One of the most counter-intuitive notions in physics is that all objects fall at the same rate, regardless of mass, aka the equivalence principle.

In test after test over many centuries, the equivalence principle has held strong.

John Philoponus, the 6th-century philosopher, was the first to contend that the velocity at which an object will fall has nothing to do with its weight (mass) and later became a major influence on Galileo Galilei some 900 years later. Galileo supposedly dropped cannonballs of varying masses off Italy's famed Leaning Tower of Pisa, but the story is probably apocryphal.

Newton also realized that the principle extended to celestial bodies, calculating that the Earth and Moon, as well as Jupiter and its satellites, fall toward the Sun at the same rate. The Earth has a core of iron, while the Moon's core is mostly made of silicates, and their masses are quite different.

Toward the end of the 19th century, Hungarian physicist Loránd Eötvös combined the pendulum approach with a torsion balance to create a torsion pendulum and used it to conduct an even more accurate test of the equivalence principle. That simple straight stick proved accurate enough to test the equivalence principle even more precisely. Torsion balances have also been employed in subsequent experiments, such as the one in 1964 that used chunks of aluminum and gold as the test masses.

Einstein cited the Eötvös experiment verifying the equivalence principle in his 1916 paper laying out the foundation for his general theory of relativity.

So physicists have been looking for violations of equivalence at those quantum scales.

One method of testing equivalence at the quantum scale is to use matter-wave interferometry.

The researchers observed the telltale interference pattern, indicating that equivalence still held to within 1 part in 10 million.