At Aalto University in Finland, two physicists believe they may have cracked a puzzle that has defied generations of scientists: how to reconcile the physics of the very big with the physics of the very small.
Mikko Partanen and Jukka Tulkki have developed a new theoretical framework that places gravity — the most elusive of the four fundamental forces — squarely within the mathematical structure of quantum field theory. This new paper proposes what they call “unified gravity,” a gauge theory that treats gravity on the same footing as the other three forces described in the Standard Model.
It’s not quite the long-sought Theory of Everything. But it is something far more tangible than the lofty name implies: a testable, renormalizable theory that blends gravity with quantum mechanics, potentially solving some of the most annoying discrepancies in physics.
“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,” Partanen said in a press release.
Gravity + Quantum Mechanics
Gravity, as Einstein described it, is a warping of space and time. This interpretation works extraordinarily well on large scales — from falling apples to orbiting planets to the motion of galaxies.
Quantum mechanics, on the other hand, governs the subatomic world, where particles flit and flicker through probability clouds. Quantum field theory, which merges quantum mechanics with special relativity, provides the backbone of the Standard Model. It has passed every experimental test so far.
But the two don’t mix. The mathematics of general relativity breaks down at the infinitesimal scales where quantum effects dominate. And efforts to plug gravity into the Standard Model — including string theory and loop quantum gravity — have largely failed to produce experimentally verifiable predictions.
That, in essence, is the problem that Partanen and Tulkki have tried to address.
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Gauge Gravity
The latest breakthrough came not from a new particle or force, but from a new symmetry.
The Standard Model is built on “gauge theories,” mathematical structures where particles interact via force-carrying fields — like photons for electromagnetism. These gauge theories hinge on specific symmetries, such as the U(1), SU(2), and SU(3) groups, that dictate how fields transform under certain operations.
Partanen and Tulkki’s insight was to describe gravity using a similar gauge structure — one involving four one-dimensional unitary symmetries (U(1)×U(1)×U(1)×U(1)), rather than the infinite-dimensional symmetries of spacetime found in general relativity.
“Instead of basing the theory on the very different kind of spacetime symmetry of general relativity,” Partanen explains, “the main idea is to have a gravity gauge theory with a symmetry that is similar to the Standard Model symmetries.”
To do this, they introduced a novel mathematical object they call the space-time dimension field. This field acts as a kind of bridge, allowing gravity to be treated like the other quantum forces — a symmetry-based interaction mediated by a gauge field.
The resulting theory, unified gravity, manages to describe gravitational interactions in flat space — specifically, in the Minkowski metric used by special relativity — without the need for spacetime curvature. Yet, with a tweak in gauge choice, it can reproduce the equations of general relativity exactly.
“So when we have particles which have energy,” says Tulkki, “the interactions they have just because they have energy would happen through the gravitational field.”
But perhaps most important, the new theory appears to be renormalizable — at least at one-loop order.
Renormalization refers to a mathematical procedure in quantum field theory. It allows physicists to tame the infinite results that pop out of their equations when calculating how particles interact. The gauge theories of the Standard Model are renormalizable. Gravity, in its usual quantum formulations, is not.
Partanen and Tulkki claim their approach sidesteps this problem. In unified gravity, the infinities can be absorbed into a finite number of parameters — just as in quantum electrodynamics or quantum chromodynamics.
The authors emphasize that they’ve only completed the proof for first-order terms. Higher-order loop corrections remain untested. But they are optimistic.
“We still have to make a complete proof, but we believe it’s very likely we’ll succeed,” says Tulkki.
What’s Next?
The work is theoretical and highly mathematical. There are no predictions yet that could be tested in a lab. But the implications — if the theory holds up — are profound.
A quantum theory of gravity is essential to understand what happens inside black holes, or what happened in the first moments after the Big Bang. Without such a theory, those extremes of the cosmos cannot be explained with high confidence.
And while this research won’t produce a new app for your phone, it does underscore how fundamental physics underlies modern life. GPS, for instance, relies on Einstein’s general relativity. Quantum mechanics powers everything from transistors to lasers.
If this new theory pans out, it could lay the groundwork for future generations of physics and technology.
“Like quantum mechanics and the theory of relativity before it, we hope our theory will open countless avenues for scientists to explore,” Partanen concludes.
For now, the authors are inviting the wider scientific community to scrutinize their model, explore its consequences, and try to extend it. If unified gravity withstands the coming tests, it could mark the next great unification in physics.
The findings appeared in the journal Reports on Progress in Physics.
