A trail of two satellites tests our gravity theories

Two physicists have caught a pair of satellites doing the Einstein twist. This dance revolves around Einstein's general relativity theory, which predicts that our spinning planet should twist space and time in a way that subtly distorts satellite orbits. After sifting through more than 100 million position measurements of these satellites, the physicists have found this minuscule effect.

The analysis has a 10 percent margin of error, according to the physicists, Ignazio Ciufolini at the University of Lecce in Italy and Erricos Pavlis with the Goddard Space Flight Center of the National Aeronautics and Space Administration. But it is precise enough to support Einstein's theory of gravity. It also illustrates how physical law can point scientists toward unanticipated discoveries that help us understand the natural world even when all the details aren't worked out.

Isaac Newton didn't even dream of the space-time twist, let alone put it into his gravity theory - a theory still widely used today. Einstein reformulated gravity theory and the twist appeared.

The two satellites in the experiment - LAGEOS and LAGEOS2 - are uninstrumented heavy spheres covered with small reflectors. Laser beams reflected from them measure the satellites' positions with high accuracy. Describing their work in Nature last month, the physicists say they look forward to reducing their error margin to about 1 percent using the new Gravity Probe B satellite. It will detect the twist using gyroscopes on board.

The great 20th-century physicist Richard Feynman explained the essence of physical law in a lecture he gave 40 years ago. He noted that there is "a rhythm and pattern between the phenomena of nature which is not apparent to the [public] eye, but only to the eye of the analyst, and it is these rhythms and patterns which we call physical laws."

He added that these laws are never perfect. "There is always an edge of mystery, always a place where we have some fiddling around to do."

The space-time twist was beyond that edge until Einstein fiddled around with gravity theory.

Physical law can also share common ground with mythology. While mythical characters are fictitious, their actions reflect underlying truth. The concept of force is the leading character in the myth of Newton's mechanics.

Yet today "the concept of force is conspicuously absent from our most advanced formulations of the basic laws," Frank Wilczek, a physicist at the Massachusetts Institute of Technology, explain-ed in Physics Today.

Einstein's relativity theory treats gravity as a phenomenon of the geometry of space-time. No forces needed.

Nevertheless, the concept of force is alive and well in the 21st century. Dr. Wilczek, one of this year's Nobel Prize winners, says that trying to do practical physics using only force-free laws would be overwhelmingly complex. The concept of force remains popular, he says, because "it is much easier to work with" even though it is "vaguely defined, limited in scope, and approximate." It reflects the basic laws well enough to be widely practical.

In other words, the myth of Newtonian forces works as a good myth should. But Wilczek adds that another "big part of the explanation for its continued use is no doubt intellectual inertia."

Finding a cure for that is a continuing challenge as scientists deal with subjects that the current myths of physical law cannot handle. Physicists feel that challenge keenly as they try to harmonize the theories of gravity and quantum physics.

But that's a whole different twist.

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