GravitGravity y

Probe Probe BB

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

massive objects.massive objects.

What is Gravity Probe What is Gravity Probe B?B?

What is a gyroscope ?What is a gyroscope ?

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

How does Gravity Probe B How does Gravity Probe B use gyroscopes to prove use gyroscopes to prove

General Relativity? General Relativity? Gravity Probe B is a "Relativity Gyroscope Satellite" Gravity Probe B is a "Relativity Gyroscope Satellite"

that will be launched into a polar earth orbit at a that will be launched into a polar earth orbit at a 400 mile altitude. Four gyroscopes in this system 400 mile altitude. Four gyroscopes in this system

should be able to measure how space and time are should be able to measure how space and time are warped by the presence of our tiny earth. The warped by the presence of our tiny earth. The earth's mass is insignificant in comparison to earth's mass is insignificant in comparison to

neutron stars, black holes, and even our own sun. It neutron stars, black holes, and even our own sun. It does, however, still distort the spacetime around it. does, however, still distort the spacetime around it.

There are two effects due to this distortion that There are two effects due to this distortion that Gravity Probe B proposes to look for: Gravity Probe B proposes to look for:

1.1. Frame-DraggingFrame-Dragging2.2. The Geodetic EffectThe Geodetic Effect

Frame-DraggingFrame-Dragging Not only is the earth a massive body, Not only is the earth a massive body,

but it is also rotating. The prediction but it is also rotating. The prediction is that as it spins it actually pulls the is that as it spins it actually pulls the fabric of spacetime around with it. fabric of spacetime around with it. The application of the gyroscope is to The application of the gyroscope is to see how the pull of earth's warpage see how the pull of earth's warpage of spacetime changes the angle of of spacetime changes the angle of the gyroscope. In an entire year this the gyroscope. In an entire year this should still be an angle of only 42 should still be an angle of only 42 milliarc-seconds (one arc is milliarc-seconds (one arc is 0.0001167 degrees). 0.0001167 degrees).

The Geodetic EffectThe Geodetic Effect

The direction in which the gyroscope is The direction in which the gyroscope is spinning will also be changed simply spinning will also be changed simply by its motion through the spacetime by its motion through the spacetime curvature caused by earth's mass. curvature caused by earth's mass. This angle, over one year, should be This angle, over one year, should be a total of 6,600 milliarc-seconds a total of 6,600 milliarc-seconds (0.0183 degrees). (0.0183 degrees).

Gravity Probe BGravity Probe B

View of a dime next to the Detector View of a dime next to the Detector Mount Assembly. The Assembly is used Mount Assembly. The Assembly is used to detect exactly how much starlight is to detect exactly how much starlight is coming through different beams from coming through different beams from the beam splitter in the telescope. The the beam splitter in the telescope. The measurements from the tiny chips measurements from the tiny chips inside are what keeps GP-B always inside are what keeps GP-B always aimed at the guide star. Image credit aimed at the guide star. Image credit to Paul Ehrensberger, Stanford to Paul Ehrensberger, Stanford University. 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 0.042 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.

Work CitedWork Citedhttp://www.macalester.edu/astronomy/people/chrissy/Links/http://www.macalester.edu/astronomy/people/chrissy/Links/gpb.htmlgpb.htmlhttp://einstein.stanford.edu/ http://www.ksc.nasa.gov/elvnew/gpb/ http://www.nasa.gov/missions/highlights/launch_update_gpb.html http://aries.phys.yorku.ca/~bartel/GPBmovie.html http://einstein.stanford.edu/ http://apnews.myway.com/article/20040420/D822NRO80.html http://www1.msfc.nasa.gov/news/ http://space.skyrocket.de/index_frame.htm?http://space.skyrocket.de/doc_sdat/gp-b.htmhttp://www.xs4all.nl/~carlkop/gravpro.htmlhttp://www.xs4all.nl/~carlkop/gravpro.htmlhttp://ilrs.gsfc.nasa.gov/ilrs/satellite_missions/list_of_satellites/http://ilrs.gsfc.nasa.gov/ilrs/satellite_missions/list_of_satellites/gravity_probe/gravity_probe/

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