NASA Home Page JPL Home Page Caltech Home Page
NASA - Jet Propulstion Laboratory + View the NASA Portal  
JPL Home JPL - Earth JPL - Solar System JPL - Stars and Galaxies JPL - Technology
Shell image/ Home button Fundamental Physics in Space
Technical Details
Shell image / home button
The Story of our Search
Adventures in Science
Technical Details
Gravitational Relativistic Physics (GRP)

Gravitational and Relativistic Physics (GRP)

    FUTURE: Gravity Probe B Seperator Dot AMS Seperator Dot STEP Seperator Dot LISA Seperator Dot SUMO
    PAST: Gravity Probe A Seperator Dot Viking Seperator Dot Lunar Laser Ranging Seperator Dot LAGEOS I & II
PRESENT: Ongoing Research
Ongoing Research

Gravitational and relativity physics is perhaps the most fundamental area of physics. Gravity is the weakest of the four known forces, yet paradoxically it is the most dominant because it acts from very large distances. In fact, every bit of matter in the universe is under the gravitational influence of every other bit of matter, even if infinitesimally so.

Relativity theories propose that gravitational forces apply equivalently to all bodies. Furthermore, Einstein's theory of general relativity puts gravity at the heart of the structure of the universe, proposing that even the orderly space-time structure of the universe can be "warped" near a body of large mass, such as the Sun or even the Earth. This warp near massive bodies affects even clocks. While these changes to the very fabric of space and time near a large body are dramatic in their importance, they are also very subtle and difficult to measure accurately. Yet, they must be taken into account even in routine astronomy observations and in measuring the position of satellites and planets. Advanced technologies must be applied to detect and characterize these minute changes so that the corrections caused by relativistic phenomena can be precise.

Several missions and experiments presently under development to improve the accuracy of measurements of gravity effects are described briefly in this section. All of these experiments make use of the nearest large body, the Earth, to measure the distortion of space-time.

  • Gravity Probe B measures how Earth distorts the space around it, and how Earth drags nearby space as it rotates. GP-B is scheduled to be launched May 1, 2002. Learn more about the science objectives, and about the many technology developments that have been necessary for this experiment, at its web site Be sure to follow the progress of this pioneer relativity experiment.
  • Satellite Test of the Equivalence Principle (STEP) will compare a body's inertial mass to its gravitational mass to the remarkable resolution of a part in 1018! For more information on STEP visit
  • The Superconducting Microwave Oscillator (SUMO) will place a high-stability clock in orbit around the Earth to test Einstein's prediction that time varies depending on the strength of gravity, the 'gravitational red shift.' SUMO has a web site -
  • The Primary Atomic Reference Clock in Space (PARCS) experiment will place a precise laser-cooled atomic clock in orbit to measure the red shift and to detect other effects of Einstein's theory of general relativity.
  • The Satellite Energy Exchange (SEE) experiment will measure G and its change with time by tracking the motions of two bodies in Earth orbit. The gravity attraction of the larger test mass, the 'shepherd,' causes the smaller mass to approach, then recede from the shepherd. To learn more go to http://GRAVITY.PHYS.UTK.EDU/see/.

Last Updated:

link to First Gov NASA logo