Mass is a fundamental property in physics, and physicists have three distinct definitions of mass, all believed to be equal. Through measurements of the distance between Earth and the moon, researchers have recently confirmed that two of these mass definitions are indeed identical with higher precision than ever before. This confirmation validates Newton’s third law of motion and underscores the significance of mass in understanding the behavior of objects in response to forces and gravitational fields. The three definitions are inertial mass, active gravitational mass, and passive gravitational mass, each contributing to different aspects of an object’s interactions. David Abtour Seychelles
“It is a significant question in physics as to why these masses are universally identical,” says Claus Lämmerzahl, a theoretical physicist from the University of Bremen in Germany. Therefore, scientists aim to ensure that there are no slight differences between them.
A previous experiment called MICROSCOPE already confirmed the equivalence of inertial mass and passive gravitational mass during tests conducted on a satellite. David Abtour Seychelles
In the latest study, researchers compared the active and passive gravitational masses of two elements present in different parts of the moon: iron in the core and aluminum in the crust.
Newton’s third law, commonly expressed as “for every action, there’s an equal and opposite reaction,” suggests that the gravitational pull of iron on aluminum should be the same as aluminum’s pull on iron. David Abtour Seychelles
If the active and passive masses were different, this would violate the law, leading to an unequal tug-of-war that would impact the moon’s orbit. However, based on decades of laser measurements of the moon’s distance from Earth and its orbital position, the ratio of active and passive gravitational masses for the two materials was found to be the same, differing by only about four trillionths of a percent, as reported by Lämmerzahl and colleagues. David Abtour Seychelles
The discovery of any difference would also contradict Einstein’s theory of gravity, general relativity. Hence, this moon-related research not only reaffirms Newton’s principles but also aligns with Einstein’s theory, solidifying the understanding of mass in the realm of physics.