# Gravitoelectromagnetism.

What a word! Up there with tetragrammaton in my list of favourite words.. but also what this post is about. The two things that have always fascinated me about physics are gravity and magnetism so when I came across this word I knew it was research time!

Gravitoelectromagnetism (GEM)
Also known as gravitomagnetism it is actually not about magnetism, it gets it’s name from the mathematical analogs between Maxwell’s field equations (for electromagnetism) and Einstein’s field equations (for gravity). GEM predicts two main testable things which are the geodesic effect and frame-dragging. Frame-dragging is quite simple to picture, here, see this simple picture:

A rotating mass warps the local space-time fabric in the direction of the spin. It doesn’t continue to warp, depending on the mass and spin of the object, it reaches a limit.

The geodesic effect is a little harder to grasp, the best video explination for it I have found is in this overview of the entire mission talked about below (20min two-part youtube video). Basically: a free-falling object near a massive rotating object will itself rotate.

A special satellite called Gravity Probe B was sent up in April 2004 then finally results were ready to be released in may 2011. When asked if this had proved Einstein right Francis Everitt answered, with a smile on his face, that “This is a question of relativity” and goes on to say “you can prove he’s more right than Newton as these effects aren’t predicted in Newtonian physics”. (The geodesic effect was proved within a precision of ~0.5%)
The test was extremely simple and infact proposed long ago, but the equipment needed didn’t exist. Later, using the star IM Pegasi for a reference, multiple gyros’ spin-axis were lined up with the same reference. During a polar-orbit  the geodesic effect and frame-dragging are 90° from each other so can be easily recorded as a vector change.

Fun Fact
The satellite used gyroscopes, at the time they were the closest thing to perfect spheres made by humans at 40-atoms off perfect thickness. Not only this, they were coated with a thin layer of niobium which is a superconductive material used in the quantum processors of D-wave’s Quantum Computers. With the gyroscopes, their spin axes were sensed by monitoring the magnetic field of the superconductive niobium layer.