Ultra-precise clocks on the International Space Station and other space missions may determine whether Albert Einstein's Special Theory of Relativity is correct and could dramatically change our understanding of the universe.

The theory, introduced in 1905, holds that if an observer moves at a uniform speed, no matter how fast or in what direction, the laws of physics and the speed of light are always the same. For example, if you stand still and drop a coin, it will fall straight down. Similarly, if you drop a coin inside a car while you're driving down the freeway at a steady speed, it will also fall straight down.

However, recent theories attempting to combine gravity and particle physics suggest that relativity might not always apply; changes in space and time may occur that could not be measured easily on Earth.

The rotating Earth orbits the Sun along the green ellipse. A spacecraft such as the International Space Station is represented by the white square moving around Earth. The white arrow on it shows the direction of an onboard atomic clock. The red arrows show one possible kind of violation of the Theory of Relativity: whereas the theory holds that there is no up or down in space, recent theories predict that there are, in fact, directions in space. Under this premise, as the spacecraft orbits Earth, the orientation of the atomic clock changes in relation to the red arrows, which ultimately leads to changes in its ticking rate.

"The International Space Station will have ultra-sensitive clocks on board, and it is a good place to test the theory," said Dr. Alan Kostelecky, professor of physics at Indiana University, Bloomington. "By comparing extremely precise clocks that can operate under zero gravity, minuscule changes in the ticking rate might be found as the spacecraft moves around Earth." This would violate Einstein's theory, which says there should be no change if different clocks in the same gravity environment are compared.

"Finding such changes would cause an upheaval in the science community and revolutionize our thinking about the fundamental structure of space and time," he added. "It would lead to insight about how our universe formed and how nature operates."

Measurements in space have several advantages over ones on Earth because the Earth's rotation axis and its rotation rate are fixed. In space the orbital axis of a satellite and its rotation rate can be different, and higher speeds are possible. Measurements in space would therefore be more sensitive to minute changes that would violate Einstein's Theory of Relativity.

Kostelecky and his colleagues Robert Bluhm of Colby College, Waterville, Me.; Charles Lane of Berry College, Mount Berry, Ga.; and Neil Russell of Northern Michigan University, Marquette, propose using specific types of clocks on the space station. For example, one type would use a maser, a cousin of the laser. Instead of emitting light, like a laser, the maser emits microwave energy at a specific frequency, which produces a very specific ticking.

Other types of clocks already planned for flight on the International Space Station could be used too. Upcoming missions include the Primary Atomic Reference Clock in Space, the Rubidium Atomic Clock Experiment and the Superconducting Microwave Oscillator. All three are part of NASA's Fundamental Physics Program. In addition, the Atomic Clock Ensemble in Space will be flown on the International Space Station by the European Space Agency.

Kostelecky says clock experiments in space may yield other intriguing results. For example, they might provide evidence for string theory. Traditionally, scientists have believed that the smallest units in the universe are particles. However, advocates of string theory believe the smallest units are elongated, like tiny pieces of string. In some string theories, empty space has an intrinsic direction. This could cause the clocks on the space station to tick at changing rates, depending on their orientation.

In addition to the International Space Station, other future missions may also test the Theory of Relativity. The proposed SpaceTime mission would fly three clocks past Jupiter, then would drop the spacecraft rapidly in toward the Sun, like an extreme version of an amusement park freefall ride. The high speed of this NASA mission would make possible new kinds of sensitive tests.

The paper by Kostelecky and his colleagues appeared in the March 4 issue of the Physical Review Letters. It is available online at: http://prola.aps.org volume 88, article 090801 for 2002.

Kostelecky conducts research under NASA's Fundamental Physics in Microgravity Research Program, part of NASA's Office of Biological and Physical Research, Washington. NASA'S Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology in Pasadena, manages the Fundamental Physics program.