Scientists used an Earth-orbiting satellite to conduct an ultra-precise test of a core premise of Einstein’s general theory of relativity, which is the modern theory of gravity. The question is whether two different kinds of mass – gravitational and inertial – are identical. The researchers found that two objects aboard the satellite fell toward Earth at the same speed, with an accuracy of one part in a quadrillion. This successful test of Einstein’s theory has significant implications for current cosmic mysteries—for example, the question of whether dark matter and dark energy exist.
Fool the old
Gravity is the force that holds the universe together, pulling in distant galaxies and leading them in an eternal cosmic dance. Gravity is partly controlled by the distance between two objects, but also by the mass of objects. An object with more mass experiences more gravity. The technical name for this type of mass is “gravitational mass”.
The mass has another property which can be called inertia. This is the tendency of an object to resist changes in motion. In other words, more massive things are harder to move: it’s easier to push a bicycle than a car. The technical name for this type of mass is “inertial mass.”
There is no reason first to assume that gravitational mass and inertial mass are equal. One controls gravity while the other controls movement. If they were different, heavy and light objects would fall at different speeds, and indeed philosophers in ancient Greece observed that a hammer and a feather fall differently. Heavy objects certainly seem to fall faster than light ones. We now know that drag is the culprit, but it was hardly obvious before.
The situation was resolved on the 17thth century, when Galileo performed a series of experiments using ramps and spheres of different mass to show that objects of different mass fall at the same speed. (His oft-cited experiment of dropping balls from the Leaning Tower of Pisa is probably apocryphal.) And in 1971, astronaut David Scott convincingly repeated Galileo’s experiment on the airless Moon when he dropped a hammer and a feather and they fell identically. The ancient Greeks had been fooled.
The claim that inertial and gravitational mass are the same is known as the principle of equivalence, and Einstein introduced equivalence into 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.
Subscribe to counter-intuitive, surprising and impactful stories delivered to your inbox every Thursday
But “most” circumstances do not mean “all,” and astronomical observations have revealed some perplexing mysteries. First, galaxies rotate faster than their stars and the gases in them can explain or than Einstein’s theory of gravity can explain. 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 full of a repulsive form of gravity called dark energy.
However, these are matters of informed guesswork. It may be that we do not fully understand gravity or the laws of motion. Before we can have confidence that dark matter and dark energy are real, we need to validate Einstein’s general theory of relativity with very high precision. To do that, we need to show that the equivalence principle is true.
While Isaac Newton tested the equivalence principle back in the 17th century, modern efforts are much more precise. In the 20th century, astronomers fired lasers from mirrors left on the moon by Apollo astronauts to show that inertia and gravitational mass are the same to an accuracy of one part in 10 trillion. That performance was impressive. But the latest experiment went even further.
General relativity passes yet another test
A group of researchers called the MicroSCOPE collaboration launched a satellite into space in 2016. Cylinders of titanium and platinum were on board, and the researchers intended to test the principle of equivalence. By placing their devices in space, they isolated the equipment from vibrations and small gravity differences created by nearby mountains, underground oil and mineral deposits, and the like. The researchers monitored the position of the cylinders using electric fields. The idea is that if the two objects orbited differently, they would need two different electric fields to keep them in place.
What they found was that the required electric fields were the same, which allowed them to determine that any difference in inertial and gravitational mass came out to less than one part in a quadrillion. In essence, they made a precise validation of the equivalence principle.
Although this is an expected result from general relativity, it has very significant consequences for the study of dark matter and dark energy. While these ideas are popular, some researchers believe that the rotational properties of galaxies can be better explained by new theories of gravity. Many of these alternative theories suggest that the equivalence principle is not quite perfect.
The MicroSCOPE measurement saw no violation of the equivalence principle. Its findings rule out some alternative theories of gravity, but not all of them. Scientists are preparing another experiment, called MicroSCOPE2, that should be about 100 times more accurate than its predecessor. If it sees deviations from the equivalence principle, it will provide scientists with crucial guidance in developing new and improved theories of gravity.