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Report by NASA's Goddard Space Flight Center, Greenbelt, MD. November 1997.
"Astronomers using NASA's Rossi X-ray Timing Explorer (RXTE) spacecraft reported today that they have observed a black hole that is literally dragging space and time around itself as it rotates. This bizarre effect, called "frame dragging," is the first evidence to support a prediction made in 1918 using Einstein's theory of relativity. Fig. 1 below illustrates the effect.
Fig 1: Frame dragging illustrated: 
Drawing Credit & Copyright: J. Bergeron, Sky & Telescope Magazine.
What you see is the accretion disk around a rotating (Kerr) black hole, with the radial paths of photons being offset and bent. Nothing, not even light, can fall "straight in" - the 'frame dragged' spacetime drags everything around with it.
"The phenomenon is distorting the orbit of hot, X-ray emitting gas near the black hole, causing the X-rays to peak at periods that match the frame-dragging predictions of general relativity. The research team, led by Dr. Wei Cui of the Massachusetts Institute of Technology, is announcing its results in a press conference today during the American Astronomical Society's High Energy Astrophysics Division (HEAD) meeting in Estes Park, CO. Collaborators in the research include Dr. Wan Chen of NASA's Goddard Space Flight Center, Greenbelt, MD, and Dr. Shuang N. Zhang of NASA's Marshall Space Flight Center, Huntsville, AL.
"If our interpretation is correct, it could demonstrate the presence of frame dragging near spinning black holes," said Cui. "This observation is unique because Einstein's theory has never been tested in this way before."
"The research team used these X-ray emissions to determine if frame dragging was present. The team found that the X-ray emissions were varying in intensity. By analyzing this variation, they found a pattern, or repetition, that was best explained by a perturbation in the matter's orbit. This perturbation, called a precession, occurs when the orbit itself shifts around the black hole. This is evidence for frame dragging because as the matter orbits the black hole, the space-time that is being dragged around the black hole drags the matter along with it. This shifts the matter's orbit with each revolution."
I must caution Blog readers not to confuse this effect with the "ordinary" precession (or perihelion shift) around non-spinning masses. This precession is 'over and above' - it adds to the precession of elliptical orbits experienced by orbits around static black holes.
In a way the frame dragging precession is easier to understand than, e.g., the perihelion shift of planet Mercury, which is not related to the rotation speed of the Sun at all. (Hmmm… this latter type of precession could be the subject of another Blog post...)
In a future post I will attempt some quantitative data (calculations) on frame dragging - it's pretty hard to get hold of accessible (quantitative) information on strong field frame dragging, so watch this space!
Regards, Jorrie
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