Einstein wins again: Space satellite confirms weak equivalence principle

Zoom / There is a long tradition of empirical testing of the weak equivalence principle – the basis of Albert Einstein’s general theory of relativity.


One of the most counter-intuitive concepts in physics is that all objects fall at the same rate, regardless of mass, aka the equivalence principle. This was most memorably illustrated in 1971 by NASA Apollo 15 astronaut David Scott while walking on the moon. He tossed a hawk feather and a hammer at the same time over a live TV broadcast, and the two bodies hit the dirt simultaneously.

There is a long tradition of empirical testing of the weak equivalence principle, which forms the basis of Albert Einstein’s general theory of relativity. In test after test over many centuries, the equivalence principle has remained strong. And now the MICROSCOPE (MICROSatellite pour l’Observation de Principe d’Equivalence) mission has achieved the most accurate test of the equivalent principle yet, confirming Einstein once again, according to a recent research paper published in Physical Review Letters. (Additional, related papers have appeared in a special issue of Classical and Quantitative Allure.)

Test, 1, 2, 3

John Philoponus, the sixth century philosopher, was the first to assert that the speed at which an object falls has nothing to do with its weight (its mass) and later became a major influence on Galileo Galilei some 900 years later. It is assumed that Galileo dropped artillery shells of various groups off the famous Leaning Tower of Pisa in Italy, but the story is probably fabricated.

Galileo she did The balls roll below inclined planes, ensuring that the balls roll 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, which makes their masses different. Lacking an accurate clock, Galileo is said to have timed the travel of the balls with his own pulse. And like Philoponus, he found that regardless of the tilt, the balls would move at the same rate of acceleration.

Galileo later refined his approach with a pendulum apparatus, which involved measuring the period of oscillation of pendulums of different mass but of identical length. This was also the method favored by Isaac Newton around 1680, and later in 1832, by Friedrich Bessel, both of which greatly improved the accuracy of measurements. Newton also recognized that the principle extends to the celestial 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 has an iron core, while the Moon’s core is mostly made of silicates, and their mass is completely different. However, NASA’s lunar laser range experiments confirmed Newton’s calculations: they are indeed falling around the sun at the same rate.

Towards the end of the 19th century, Hungarian physicist Lorand Etvos combined the pendulum approach with torsion equilibrium to create a torsion pendulum and used it to make a more accurate test of the equivalence principle. That simple straight stick proved accurate enough to test the equivalence principle more accurately. Torsion scales were also used in later experiments, such as the one in 1964 that used pieces of aluminum and gold as test blocks.

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


Einstein cited the Eötvös experiment to verify the equivalence principle in his 1916 paper that laid the foundation for his general theory of relativity. But general relativity, while working well at the macro level, breaks down on the subatomic scale, which is where the rules of quantum mechanics begin. So physicists have been looking for violations of parity at those quantum scales. This would be evidence of potential new physics that could help unite the two into one grand theory.

One way to test for equivalence on the quantum scale is to use material wave interferometry. It’s about the classic Michaelson-Morley experiment trying to detect the motion of the Earth through a medium called the luminous aether, which physicists at the time thought permeated space. In the late nineteenth century, Thomas Young used such an instrument in 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 the article.

Previous experiments using matter-wave interferometry measured the free fall of two isotopes of the same atomic element, hoping to unsuccessfully detect subtle differences. In 2014, a team of physicists thought there might not be enough difference between their formulations to achieve maximum sensitivity. So they used isotopes of different elements in their version of those experiments, which are rubidium and potassium atoms. The laser pulses ensured that the atoms fell on separate paths before recombination. The researchers observed the telltale interference pattern, indicating that the valency was still maintained within 1 part in 10 million.

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