Satellite Observations Of Black Holes Confirm "Frame-Dragging" Effect 80 Years After Prediction By Einstein

November 06, 1997

The next time you feel like you're barely dragging along, blame relativity. You'll be stretching the point, but it appears that Einstein was right: space and time get pulled out of shape near a rotating body.

Einstein predicted the effect, called "frame dragging," 80 years ago. Like many other aspects of Einstein's famous theories of relativity, it's so subtle that no conventional method could measure it.

Using recent observations by X-ray astronomy satellites, including NASA's Rossi X-ray Timing Explorer, a team of astronomers is announcing that they see evidence of frame dragging in disks of gas swirling around a black hole. The discovery will be announced today at a meeting of the High Energy Astrophysics Division of the American Astronomical Society in Estes Park, Colo., by Dr. Wei Cui of the Massachusetts Institute of Technology, and his colleagues, Dr. Nan Zhang, working at NASA's Marshall Space Flight Center, and Dr. Wan Chen of the University of Maryland in College Park.

Frame dragging is one of the last frontiers in relativity. More familiar and already proven are the conversion of mass into energy (as seen in atomic bombs and stars) and back, the Lorentz transformations that make objects near the speed of light grow thinner and heavier and stretch time, and the warping of space by gravity (as seen when light is bent by a massive object).

Einstein also predicted that the rotation of an object would alter space and time, dragging a nearby object out of position compared to predictions by the simpler math of Sir Isaac Newton.

The effect is incredibly small, about one part in a few trillion, which means that you have to look at something very massive, or build an instrument that is incredibly sensitive and put it in orbit.

Cui, Zhang, and Chen followed the first path, looking at radiation coming from around black holes, once-massive stars compacted by an explosion to a diameter of a few kilometers. The gravitational field is so strong that everything -- including light -- goes in.

While we cannot see black holes directly, we can detect them by the light emitted by gas as it spirals inward to the black hole's point of no return, called the "event horizon". Like an unruly crowd jamming a stadium entrance, the gas becomes heated as it gets closer to the event horizon, and gives off radio waves, visible light, and -- just before it disappears -- x-rays.

The best place to look for black holes is in binary star systems where a normal star feeds the disk of accreting matter crowding its way into the accompanying black hole. Cui's team looked at two black holes, called GRS 1915+105 and GRO J1655-40, that also emit superluminal gas jets.

In an earlier paper authored by Nan Zhang, the team measured the spin of these black holes based on the highest energy radiation which would also come from the innermost, and last possible orbit before the event horizon.

With the spin measured, the team turned to quasi-periodic oscillations, slight changes in the timing of the signals from the superluminal jets, and in other black holes.

What Cui, Zhang, and Chen found was a precession far greater than a simple mechanical effect could explain. In the case of GRO J1655-40, which is about 7 times more massive than our sun and spinning near the maximum allowed rate, the accretion disk precesses 300 times a second! GRS 1915+105, also spinning near the maximum rate, the disk precesses a mere 67 times per second.

Two other black holes have very slow and variable precession rates, indicating that their accretion disks have not settled into a stable formation.

Someone observing a safe distance away from the black hole binary system would see the accretion disk appear to wobble like a top out of balance.

Zhang said the team is confident of its findings because spin calculations made in this work give the same answer as earlier spin calculations using a different approach.

And speaking of different approaches, what of the second method for measuring frame dragging?

Zhang said that it remains as important as ever. NASA is developing it as Gravity Probe-B, a satellite containing precision gyroscopes inside a liquid helium bath. GP-B will point at a selected star, and sensitive instruments will measure how much the gyros precess after conventional effects are nullified. The leftover effects should provide a precise measure of frame dragging.Zhang pointed out that the Rossi satellite observations are not a controlled experiment. The exact mass of the star and other effects around it are not known with great detail. Gravity Probe-B, though, will be the controlled experiment which gives physicists the precision they need for filling it blank spots in our understanding of how the universe works.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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