In the realm of physics, there are recognized to be four fundamental interactions: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. However, for more than a century, researchers have harbored suspicions that this list is incomplete. The quest for a mysterious “fifth force” ebbs and flows, sometimes fading, only to reignite with renewed vigor. This saga began with seemingly mundane measurements taken by a Hungarian nobleman, escalated into scientific frenzy in the 1980s, and continues today as physicists again report anomalies that do not conform to current theoretical frameworks.
The Experiment of Baron Eötvös
Toward the close of the nineteenth century, Hungarian Baron Loránd Eötvös set out to test a cornerstone of physics: the equivalence principle. He sought to verify whether gravity truly acts uniformly on various materials, irrespective of their specific chemical makeup. While Newton had established this to an accuracy of $10^{-3}$, Eötvös aimed for much greater precision.
The Baron was no mere academic; he was a master of precision mechanics. He engineered a torsion balance of extraordinary sensitivity.
This apparatus featured a beam suspended from an exceptionally fine thread.
The key innovation: the masses fixed to either end of the beam were suspended at slightly different elevations.
This configuration allowed the instrument to register minute variations in gravitational pull and the Earth’s rotation.
To eliminate external interference, Eötvös conducted his work nocturnally, when city vibrations from traffic subsided. The instrument was shielded within a double casing to guard against drafts. The findings from his years of labor, published posthumously in 1922, concluded: the masses were equivalent, Newton was correct, and no deviations existed, achieving a precision of $10^{-9}$.
Einstein was thrilled, as this empirical data formed a crucial pillar for his General Theory of Relativity.
The Reexamination by Fischbach
Six decades later, in 1986, American physicist Ephraim Fischbach revisited Eötvös’s historical records. He was searching for signatures of interactions that might unify quantum mechanics with gravity. Fischbach discerned patterns that others had dismissed merely as experimental noise. Within Eötvös’s data, he identified subtle discrepancies that arranged themselves into a clear structure. It appeared that the gravitational attraction exerted on the test masses varied based on their chemical composition, specifically correlating with the baryon number (the total count of protons and neutrons).
For instance, the combination of “water–copper” yielded a significant deviation, while “platinum–magnalium” showed nearly perfect agreement. Fischbach publicly asserted: a fifth force must exist in nature.
This force was characterized as a weak repulsive interaction, termed the “hypercharge.”
It was hypothesized to operate over distances spanning 100 to 200 meters.
Its strength was estimated to be roughly 1% of the force of gravity.
This announcement made front-page headlines in the New York Times. The physics community, previously placid, was thrown into turmoil.
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The Hunt for the Ghost
An intense period of experimental pursuit immediately commenced. To detect this putative fifth force, scientists needed to measure gravity where conditions deviated significantly from standard laboratory settings.
Television Towers. A team from the U.S. Air Force Geophysical Laboratory measured gravity at various heights on tall broadcasting towers, searching for deviations from Newtonian law as altitude changed. Initially, they reported positive findings!
Greenland Ice Sheet. Researchers lowered sensitive instruments into deep boreholes drilled into the ice, assuming the ice offered a perfectly uniform medium.
The Edge of a Cliff. Peter Thieberger conducted an elegant experiment near the Palisades cliff. He placed a copper sphere in a water basin. If the fifth force were active, the rock mass would attract the copper and the water unequally, causing the sphere to drift toward the cliff edge. Indeed, it drifted!
It seemed the discovery of the century was on the verge of realization.
Geology Versus Physics
By the early 1990s, the initial euphoria gave way to disillusionment. More precise follow-up experiments conducted by the Eötvös-Wash (Eöt-Wash) group, named in homage to Baron Eötvös, reported null results. Where had the fifth force vanished?
It turned out that the researchers had underestimated the complexities of the Earth itself. The observed “anomalies” were eventually attributed to unaccounted-for geological features distorting the gravitational field:
Beneath the television towers, varying densities of subsurface rock formations were found to skew the local gravitational field.
In Greenland, the bedrock topography beneath kilometers of ice was creating unexpected gravitational gradients.
Thieberger’s sphere moved not due to a new fundamental interaction, but because of microscopic water currents induced by thermal variations, not a fundamental force.
The fifth force was, once again, effectively “buried.” Physicists concluded that Fischbach had erred, and Eötvös’s original deviations were artifacts of early twentieth-century instrumentation imperfections.
The X17 Anomaly
In 2016, a new sensation emerged from the same region where the quest began—Hungary. A research team at the Institute of Nuclear Research (ATOMKI) in Debrecen announced they had potentially rediscovered a fifth force, but this time, they were investigating nuclear decay rather than macroscopic gravity.
While examining the radioactive decay of Beryllium-8 isotopes, the scientists noted a peculiar excess: positrons and electrons were emitted from the nucleus at an angle of 140 degrees more frequently than theoretically predicted. The statistical significance reached 6.8 sigma—a high threshold suggesting the result is not attributable to random chance or error. The researchers hypothesized that an unknown particle with a mass around 17 MeV was responsible for this effect, subsequently naming it the X17 boson.
In 2019, the same phenomenon was observed during the decay of Helium nuclei. Theorists suggest X17 is a “protophobic” particle; it avoids interaction with protons but does interact with neutrons, explaining why it evaded detection in massive colliders that primarily smash protons together. Conceivably, this particle could serve as a crucial link toward comprehending dark matter.
The contemporary scientific community has absorbed the bitter lessons of the 1980s. Independent verification efforts are currently underway. As of 2025, the MEG II experiment in Switzerland has not definitively confirmed the particle’s existence, nor has it completely ruled it out.
Physics now stands once again at a juncture of uncertainty. Either the X17 boson represents another misinterpretation of data and apparatus errors, or the world is on the cusp of a discovery that will reshape our understanding of the cosmos, just as Einstein’s theory once did.
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