A Levitating Sphere Measures Invisible Forces express
A magnetic sphere hangs in a vacuum above a cold metal plate. No strings, no stands — only the invisible field of a superconductor keeps it from falling. Give the sphere a gentle nudge, and it starts to oscillate up and down; the frequency of those oscillations depends on how strongly the plate pulls on it.
That's the idea behind the new force sensor. Instead of a spring or a cantilever, there's a levitating sphere; instead of measuring displacement directly, it tracks the shift in resonant frequency picked up by an ultra-sensitive microwave circuit. The trick is that the larger the sphere, the more its motion affects the signal, so fewer photons are needed for a precise measurement. This opens the door to quantum metrology with macroscopic objects.
Such precision is needed to test whether Newton's law of universal gravitation breaks down at distances of fractions of a micrometer. Some theories predict that extra forces, called Yukawa forces, appear at these scales. The sensor could either detect them or place new constraints on them. It could also measure the Casimir effect, the attraction arising from quantum fluctuations of the vacuum between closely spaced surfaces.
The plate is cooled to near absolute zero, and the sphere is coated in gold to suppress stray electric fields. The gold, however, creates eddy currents that damp the motion, so there's a trade-off to balance. But if it all works, we'll get our first look at how a macroscopic object behaves at the boundary of the quantum world.
A superconductor expels magnetic fields, so the sphere levitates without any support, as if resting on an invisible cushion.
In Arthur C. Clarke's novel The Fountains of Paradise, levitating building blocks are described for a space elevator: this sensor uses a similar principle of magnetic levitation for scientific measurements.