Following the death of Baron Loránd von Eötvös in 1919, his collaborators Desiderius Pekar and Eugen Fekete co-authored a paper in 1922 containing the results of a series of earlier experiments testing the identity of inertial and gravitational mass, the Weak Equivalence Principle (WEP). Although the so-called "EPF" paper made no claim for any WEP violations, a subsequent 1986 reanalysis of the EPF paper revealed a pattern in their data suggesting the presence of a new 5th force in nature. Although the EPF data, and the 1986 reanalysis of these data, present fairly compelling evidence for such a 5th force, many contemporary experiments have failed to detect its presence. Here we summarize the key elements of this "Eötvös paradox," and suggest some possible paths to a resolution. Along the way we also discuss the close relationship between Eötvös and Einstein, and consider how their respective contributions may have been influenced by the other's. We conclude with a discussion of a possible connection between the recently detected "X17" particle and the 5th force.

Ephraim Fischbach is a professor in the Department of Physics and Astronomy, and is a Fellow of the American Physical Society. His interests include theoretical elementary particle and nuclear physics, general relativity and astrophysics. His work was featured in an episode of “Through the Wormhole” with Morgan Freeman on July 13, 2011.

Department of Physics General Colloquia

Researchers should cite this work as follows:

• Ephraim Fischbach (2020), "The Eötvös Paradox: The Enduring Significance of Eötvös' Most Famous Experiment," https://nanohub.org/resources/34059.

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1. Baryon number
2. casmir forces
3. material properties
4. submicron forces