Quantum theory challenged by new test of equivalence principle

One of the fundamental assumptions in physics is the consistent relationship between various properties of mass, such as weight, inertia, and gravitation. This principle, known as the equivalence principle, is crucial for Einstein’s theory of relativity and forms the foundation of our current understanding of physics. However, quantum theory suggests the possibility of a violation of this principle, which creates an inconsistency between Einstein’s gravitational theory and modern quantum theory.

To address this discrepancy, precise tests of the equivalence principle become increasingly important. A collaborative team from the Center of Applied Space Technology and Microgravity (ZARM) at the University of Bremen and the Institute of Geodesy (IfE) at Leibniz University Hannover recently conducted groundbreaking research in this field. Their aim was to provide evidence, with a significantly higher level of accuracy, that passive gravitational mass and active gravitational mass always maintain equivalence, regardless of the specific composition of the masses involved.

This research was carried out as part of the Cluster of Excellence “QuantumFrontiers.” Today, the team has published their remarkable findings in Physical Review Letters, presenting their work as a highlights article.

Physical context

The concept of inertial mass involves the resistance to acceleration experienced by an object, such as the sensation of being pushed into your seat when a car starts moving. On the other hand, passive gravitational mass relates to how an object responds to gravity, determining its weight on Earth. Active gravitational mass, in turn, refers to the strength of an object’s gravitational field or the gravitational force it exerts.

The equivalence of these mass properties holds immense significance for the theory of general relativity. Hence, scientists are actively engaged in conducting ever more precise tests to confirm the equivalence of both inertial and passive gravitational mass, as well as the equivalence between passive and active gravitational mass. By scrutinizing these equivalences, researchers aim to further validate and refine our understanding of the fundamental principles governing the universe.

What was the study about?

Suppose we entertain the notion that the passive and active gravitational masses are not equivalent and their ratio depends on the material composition. In such a scenario, objects made of different materials with distinct centers of mass would exhibit self-acceleration. For instance, the moon, comprised of an aluminum shell and an iron core with offset centers of mass, would experience acceleration. This intriguing hypothetical change in velocity could be measured with remarkable precision using a technique called “Lunar Laser Ranging.”

Lunar Laser Ranging involves directing lasers from Earth towards retroreflectors positioned on the moon during the Apollo missions and the Soviet Luna program. The round-trip travel times of these laser beams are meticulously recorded. In the current study, a team of researchers scrutinized the “Lunar Laser Ranging” data collected over a span of five decades, from 1970 to 2022, with the objective of investigating the potential effects arising from the disparity in mass.

Remarkably, their analysis yielded no evidence of such effects. This remarkable finding implies that the passive and active gravitational masses are equivalent to an astonishing precision of approximately 14 decimal places. This estimation is a hundred times more accurate than the most precise previous study, which dates back to 1986.

The Institute of Geodesy at Leibniz University Hannover (LUH), renowned for its expertise in laser distance measurements to the moon, is one of only four centers worldwide dedicated to the analysis of such data. With their unique proficiency, the institute conducted an in-depth examination of the Lunar Laser Ranging measurements, encompassing comprehensive error analysis and the interpretation of the results, particularly with regards to the testing of general relativity.

Source: Leibniz University Hannover

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