It is often possible to use data collected for one purpose to study something else. For example, A Recent paper It used half a century’s worth of observations of the Moon’s orbit to make careful and precise tests of the nature of gravity, as well as some of the fundamental assumptions that contributed to Einstein’s theory of general relativity.
One block, three posts
Mass can be thought of as the amount of “matter” that an object is made of. This causes the force of gravity between two objects, which makes the objects difficult to move. Basically, it serves three different functions. First, the mass generates a gravitational field that will forcefully affect other objects, so we can call this “active gravitational mass.” Second, the mass can feel the gravitational effects of objects around it, and we can call this “negative gravitational mass.” Third, mass resists changes in motion – which is why pushing a large rock is difficult – so we can call this “inertial mass.”
In introductory physics classes, and even in Einstein’s more advanced theory, the three “types” of mass are thought to be the same thing. But there is no fundamental reason why this should happen. Each one can be different. Since the idea of their similarity is a fundamental assumption of Einstein’s theory of gravity, it is necessary to test this conjecture.
This is what the researchers did. On July 21, 1969, Apollo 11 astronauts placed a laser reflector on the lunar surface, followed by additional reflectors on subsequent lunar missions. Since then, researchers have been able to monitor the distance between the Earth and the Moon. This is an incredibly precise measurement, accurate to within a millimeter, which is similar to measuring the distance between New York and Los Angeles with an accuracy equivalent to the width of a human hair. One result was that scientists determined that the Moon is moving away from Earth at a rate of 1.5 inches (3.8 cm) per year.
Active gravitational mass versus passive gravitational mass
The most recent paper investigated whether active mass and passive mass are the same. To do this, researchers took advantage of the moon’s geological advantage. The Apollo missions determined that the maria (the large dark spots on the face of the Moon) were rich in iron, while the lunar highlands were rich in aluminium. Because Maria was formed from lava coming from the moon’s interior, researchers assumed that the moon’s mantle was rich in iron. The highlands, being the outer part of the Moon, should reflect the chemical composition of the aluminum-rich lunar crust.
In the case where the active and passive gravitational masses are the same, the gravitational force of iron on aluminum should be the same as that of aluminum on iron. However, if the mass of active and passive gravity is different, there must be a net force. This net force will be similar to the tidal movement on Earth and will speed up or slow down the Moon’s orbit.
Using more than half a century of measurements of the Moon’s position (from April 1970 to April 2022), astronomers were able to determine that the Moon’s orbit was slowing down by as little as 25.8 arcseconds per century. At this rate, it would take more than 14,000 years for the Moon to move back one degree from where it would be if it continued to travel at the speed it is moving today.
From this precise measurement, the researchers concluded that the active and passive forms of mass were essentially identical. If they were different, the difference would be less than one part in 26 trillion.
This measurement is crucial to confirming the validity of one of the assumptions of Einstein’s theory of gravity. If not, it would have called into question many aspects of modern astronomy, including dark matter and dark energy. However, the latter measurement confirms the validity of our current theoretical framework. Once again, it shows that Einstein was right.
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