Bald Black Holes
While pulsars are certainly compact objects, the true gravitational heavyweights are black holes. Dimitrios Psaltis, an astrophysicist at the University of Arizona, is helping to test what may be general relativity’s most extreme prediction: that large-enough stars will eventually collapse under their own gravity to form these infinitely dense objects. Despite decades of data implying the existence of black holes, all the evidence is circumstantial, based on observations of their effects on light or other objects, Psaltis says. Black holes themselves have yet to be directly observed.
Seeing is believing, so Psaltis and his colleagues hope to take a direct photo of Sagittarius A*, the monstrous black hole that astronomers suspect lurks at the center of our galaxy. To do this, the researchers will use the Event Horizon Telescope (EHT) — a combination of more than 10 radio telescopes and telescope arrays scattered across the planet — which should be able to see all the way to the edge of Sagittarius A*, some 26,000 light-years away. Psaltis and his colleagues suspect the black hole will cast a circular shadow amid a radio-wave background.
In addition to proving that black holes exist, Psaltis says the EHT should also confirm or challenge another key tenet of relativity, the no-hair theorem. The eminent theoretical physicist John Archibald Wheeler quipped that “black holes have no hair,” meaning they’re all identical except for three key distinguishing characteristics: mass, rotation and a vanishingly small electric charge. Any “hair” — essentially, any specific information about what goes into the black hole, like an object’s chemical composition or molecular structure, or even just shape and size — is lost forever within the black hole’s event horizon.
With the EHT, Psaltis and his team plan to study the size and shape of the shadow that Sagittarius A* casts. General relativity’s no-hair theorem predicts an almost perfectly circular shadow; modifications to general relativity in which black holes retain their hair could yield an ellipsoidal one. Also hoping to probe the theorem, University of Florida physicist Clifford Will suggested a test to track how ordinary stars near Sagittarius A* move, and Norbert Wex and Michael Kramer at the Max Planck Institute for Radio Astronomy in Germany hope to do the same with one or more pulsars, though they first need to find one that is close enough. Black hole hair would change how such objects orbit near the hole, and these changes could be detectable by telescopes that will come online within the next decade.
“If we do find that the no-hair theorem is not satisfied, that would really be a major blow to the theory, or to black holes,” says Psaltis. It would be a surprise, he acknowledges, but sooner or later, something unexpected has to show up. “And in gravity, like in most places in physics, whenever we open a window on somewhere we’ve never looked, we always find a surprise.”
Gazing at Gravity
Finally, one set of experiments will probe relativity not by collecting and analyzing light from cosmological bodies, but by looking at gravity itself. The Laser Interferometer Gravitational-Wave Observatory (LIGO) and a companion experiment named Virgo will search out those cosmic ripples, or gravitational waves, emanating from galaxies hundreds of millions of light-years away.