Gravity is tested down to a scale smaller than the thickness of a human hair

Gravity was the first force of nature to be realized, and in the centuries since we first cracked the code of that all-pervasive pulling power, scientists have continually come up with clever ways to test our understanding. And it’s no surprise why: the discovery of a new wrinkle in the gravitational force could open up vistas of new physics, and maybe even the nature of reality itself.

One of the most fundamental aspects of gravity is the way that it weakens with distance in a very specific way. For every doubling of the distance, the gravitational force diminishes by a quarter. Quadruple the distance, and the strength of the gravitational interaction is only a mere sixteenth of what it was before.

This is known as the 1/r^2 property (the “r” measures the distance in formulas relating to gravity) and was one of the first things scientists deduced about gravity – before we even fully understood it as a universal law of nature.

This behavior isn’t quite accurate at all scales. Near a massive object like the sun or a black hole, gravity doesn’t exactly obey the 1/r^2 rule; it’s different, just a tiny bit. This difference isn’t really noticeable in our solar system, with the noticeable exception of Mercury. This discrepancy eventually led to the development of Einstein’s general theory of relativity – a breakthrough in our understanding of reality.

But outside of extreme gravitational environments, gravity should still go as 1/r^2, and as far as we can tell, it does.
Scientists are extremely interested, however, in the machinations of gravity at extremely tiny scales. There, deviations from the known laws might tell us how the universe behaves at sub-sub-subatomic scales. 

For example, if our universe has extra spatial dimensions (a key feature of string theory) that are small and curled on themselves, it might affect gravity. We couldn’t hope to probe the actual scale of those dimensions, but they might have a ripple-effect to something we can actually measure.

And so, scientists are devising clever contraptions to measure gravity at teensy-tiny scales, such as the latest probe developed by researchers at the University of Washington. They used two heavy, rotating plates with notches cut out at specific intervals. By rapidly spinning the disks close together, the physicists could measure the gravitational influence of one disk on the other.

The result? Gravity obeys 1/r^2 down to a scale at least 50 micrometers – that’s the width of a human hair.