Researchers at Aalto University have made a significant stride by developing a novel quantum theory of gravity. This new framework describes gravity in a manner that is consistent with the Standard Model of particle physics, potentially leading to a more profound understanding of the universe's origins. Although theoretical physics might seem distant from practical technology, such fundamental discoveries are crucial. The study was published in Reports on Progress in Physics.
The gravity quantum field is calculated in flat spacetime. The curved classical metric is calculated using the expectation value of the gravity quantum field. Image Credit: Mikko Partanen and Jukka Tulkki / Aalto University.
The long-sought-after unified theory, aiming to integrate gravity with the other fundamental forces (electromagnetism, and the strong and weak nuclear forces) is now closer to being achieved. Generations of physicists have strived to accomplish this, facing the challenge of reconciling the inherent incompatibility between two foundational pillars of modern physics: quantum field theory and Einstein's theory of gravity. For instance, the GPS functionality in smartphones relies on Einstein's theory of gravity.
Mikko Partanen and Jukka Tulkki detail their groundbreaking new theory in the study. Lead author Partanen anticipates that this research will lead to significant breakthroughs in understanding within the next few years.
If this turns out to lead to a complete quantum field theory of gravity, then eventually it will give answers to the very difficult problems of understanding singularities in black holes and the Big Bang.
Mikko Partanen, Postdoctoral Researcher and Study Lead Author, Aalto University
“A theory that coherently describes all fundamental forces of nature is often called the Theory of Everything. 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,” said Partanen.
Reconciling the Irreconcilable
The crucial step was to formulate a description of gravity within an appropriate gauge theory—a type of theory where particles interact via a field.
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. So when we have particles which have energy, the interactions they have just because they have energy would happen through the gravitational field.
Jukka Tulkki, Professor Emeritus, Aalto University
A persistent challenge for physicists has been to develop a gauge theory of gravity that aligns with the gauge theories describing the other three fundamental forces: electromagnetism, the weak nuclear force, and the strong nuclear force. The Standard Model of particle physics, a gauge theory itself, successfully describes these three forces and possesses specific symmetries.
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.
Mikko Partanen, Postdoctoral Researcher and Study Lead Author, Aalto University
The absence of such a unified theory prevents physicists from reconciling the two most powerful frameworks: quantum field theory and general relativity. Quantum theory elucidates the world of the very small, minuscule particles interacting probabilistically, while general relativity describes the macroscopic world of everyday objects and their gravitational interactions. These are descriptions of the universe from distinct viewpoints, and both theories have been validated with remarkable accuracy, yet they remain incompatible.
Moreover, due to the weakness of gravitational interactions, greater precision is required to investigate genuine quantum gravity effects that go beyond general relativity, which is inherently a classical theory.
“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.
These extreme conditions prevail around black holes and in the universe's earliest moments, immediately following the Big Bang—environments where current physics theories cease to be effective.
Driven by a long-standing fascination with fundamental physics questions, Partanen discovered a novel symmetry-based approach to the theory of gravity and, together with Tulkki, further developed this concept. Their resulting work holds significant promise for ushering in an entirely new era of scientific understanding, much like how the comprehension of gravity ultimately paved the way for technologies like GPS.
Open Invite to the Scientific Community
While acknowledging the promise of their theory, the researchers emphasize that its formal proof is still underway. Their approach employs a mathematical technique called renormalization, which is used to manage the infinities that arise in the calculations. Partanen and Tulkki have currently demonstrated the effectiveness of this technique up to a specific level, for what are known as 'first order' terms, but they still need to confirm that these infinities can be consistently eliminated throughout the complete calculation.
“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. We still have to make a complete proof, but we believe it’s very likely we’ll succeed,” explained Tulkki.
Partanen agrees. He acknowledges there are still challenges ahead, but with time and effort, he believes they can be overcome. “I can’t say when, but I can say we’ll know much more about that in a few years.”
The team has decided to publish the theory in its current state to allow the broader scientific community to become acquainted with it, verify their findings, contribute to its further development, and use it as a foundation for future research.
“Like quantum mechanics and the theory of relativity before it, we hope our theory will open countless avenues for scientists to explore,” concluded Partanen.
Journal Reference:
Partanen, M., & Tulkki, J., (2025) Gravity generated by four one-dimensional unitary gauge symmetries and the Standard Model. Reports on Progress in Physics. doi/10.1088/1361-6633/adc82e