Re: Astro Picture of the Day (for Uncle Al?)
Posted by Uncle Al on Apr 29, 2004 at 18:42
(68.99.179.162)Re: Astro Picture of the Day (for Uncle Al?) (Mike Kremer)
Gravity Probe B is two pairs of perfect gyroscopes cooled to about 2 K, spinning at 10,000 rpm. Gravitomagnetic effects (frame-dragging or the Lense-Thirring effect) are named for the symmetry of the math. Magnetism as such has nothing to do with it.
The experiment is simple in principle. Make a rigid arrow (vector). Move it along a Euclidean plane always parallel to itself. Is the arrow always parallel to its original self and aligned the same way vs. the surface?? Sure! You have measured zero curvaure.
Variant 1: Imbed the arrow to its midpoint in the center of a Moebius band - arrow tip, shaft, then Moebius band, arrow shaft, fletching. Align the arrow normal (perpendicular) to the surface. Now carefully run it around the Moebius band and back to the starting point, always held perfectly normal to the surface. The arrow arrives upside down. Whoa!
Variant 2: Take a globe of the Earth. Place the arrow wherever and however you will on the surface. Once again, parallel transport it. If you started at the north pole with the arrow pointing into the sky, when you arrive at the South Pole it will be pointing into the ground. The mis-alignment is a measure of the curvature of the surface.
Angular momentum is conserved. A spinning gyro is an absolute reference frame. If the spinning Earth entangles spacetime, then the gyro stays pointed (parallel transport of the vector), but physical reality is curved and will not fit back at the starting point. As Gravity Probe B orbits, the local universe will go out of whack vs. the distant universe. The gyro will appear to progressively point in a different direction.
With four gyros pointing in three planes, random error should square-root cancel and net deviation should uniformly add. Orbiting at an altitude of about 650 kilometers, the superconducting gyroscope should precess 42 milliarcseconds per year as predicted. Everything else is details (middle of document).
--
Uncle Al
http://www.mazepath.com/uncleal/qz.pdf
http://www.mazepath.com/uncleal/eotvos.htm
(Do something naughty to physics)