Star Trek fans -- and physicists -- have known that space and time get distorted near certain galactic objects, but now they have more precise information about the way that distortion works near spinning black holes. Researchers led by an MIT scientist recently obtained the first observational evidence that massive, rotating black holes in our galaxy drag space and time around with them as they gather matter into their spiral, much as a twister picks up objects in its path.
This phenomenon, known as frame-dragging, was first predicted in 1918 as a natural consequence of Einstein's general theory of relativity, which describes the effects of gravity on space and time. But it had been unproved by experiments or observation until recently, when Italian researchers suggested the effect might be present near spinning neutron stars. The MIT team then applied a similar idea to several black holes in our galaxy.
"If our interpretation is correct, it could be said to prove the presence of frame-dragging near spinning black holes," said Dr. Wei Cui, a research scientist at MIT's Center for Space Research (CSR) who is lead author on a paper presented at a meeting of the High Energy Astrophysics Division of the American Astronomical Society (AAS) on November 6. His collaborators are research scientists Shuang N. Zhang of NASA's Marshall Space Flight Center, and Wan Chen of NASA's Goddard Space Flight Center.
Black holes are exceptionally compact objects with a gravitational pull so strong that no light can escape them. Since black holes cannot be seen directly, their existence can only be deduced from observations of the behavior of sister-stars thought to cohabit with black holes. The gravitational pull of the black hole forces the sister star to revolve around it.
The black hole then acquires material from the star by pulling the matter into the orbit of an accretion disk -- a ring-like disk of gas that moves around the black hole. As the matter in that disk moves closer and closer to the black hole, the matter heats up and begins to emit X-rays. These X-ray emissions are critical to the measurement of the frame-dragging effect.
Dr. Cui's team took the results of their own recent study that measured how fast black holes spin by using the inferred temperature and location of the matter rotating around them. That study, which came out earlier this year, gave the first published measurement of a black hole's spin. Using that measurement and the mass of the black hole, his team then determined how frame-dragging would affect the material in the accretion disk as it orbits the black hole.
They showed that the matter's orbit in the accretion disk would wobble, much as a child's top wobbles when it slows down. The frequency at which it would wobble, based on their calculations, turned out to be the same frequency as the actual oscillations in intensity of the X-ray emissions previously measured by other researchers. They theorized that this wobble is evidence of frame-dragging, because the matter's orbit can only wobble if the space and time in which it exists are being dragged.
Dr. Cui pointed out that they cannot claim with absolute certainty that they have proven the presence of frame-dragging. However, he noted that while there are other interpretations that work for two of the five black holes studied, none of them can be satisfactorily applied to all five.
Actually, the general theory of relativity predicts that frame-dragging should occur around any spinning body, even the Earth. And NASA's plans to launch a satellite called Gravity Probe B in 2000 could lead to a direct measurement of the Earth's frame-dragging. But the effect would be much more marked around a body with both tremendous mass and small size, like a black hole, and therefore somewhat easier to detect. Even so, its detection took nearly 80 years from the time it was first predicted.
"Although theorists predicted the frame-dragging effect, they didn't have any observational evidence to prove it before the Rossi X-ray Timing Explorer," said Dr. Cui, whose research was funded by NASA.
ROSSI RXTE
The Rossi X-Ray Timing Explorer, or RXTE, is a 6,700-pound observatory placed into orbit by NASA in December 1995 to gather information on black holes and neutron stars -- objects akin to black holes only less massive. It is named after Bruno B. Rossi, an MIT professor who was a pioneer in the field of X-ray astronomy.
Two of the instruments on board RXTE were designed by Professor Hale Bradt and colleagues at the CSR. The first, the All Sky Monitor (ASM), sweeps over 80 percent of the sky every 90 minutes, and monitors the intensities and spectra of the brightest X-ray sources. The second is the Experiment Data System (EDS), a powerful computer that crunches numbers before transmitting data back to Earth.
At the AAS meeting in Estes Park, CO, where Dr. Cui's findings were presented, the Italian researchers presented their proof of frame-dragging by spinning neutron stars. Most of the data used by both teams were obtained by RXTE.
Since the research on frame-dragging by black holes was first reported last Thursday, it has generated intense media interest, with reports published or aired by CNN, ABC News, USA Today, NPR, the Washington Post, The Los Angeles Times, the Chronicle of Higher Education and the Boston Globe, among others.
The very existence of black holes is itself the subject of considerable scientific debate. They are thought to be created when a very large star near the end of its life collapses under its own gravitational pull. Such stars are exceptionally dense because they become as small as 60 kilometers -- or 40 miles -- in diameter, while still retaining a mass many times that of our Sun. Once a star reaches this stage, its gravity is so powerful that absolutely nothing, not even light, can escape, leaving what appears to us as a black hole in space.
"Of course there are still many unanswered questions about the X-ray emission processes in these black hole systems," said Dr. Cui. "But the observations in this case seem to suggest the presence of the frame-dragging effect -- that spinning black holes do drag space and time around with them" -- something Trekkies have known for years.
A version of this article appeared in MIT Tech Talk on November 12, 1997.
