Worldbook at NASA

Close up of a clear, uncoated quartz rotor reflecting a poster of Albert Einstein.

Relativity: Einstein's two theories are the "foundation blocks" of modern physics.

› Learn More

NASA Fact: GPB

The Gravity Probe B gyroscopes are the most perfect spheres ever made by humans. If these ping pong-sized balls of fused quartz and silicon were the size of the Earth, the elevation of the entire surface would vary by no more than 12 feet.