting with the densest, most massive objects we know: the supermassive black holes at the centres of galaxies.
Every galaxy, including the Milky Way, hosts a massive dark object at its core. We only know that they are hyperdense and do not emit light, leading physicists to assume that they are black holes millions of times more massive than our sun. However, there is no direct evidence that these black holes follow the rules of relativity.
鈥淭heir measured mass is enough to say that these objects are not neutron stars or clusters of non-luminous objects, but we do not know if the space-time geometry is the one predicted by general relativity,鈥� Bambi says.
He and his colleagues have calculated what the object at the centre of our galaxy would look like if it did not obey Einstein鈥檚 laws of general relativity. They find that there should be stable regions very close to them where particles can hover for long periods of time, instead of falling directly into them (). Some gas swirling around the black holes should get trapped in these stable orbits. 鈥淭he result is a compact object with gas rings, more or less like Saturn,鈥� says Bambi.
In theory, the ideal opportunity to test Bambi鈥檚 ideas will come later this year, when the black hole at the centre of the Milky Way has its first big meal in living memory. A gas cloud dubbed G2 is set to approach the monstrous object鈥檚 edge. If Bambi is right, radio, infrared and X-rays signals from the cosmic binge could reveal ring-like structures at the black hole鈥檚 edge.
Sadly, the right telescope for the job probably won鈥檛 be ready in time. But in future, space-based gravitational wave telescopes could look for irregular bursts of gravitational waves produced when particles jump from one ring to another. And the and GRAVITY observatory, set to come online in the next 5 to 10 years, will observe the motion of plasma orbiting black holes. That means they could spot the leftovers of this summer鈥檚 cosmic binge.
鈥淲e are living in a very exciting time for black hole physics,鈥� says of the Paris Observatory, France. 鈥淔or the first time, we will see the vicinity of the event horizon predicted by general relativity.鈥� Studies like Bambi鈥檚 do the groundwork so that when these new telescopes come online, we can finally find the cracks in one of physics鈥檚 greatest pillars.
This article appeared in print under the headline 鈥淏lack hole binge could test relativity鈥�
