Image Credit: ISTOCK
Majority of the phenomena occurring in nature could be explained based on just four fundamental forces. In increasing order of their strength, these forces are gravitational force, weak nuclear force, electromagnetic force, and strong nuclear force. Although these fundamental interactions explain most of the physical events in our universe, there are some phenomena which could not be explained based on these known forces thus leading physicists to ponder whether there could be additional forces at play.
Some of the main reasons why the search for the fifth fundamental force has been propelled lately are dark mass and the agent causing the accelerated expansion of the universe, namely dark energy. Quintessence, a form of dark energy, has been speculated to be a candidate for the fifth force [1, 2]. Another fifth force probe that became famous during the 80s resulted from a reanalysis of the Eötvös experiment . As a result, researchers around the world have explored it as a plausible model .
Various other theoretical models have emerged, proposing candidates for the fifth force. These models often draw inspiration from ideas that extend beyond the standard model of particle physics. Some theories, such as the Georgi-Glashow model, suggest the existence of particles called X bosons that mediate this fifth force. These hypothetical particles could interact differently from the particles responsible for the other fundamental forces, leading to unique signatures in particle collider experiments and cosmological observations.
Despite many proposals, there has been a lack of evidence for the nature of the fifth force and its interactions though the bar has already been set high. Recent empirical results at the
Fermilab in Chicago have once again sparked interest among the physics community that might lead to the realization of the fifth fundamental force. The scientists at Fermilab in a probe named muon g-2 experiment have found that muons behave in a way that cannot be explained within the framework of the standard model of particle physics [5, 6]. Their dynamical motion has been detected to be faster than that predicted by the standard model which has led experts to believe the intervention of a novel force that might be causing this.
Comparison of empirical results with the standard model prediction. Source: Ryan Postel, Fermilab/Muon g-2 collaboration
The current result essentially backs up results from 2021 when striking features about the magnetic moment of muons were revealed which also carried conclusive evidence of new particles and/or forces that currently lack in the standard model. Individually, every muon acts akin to a miniature magnetic bar when subjected to a magnetic field—this phenomenon is referred to as its magnetic moment. Furthermore, muons possess an inherent quality known as spin. The interplay between the spin and the magnetic moment of a muon is identified as the g-factor. It is anticipated that the g-factor for both the electron and the muon will be two. Thus, it becomes crucial to ascertain whether the deviation from this expectation, denoted as g-2, registers at zero when measured. The reason why 2 is subtracted from the original value of the g-factor is to understand the contribution from the quantum foam, the concept according to which space is filled with particles and is not really empty.
Coming to the physics part of how the result opens the possibility of a new force, let us consider the experimental study undertaken at Fermilab. The muons that are essentially 200 times the mass of the electron is allowed to interact with a magnetic field of strength 1.45 Tesla. As a result, the muons vibrate analogous to a spinning top and the rate of this vibration is proportional to the field strength. The experimental facility produces countless muons which is stored in a circular magnet called the storage ring which has a diameter of 14 metres. This ring contains detectors in its interior that counts the electrons formed due to the decay of muons. An interesting fact to note here is that counting the electrons is correlated with the rate of muons’ vibrations thus more the electrons detected, greater is the precision in measurement. Physicist Paul Dirac predicted that the value of muon’s g-factor is equal to 2, however according to quantum mechanical considerations, there is a non-trivial contribution to the g-factor due to virtual particles. The goal of muon g-2 probe is to study the difference between the original g-factor and the value predicted which is 2. Now, in case the standard model lacks novel forces/particles, it would correspond to a situation wherein the rate would be either higher or lower than the predicted number but by a small margin.
Various other research groups have also joined this hunt, notably the team at CERN’s Large Hadron Collider (LHC). One of the team members is physicist Mitesh Patel who is originally based at Imperial College London. According to him, results which disagree with the standard model are very crucial. Quoting him on this, “Measuring behaviour that does not agree with the predictions of the Standard Model is the holy grail for particle physics. It would fire the starting gun for a revolution in our understanding because the model has withstood all experimental tests for more than 50 years.”
The current discrepancy between theoretical prediction and empirical result could potentially mean that there are new undiscovered particles in the universe and these particles would mediate a new force of nature. The discovery of a fifth force would be revolutionary in so many ways. It would open a whole new dimension of physics for researchers to explore, both in theory and practice.
RSF in Perspective:
The standard model of particle physics has been a successful theory and has added remarkably to our knowledge about the physical world. However, research has shown that this theory is limited if not wrong. Physicists in their pursuit of what lies beyond the standard model, have produced various new theories and models such as the MSSM and nMSSM, which are essentially based on the idea of supersymmetry. But we know that even supersymmetry is a failed idea so far as it has not yielded any empirically tested result. Due to lack of conclusive empirical evidence, various groups around the world are trying to come up with novel frameworks that could do justice to this pursuit.
Physicist Nassim Haramein has proposed a first principles-based theory called the generalized holographic model that has been successful in yielding results better than the standard model. For example, the model provided an estimate for the charge radius of the proton, the standard model value of which was in disagreement by 4% . The model has also contributed heavily to our understanding of subatomic particles, black holes, vacuum catastrophe to name a few [8, 9, 10].
The latest measurements at Fermilab which resulted in the most precise value of the magnetic moment of muon is very crucial for further developments in theoretical physics. As this result exposes the gaps in the current model, it would be interesting to see how novel frameworks such as that of Haramein accounts for the same. The new result could now be compared and as well be derived from first principles. All this would hopefully be incorporated in his upcoming paper entitled, “Scale Invariant Unification of Forces, Fields & Particles in a Quantum Vacuum Plasma”.
 Wetterich, C. "Quintessence – a fifth force from variation of the fundamental scale". Heidelberg University.
 Cicoli, Michele; Pedro, Francisco G.; Tasinato, Gianmassimo. "Natural quintessence in string theory". Journal of Cosmology and Astroparticle Physics (2012). DOI: 10.1088/1475-7516/2012/07/044
 Fischbach, Ephraim; Sudarsky, Daniel; Szafer, Aaron; Talmadge, Carrick; Aronson, S.H. "Reanalysis of the Eötvös experiment". Physical Review Letters (1986). DOI: 10.1103/PhysRevLett.56.3
 Jha, R., Sinha, K.P. “A possible model for fifth force”. Pramana - J. Phys (1988). DOI: 10.1007/BF02846963
 B. Abi et al. (Muon g-2 Collaboration), “Measurement of the Positive Muon Anomalous Magnetic Moment to 0.46 ppm”. Physical Review Letters (2021). DOI: 10.1103/PhysRevLett.126.141801
 D P Aguillard et al, “Measurement of the Positive Muon Anomalous Magnetic Moment to 0.20 ppm” (2023). https://muon-g-2.fnal.gov/result2023.pdf
 Haramein, N. Quantum Gravity and the Holographic Mass, Physical Review & Research International, ISSN: 2231-1815, Page 270-292 (2012).
 Haramein, N. The Schwarzschild Proton, AIP Conference Proceedings, CP 1303, ISBN 978-0-7354-0858-6, pp. 95-100 (2010).
 Val baker, A.K.F, Haramein, N. and Alirol, O. The Electron and the Holographic Mass Solution, Physics Essays, Vol 32, Pages 255-262 (2019).
 Haramein, N & Val Baker, A. K. F. Resolving the Vacuum Catastrophe: A Generalized Holographic Approach, Journal of High Energy Physics, Gravitation and Cosmology, Vol.05 No.02, Article ID:91083, 13 pages (2019).