Saturday, December 17, 2005

Gravity Probe B data finishes collecting data

This actually happened back at the end of September, but it was not covered much in the news (or I missed it).

Einstein's theory of general relativity predicts that a spinning object produces slightly different gravity than a non-spinning one. (It is called frame dragging, in case you care.) Gravity Probe B is a satellite designed to measure this effect for the rotation of Earth. If the results are reliable, they will either confirm relativity or blow it out of the water.

In principle, you can measure it by just putting a test mass in orbit around Earth and watching its orbit. Unfortunately for scientists, the rotation-and-gravity effect is absurdly, ridiculously tiny. The test mass must be highly isolated from anything that might affect it, like a single grain of dust, or even just sunlight. (Seriously: sunlight exerts way too much pressure.)

The measurements must be made far more meticulously than even the normal standards for scientific instruments. The test masses are the best spheres ever made by man, polished to within a few dozen molecule-widths of perfection. The masses are spun at thousands of RPMs to magnify the rotation-and-gravity effect. They are kept in a very high vacuum so aerodynamic drag will not affect them. Their container is made of superconductor to keep out magnetic and electric fields. The surroundings are chilled to within a few degrees of absolute zero to keep thermal radiation pressure from doing much. The test masses travel in their own orbits without being touched by anything, meaning the satellite has to constantly adjust to keep them from bumping into the walls of the container.

In short, it is one of the most audacious and precise projects every carried out. And so far the engineering seems to have been a success.


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