The researchers used a satellite orbiting Earth to perform an ultra-precise test of a central premise of Einstein’s theory of general relativity, which is the modern theory of gravity. The question is whether two different types of mass – gravitational and inertial – are the same. Scientists found that two objects aboard the satellite fell towards Earth at the same speed, with an accuracy of one part in a quadrillion. This successful test of Einstein’s theory has substantial implications for current cosmic mysteries – for example, the question of whether dark matter and dark energy exist.
deceive the elders
Gravity is the force that holds the Universe together, pulling distant galaxies and guiding them into an eternal cosmic dance. The force of gravity is governed partly by the distance between two objects, but also by the masses of the objects. An object with more mass experiences more gravity. The technical name for this type of mass is “gravitational mass”.
Mass has another property, which we might call inertia. It is the tendency of an object to resist changes in motion. In other words, bigger things are harder to move: it’s easier to push a bike than a car. The technical name for this type of mass is “inertia mass”.
There is no reason first assume that gravitational mass and inertial mass are the same. One governs the force of gravity, while the other governs motion. If they were different, heavy and light objects would fall at different rates, and indeed ancient Greek philosophers observed that a hammer and a feather fall differently. Heavy objects certainly seem to fall faster than light objects. We now know that air resistance is the culprit, but it was hardly obvious in the past.
The situation was clarified on the 17e century, when Galileo performed a series of experiments using ramps and spheres of different masses to show that objects of different masses fall at the same rate. (His oft-cited experience of dropping bullets from the Tower of Pisa is probably apocryphal.) And in 1971, astronaut David Scott convincingly repeated Galileo’s experience on the airless Moon, when he dropped a hammer and a feather, and they fell alike. The ancient Greeks had been duped.
The claim that inertial mass and gravitational mass are the same is known as the equivalence principle, and Einstein’s hardwired equivalence in his theory of gravity. General relativity successfully predicts how objects fall under most circumstances, and the scientific community accepts it as the best theory of gravity.
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However, “most” circumstances do not mean “all”, and astronomical observations have revealed puzzling mysteries. For one thing, galaxies spin faster than their stars, and the gases they contain may explain or explain Einstein’s theory of gravity. The most accepted explanation for this discrepancy is the existence of a substance called dark matter – matter that does not emit light. Another cosmic puzzle is the observation that the expansion of the Universe is accelerating. To explain this oddity, scientists have postulated that the Universe is filled with a repulsive form of gravity called dark energy.
However, these are matters of educated guesswork. We may not fully understand gravity or the laws of motion. Before we can have any certainty that dark matter and dark energy are real, we need to validate Einstein’s theory of general relativity with very high precision. To do this, we need to show that the equivalence principle is true.
While Isaac Newton tested the principle of equivalence in the 1600s, modern efforts are much more accurate. In the 20th century, astronomers bounced lasers off mirrors left on the moon by Apollo astronauts to show that inertial and gravitational masses are identical with an accuracy of one part in 10 trillion. This achievement was impressive. But the most recent experiment has gone even further.
General relativity passes another test
A group of researchers called the MicroSCOPE Collaboration launched a satellite into space in 2016. Titanium and platinum cylinders were on board, and the scientists’ intention was to test the principle of equivalence. By placing their device in space, they isolated the equipment from vibrations and small gravitational differences created by nearby mountains, underground oil and mineral deposits, etc. The scientists monitored the location of the cylinders using electric fields. The idea is that if the two objects rotated differently, they would need to use two different electric fields to hold them in place.
What they discovered was that the electric fields required were the same, which allowed them to determine that any difference in inertial and gravitational mass was less than one part in a quadrillion. Essentially, they made a precise validation of the principle of equivalence.
Although this is an expected result from the point of view of general relativity, it has very important consequences for the study of dark matter and dark energy. Although these ideas are popular, some scientists believe that the rotating properties of galaxies can be better explained by new theories of gravity. Many of these alternative theories imply that the principle of equivalence is not quite perfect.
The MicroSCOPE measurement found no violation of the principle of equivalence. His results rule out some, but not all, alternative theories of gravity. The researchers are preparing a second experiment, called MicroSCOPE2, which should be around 100 times more precise than its predecessor. If it finds deviations from the principle of equivalence, it will give scientists crucial guidance for developing new and improved theories of gravity.
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