JumpStart - Physical Science

Gravitational and Relativistic Physics*

This illustration shows how the space-time field is warped by Earth's gravity. The graph-like lines represent the space-time field as a giant mesh sheet. Gravity distorts the space-time field near the planet, causing distances in space to be slightly different than those one might otherwise expect.

Einstein described gravity as a disturbance in the curvature of space and time. This theory is called general relativity. To better understand its effects, one may consider the space-time field as a giant mesh sheet. The gravitational field of each planet, star, or other massive body creates a depression in the mesh, like a marble lying on a taut bed sheet. The result is a slight distortion, or stretching, of the mesh squares near the body. Taking the Earth as one example, this distortion, called the geodetic effect, causes distances in space to be slightly different than those one would expect if the Earth were not there. Einstein also predicted a frame-dragging effect, caused by rotating bodies, that creates another slight disturbance in measurements of space and time. To visualize this effect, imagine that as Earth rotates, it twists the theoretical mesh sheet of space-time and drags it around with the Earth, creating a very slow whirlpool effect. This whirlpool effect causes a disturbance in measurement of space and time, such as when radio transmissions from a satellite passing near a massive body become slightly altered or distorted. These effects are difficult to measure because the instruments must be extremely sensitive and because the measurements must be made in a microgravity environment, which allows an increase in the magnitude of the effects. NASA-funded researchers are designing a drag-free satellite with minimal vibration disturbances to measure the geodetic effect with much greater precision and to calculate the frame-dragging effect for the first time.

Because our concepts of the universe rely fundamentally on theories of general relativity, scientists are driven to put Einstein's theories of gravitation to the most precise tests possible. Modern technology now enables us to make significant leaps in the precision of such tests. The development of drag-free satellite technology will assist the fundamental physics community in its goal of investigating the weak equivalence principle, which asserts that any two objects, regardless of their composition, will experience the same acceleration due to the force of gravity. The most famous test of this principle was performed by Galileo, who allegedly dropped a cannonball and a musket ball from the Leaning Tower of Pisa and found that they hit the ground "within a handsbreadth" of each other. A proposed modern test of this principle would use a drag-free satellite in low Earth orbit to drop four pairs of masses (each pair consisting of concentric hollow cylinders of different materials) at the same time.


This experiment design has two distinct advantages over the original experiment. The first advantage is that the masses can be dropped over a much greater distance, which will allow more time for any variation in relative position of the masses to be observed; instead of the 180-foot height of the Tower of Pisa, the equivalent vertical drop distance of the satellite in orbit is approximately 2,000 kilometers. The other advantage is that during the several months that the satellite will be in orbit, the test of the weak equivalence principle can be repeated thousands of times, accumulating a tremendous amount of data. Other benefits of the experiment design include the reduction of thermal noise (heat fluctuation affecting the accuracy of the data) achieved by conducting the experiment at a very low temperature and the elimination of potential vibration disturbances. This modern test of the weak equivalence principle will enable researchers to measure the difference in the relative position of the objects to within 10-15 m (approximately five-billionths of the width of a human hair). Any measured difference in this relative position will necessitate re-evaluation of the current fundamental hypotheses about relativity.