For over a century, the search for particles with pure magnetic charge, called magnetic monopoles, has fascinated and intrigued the scientific community. Unlike common magnets, which necessarily have two poles, north and south, these hypothetical particles would exhibit a single magnetic pole, thus offering a unique point-like magnetic charge. This singularity raises fundamental issues in physics, particularly regarding the symmetry of the equations describing electromagnetic fields and the unification of fundamental forces. In 2025, despite major experimental advances, their direct observation remains elusive, yet they inform theoretical models and high-energy experiments like ATLAS at the Large Hadron Collider.
Magnetic monopoles are not just a theoretical fantasy; they embody a profound ambition of particle physics: to understand the very nature of charges and forces. Since Paul Dirac’s demonstration in 1931, which linked the existence of these particles to the quantization of electric charge, to the integration of them into modern unification theories of interactions, the quest, though unsuccessful, has renewed the perspective on magnetism and electromagnetism. It also illustrates the delicate symmetry that science seeks to attain between electrons and monopoles, between electric and magnetic fields.
Recent work by the ATLAS collaboration, analyzing countless data from high-energy collisions at the LHC, imposes strict constraints on the mass and production of monopoles. This research pushes the boundaries and directs future experiments towards uncharted horizons. Their innovative method, based on the extreme ionization that these particles would induce on detectors, helps progressively eliminate many hypotheses while refining the understanding of interactions related to pure magnetism. Thus, magnetic monopoles remain a captivating mystery, at the crossroads of theoretical and experimental physics.
In short:
- Magnetic monopole: hypothetical particle possessing a single magnetic pole, unlike classic dipolar magnets.
- The physicist Paul Dirac formalized in 1931 the possibility of these particles within the framework of quantum theory, linking their existence to the quantization of electric charges.
- ATLAS experiments at the LHC in 2025 examine proton-proton collisions to detect monopoles up to 4 tera-electronvolts, with no positive results to date.
- The presence of a monopole would explain the symmetry of Maxwell’s electromagnetism equations and provide keys to the unification of fundamental forces.
- Quasiparticles resembling magnetic monopoles have been observed in solid-state physics, creating new perspectives, particularly for spintronics and quantum computing.
Theoretical foundations of magnetic monopoles in particle physics
The notion of magnetic monopole fits within particle physics as an exciting hypothesis capable of overturning the current understanding of magnetism and electromagnetism. Originally, Maxwell’s equations governing these phenomena showed a fundamental asymmetry: the electric field is generated by point-like charges, which leads to a non-zero divergence, while the magnetic field theoretically never has point-like sources, maintaining a strictly zero divergence. This latter aspect reflects the absence of monopole magnets in nature as perceived in everyday life.
It was in 1931 that Paul Dirac introduced the idea that the possible existence of magnetic monopoles would naturally explain the observed quantization of electric charges. This hypothesis remarkably has the advantage of rebalancing the symmetry between electric and magnetic fields in fundamental equations. By associating a magnetic charge with each monopole, Dirac demonstrated that the quantization condition compels the elementary electric charge to be an inversely proportional fraction of this magnetic charge. Practically, this means that the discovery of a single monopole particle would suffice to justify why all observed electric charges are multiples of an elementary charge.
Beyond this quantum framework, several advanced theories seek to integrate magnetic monopoles into models of unification of fundamental forces. These historical theories, emerging in the 1970s and beyond, such as grand unification models (GUT), predict the existence of these particles at extremely high energies, often inaccessible to current experiments. Joseph Polchinski, a major figure in modern theoretical physics, has thus described the existence of magnetic monopoles as “one of the safest bets” on physics beyond the Standard Model, despite their lack of experimental observation so far.
This context highlights the importance of experimental research, particularly through high-energy experiments like those conducted at the LHC. Indeed, the detection of particles with pure magnetic charge would not only question the structure of electromagnetic interactions but also the fundamental mechanisms governing matter at its most elementary level. It would also pave the way for modifications of current theories and the integration of a new symmetry known as electric-magnetic duality.
Modern experimental research and constraints imposed by the ATLAS experiment
The ATLAS experiment, located at the Large Hadron Collider (LHC), currently represents the most advanced method for detecting magnetic monopoles produced in very high energy collisions. The principle relies on the ability of these hypothetical particles, carrying a magnetic charge equivalent to 68.5 times the elementary electric charge, to strongly ionize the matter they pass through. These ionizing traces are detected via sophisticated systems like the transition radiation tracker and the liquid argon electromagnetic calorimeter.
ATLAS physicists have focused their research on pairs of magnetic monopoles, exploring a mass range of up to 4 tera-electronvolts (TeV) and magnetic charges of 1gD and 2gD (gD representing Dirac’s magnetic charge). The production mechanisms studied include production via the Drell-Yan process or photon fusion, both generating these monopoles during proton-proton collisions at 13 TeV.
The collection and analysis of data from the second operational period of the LHC (2015-2018) have established very strict limits. Indeed, no direct detection or trace compatible with a magnetic monopole has been observed. This absence of expert evidence, however, serves to strengthen constraints on the production rate and the minimum possible mass of these particles, thereby contributing to refining theoretical scenarios.
Meanwhile, research extends to high electric charge objects (HECO), with charges ranging from 20e to 100e, thus deepening the potential scope of discoveries. The ATLAS detector offers remarkable sensitivity within the global scientific community in this field, surpassing other experiments such as MoEDAL-MAPP, which are more compact but take complementary approaches, particularly for finite-sized monopoles.
