Gravity Probe B

Gravity Probe B is the relativity gyroscope experiment being developed by NASA and Stanford University to test two extraordinary, unverified predictions of Albert Einstein's general theory of relativity that states space and time are very slightly distorted by the presence of massive objects. What is Gravity Probe B?

What is a gyroscope ? gyroscope The first, invented in 1852 by the French physicist J. B. L. Foucault, was an instrument for studying the Earth's rotation by means of a freely suspended flywheel. Since then gyroscopes have found many applications, especially in navigation, and many types exist. The ones for Gravity Probe B are not flywheels but electrically supported spheres, spinning in a vacuum. Others utilize the spins of atomic nuclei, circulating sound waves, even circulating laser beams. In all gyroscopes the underlying principle is that rotating systems, free from disturbing forces, should stay pointing in the same direction in space.

How does Gravity Probe B use gyroscopes to prove General Relativity? Gravity Probe B is a "Relativity Gyroscope Satellite" that will be launched into a polar earth orbit at a 400 mile altitude. Four gyroscopes in this system should be able to measure how space and time are warped by the presence of our tiny earth. The earth's mass is insignificant in comparison to neutron stars, black holes, and even our own sun. It does, however, still distort the spacetime around it. There are two effects due to this distortion that Gravity Probe B proposes to look for: 1. Frame-Dragging 2. The Geodetic Effect

Frame-Dragging Frame-Dragging Not only is the earth a massive body, but it is also rotating. The prediction is that as it spins it actually pulls the fabric of spacetime around with it. The application of the gyroscope is to see how the pull of earth's warpage of spacetime changes the angle of the gyroscope. In an entire year this should still be an angle of only 42 milliarc-seconds (one arc is degrees). Not only is the earth a massive body, but it is also rotating. The prediction is that as it spins it actually pulls the fabric of spacetime around with it. The application of the gyroscope is to see how the pull of earth's warpage of spacetime changes the angle of the gyroscope. In an entire year this should still be an angle of only 42 milliarc-seconds (one arc is degrees).

The Geodetic Effect The direction in which the gyroscope is spinning will also be changed simply by its motion through the spacetime curvature caused by earth's mass. This angle, over one year, should be a total of 6,600 milliarc-seconds ( degrees).

Gravity Probe B

View of a dime next to the Detector Mount Assembly. The Assembly is used to detect exactly how much starlight is coming through different beams from the beam splitter in the telescope. The measurements from the tiny chips inside are what keeps GP-B always aimed at the guide star. Image credit to Paul Ehrensberger, Stanford University. View of a dime next to the Detector Mount Assembly. The Assembly is used to detect exactly how much starlight is coming through different beams from the beam splitter in the telescope. The measurements from the tiny chips inside are what keeps GP-B always aimed at the guide star. Image credit to Paul Ehrensberger, Stanford University.

A spinning ball of electrical charge produces a well- prescribed static magnetic field, and correspondingly a spinning mass such as the Earth is expected to produce a static gravitomagnetic field.

The predicted drift rates are 6.6 arc sec per year for the geodetic effect and arc sec per year for the frame dragging.

A milliarc-second is 1/1,000 of a arc-second and there are 3,600 arc-seconds in a single angular degree. This tiny angle corresponds to the width of a human hair as seen from ten miles! Miniscule as that is, the Gravity Probe B gyroscopes are expected to measure even smaller angles, checking the frame dragging effect to 1% and the geodetic effect to one part in 10,000.

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