# Gravity Probe

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Published on January 7, 2009

Author: pixiejen

Source: slideshare.net

## Description

Advanced Lab (a class) Presentation on the Gravity Probe.
(2nd Power point ever dont laugh)

By Jennifer Nalley Gravity Probe B

Gravity Probe B tested Einstein's ideas about space and time and how the Earth distorts them.

The Geodetic Effect: The amount by which the Earth warps the local space time in which it resides. The Frame Dragging Effect: The amount by which the rotating Earth drags local space time around with it. Although it had been dreamed of since 1959.... scientists lacked the technology that would allow them to verify two specific theoretical consequences of Einsteins General Theory of Relativity. A perfect plan.... that had to wait for technology to catch up.... And finally it did!!! Two Effects General Relativity Predicted The Frame Dragging Effect: The amount by which the rotating Earth drags local space time around with it.

Einstein Right Again? Gravity Probe B is the relativity gyroscope experiment developed by NASA and Stanford University to test two unverified predictions of Albert Einstein's general theory of relativity.

How Gravity Probe B Works Place a gyroscope and a telescope in a polar-orbiting satellite, 642 km above the Earth. (GP-B actually uses four gyroscopes for redundancy.) At the start of the experiment, align both the telescope and the spin axis of the gyroscope with a distant reference point—a guide star. Keep the telescope aligned with the guide star for a year,and measure the precession change in the spin axis alignment of the gyros over this period in both the plane of the orbit (the geodetic precession), and orthogonally in the plane of the Earth’s rotation (frame-dragging precession). (Quoted from Stanford Publication)*4

Place a gyroscope and a telescope in a polar-orbiting satellite, 642 km above the Earth. (GP-B actually uses four gyroscopes for redundancy.)

At the start of the experiment, align both the telescope and the spin axis of the gyroscope with a distant reference point—a guide star.

Keep the telescope aligned with the guide star for a year,and measure the precession change in the spin axis alignment of the gyros over this period in both the plane of the orbit (the geodetic precession), and orthogonally in the plane of the Earth’s rotation (frame-dragging precession). (Quoted from Stanford Publication)*4

Gravity Probe B Quick Facts Measurements Predicted Drift-Geodetic Effect 6.61 arcseconds (1.84x10 -3 degrees) per year Predicted Drift -Frame-Dragging 0.041 arcseconds (1.14x10 -5 degrees) per year Required Accuracy Better than 0.5 milliarcseconds (1.4x10 -7 degrees) per year Gyroscopes (4) Shape Spherical (Sphericity < 40 atomic layers from perfect) Diameter 3.8 centimeters (1.5 inches) Composition Homogeneous fused quartz Coating Niobium (uniform layer 1,270 nanometers thick) Spin Rate 72 Hz (4,300 rpm) average Drift Rate L Exceptional Gyroscopes In order for Gravity Probe B to measure any “twist” or curvature of local space-time, it needed a nearly perfect gyroscope... One that would not wobble or drift more than 0.000000000001 degrees in an hour while it is spinning. Such precision was needed, because the predicted twist of local space-time is smaller than 0.0000001 degree each year! The World's Roundest Object !

In order for Gravity Probe B to

measure any “twist” or curvature of local space-time, it needed a nearly perfect gyroscope...

One that would not wobble or drift more than 0.000000000001 degrees in an hour while it is spinning.

Such precision was needed, because the predicted twist of local space-time is smaller than 0.0000001 degree each year!

FAST FACTS Gyro Size 1.5 inches of fused quartz Sphericity < 40 atomic layers from perfect Quartz Purity Within 2 parts per million Gyro Spin Rate ~10,000 rpm Gyro Drift Rate < 10 -12 degrees/hour The Gravity Probe B Gyroscope...... is a 1.5-inch sphere of fused quartz, polished and “ lapped” to within a few atomic layers of perfect sphericity. A scan of its surface shows that only . 01 microns separate the highest point from the lowest point.

Time-line

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