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 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

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

Animation of the proposed space station experiments is
available at:

More information on the experiments and NASA’s Fundamental
Physics Program can be found at the following Web sites:




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.