purpose of the PARCS project is to place an advanced laser-cooled cesium
atomic clock in orbit and
utilize it to test a variety of predictions of the Theory
of Relativity. One of these predictions, made by Albert
Einstein in 1915, is that clocks tick slower in strong gravity
than they do in weak gravity.
An orbiting satellite might place PARCS at
an altitude of 220 miles (360 kilometers), where gravity
is slightly weaker than that found at the Earth's surface.
Thus the PARCS clock aboard the satellite ticks faster than
a clock on the surface of the Earth by about 1 second in
every 10,000 years.
We Hope to Find Out:
tiny shifts have already been observed in previous experiments-the
aim of PARCS is to measure them about a hundred times more
accurately than ever before. To observe such a small change in clock rate
requires extremely accurate clocks, both in orbit and on the ground.
PARCS will be the most accurate clock ever built, keeping
time to within 1 second in 300 million years. It will be
compared to the master clock of the United States, which
is at the National Institute of Standards and Technology
(NIST) in Boulder, Colorado. Principal Investigators for
the PARCS project are from NIST and the University of Colorado.
clock consists of two major components: an oscillator, which
produces a stable frequency (in other words, something that
produces a steady series of "ticks"), and a "frequency
checker", which compares that frequency to the natural
frequency of an atom. For PARCS, the oscillator will itself
be a highly stable atomic clock, a hydrogen maser built
by the Smithsonian Astrophysics Observatory. The frequency
checker part of the apparatus consists of a beam of very
cold cesium atoms, which pass through a pair of microwave
cavities, which are used to very accurately measure the
natural frequency difference between two internal energy
levels of an atom. The hydrogen maser frequency is checked
against this frequency and then compared to that of a clock
on the ground. Because every cesium atom of the same isotope
is identical, we can be sure that any differences in frequency
that we see between a clock on the ground and one in orbit
are due to relativistic effects.
the very high accuracy required, PARCS will use atoms that
have been cooled to a temperature of just 1 millionth of
a degree above absolute zero. This is achieved using a technique
called "laser cooling." Photons from several laser
beams, each coming from a different direction, bounce off
of the atoms, giving the atoms a small push with each bounce.
These small pushes serve to slow down the atomic motion,
resulting in dramatically cooler temperatures. These lower
temperatures allow the natural frequency of the atom to
be measured much more accurately. Further improvements can
be made by performing the experiment in space. Because objects
in orbit are all freely falling (this is what produces the phenomena
the atoms can be observed for a longer time before they
hit the walls of the container. The longer measurement times
yield more precise clocks. As Werner Heisenberg showed
in 1927, this longer observation time allows for a more
precise measurement of an energy level (this is called "the
uncertainty principle for time and energy").
We'll Conduct Our Experiment:
To compare the measurement of time by
the PARCS experiment in orbit and an accurate clock on the
ground, the Global Positioning System (GPS) is used. Each
clock compares its frequency to that transmitted by the
GPS satellites. By knowing each of these frequency differences,
one can calculate the frequency difference between the ground
clock and the space clock.
- See the RACE experiment information