An ultra-precise test of a basic premise of Einstein's theory of general relativity, which is the contemporary theory of gravity, was performed using a satellite circling the Earth. The question is whether two types of mass — gravitational and inertial — are equivalent. The scientists discovered that two objects aboard the spacecraft descended toward Earth at the same rate, with a one-part-in-a-quadrillion accuracy. This successful test of Einstein's theory has significant implications for present cosmic puzzles, such as the existence of dark matter and dark energy.
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Imitating the ancients
Gravity is the force that keeps the Universe together, tying distant galaxies together and guiding them in an everlasting cosmic dance. The strength of gravity is determined in part by the distance between two objects, but also by their masses. Gravity increases when an object's mass increases. This form of mass is known scientifically as "gravitational mass."
Mass also has another feature known as inertia. This is the tendency of an object to resist changes in motion. In other terms, larger objects are more difficult to move: a bicycle is easier to push than a car. This form of material is referred to as "inertial mass."
There's no reason to believe gravitational mass and inertial mass are the same thing. The one governs gravity, whereas the other governs motion. Heavy and light items would fall at different rates if they were different, and philosophers in ancient Greece recognised that a hammer and a feather fall at different rates. Heavy things appear to fall faster than light ones. We now know that air resistance is to blame, but this was not always clear.
The situation was resolved in the 17th century, when Galileo demonstrated that objects of different masses fall at the same rate using ramps and spheres of varied masses. (His famous experiment of dropping balls from the Tower of Pisa is most likely fictitious.) In 1971, astronaut David Scott impressively replicated Galileo's experiment on the airless Moon by dropping a hammer and a feather, which fell exactly. The ancient Greeks were duped.
Dark speculation
The equivalent principle states that inertial and gravitational mass are the same, and Einstein built equivalence into his theory of gravity. General relativity accurately predicts how objects fall in most situations, and the scientific community regards it as the best gravity theory.
However, "most" does not imply "all," because astronomical observations have uncovered several puzzling mysteries. For one thing, galaxies rotate faster than their stars and the gases within them can account for or explain using Einstein's theory of gravity. The most widely accepted explanation for this disparity is the existence of dark matter – something that does not radiate light. Another cosmic puzzle is the fact that the Universe's expansion is speeding. To explain this anomaly, physicists hypothesise that the Universe is filled with a repulsive kind of gravity known as dark energy.
However, they are only educated guesses. It's possible that we don't fully comprehend gravity or the laws of motion. Before we can be certain that dark matter and dark energy exist, we must validate Einstein's theory of general relativity with extreme precision. To do so, we must demonstrate the equivalence principle's validity.
Although Isaac Newton tested the equivalence principle in the 1600s, recent efforts are far more precise. Astronomers bounced lasers off mirrors left on the moon by Apollo astronauts in the twentieth century to demonstrate that inertial and gravitational mass are the same to one part in 10 trillion. That was quite an accomplishment. However, the most recent experiment went even further.
Another test is passed for general relativity
In 2016, the MicroSCOPE consortium of researchers launched a satellite into space. The scientists intended to test the equivalency principle with titanium and platinum cylinders on board. They insulated their equipment from vibrations and tiny gravitational changes caused by neighbouring mountains, subterranean oil and mineral resources, and the like by putting it in space. Using electric fields, the scientists tracked the location of the cylinders. If the two objects orbited in opposite directions, they would need to use two different electric fields to maintain them in place.
They discovered that the requisite electric fields were identical, allowing them to calculate that any discrepancies in inertial and gravitational mass were smaller than one part in a quadrillion. They essentially validated the equivalence principle precisely.
While this is a predicted result from the standpoint of general relativity, it has far-reaching implications for the research of dark matter and dark energy. While these views are popular, other scientists feel that new gravity theories can better explain the spinning features of galaxies. Many of these alternate ideas suggest that the equivalence principle isn't flawless.
The equivalence principle was not violated during the MicroSCOPE measurement. Its findings rule out certain alternative gravity theories, but not all of them. Researchers are planning a second experiment, termed MicroSCOPE2, that will be 100 times more precise than the first. If it detects violations from the concept of equivalence, it will provide scientists with critical assistance in constructing new and improved gravity theories.
Reference(s): APS
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