Black hole as a peephole

The smearing of starlight near the Milky Way’s central black hole could provide a new window into extra dimensions.

Black holes have been praised as potential extradimensional peepholes before. Gravity is surprisingly weak at macroscopic scales, especially compared to the other fundamental forces, and some physicists think gravity could be leaking from the three-dimensional world humans inhabit into extra, unobserved dimensions. Observations of black holes could help (SN: 9/26/09, p. 22): Shrinking black holes could be evaporating into other dimensions, and any tiny black holes that might be produced in the Large Hadron Collider would be the result of extra-strong gravity at micrometer length scales, meaning gravity’s relative weakness can be blamed on extra dimensions.

Those tests rely on black holes ranging in mass from many times the mass of the sun to many times smaller than an atomic nucleus. But a new technique proposed February 14 at the American Physical Society meeting in Washington would make use of the most massive object in the Milky Way galaxy: the supermassive black hole at the galactic center.

Weighing in at about 4 million solar masses, that black hole exerts enough gravitational oomph to bend the light of stars that orbit near it for observers on Earth to see a phenomenon known as gravitational lensing.

“A gravitational potential…does several things to the light,” said graduate student Amitai Bin-Nun of the University of Pennsylvania. “It can split the light into multiple images, it can distort those images, and most importantly, those images either are brighter or less bright than the original light coming from the source.”

Bin-Nun chose a particular star whose orbit brings it close enough to the supermassive black hole to distort its light. He ran simulations of how its brightness will change over the next 10 years assuming two different shapes for the spacetime around the black hole: one predicted by a four-dimensional (three space and one time) universe, and one that includes the effects of a fifth dimension.

“This is a test of what gravity looks like around the black hole,” he said.

The simulations showed that at the star’s peak brightness, which it will reach in early 2018, the star will be about 44 percent brighter if the universe has five dimensions than if it has four.

Unfortunately, in either case the star would be too dim for most current ground-based telescopes to see. “This is observationally challenging,” Bin-Nun said, which may be an understatement. “But it is still a statistically and scientifically significant result.” He pointed out that a telescope called MICADO that was proposed to come online in the next decade should be able to detect the star, and other stars that have not yet been discovered might be better candidates.

If such a brightness difference was observed, it would be “incredibly important,” comments Tom Murphy of the University of California, San Diego. “It gets at the very nature of space and time.” But he’s skeptical that the observations are even possible. “It’s not a useful test if it can’t be done,” he says.