The question of whether the proton—one of the fundamental building blocks of matter—decays has fascinated physicists for decades. Early predictions based on Grand Unified Theories (GUTs) in the 1970s and 1980s proposed that protons might eventually decay, albeit at an extraordinarily slow rate. However, experimental findings have put strict constraints on the proton’s lifetime, estimating that it must exceed 10^34 years—an unfathomably long time, far longer than the age of the universe itself. Despite these findings, new research is now emerging that revisits this long-standing question with a fresh perspective: could the proton decay at a different rate in other regions of space or at different times in the universe’s history?
Rethinking Proton Decay: New Questions and Hypotheses
In a recent study published in Physical Review D, two physicists, Peter Denton and Hooman Davoudiasl from Brookhaven National Laboratory, set out to investigate whether proton decay could vary depending on the location or the era in which it occurs. They proposed that under certain conditions, the proton could decay much faster than previously thought—potentially on a timescale of around 10^18 years, which is still extremely long but much shorter than the previously accepted limit. Their work is not just a theoretical exercise; it opens up new avenues for understanding the fundamental nature of matter and the forces that govern the universe.
Denton and Davoudiasl’s research challenges the assumption that the proton’s stability is uniform across time and space. They suggest that while Earth-based experiments have established the proton as incredibly stable, we cannot be certain that the same holds true everywhere in the cosmos or at all points in the universe’s past. Their work is particularly focused on understanding whether the proton’s lifetime might have been different in the distant past or whether it could decay more rapidly in different regions of space. Could the heat generated by proton decay affect cosmic phenomena such as the Earth’s iron core or neutron stars? And could such heat production provide clues to the proton’s actual lifetime?
The Proton Decay Channel: The Role of Dark Fermions
To explore these questions, Denton and Davoudiasl considered different possible decay modes for the proton. In particular, they focused on one potential decay channel in which a proton could decay into a positive pion (a type of meson) and a “dark fermion.” Here, the term “dark” refers to a particle that interacts very weakly with the ordinary matter we are familiar with—i.e., it does not participate in the electromagnetic force. If this dark fermion is less massive than the proton minus the mass of the pion, proton decay could happen far more rapidly than current experimental results suggest.
This hypothesis turns on the idea that proton decay, if it occurs, might release energy in the form of heat. If this heat were to accumulate over time, it could have observable effects on cosmic objects. For example, one place where this energy could show up is in the Earth’s solid iron core. Over the past billion years, the core has solidified, and if proton decay were occurring at the rates suggested by the researchers, this heat production would have been enough to prevent the core from solidifying entirely. In other words, if the proton’s lifetime were shorter than 10^34 years, the heat from proton decay would have been significant enough to keep the Earth’s core molten. Since we know the Earth’s core is now solid, this suggests that the proton’s lifetime must exceed a certain threshold.
Proton Decay and the Earth’s Core
The authors calculated that, based on current geological understanding, if protons decayed at the rate they are proposing, the heat generated by this decay would have been roughly four times the current amount radiated by the Earth’s inner core. The inner core, which radiates about 10^13 watts as a blackbody at a temperature of 6,200 K, would have been molten if the proton decay rate were too high. As a result, the researchers concluded that the proton’s lifetime must be at least 2 × 10^18 years to avoid heating the Earth’s core beyond its current state.
The idea that proton decay could impact the Earth’s internal processes is intriguing, but Denton and Davoudiasl didn’t stop there. They expanded their investigation to other cosmic objects, such as neutron stars, to see if proton decay could help explain certain observed phenomena.
Proton Decay and Neutron Stars
Neutron stars are some of the densest objects in the universe, formed when massive stars collapse under their own gravity after running out of fuel. Denton and Davoudiasl turned their attention to a particular neutron star known as PSR J2144–3933, the coldest known pulsar, which has a temperature of less than 42,000 K and is roughly 0.3 billion years old. This neutron star, composed predominantly of neutrons, also contains a substantial number of protons.
Using the known properties of neutron stars—such as their mass, temperature, and composition—the authors calculated the proton decay rate that would be required to account for heat generation within the star. They found that if proton decay were occurring at a faster rate than previously assumed, it could contribute to the star’s thermal emission. Based on these calculations, they concluded that the proton’s lifetime would need to be greater than 1.5 × 10^18 years—again, close to the lower limit inferred from the behavior of the Earth’s core.
Cosmic Constraints on Proton Decay
The researchers didn’t limit their investigation to just two objects in the universe. They also considered other potential sources of evidence that could help constrain the proton’s lifetime. One such source is the Cosmic Microwave Background (CMB), the faint radiation left over from the Big Bang. By examining the CMB and its relationship to dark matter, they derived an additional constraint, suggesting that the proton’s lifetime must be greater than 2 × 10^17 years.
Further constraints could potentially come from the study of natural gas on Earth, particularly the ratio of carbon isotopes found in the gas. The ratio of carbon-14 to carbon-12 in natural gas is extremely low, and if proton decay were occurring more frequently in the past, it could have contributed to an increase in the amount of carbon-14 in the Earth’s atmosphere over geological timescales. This could provide an additional lower bound on the proton’s lifetime, estimated at around 10^19 years.
The Role of Hypothetical Forces in Proton Decay
One of the more speculative aspects of Denton and Davoudiasl’s work involves the possibility of a long-range force that could influence the proton’s lifetime. This hypothetical force could cause variations in the proton’s stability depending on its location in the universe or its proximity to certain cosmic phenomena. For example, as the Sun orbits the center of the Milky Way, it could encounter regions where this long-range force is more prominent, potentially shortening the proton’s lifetime in those areas.
While this idea is still very much in the realm of theoretical speculation, it highlights the potential for new forces or interactions that might influence proton decay in unexpected ways. Such forces could vary over time and space, leading to fluctuations in the proton’s lifetime.
The Future of Proton Decay Research
Denton and Davoudiasl’s work challenges our conventional understanding of proton decay by suggesting that the proton’s lifetime may not be constant across time and space. This opens up a host of intriguing possibilities for future research, both in the lab and in cosmic observations. It also raises the exciting prospect that experiments conducted on distant planets or in different epochs of the universe could reveal new insights into the stability of matter itself.
As technology advances, particularly in the fields of astrophysics and particle physics, we may be able to observe proton decay more directly, test these new hypotheses, and explore whether the fundamental forces of the universe are as immutable as we once thought—or whether they are, in fact, more variable than we ever imagined.
The question of whether protons decay remains one of the great mysteries of modern physics. While current experiments have set stringent limits on its lifetime, new theoretical work continues to push the boundaries of what we know. As we look further into space and deeper into the past, the discovery of proton decay—if it happens—could unlock profound new understandings of the universe’s most fundamental laws.
Reference: Hooman Davoudiasl et al, How fast can protons decay? Physical Review D (2025). DOI: 10.1103/PhysRevD.111.035026