Einstein's test of general relativity has major implications

Einstein’s test of general relativity has major implications

Researchers have used an Earth-orbiting satellite to conduct an ultra-precise test of a basic 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 identical. Scientists found that two objects aboard the satellite fell toward Earth at the same rate, with a precision 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.

Fooling the Ancients

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 in part 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 might 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 the 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 the philosophers of ancient Greece observed that a hammer and a wedge fall differently. Certainly, heavy objects seem to fall faster than light ones. We now know that air resistance is to blame, but this was not obvious in the past.

The situation was clarified on the 17thth 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 experiment of throwing marbles 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 wedge, and they fell identically. The ancient Greeks had been fooled.

The dark conjecture

The statement that inertial and gravitational mass are the same is known as the principle of equivalence, and Einstein wired the 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.

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However, “most” circumstances do not mean “all”, and astronomical observations have revealed some puzzling mysteries. First, galaxies rotate faster than their stars and the gas inside them can explain or than Einstein’s theory of gravity can. The most accepted explanation for this discrepancy is the existence of a substance called dark matter – matter that does not emit light. Another cosmic conundrum 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 confidence 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 equivalence principle back in the 1600s, modern efforts are much more precise. In the 20th century, astronomers shone lasers at mirrors left on the moon by the Apollo astronauts to show that inertial and gravitational mass are the same to an accuracy of one part in 10 trillion. That achievement was impressive. But the latest experiment went further.

General relativity passes another test

A group of researchers called the MicroSCOPE collaboration launched a satellite into space in 2016. On board were cylinders of titanium and platinum, and the scientists’ intention was to test the principle of equivalence. By placing their device in space, they isolated the equipment from the vibrations and small gravitational differences created by nearby mountains, underground oil and mineral deposits, and the like. The scientists monitored the location of the cylinders using electric fields. The idea is that if the two objects orbited differently, they would have to use two different electric fields to hold them in place.

What they found was that the required electric fields were the same, allowing them to determine that any difference in inertial and gravitational mass resulted to less than one part in a quadrillion. In essence, 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 substantial consequences for the study of dark matter and dark energy. Although these ideas are popular, some scientists believe that the rotational 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 alternative theories of gravity, but not all. The researchers are preparing a second experiment, called MicroSCOPE2, which 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 for developing new and improved theories of gravity.

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