Einstein wins again: Space satellite confirms weak equivalence principle

There is a long tradition of experimentally testing the weak equivalence principle - the basis of Albert Einstein's general theory of relativity.
Enlarge / There is a long tradition of experimentally testing the weak equivalence principle – the basis of Albert Einstein’s general theory of relativity.


One of the most counterintuitive notions in physics is that all objects fall at the same rate regardless of their mass, also known as the equivalence principle. This was memorably illustrated by NASA Apollo 15 astronaut David Scott during a lunar walk in 1971. He dropped a hawk feather and a hammer simultaneously over a live televised broadcast, and the two objects hit the ground at the same time.

The weak equivalence principle, which forms the basis of Albert Einstein’s general theory of relativity, has a long tradition. Test for test over many centuries, the equivalence principle has proven itself. And now the MICROSCOPE (MICROSatellite pour l’Observation de Principe d’Equivalence) mission has performed the most precise test yet of the equivalence principle, reaffirming Einstein, according to a recent article in the journal Physical Review Letters. (Other related articles appeared in a special issue of Classical and Quantum Gravity.)

testing, 1,2,3

John Philoponus, the 6th-century philosopher, was the first to claim that the speed at which an object falls had nothing to do with its weight (mass), and later became a major influence on it some 900 years later Galileo Galilei. Galileo is said to have dropped cannonballs of varying masses from Italy’s famous Leaning Tower of Pisa, but the story is probably apocryphal.

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Galileo did Rolling balls down inclined planes caused the balls to roll at much slower speeds, making their acceleration easier to measure. The spheres were similar in size, but some were made of iron and some were made of wood, which made their mass different. In the absence of an accurate clock, Galileo reportedly timed the bullets’ journey with his pulse. And like Philoponus, he found that the balls would travel at the same rate of acceleration regardless of the incline.

Galileo later refined his approach with a pendulum apparatus in which the period of oscillation of pendulums with different masses but the same length was measured. This method was also favored by Isaac Newton around 1680 and later, in 1832, by Friedrich Bessel, both of whom greatly improved the accuracy of the measurements. Newton also recognized that the principle extended to celestial bodies, calculating that the earth and moon, and Jupiter and its satellites, fall on the sun at the same speed. The Earth has an iron core, while the Moon’s core is mostly silicates, and their masses are quite different. But NASA’s laser moon distance experiments have confirmed Newton’s calculations: they do indeed fall around the sun at the same speed.

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Towards the end of the 19th century, the Hungarian physicist Loránd Eötvös combined the pendulum approach with a torsion balance to form a torsion pendulum and thus carried out an even more precise test of the equivalence principle. This simple straight stick proved accurate enough to test the equivalence principle even more closely. Torsion balances were also used in later experiments, such as that of 1964, which used chunks of aluminum and gold as test masses.

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


Einstein cited the Eötvös experiment to confirm the equivalence principle in his 1916 paper in which he laid the basis for his general theory of relativity. But although general relativity works reasonably well at the macro scale, it breaks down at the subatomic level, where the rules of quantum mechanics apply. Therefore, physicists have looked for violations of equivalence on these quantum scales. That would be evidence of possible new physics that could help unify the two into one grand theory.

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One method to test equivalence at the quantum scale is to use matter-wave interferometry. It is related to the classic Michaelson-Morley experiment, which attempted to capture the motion of the Earth through a medium called luminous ether, which physicists at the time believed permeated space. At the end of the 19th century, Thomas Young used such an instrument 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 same applies to 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 possible sensitivity. So in their version of these experiments they used isotopes of different elements, namely rubidium and potassium atoms. Laser pulses caused the atoms to fall down two separate paths before recombination. The researchers observed the telltale interference pattern, indicating the equivalence was still within 1 part in 10 million.

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