Science NOW | 24 Nov 2010
Gravitational lensing of stars orbiting Sgr A* as a probe of the black hole metric in the Galactic center - AY Bin-NunCould space have dimensions beyond the three that we all know and love? Some theories in particle physics speculate that it might, although these dimensions would be curled up in loops so small, they could probably be probed only in high-energy particle collisions. Now, however, one theorist suggests that, at least in principle, these hypothesized dimensions might reveal themselves in another subtle way. If there are extra dimensions, then the gravity from the black hole in the center of our galaxy might dramatically brighten the images of stars beyond it. Others see problems in making the scheme work, however.
The idea rests on two assumptions. First, that space has extra dimensions. That's a central tenet of string theory, which posits that every fundamental bit of matter is really an infinitesimal string vibrating in one way or another. According to string theory, space has six extra dimensions that we don't see because they're curled and tangled up at very small length scales.
The second assumption is the warping of space by a black hole. According to Einstein's theory of gravity, matter warps space and time, or spacetime. This warping changes the paths that free-falling objects follow, making, for example, planets orbit the sun when they would otherwise travel at constant speeds in straight lines into space—in other words, creating what we think of as gravity.
In Einstein's theory, warped spacetime even affects light. When light from a star passes near a galaxy, for example, its path bends, changing where the star appears in the sky—the so-called gravitational lensing effect. If a star is just behind a gravitational lens such as a black hole, the effect can produce multiple images of a star or even a so-called Einstein ring that encircles the lens. Just like a telescope does, gravitational lensing also makes stars appear larger and hence brighter.
Now, physicist Amitai Bin-Nun of the University of Pennsylvania argues that gravitational lensing around Sagittarius A*, the supermassive black hole thought to be at the center of the Milky Way, might provide a way to search for extra dimensions. In one version of the extra-dimensions scenario, gravity is much stronger near the black hole than it would be without the extra dimensions, so that images of the stars appear larger and brighter than they otherwise would. Bin-Nun used numerical simulations to show that, in a world with extra dimensions compared with one without, a star known as S2 could be as much as 44% brighter when it reaches its peak brightness in 2018. If S2 were that bright, it could be evidence for extra dimensions or at least evidence that our understanding of gravity should be modified.
Bin-Nun says that the approach, published in Physical Review D, could be a good way to probe black hole properties, but he acknowledges practical and theoretical concerns. Using gravitational lensing to search for extra dimensions "depends on a telescope being able to see a very faint object" at the galactic center, he says, which may be impractical given current technology. Still, if astrophysicists observed the brightness Bin-Nun predicts, it could be a sign that some of those extra dimensions are coiled loosely enough to have a detectable effect.
Harvard University physicist Abraham Loeb says that "overall, it's a good idea to test modifications of gravity" using black holes. But he says Bin-Nun's hypothesis will be hard to validate and that observations of the orbits of planets in our solar system might already rule out such large changes in the strength of gravity. Furthermore, Loeb says, if the black hole is rotating, it would produce effects that could easily be confused with Bin-Nun's predictions.
- Physical Review D 82(6) 064009 (07 Sep 2010) DOI: 10.1103/PhysRevD.82.064009