Increase / There is a long tradition of experimental verification of the principle of weak equivalence, the basis of Albert Einstein’s general theory of relativity.


One of the most absurd notions in physics is that all objects fall at the same speed, regardless of their mass. principle of equivalence. This was memorably illustrated in 1971 by NASA Apollo 15 astronaut David Scott during his moonwalk. He fell out the falcon feather and the hammer at the same time via live television, and the two objects hit the dirt at the same time.

There is a long tradition experimental verification of the principle of weak equivalence, which is the basis of Albert Einstein’s general theory of relativity. In trial after trial over the centuries, the principle of equivalence has held strong. And now MICROSCOPE (MICROSatellite pour l’Observation de Principe d’Equivalence) mission achieved the most accurate test of the equivalence principle to date, reaffirming Einstein’s for a a recent paper published in Physical Review Letters. (Additional related papers appeared in a special issue of Classical and Quantum Gravity.)

Testing, 1,2,3

John Philoponus, a 6th century philosopher, was the first to argue that the speed of an object’s fall has nothing to do with its weight (mass), and later greatly influenced Galileo Galilei some 900 years later. Galileo is believed to have dropped cannonballs of various masses from the famous Leaning Tower of Pisa in Italy, but this story is probably apocryphal.

Galileo did roll the balls down inclined planes, which ensured that the balls rolled at much slower speeds, making it easier to measure their acceleration. The balls were the same size, but some were made of iron, others of wood, which made their masses different. Not having an accurate clock, Galileo reportedly timed the movement of the balls by his pulse. And, like Philoponus, he discovered that regardless of the inclination, the balls will move with the same acceleration rate.

Galileo later refined his approach using a pendulum apparatus, which involved measuring the period of oscillation of pendulums of different masses but of the same length. It was also the method favored by Isaac Newton around 1680 and later by Friedrich Bessel in 1832, both of whom greatly improved the accuracy of measurements. Newton also realized that this principle extended to the heavenly bodies, calculating that the Earth and the Moon, as well as Jupiter and its moons, fall toward the Sun at the same rate. The Earth’s core is made of iron, while the Moon’s core is mostly made of silicates, and their masses are quite different. Yet NASA laser experiments on lunarology confirmed Newton’s calculations: they really fall around the Sun at the same speed.

At the end of the 19th century, the Hungarian physicist Lorand Etvös combined the pendulum approach with rotating weights to create a rotating pendulum and used it to perform an even more precise test of the equivalence principle. This simple straight stick proved to be accurate enough to test the principle of equivalence even more precisely. Torsion scales were also used in later experiments, such as in 1964, which used pieces of aluminum and gold as test masses.

Illustration of the MICROSCOPE satellite mission.
Increase / Illustration of the MICROSCOPE satellite mission.


Einstein referred to Etvösch’s equivalence principle experiment in his 1916 paper, laying the foundation for his general theory of relativity. But general relativity, while it works quite well on the macroscale, breaks down on the subatomic scale, where the rules of quantum mechanics apply. So physicists look for equivalence violations on these quantum scales. This would be evidence of potential new physics that could help unify the two theories into one grand theory.

One method of testing equivalence at the quantum scale is the use of matter-wave interferometry. This is related to the classic Michelson-Morley experiment, which attempts to detect the motion of the Earth through a medium called the light-bearing aether, which physicists at the time believed permeated space. At the end of the 19th century, Thomas Young used such a tool for his famous double-slit experiment to test whether light is a particle or a wave—and, as we now know, light is both. The the same is true for matter.

Previous experiments using matter-wave interferometry measured the free fall of two isotopes of the same atomic element, in vain hoping to detect subtle differences. In 2014, a team of physicists found that there might not be enough of a difference between their compositions to achieve maximum sensitivity. So they are isotopes are used different elements in his version of these experiments, namely rubidium and potassium atoms. The laser pulses caused the atoms to fall along two separate paths before recombination. The researchers observed a notable interference pattern, showing that equivalence still holds to within 1 part in 10 million.

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