Awesome experimental physics

Once upon a time, there was a theory called Einstein's theory of general relativity. It made a lot of predictions about how the world works, and some of them were a little tricky to verify. So a group of physicists set out on what can only be described as an extraordinary experimental journey. They called it Gravity Probe B, and it was designed to measure two general relativistic effects: the warping of spacetime around the Earth, and the dragging of nearby spacetime along with the Earth's rotation. These required an unprecedented degree of accuracy (the frame-dragging effect apparently resulted in a precession of less than 40 milliarcseconds over the course of a year!), and produced some fantastic detectors. 

The essence of the experiment is that a telescope and four gyroscopes (for redundancy - theoretically only one is required) are aligned on a guide star. As the spacecraft (did I mention this was an experiment in orbit? Talk about complicated mission control) orbited the Earth, the telescope stayed aligned on the guide star, and the gyroscopes rotated happily onward without changing their alignment - relative to the spacetime around them. Then by measuring the angle between the telescope and gyroscopes' alignments, the experimenters could deduce properties of the warped spacetime around the Earth.

One of the many tricky bits of this experiment is that the gyroscopes had to be pretty much perfect. Any imperfections would cause them to precess differently, which would result in systematic errors in the final analysis. They also needed to be suspended stably without any friction, while still allowing for readout. The solution to the first problem was to manufacture the most perfectly spherical manmade objects ever - fused quartz balls with less than 25 nanometers of variation from the tallest peak to the deepest valley. The group doing the experiment actually had to create a whole new polishing method in order to attain this, and the gyroscopes still hold the record for the most spherical manmade objects in the world. The solution to the suspension problem was to coat these quartz balls in a thin layer of superconducting niobium. This was suspended by an electrical casing that never actually contacted the gyros, which continuously adjusted the power to six electrodes according to their simultaneous readout to keep the gyro centered in the casing. 

Gyro readout is another interesting issue the group had to deal with. First off, there's the issue of reading out the direction of the spin axis of a perfectly uniform sphere with no distinguishing markings. Then there's the issue of doing so without interfering with the sphere's rotation. Both of these are nontrivial technical challenges, and both are solved by strange properties of superconductivity. There's this phenomenon called the London moment, in which a spinning superconductor produces a magnetic moment along its spin axis. Gravity Probe B needed to measure that without generating a magnetic field to cause a torque on the magnetic moment, and they needed to do it with the remarkable precision mentioned above. Once again, superconductivity came to the rescue. There's this fancy thing called a SUperconducting Quantum Interference Device (because who doesn't like SQUIDs?) which is able to measure tiny magnetic fields over time. And certain feedback mechanisms in the circuit were able to cancel any magnetic field change caused by the readout, so the gyros could spin happily away completely undisturbed. 

In the end, Gravity Probe B was a successful mission, and it came up with spectacular agreement with theory. Another win for Einstein.