Scientists Find New Proof of Black Holes

Harvard researchers have found what they call the most compelling evidence yet for the existence of black holes and the "event horizons" that surround them, confirming with actual observations a result predicted 86 years ago by Einstein's theory of general relativity.

The findings, announced at the annual meeting of the American Astronomical Society last week, confirmed the existence of event horizons, the point of no return beyond which black holes consume matter.

Once matter crosses that point, nothing can escape--not even light. That means that black holes really are black after all, as theory says they should be.

"Showing that they're black hadn't been done before. It's a fundamental property of black holes," said Michael R. Garcia, a researcher at the Harvard-Smithsonian Center for Astrophysics.

Researchers looked at x-ray novae, which are pairs of stars that rotate quickly and very close to each other. One of the stars in an x-ray nova is a normal star, like the sun, and the other one is a dense, compact star.

In some x-ray novae, the dense star is a neutron star. In these situations, gas and other matter flow away from the normal star and eventually collide with the surface of the neutron star--causing an explosion that releases a bright burst of energy.

In other x-ray novae, the dense star is actually a black hole, a place where all of the star's mass has collapsed to a single point with no volume but an infinitely strong gravitational pull.

In this case, matter from the normal star approaches a so-called "event horizon" that surrounds the black hole. Instead of causing a bright explosion, the matter gets devoured by the black hole. Once something crosses the horizon nothing can escape--not even light.

Einstein's 1915 general relativity theory has predicted the existence of black holes and event horizons. But until recently, telescopes were too imprecise to tell the difference between dim neutron stars and even dimmer black holes.

Now, using a telescope with finer resolution, Garcia and his colleagues have found that black holes are 100 times darker than neutron stars.

Even four years ago, Garcia said, the data on black holes and event horizons would have failed the informal "thumb rule" in science: "If you can stick your thumb on it and blot out the important data point, then you're really in trouble."

Up to that time, technical limitations meant scientists had only identified one x-ray nova as definitely containing a black hole. But new technologies have shown Garcia and his fellow researchers three more x-ray novae with black holes.

"The old results were like that. You could have blotted out that one point," he said. "Now you can't. You can't throw out a point and say it's an oddball."

According to relativity theory, very massive stars--about three times heavier than the sun--eventually collapse to become black holes. In the past, astronomers had assumed that the extremely massive objects in x-ray novae were black holes.

By demonstrating the presence of event horizons, the Harvard researchers say that they have supplied more conclusive evidence that black holes resemble the predictions of Einstein's theory.

"Technically you cannot say you have discovered a black hole until you show that what you have discovered an object that is massive and that has an event horizon," said astronomy professor Ramesh Narayan, one of the researchers.

Narayan called the evidence that he and his colleagues had for event horizons "fairly compelling" but "not conclusive." For instance, other scientists say the difference in brightness could be caused by less matter flowing toward dense neutron stars, not by black holes devouring the matter.

"The specific details of our theory may or may not be correct," Narayan said, "but whatever explanation one comes up with will very likely have to invoke the event horizon to explain this dramatic difference in the brightness of these two systems."

Researchers used the sophisticated Chandra X-ray Observatory, an orbiting satellite launched about a year and a half ago by the National Aeronautics and Space Administration (NASA).

"It's a billion dollar mission. It's a technological marvel," Narayan said.

Chandra has cleaner and more precisely aligned mirrors than any other telescope. According to Garcia, the improvement means Chandra can see objects that are 100 times dimmer than what previous telescopes could see--and that difference is enough to detect the factor of 100 in brightness between neutron stars and black holes.

The namesake of the Chandra observatory--Subrahmanyan Chandrasekharan, an Indian astronomer who won the Nobel Prize in 1983 for his work on the evolution of stars--also studied black holes.

"The black hole is, I think, one of the most amazing objects we study in astrophysics," Narayan said.

--Staff writer Andrew S. Holbrook can be reached at holbr@fas.harvard.edu.