A key tenet of Einstein’s general theory of relativity has passed the most rigorous tests.
Using a specially designed satellite, an international team of scientists measured the acceleration of a pair of freely falling objects in Earth’s orbit.
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The results, based on five months of data, show that the acceleration did not differ by more than one part in 1015, ruling out violations of the weak equivalence principle by that measure.
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The Weak Equivalence Principle is relatively easy to observe and states that all objects, regardless of their mass or composition, accelerate equally in the same gravitational field when unaffected by other influences.
Perhaps most famously, the dramatic effect was demonstrated in 1971 when astronaut Dave Scott dropped a hammer and feather from the same height while he was standing on the moon.
If there were no air resistance to slow down the plumes, both objects would fall to the lunar surface at the same speed. The new trial, called MICROSCOPE and drove by the late physicist Pierre Touboul, was fairly more thorough than Scott’s exhibition.
Includes satellites in orbit around the Earth from April 25, 2016 to inactive on October 18, 2018. During this time, the team performed several experiments using a suspended mass in free fall, providing a total of five months of data. Two-thirds of these data refer to test mass pairs of different compositions, titanium and platinum alloys.
The remaining third contained pairs of reference masses of the same platinum composition. The experimental team used electrostatic forces to keep the two test masses in the same position relative to each other.
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When there is a difference in acceleration (a metric known as the Eötvös ratio), the instrument records the change in electrostatic force that holds the mass in place.
Initial results published in 2017 were promising, with no violation of the weak equivalence principle up to the Eötvös parameter of −1±9 x 10−15. However, the satellite was still active and generating data, so the job was not done. The entire data set solidifies these initial results by limiting the Eötvös parameter to 1.1 x 10−15.
This is by far the strictest limit of the weak equivalence principle and is unlikely to be exceeded in the short term. This means that scientists can continue to trust general relativity more confidently than ever before and place new constraints on the intersection between general relativity and quantum mechanics.
This is surprising given 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 team can use the results to design much more rigorous tests.
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These tests will help examine the limitations of the general theory of relativity, a framework that describes gravity in physical space-time. But at the atomic and subatomic scales, general relativity breaks down and quantum mechanics takes over.
Scientists have been trying to work out the differences between the two for quite some time. One way is to point out where the general theory of relativity breaks down.
We now know that for weak equivalence, a one part failure does not occur in 1015.
Certain improvements that can be made in the next iteration of the satellite could be analyzed as a fraction of the year 1017.
“For at least 10 to 20 years, we haven’t seen any improvement with our space satellite experiments,”
said Manuel Rodrigues, a physical engineer at the French National Center for Aeronautical and Space Research (ONERA).
However, we believe that these results will suffice for the time being.
The team’s incredible work has been published in a special issue of Physical Review Letters and Classical and Quantum Gravity.
