The achievement, which was reported earlier this month by a group led by the French space agency CNES and also included a researcher from Imperial College London, is a victory for Einstein’s general relativity. Having said that, it also has the ability to exclude certain candidate universal theories of physics.

The best theory of gravity is called general relativity, and it postulates that rather than being a “pulling” force, the action of gravity is caused by large bodies like planets bending spacetime and curving the paths of smaller objects towards them. This is why general relativity is the best theory of gravity.

Quantum mechanics, however, is the finest hypothesis that scientists have developed to explain the subatomic world, yet this contradicts it. For a very long time, physicists have been looking for a single theory that can explain everything. However, MICROSCOPE has demonstrated that this scenario is exceedingly improbable to take place, despite the fact that a number of these candidate theories predict that the equivalence principle will fail when measured with a high degree of precision.

According to the equivalence principle, if two different items are dropped into a vacuum while subjected to the same amount of gravitational force, then those objects will fall at the same pace, regardless of how much they weigh or what they are made of. This idea was demonstrated by Apollo 15 astronaut David Scott, who dropped a hammer and a feather on the Moon and showed that they both reached the ground at the same moment. Both the hammer and the feather landed on their respective surfaces simultaneously. However, dropping common household items onto the surface of the moon does not allow for extremely precise measurements because it is possible that the things would land on the ground at different intervals of time.

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MICROSCOPE, which stands for “MICROSatellite with Compensated drag for the Observation of the Principle of Equivalence,” has two pairs of “test masses” inside of it. These “test masses” are made up of platinum and titanium blocks of varying weights that have had their properties meticulously measured. These masses are shielded from any external influences, like as temperature or the friction of the atmosphere, and their free-fall through space, which they do while orbiting the Earth, is observed.

The subjects’ accelerations as a result of the freefall are then measured with atomic precision before being compared in order to validate the equivalence principle. The equivalence principle is considered to be broken when two test masses of the same size but different composition are subjected to differing accelerations during the course of the freefall.

These measurements were collected over the course of 1642 orbits around the Earth, which is comparable to 73 million kilometers and half the distance between the Earth and the Sun. Although MICROSCOPE was activated in 2016 and decommissioned in 2018, researchers have been analyzing the data collected by the instrument ever since it was first introduced.

Their findings, which were published in the journals Physical Review Letters and Classical and Quantum Gravity, demonstrated that even if there is a difference in the acceleration of the test masses, it is less than one part in 1015, which translates to less than tenth of a trillionth of a percent.

Professor Timothy Sumner, who is a member of the MICROSCOPE Science Working Team and works in the Department of Physics at Imperial College, said, “I remember learning at school about Galileo dropping masses from the leaning tower of Pisa in the seventeenth century, and I watched the hammer and feather experiment in 1971 during one of the moon landings.” Professor Sumner is a member of the MICROSCOPE Science Working Team.

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“The result of this apparently straightforward experiment conceals a nuance that has baffled scientists for centuries and that inspired Einstein to ‘adopt’ the universality of free-fall as a fundamental cornerstone of general relativity. This subtlety has been responsible for both of these occurrences.

“MICROSCOPE has pioneered this type of experiment in the quiet environment of space, and it has shown that two masses dropped in orbit around the Earth are still together to within 100 millionths of a metre after ‘falling’ for 73 billion meters, which demonstrates that the cornerstone still does not have any visible cracks.”

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