One of the most counter-intuitive notions in physics is that all objects fall at the same rate, regardless of their 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’s feather and a hammer at the same time via a live TV stream, and both 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. Trial after trial for many centuries, the principle of equivalence held firm. And now, the MICROSCOPE (MICROSatellite pour l’Observation de Principe d’Equivalence) mission has performed the most accurate test of the equivalent principle to date, once again confirming Einstein, according to a recent paper published in the journal Physical Review Letters. . (Additional related articles appeared in a special issue of Classical and Quantum Gravity.)
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. late. Galileo allegedly dropped cannonballs of varying masses off the famous Leaning Tower of Pisa in Italy, but the story is likely apocryphal.
Galileo did rolling the balls on inclined planes, which ensured that 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, which made their masses different. Having no precise clock, Galileo would have timed the travel of the bullets with his pulse. And like Philoponus, he discovered that regardless of the inclination, the balls would move at the same rate of acceleration.
Galileo then refined his approach using a pendulum apparatus, which involved measuring the period of oscillation of pendulums of different mass but identical length. It 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 celestial bodies, calculating that the Earth and the Moon, as well as Jupiter and its satellites, fall towards the Sun at the same rate. The Earth has an iron core, while the Moon’s core is mostly made up of silicates, and their masses are quite different. Yet NASA’s lunar laser ranging experiments confirmed Newton’s calculations: they are indeed falling around the Sun at the same speed.
Towards the end of the 19th century, 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. This simple straight stick proved sufficiently accurate to test the principle of equivalence even more precisely. Torsion balances were also used in later experiments, such as 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 laying the foundations 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 have been looking for equivalence violations at these quantum scales. This would be evidence of potential new physics that could help unify the two into one big theory.
One method of quantum-scale equivalence testing is to use matter-wave interferometry. It’s related to the classic Michaelson-Morley experiment that attempted to detect the Earth’s motion through a medium called luminiferous aether, which physicists at the time believed to permeate space. At the end of the 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 there might not be enough difference between their compositions to achieve the highest sensitivity. So they used isotopes of different elements in their version of these experiments, namely rubidium and potassium atoms. Laser pulses ensured that the atoms followed two separate paths before recombining. The researchers observed the telltale interference pattern, indicating that the equivalence was still maintained at 1 part in 10 million.
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