Einstein’s general theory of relativity passes another test with implications for dark matter and dark energy

The researchers used a satellite orbiting the Earth to conduct an ultra-precise test of a core thesis of Einstein’s general theory of relativity, the modern theory of gravity. The question is whether two different types of mass – gravitational and inertial – are identical. The scientists found that two objects on board the satellite fell to 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 – such as whether dark matter and dark energy exist.

The elders deceive

Gravity is the force that holds the universe together, tugging at distant galaxies and leading them in an eternal cosmic dance. The strength of gravity is determined in part by the distance between two objects, but also by the mass of the objects. An object with more mass experiences more gravity. The technical name for this type of mass is “gravitational mass”.

Mass has yet another property that could be called inertia. This is an object’s tendency to resist changes in motion. In other words, heavier things are harder to move: a bike is easier to push than a car. The technical name for this type of mass is “inertial mass”.

There is no reason a priori 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 rates, and indeed philosophers in ancient Greece observed that a hammer and a feather fall differently. Heavy objects actually seem to fall faster than light ones. We now know that drag is to blame, but that was hardly obvious in the past.

The situation became worse in the 17^th In the 19th century, Galileo conducted a series of experiments using ramps and spheres of different masses to show that objects of different masses fall at the same speed. (His oft-cited experiment of dropping balls from the Tower of Pisa is probably apocryphal.) And in 1971, astronaut David Scott convincingly repeated Galileo’s experiment on the vacuum moon when he dropped a hammer and feather, and they fell identically. The ancient Greeks had been fooled.

Dark guess

The assertion that inertial and gravitational masses are the same is known as the equivalence principle, and Einstein built equivalence firmly into his theory of gravitation. General relativity successfully predicts how objects fall under most circumstances, and the scientific community accepts it as the best theory of gravity.

However, “most” circumstances do not mean “all,” and astronomical observations have revealed some perplexing mysteries. For one thing, galaxies rotate faster than their stars and the gases within 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 mystery 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 could be that we don’t fully understand gravity or the laws of motion. Before we can be confident that dark matter and dark energy are real, we need to validate Einstein’s general theory of relativity with very high precision. To do this, we have to show that the equivalence principle holds.

While Isaac Newton tested the equivalence principle as early as the 16th century, 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 mass are equal to within one part in 10 trillion. This achievement was impressive. But the latest experiment went even further.

General relativity passes another test

A research group called the MicroSCOPE Collaboration launched a satellite in 2016. Titanium and platinum cylinders were on board, and the scientists’ intention was to test the principle of equivalence. By taking their apparatus into space, they isolated the equipment from vibrations and small gravitational differences caused by nearby mountains, underground oil and mineral deposits, and the like. The scientists monitored the position of the cylinders using electric fields. The idea is that if the two objects orbit differently, they would have to use two different electric fields to keep them in place.

What they found was that the electric fields required were the same, allowing them to determine that any differences in inertial and gravitational mass were less than one part in a quadrillion. In essence, they have performed an accurate validation of the equivalence principle.

While this is an expected result from the perspective of general relativity, it has very significant implications for the study of dark matter and dark energy. While these ideas are popular, some scientists believe that the rotational properties of galaxies can be better explained by new gravitational theories. Many of these alternative theories imply that the equivalence principle is not entirely perfect.

The MicroSCOPE measurement saw no violation of the equivalence principle. His results rule out some alternative theories of gravity, but not all. Researchers are preparing a second experiment called MicroSCOPE2, which is said to be about 100 times more accurate than its predecessor. If it detects deviations from the equivalence principle, it will provide scientists with crucial guidance for the development of new and improved theories of gravity.