Einstein wins again: the space satellite confirms the principle of weak equivalence

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


One of the most counterintuitive notions in physics is that all objects fall at the same speed, regardless of mass, which is 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 simultaneously via a live television feed, and the two objects hit the ground simultaneously.

There is a long tradition of experimental verification of the weak equivalence principle, which forms the basis of Albert Einstein’s theory of general relativity. Test after test over many centuries, the principle of equivalence has stood firm. And now the MICROSCOPE (MICROSatellite pour l’Observation de Principe d’Equivalence) mission has obtained the most accurate test of the equivalent principle to date, confirming Einstein once again, according to a recent article published in the journal Physical Review Letters. (Additional related documents have appeared in a special issue of Classical and Quantum Gravity.)

Proof, 1,2,3

John Philoponus, the 6th century philosopher, was the first to argue that the speed 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 allegedly launched cannonballs of various masses from the famous Leaning Tower of Pisa in Italy, but the story is probably apocryphal.

Galileo done rolling the balls on inclined planes, which ensured that the balls rolled at much slower speeds, making their acceleration easier to measure. The balls were similar in size, but some were made of iron, others of wood, which made their masses different. In the absence of a precise clock, Galileo would have timed the journey of the balls with his beating. And like Philoponos, he discovered that regardless of the inclination, the balls would travel at the same acceleration rate.

Galileo later perfected his approach using a pendulum, which involved measuring the period of oscillation of pendulums of different mass but identical length. This was also Isaac Newton’s preferred method around 1680, and later, in 1832, by Friedrich Bessel, who greatly improved the accuracy of measurements. Newton also understood that the principle extended to celestial bodies, calculating that the Earth and the Moon, as well as Jupiter and its satellites, fall towards the Sun at the same speed. The Earth has an iron core, while the Moon’s core is mainly composed of silicates, and their masses are quite different. Yet NASA’s lunar laser beam experiments have confirmed Newton’s calculations: in fact, they fall around the Sun at the same speed.

Towards the end of the 19th century, the Hungarian physicist Loránd Eötvös combined the pendulum approach with a torsion equilibrium to create a torsion pendulum and used it to conduct an even more accurate test of the equivalence principle. That simple straight stick turned out to be accurate enough to test the equivalence principle even more accurately. Torsion balances were also employed in later experiments, such as the one in 1964 which used bits of aluminum and gold as test masses.

Illustration of the MICROSCOPE satellite mission.
Zoom in / Illustration of the MICROSCOPE satellite mission.


Einstein cited Eötvös’s experiment testing the equivalence principle in his 1916 paper which laid the foundation for his theory of general 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 have been looking for equivalence violations on those quantum scales. This would be evidence of a potential new physics that could help unify the two into one great theory.

One method for testing quantum-scale equivalence is to use matter wave interferometry. It is related to the classic Michaelson-Morley experiment that attempts to detect the movement of the Earth through a medium called the luminiferous aether, which physicists of 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 have 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 perhaps there wasn’t enough difference between their compositions to achieve maximum sensitivity. So they used isotopes of different elements in their version of those experiments, namely rubidium and potassium atoms. The laser pulses ensured that the atoms fell along two separate paths before recombining. The researchers looked at the telltale interference pattern, indicating that the equivalence still remained within 1 part in 10 million.

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