Einstein is looking at you through a near perfect glass sphere. In fact this is the most precise sphere that humans have ever created. The surface of this little marble is so smooth that any bumps or scratches are no higher than 40 atoms. Cool! But why?
Physicists created four of these precise fused quartz spheres to test predictions made by Einstein’s theory of general relativity. This theory completely changed the way we think about space and time. Instead of objects sitting in space as time tics and tocs, objects distort and interact with space and time together. NASA’s Gravity Probe B satellite is using these near perfect spheres to test a couple of really strange predictions about how objects interact with spacetime –namely the geodetic effect and frame dragging.
This geodetic effect predicts that as a satellite orbits the Earth, it will slowly rotate. For instance the glass spheres in Gravity Probe B are spinning rapidly at 3,500 rpm. Recall that when a gyroscope is set spinning, the axis of rotation tends to point in the same direction as it started. This is due to conservation of angular momentum. The satellite was initially put into a polar orbit with the axis of the gyroscopes pointed at a particular star. However, after many orbits around the Earth, the geodetic effect caused the glass sphere gyroscopes to point slightly away from the star.
Gravity Probe B will also test the phenomenon of frame dragging. As the Earth rotates, it gently drags space and time with it. This caused the satellite to rotate in the same direction as the Earth’s rotation. Frame dragging also answers the famous question: If the Earth stood still and the rest of the universe rotated around it instead, would its equator still bulge? According to general relativity and Gravity Probe B, the answer is YES. It doesn’t matter if you are spinning or if the universe is revolving around you. Both situations are equivalent.
Learn more about how Gravity Probe B works.
Watch a great video about how to demonstrate the geodetic effect with paper.