Einstein’s general theory of relativity recently passed its toughest test yet.
Using a specially designed satellite, an international team of scientists measured the acceleration of two objects in free fall in Earth’s orbit. Results based on five months of data indicate that the accelerations do not differ from each other by more than one part in 1015, ruling out any violation of the weak equivalence principle on such a scale.
The principle of weak equivalence is relatively easy to observe and states that all objects, regardless of their mass and composition, accelerate identically in the same gravitational field when they are not acted upon by anything else.
Perhaps the most famous demonstration of this dramatic effect occurred in 1971, when astronaut Dave Scott, standing on the surface of the moon, dropped a hammer and a feather simultaneously from the same height. Because air resistance did not slow down the plume, the two objects fell to the surface of the moon at the same speed.
A new experiment called MICROSCOPE, led by the recently deceased physicist Pierre Touboul was much more rigorous than Scott’s demonstration on the moon. It involved an artificial satellite that orbited the Earth from April 25, 2016, until its deactivation on October 18, 2018.
At the time, the research team ran a series of experiments using suspended masses in free fall, totaling five months of data. Two-thirds of the data included pairs of test masses with different compositions, titanium and platinum alloys. The remaining third consisted of pairs of masses of the same platinum composition.
The experimental equipment used electrostatic forces to keep the two test masses in the same position relative to each other. If there was any difference in acceleration—a measure called the Etwesch ratio—the equipment had to detect changes in the electrostatic forces holding the masses in place.
Early results published in 2017 were promising, failing to detect any deviation from the weak equivalence principle to the Etwesch parameter, −1±9 x 10−15. However, the satellite was still operational and producing data, so the paper was not complete. A complete set of data supports earlier findings, constraining the Etwesch parameter to 1.1 x 10−15.
This is the tightest limit of the weak equivalence principle to date, which is unlikely to be overcome in the near future. This means that scientists can rely on general relativity with more confidence than ever before; At the same time, they put new constraints on the intersection between general relativity and quantum mechanics—two regimes that operate by different rules.
“We have new and much better constraints for any future theory, because these theories must not violate the equivalence principle at this level,” explains astronomer Gilles Metre of France’s Côte d’Azur observatory.
The results are impressive considering that equipment designed to operate in the microgravity environment of Earth orbit cannot be tested before launch. Now that the MICROSCOPE project has been successfully completed, the research team can use its results to develop a more rigorous test.
These tests will help scientists test the constraints of general relativity—the framework that describes gravity in spacetime. However, at the atomic and subatomic scales, general relativity breaks down and is replaced by quantum mechanics. Scientists have been trying to solve the differences between these two for a long time. One way to do this might be to determine exactly where general relativity breaks down.
We now know that such a violation for weak equivalence does not occur below one part of 1015. Further experiments may push this limit to a fraction of 1017.
“It takes at least ten or twenty years for this kind of improvement in space satellite experiments,” says Manuel Rodríguez, a physicist at the French National Center for Aerospace Studies (ONERA).
The research was published in the journals Physical Review Letters and Classical and Quantum Gravity.
Prepared by eurekalert.org