The Gravity Probe B satellite … probably the most precise measurement you’ve never heard of.
In 1918, 2 years after Einstein unveiled his theory of General Relativity, two Austrian physicists (Josef Lense and Hans Thirring) noticed an intriguing side effect: Einstein’s equations predicted that large, spinning bodies (like Earth) should actually drag spacetime with them as they spun. It wasn’t until GP-B came along, 86 years later and 44 years after the experiment was first proposed, that frame-dragging could actually be measured.
The frame dragging effect, even after spending a year in Earth orbit 400 miles up, is tiny: it’s like measuring the width of a human hair from a quarter mile away. How do you do that? With four of the roundest objects ever made by mankind: the superconducting niobium coated fused quartz spheres. Second to neutron stars, these are probably the roundest objects in the universe.
Spun at 4,000 RPMs, these things became the world’s best gyroscopes (a million times better than navigational systems) and Did. Not. Move. The satellite pointed itself at a faraway star, spun up the gyroscopes, and watched as spacetime stretched underneath it by the slightest of degrees as the gyroscopes stayed in their original orientation.
There was a problem, however. There were “hot spots” (my term) where some charge would build up on the spheres and on the sphere housing. These hot spots would randomly cause a torque that would jerk the sphere left or right – only by a tiny amount, but even that tiny amount was a hundred times greater than the effect they were looking for. The team spent a frenzied 18 months trying to analytically account for these random jumps of the spheres. They could never reduce all of them.
Why the “B” in Gravity Probe B? Because Gravity Probe A was launched in 1976, and measured how time goes slower close to a massive object like Earth. GP-A went about 6,200 miles straight up, and confirmed that up there, clocks run about 4 parts in 10 billion faster – just like Einstein said they would.
If you want to dive waaaay down the rabbit hole of information, there is a technical journal called “Classical and Quantum Gravity” that devoted an entire issue to Gravity Probe B. You can find a great summary and all of the technical papers on their website.
For a slightly more digestible summary, Stanford has a good writeup with lots of pictures. Happy reading.