Is There A Fifth Force Of Nature?
In the quest to understand the fundamental forces that govern our universe, the Standard Model of particle physics has long stood as the cornerstone, explaining the electromagnetic, strong, and weak interactions among observable particles. This model describes three of the four known forces: the electromagnetic force, which only governs the behavior of charged particles; the strong force, which glues subatomic particles together; and the weak force, which dictates radioactivity and decay.
Yet, the Standard Model intriguingly omits gravity, the weakest of the four and the one that applies universally to all objects in the universe. This omission paves the way for theories like string theory and fuels the pursuit of a more unified framework, potentially indicating gaps in our understanding and hinting at undiscovered forces.
Recent experimental discrepancies, such as those revealed in Fermilab’s Muon g-2 experiment, have stirred the physics community, suggesting that the muon’s behavior under magnetic fields might not fully align with Standard Model predictions. This anomaly raises profound questions about the possible existence of a fifth force of nature, one not accounted for by current theories.
The Standard Model and Its Limitations
Particle physics is underpinned by what is referred to as the Standard Model, which governs electromagnetic, strong, and weak interactions for all of the observed particles in our universe. In a race of theoretical and experimental discoveries, the Model was assembled starting from the early fifties to the late seventies of the 20th century.
However, gravity remains the missing link in this analysis, although there exists more complicated completions of the Standard Model such as string theory which necessitate gravitation.
The Standard Model is built under the assumption of symmetries, which are rules that dictate the behavior of one force. These symmetries help to explain the interactions and transformations of particles under the three forces. There are gaps that the Standard Model cannot fill, such as the behavior of dark matter and anomalies in particle decays. A natural way of extending the Standard Model is by fitting these symmetries into a larger one. This is the impetus of the effort to find Grand Unified Theories which unify the three forces of the Standard Model into one.
The forces, including gravity, are distinguished by four main categories: strength, range, particles that mediate the interactions, and the particles on which they act.
A Table Detailing the Four Fundamental Forces: Their Strength, Range, Mediators, and Target Particles
| Force | Strength (logarithmic scale where gravity is normalized to one) | Range in meters | Mediator | Particles acted on |
|---|---|---|---|---|
| Gravity | 1 | Infinite | Graviton | Everything |
| Electromagnetism | 1036 | Infinite | Photon | Charged particles |
| Strong | 1038 | 10-15 | W and Z bosons | Quarks and gluons |
| Weak | 1025 | 10-18 | Gluons | Quarks and leptons |
Quarks make up the proton and neutron, and gluons hold them together. Electrons and muons (which is a heavier cousin of the former) are examples of leptons that do not participate in the strong force.
The Muon g-2 Experiment
The muon is a heavier version of the electron. Quantum electrodynamics, which is one part of the Standard Model that has been matched to experiment to a significant degree, predicts that its magnetic moment is very close to two. Magnetic moment dictates how the muon aligns when subjected to a magnetic field. Recently, Fermilab, a United States national laboratory, showed that the muon's magnetic moment does not agree with the theory. This has sent the community of physicists on a hunt to patch up the Standard Model and served as a motivator for Beyond the Standard Model physics.
Why is this quantity and its discrepancy with theory related to the potential existence of a fifth force of nature? Supposing that the muon is subject to some other interactions that the Standard Model is blind to, then it can potentially receive corrections to its magnetic moment, thereby fixing the mismatch with the experiment.
The status of this is unclear and has opened up many pathways of moving forward. One potential candidate is the dark sectors. These are parts of the extension of the Standard Model that do not interact with all particles but only the muon. Dark sectors work in two main ways: either the interactions are governed by gravity or through the introduction of new particles that mediate exotic interactions that are not part of the conventional Standard Model. Examples of the latter include dark photons which only interact electromagnetically, sterile neutrinos which only interact gravitationally, and the axion which only interacts via the strong force. While this seems ad-hoc, it remains a theoretical possibility which has not yet been ruled out.
Hints of a Fifth Force
Where could the potential fifth force of nature be coming from? As we saw above, the muon problem hints at the possibility of this force. However, it is not the only phenomenon that suggests the existence of an additional force. There are two other scientific observations and experimental results that also point towards this intriguing possibility:
Dark Matter and Dark Energy
One avenue for the emergence of the fifth force comes from dark matter and dark energy, which are responsible for the majority of the makeup of the universe and its expansion, respectively. These objects continue to puzzle scientists; if dark matter interacts through a fifth force within a dark sector, it might explain why it doesn’t interact electromagnetically, while a fifth force tied to dark energy could help elucidate the accelerated expansion of the universe. An explanation for the overabundance of dark matter relative to regular matter could be a fifth force which produces the former and thus interacts differently with the regular matter we observe in our universe.
The X17 Particle
Another comes from the mysterious X17 particle observed in an experiment by Hungarian physicists, where unusual particle decay patterns indicated the possible existence of a new light particle that could mediate a force beyond the four known ones. The X17 particle functions much like the photon except that it has a mass. Typically, such particles mediate a force just as the photon mediates electromagnentic interactions. However, due to its mass it is expected to govern a short-ranged force. Scientists hypothesize that the force mediated by particle X17 is a fifth force which interacts weakly with matter, which may explain why it took so long to detect it.
Together, these hints paint an exciting picture of new physics and inspire ongoing experiments aimed at uncovering potential interactions that go beyond current models.
