One of the most counter-intuitive notions in physics is that all objects fall at the same rate regardless of mass, i.e. the principle of equivalence. This was memorably illustrated in 1971 by NASA Apollo 15 astronaut David Scott during a moonwalk. He dropped a hawk feather and a hammer at the same time via a live television feed, and the two objects hit the ground simultaneously.
There is a long tradition of experimental testing of the weak equivalence principle, which forms the basis of Albert Einstein’s general theory of relativity. In test after test over several centuries, the principle of equivalence has held strong. And now the MICROSCOPE mission (MICROSatellite pour l’Observation de Principe d’Equivalence) has performed the most precise test of the equivalent principle to date, confirming Einstein once again, according to a recent paper published in the journal Physical Review Letters. (Other related papers appeared in a special issue of Classical and Quantum Gravity.)
John Philoponus, the 6th century philosopher, was the first to argue that the speed with which an object will fall has nothing to do with its weight (mass) and later became a major influence on Galileo Galilei about 900 years later. Galileo is supposed to have thrown cannonballs of various masses from the famous Leaning Tower of Pisa in Italy, but the story is probably apocryphal.
Galileo made spin the balls in inclined planes, which ensured the balls rolled at much lower speeds, making their acceleration easier to measure. The balls were similar in size, but some were made of iron, others of wood, making their masses different. Lacking an accurate clock, Galileo timed the balls’ journey with his pulse. And like Philoponus, he discovered that regardless of the inclination, the balls will travel at the same rate of acceleration.
Galileo later refined his approach using a pendulum apparatus, which involved measuring the period of oscillation of pendulums of different mass but identical length. This was also the method favored by Isaac Newton around 1680 and later in 1832 by Friedrich Bessel, both of whom greatly improved the accuracy of measurements. Newton also realized that the principle extended to the heavenly bodies, calculating that the Earth and the Moon, as well as Jupiter and its satellites, fall toward the Sun at the same rate. The Earth has an iron core, while the Moon’s core is made up mostly of silicates, and their masses are quite different. However, NASA’s lunar laser experiments confirmed Newton’s calculations: they do indeed fall around the Sun at the same rate.
Towards the end of the 19th century, the Hungarian physicist Loránd Eötvös combined the pendulum approach with a torsion balance to create a torsion pendulum and used it to perform an even more precise test of the equivalence principle. That simple straight stick proved accurate enough to test the principle of equivalence even more precisely. Torsion balances were also used in later experiments, such as the one in 1964, which used pieces of aluminum and gold as test masses.
Einstein cited the Eötvös experiment verifying the equivalence principle in his 1916 paper that laid the foundation for his general theory of relativity. But general relativity, while it works quite well on the macro scale, breaks down on the subatomic scale, where the rules of quantum mechanics come into play. So physicists looked for equivalence violations at those quantum scales. This would be evidence of a potential new physics that could help unify the two into one grand theory.
One method of testing quantum-scale equivalence is using matter-wave interferometry. It is related to the classic Michaelson-Morley experiment which attempts to detect the motion of the Earth through a medium called the luminiferous aether, which physicists at the time believed permeated space. In the late 19th century, Thomas Young used such an instrument for his famous double-slit experiment to test whether light was a particle or a wave—and as we now know, light is both. The same goes for matter.
Previous experiments using matter wave interferometry measured the free fall of two isotopes of the same atomic element, hoping in vain to detect minute differences. In 2014, a team of physicists thought that maybe there wasn’t enough of a difference between their compositions to achieve the greatest sensitivity. So they used isotopes of different elements in their version of those experiments, namely atoms of rubidium and potassium. The laser pulses ensured that the atoms fell down two separate paths before recombination. The researchers observed the interference pattern, indicating that equivalence still holds to one part in 10 million.