A long-elusive “theory of everything,” also known as a unified field theory, may finally be within reach as Finnish researchers at Aalto University unveil a new quantum theory of gravity that is compatible with the standard model of particle physics.
Physicists have long struggled to understand how our universe fits together due to discrepancies between quantum field theory and Einstein’s theory of relativity—two foundational pillars of modern physics that each work well independently, but fail to align.
Modern technologies, from airplanes in flight to the GPS systems that track them, rely heavily on physics, and could be significantly advanced if the relationship between quantum mechanics and gravity were finally understood.
Producing a Theory of Everything
“A theory that coherently describes all fundamental forces of nature is often called the Theory of Everything,” said lead author Mikko Partanen. “Some fundamental questions of physics still remain unanswered. For example, the present theories do not yet explain why there is more matter than antimatter in the observable universe.”
The Finnish researchers had to walk a fine line to develop a theory where particles interact through a field, known as a gauge theory.
“The most familiar gauge field is the electromagnetic field. When electrically charged particles interact with each other, they interact through the electromagnetic field, which is the pertinent gauge field,” explained co-author Jukka Tulkki. “So when we have particles which have energy, the interactions they have just because they have energy would happen through the gravitational field.”
Physicists have long struggled to develop a gauge theory of gravity that integrates with the gauge theories for electromagnetism, the weak nuclear force, and the strong nuclear force. The Standard Model of particle physics unifies the gauge theories describing those three fundamental forces.
“The main idea is to have a gravity gauge theory with a symmetry that is similar to the Standard Model symmetries, instead of basing the theory on the very different kind of spacetime symmetry of general relativity,” said Partanen.
From the Smallest to the Largest
The absence of such a unifying theory has made understanding how the universe behaves at all scales difficult. Quantum theory governs the smallest, probabilistic particle interactions, while general relativity describes large-scale gravitational phenomena. Although both models are independently successful, they remain incompatible, necessitating a quantum theory of gravity to bridge the gap.
“A quantum theory of gravity is needed to understand what kind of phenomena there are in cases where there’s a gravitational field and high energies,” said Partanen.
Such scenarios include black hole singularities and the very early universe, where current models begin to break down.
Partanen and Tulkki’s theory still needs to be fully proven. Having already resolved the first-order terms, they are currently addressing the challenge of renormalization, dealing with the infinities that can arise in quantum theories.
“If renormalization doesn’t work for higher-order terms, you’ll get infinite results. So it’s vital to show that this renormalization continues to work,” explained Tulkki. “We still have to make a complete proof, but we believe it’s very likely we’ll succeed.”
“I can’t say when, but I can say we’ll know much more about that in a few years,” added Partanen.
The researchers have released the current version of their theory to the scientific community for feedback and further development.
“Like quantum mechanics and the theory of relativity before it, we hope our theory will open countless avenues for scientists to explore,” Partanen concluded.
The paper, “Gravity generated by four one-dimensional unitary gauge symmetries and the Standard Model” appeared in National Science Review on April 25, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.