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Is There a Fifth Fundamental Force in Physics?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

Learn about the four forces of nature and the existence of a potential fifth force.

Learn about the four forces of nature and the existence of a potential fifth force.

A Potential Fifth Force of Nature

The four known forces of the Universe are the strong and weak nuclear forces, electromagnetism, and of course gravity. But that last one has stubbornly refused to cooperate in our search for a fundamental theory of everything, uniting all the forces into one. We know how to relate the other three together, but gravity eludes us. But something inherent in the search for everything is that we know all the forces that exist. Do we? Evidence exists for a potential fifth force of nature, and gravity provides clues to its potential existence.

Chasing Ghosts

The initial hunt for a fifth force stems from our understanding of what mass is. You see, we can think of its gravitational aspects and its inertial ones as well. Historically, these have been the same in nature and therefore transitory, and Einstein’s relativity makes it explicit with the weak equivalence principle. It is at the heart of relativity and without it then inertial frames conveying the same information would fall into jeopardy. Falling objects would vary at the rate they fall because of their chemical composition (which we intuitively feel is true). So, are inertial masses and gravitational ones the same? (Ball)

We have many experiments that confirmed the mass of an object does not impact the rate at which it falls. One of the first was done by Simon Stevin, who in 1586 dropped objects from a clock tower in Delft and noted their fall times were practically identical. Galileo of course conducted similar experiments. And Isaac Newton noted the simple pendulum clock did not have a period that depended on the mass of the swinging object but on the length of the swinging arm instead (Ibid).

Of course, more refined methods for testing the equivalence of accelerating and gravitational fields were eventually devised. In 1889, Baron Lorand Eotvos used what is known as a torsion balance to compare the properties of two different objects with the same mass. A torsion balance is simply a horizontal rod with a standard mass (in this case platinum) on one end and another object on the opposite side. Gravity should cause this to be balanced, but a rotation could occur if the rotation of Earth affects the two objects differently. That is, the inertia of the Earth’s movement would produce a detectable swivel if the inertial masses are different.

To see these differences, one could place a mirror at the end of the rod and use light to spot the rotation. However, one difficulty would be the local variations of gravity due to the height and density of the surrounding material, so if you test a north-south and then east-west configuration you have resolved this issue, but rather sneakily. Nothing is different gravitationally, but the orientation would vastly impact the rotation if the inertial masses were not equivalent.

So, if the two orientations produce the same effects then you have shown the two masses do indeed respect the weak equivalence principle. And to prevent any thermally induced convective issues disturbing the rig, the experiment was done in a vacuum as well as in the dark (because light also generates heat) (Ibid).

The torsion experiment.

The torsion experiment.

The experiment was done with a variety of objects and to no one’s surprise, the results indicated no differences up to 1 part in 20 million. Then in 1905 relativity made its appearance on the stage and people again wondered if the mass under gravity was any different than the inertial mass of an object, especially when at great speeds that relativity was most consequential.

Eotvos returned to his experiment and along with his students, Dezso Peker and Jeno Fekete, tested even more materials against his platinum standard mass. Though Eotvos died in 1919, three years later saw new results published and confidence in them grew, with any potential error being 1 part in 200 million. And in 1935, Janos Renner (another Eotvos student) was able to refine and get the error of the results down to 1 part in 2.5 billion.

It seemed like a closed case: No differences are out there, and the weak equivalence principle holds true. No strange mysterious new force is out there that would cause differences to be seen (Ibid)

Results for different materials.

Results for different materials.

Making Ratios

Many years later, Ephraim Fischbach became interested in a potential fifth force when he heard about an experiment conducted by Roberto Colella and his team in 1975. In it, the effects of Newtonian gravity on subatomic particles were examined. That got Fischbach thinking about the consequences of Einstein’s gravity on such particles and what those dynamics would even look like. And he was thinking any such new behavior could explain the particle paths of kaons and their anti-matter counterparts, which hinted at some relationship with their baryonic numbers. That, people, is simply a count of all the quarks and anti-quarks present in our object.

Somehow, the different behavior seemed to correlate to this value. And if the baryonic number is related to composition, its got a chemical dependence…hinting at some inertial considerations. If you compare the ratio of the baryonic number to the mass of the object, you may be able to tease out some differences. But shouldn’t that ratio be consistently the same, no matter what we look at? You would think so, but pesky things like binding energy holding atoms together do vary atom-to-atom, so the ratio does have some differences (Ibid).

In 1985, Fischbach along with student Carrick Talmadge went over these ratios for the materials used in Eotvos’s experiment and compared them to the data of the experiment. Shockingly, a direct correlation was spotted between the ratios and the supposed error measurements of the differences in gravitational acceleration that Eotvos found. What did it mean? Scientists got more and more precise with the torsion balance until they were able to reduce those errors and show that no true relationship existed and that the fifth force likely doesn’t exist…on small scales (Ball, Letzer).


A different avenue in pursuing a fifth force ironically involves gravity again…sort of. Cosmology has many mysteries to it that can frustrate people. Questions like what started inflation, what is dark matter and dark energy comprised of, and such are all current devices to explain data we have but that lack proper explanations themselves. Gravity has driven the pursuit of many of these mysteries, but what if something more quantum in origin is required? Are there any signs of such a mystery particle (Cossins 33)?

In 2015, Attila Krasznahorkay and team from the Institute for Nuclear Research at the Hungarian Academy of Sciences were looking at Be-8, an unstable isotope of beryllium, and noticed some strange things about its decay into constituent particles. The pattern seen implied a “shorter lived, slow-moving particle” of about 17 mega-electron volts, or about 30 electron masses. Nothing in the Standard Model of particle physics predicts this, but it does fit the bill as a dark matter candidate based on that mass.

Specifically, it could be a dark photon or the force-carrying particle of dark matter. Attila did a follow-up experiment but with helium nuclei and spotted the same strange decay, point to a “photophobic X boson” or force-carrying particle that is in a strange Goldilocks zone of not being a viable force carrier. That is, it's not big enough nor small enough to fit into the Standard Model’s theory for a force carrier. What could it be? (Ibid).

Maybe it’s a chameleon force, something that would be very hard to spot here on Earth. Its mass would fluctuate according to the local density of matter present and thus be hard to see. Otherwise, we would have noticed it before all this specialized experimentation. So on a local scale, it would have to be small, but say on a galactic scale, it could mime the behavior we attribute to dark energy. But how could we test for something like this here? (33-4)

A test done by Burrage and his team at the Imperial College of London attempted to nullify as many possible forces as possible in a given area and see if weird behavior was spotted. A roughly 1-foot-in-diameter vacuum chamber was used with a roughly marble-sized metal sphere in the center producing effects to cancel out forces. Atoms were dropped on it and examined to see if they demonstrated any weird behavior deviating from known theory, but no dice. Other models now need to be tested out to explain the mysterious boson (35).

Hey, if a fifth force was easy to spot, wouldn’t it have been seen by now?

Works Cited

Ball, Phillip. “The Fifth Force of Physics is Hanging by a Thread.” Nautil.us. NautilisThink Inc., 16 Mar. 2017 Web. 21 Jan. 2021.

Cossins, Daniel. “May the fifth force be with you.” New Scientist. New Scientist Ltd., 16 May 2020. Print. 33-5.

Letzter, Rafi. “Physicists who disproved ‘5th force’ win $3 million ‘Breakthrough’ prize.” Livescience.com. Future US, Inc., 12 Sept. 2020. Web. 22 Jan. 2021.

© 2021 Leonard Kelley

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