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The Long Arm of Einstein Guides My Steering Wheel

Published: August 16, 200

I was driving in Los Angeles a few weeks ago and relying on a global positioning system receiver attached to my hand-held organizer, which was verbally guiding me through a maze of freeways and unfamiliar streets.

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J. Emilio Flores/The New York Times, top; Jon Levy/AFP

Global positioning systems link traffic in Los Angeles to an Einstein theory.

I had plenty of time stopped in traffic jams to ponder the scientific breakthroughs that made this technology possible.

What I thought about were not the triumphs of Silicon Valley and microchips. I thought about Albert Einstein.

The United Nations and several other international organizations have designated 2005 as the World Year of Physics in honor of Einstein's "miracle year" in 1905, when he wrote five seminal papers that changed the way we think about the world. But his greatest achievement occurred a decade later, when he finished his general theory of relativity.

Einstein revolutionized our understanding of space, time and the nature of gravity. He described a universe in which matter and energy could curve space, which in turn would affect the dynamics of matter and energy, which then could affect the geometry of space, and so on.

It took more than three decades to develop the first direct terrestrial experimental tests of his esoteric ideas. But, esoteric or not, without general relativity, I couldn't precisely navigate through Los Angeles.

Global positioning systems rely on careful measurement differences between signals sent from several satellites thousands of miles apart.

The time differences between the signals sent to the device in my car must be measured to an accuracy of about a billionth of a second to distinguish my position to within a few feet.

But general relativity predicts that the relative clicking of clocks changes depending on their position in a gravitational field. Satellites located high above Earth are moving in a gravitational field that is slightly weaker than what we experience on the ground.

As a result, their internal clocks tick at a different rate than those on Earth.

The effect is extremely small, but it is comparable to the accuracy needed to distinguish the position of objects on the scale that is otherwise possible with needed by global positioning devices. Without correcting for it, the systems would give results that are ever so slightly off.

Einstein didn't develop general relativity because he wanted to find a better way to track his own position. He wanted to address fundamental questions about the universe, and even if he had been so inclined he probably could not have foreseen in 1915 the technology that makes G.P.S. measurements possible today.

This is a timely example of the cross-germination of fundamental scientific investigation and modern technological innovation than this.

The insights that Einstein's theories have given us about the basic workings of the cosmos, on scales as large as the visible universe itself, and going back to the earliest moments of the Big Bang, are invaluable to our understanding of nature and themselves should justify the support we provide for continuing research in general relativity and cosmology.

But in a time when many areas of fundamental research are facing drastic budget threats in favor of more targeted programs with short-term goals, it is worth remembering that even the most esoteric scientific ideas can ultimately affect one's daily life.

It is something worth thinking about the next time you are caught in a traffic jam.

Lawrence M. Krauss is director of the Center for Education and Research in Cosmology and Astrophysics at Case Western Reserve University. His new book,"Hiding in the Mirror," will appear this fall.