According to Einstein's general relativity theory, material mass bends space and distorts time. If that mass is rotating, the theory says, it also drags nearby space-time around with it. NASA's Rossi X-ray observing satellite now is tantalizing astronomers with hints that it is seeing space being stirred, as well as bent, by fast-spinning superdense objects called neutron stars.
But what really turns astrophysicists on is the prospect that they at last can find out if Einstein was right about the way matter responds to strong gravity.
"Einstein's theory of how matter moves in strongly curved space-time has not yet been verified," says Dutch astrophysicist Michiel van der Klis. He explains that Rossi "is the first instrument that has allowed us to actually see" that movement. The neutron star X-rays "carry that message," he says.
Dr. Van der Klis and colleagues Peter Jonker and Mariano Mendez at the University of Amsterdam have published some of the latest analysis of this "message" in the letters section of September's Astrophysical Journal. They have identified a narrow band of frequencies in the X-ray emissions that reflect the way gas clumps are orbiting three neutron stars. What they see could be due, in part, to the way the stars drag space as they spin. But that conclusion is speculative.
Their work is part of a fast-paced effort by several research groups which began after the Rossi X-ray Timing Explorer was orbited nearly five years ago. It was specifically designed to give astronomers their first window on the world of super-strong gravity. That means the space around collapsed massive stars, including neutron stars. As Van der Klis has noted in a review paper in Science, "They have the strongest gravity, and hence curve space-time the tightest, of all known objects in the universe."
Neutron stars pack between 1.4 to 3 solar masses of matter - 1 solar mass being equal to the mass of the sun - in a space only a few miles across. They are as dense as an atomic nucleus. If they accompany an ordinary star in a binary system, they can pull in matter from their companion. That matter gives off X-rays as it spirals around and then impacts the neutron star. Rossi's X-ray telescope is designed to detect those X-rays with enough detail for astronomers to decipher the motions of the matter that produces them.
Jean Swank, Rossi project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md., says that Rossi data "launched a swirl of intense theoretical work that has produced several possible explanations" of what is going on. This has inspired several news releases over the past four years that featured speculation, but no definite conclusion, about possible space dragging. The Dutch team's "newly discovered signals may be the key that unlocks the door, so we can see what the right explanation is," Dr. Swank says.
That "right explanation" could have far-reaching implications. Einstein's relativity theory underlies much of modern physics. Were his gravity theory to be found faulty, it would rattle physics's foundations. The theory depends on a seemingly simple fundamental principle. Einstein said that the kind of mass that gives matter inertia - that makes moving objects resist efforts to change their motion - is the same as the mass that creates gravity. This so-called equivalence principle requires a theory of gravity that is purely geometric.
In other words, a massive body, such as a neutron star, warps surrounding space. Other matter, such as orbiting gas clumps, moves through that space following its distorted geometry. There is no gravitational force, as such, involved.
If Rossi data were to show that matter around neutron stars does not move as general relativity predicts, theorists would scurry to find another geometric theory. They might even wonder if the kind of mass that creates gravity really is the same as the mass that creates inertia. In either case, if general relativity were to fail in any aspect, its entire structure would unravel, as Einstein himself explained.
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