The quest continues with renewed attention to the third operational period of the LHC, through the refinement of techniques and the development of strategies aimed at uncovering these particles with atypical properties. These efforts illustrate how the detection of magnetic monopoles would constitute a major advancement, shaking our certainties in particle physics while providing explanations for the mysteries related to charge and magnetism.
Symmetry and duality in electromagnetism: implications of the existence of monopoles
The classical framework of electromagnetism, codified in Maxwell’s equations, presents an intrinsic asymmetry between electricity and magnetism. The observed non-existence of magnetic monopoles explains why the magnetic field always has a zero divergence, unlike the electric field. Were we to admit the existence of monopoles, these equations would regain a perfectly symmetric form, known as electric-magnetic duality.
This perfect symmetry would have profound repercussions on the understanding of fields and interactions. On one hand, the introduction of magnetic charges would allow for direct sources for both electric and magnetic fields, modifying the standard terms of the equations. On the other hand, this would also predict the existence of magnetic currents associated with these charges, a concept absent from classical formalism.
The theoretical implications far exceed mere mathematical elegance. Indeed, the presence of monopoles would support theories proposed to unite the electromagnetic force with the strong nuclear, weak nuclear, and gravitational forces. By bringing greater coherence to the observed phenomena, this could represent a decisive step towards a unification of forces in modern physics.
A 2013 paper renewed the reflection on the zero divergence of the magnetic field, associating this property with the total conservation of momentum in electromagnetic systems with massive particles. This perspective also opens the door to a crucial question: if zero divergence is a condition resulting from conservation symmetries, the possibility that these symmetries are locally broken or modified by the existence of monopoles becomes conceivable.
In light of these considerations, the hypothesis of magnetic monopoles appears not only compatible with current knowledge but also drives active research at the interface between experiments, theoretical physics, and new detection technologies.
Indirect observation: magnetic monopoles in solid-state physics and technological advances
While direct detection of elementary monopoles remains a challenge, solid-state physics offered in 2009 a form of unprecedented observation of quasiparticles mimicking certain properties of magnetic monopoles. By artificially recreating synthetic monopoles in specific crystals such as holmium and dysprosium titanate, researchers have been able to observe analogous phenomena, notably finite Dirac string ends, essential elements in describing these quasiparticles.
These discoveries, though limited to a microscopic framework, generate considerable excitement due to their potential applications. Analyses revealed the presence of associated magnetic currents, suggesting a new way to manipulate magnetism in materials. This phenomenon will likely fuel advances in microelectronics, particularly in spintronics, where control over electronic spin opens new resources for quantum information and processing.
However, it is crucial to differentiate these quasiparticles from the notion of elementary monopole arising from particle physics. These objects do not possess a point-like elementary magnetic charge but exhibit comparable behaviors under controlled conditions, illustrating the extraordinary capacity of condensed matter to simulate complex quantum phenomena.
The emergence of these results has encouraged the continuation of fundamental research on magnetism while establishing tangible perspectives for the development of quantum technologies, particularly quantum computing, where the management of new forms of pure magnetism could play a decisive role in the future.
Quiz: Magnetic Monopoles
Test your knowledge of magnetic monopoles: particles with pure magnetic charge.
Key points on the search for magnetic monopoles in 2025
- Essential characteristic: monopoles are particles with a unique point-like magnetic charge, unlike classical dipolar magnets.
- Experimental constraints: limits imposed by ATLAS exclude the production of magnetic monopoles with mass below 4 TeV in 13 TeV collisions for certain charges.
- Production mechanisms: via the Drell-Yan process and photon fusion in high-energy collisions at the LHC.
- Fundamental theoretical role: the existence of monopoles explains the quantization of electric charges and offers perfect symmetry to Maxwell’s equations.
- Indirect observation: solid-state physics reproduces certain properties in quasiparticles, enriching practical understanding of magnetism and technological applications.
- Future perspectives: revitalization of experimental research with future operational phases of the LHC and the development of increasingly sensitive detectors.
| Aspect | Description | Importance in physics |
|---|---|---|
| Magnetic charge 1gD | 68.5 times the elementary electric charge | Basis for the quantization of charges |
| Maximal mass sought | ~4 tera-electronvolts | Limit of current LHC capabilities |
| Production mechanisms | Drell-Yan and photon fusion | Key processes for generation in collision |
| Detectors used | Transition radiation tracker, liquid argon electromagnetic calorimeter | Allow detection of intense ionizations |
| Experimental limits | Absence of detection on data from 2015-2018 | Strongly constrains theoretical models |
What is a magnetic monopole?
A magnetic monopole is a hypothetical particle that possesses a unique point-like magnetic charge, meaning one north or south pole, unlike usual magnets that have two poles.
Why is the search for magnetic monopoles important?
Because it would help explain the quantization of electric charges and lead to a perfect symmetry of Maxwell’s equations, thereby contributing to the unification of fundamental forces.
What methods are used to detect a magnetic monopole?
Experiments use detectors capable of identifying massive ionization in matter, notably the transition radiation tracker and the electromagnetic calorimeter, by observing high-energy collisions produced in accelerators such as the LHC.
Have there been concrete observations of magnetic monopoles?
No elementary monopole particle has been detected so far, but some quasiparticles in crystalline materials have shown similar properties to magnetic monopoles.
What are the future perspectives for research on magnetic monopoles?
Future operational periods of the LHC and the refinement of detectors will allow exploration of wider energy domains and refinement of the limits of magnetic monopole production.