At long last, a unified theory combining gravity with the other fundamental forces—electromagnetism and the strong and weak nuclear forces—is within reach. Bringing gravity into the fold has been the goal of generations of physicists, who have struggled to reconcile the incompatibility of two cornerstones of modern physics: quantum field theory and Einstein's theory of gravity.
Researchers at Aalto University have developed a new quantum theory of gravity which describes gravity in a way that's compatible with the Standard Model of particle physics, opening the door to an improved understanding of how the universe began. While the world of theoretical physics may seem remote from applicable tech, the findings are remarkable. Modern technology is built on such fundamental advances — for example, the GPS in your smartphone works thanks to Einstein's theory of gravity.
Mikko Partanen and Jukka Tulkki describe their new theory in a paper just published in Reports on Progress in Physics. Lead author Partanen expects that within a few years, the findings will have unlocked critical understanding.
'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,' he says.
'A theory that coherently describes all fundamental forces of nature is often called the Theory of Everything,' says Partanen, although he doesn't like to use the term himself. '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.'
Reconciling the irreconcilable
The key was finding a way to describe gravity in a suitable gauge theory — a kind of theory in which particles interact with each other through 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,' explains Tulkki. 'So when we have particles which have energy, the interactions they have just because they have energy would happen through the gravitational field.'
A challenge long facing physicists is finding a gauge theory of gravity that is compatible with the gauge theories of the other three fundamental forces — the electromagnetic force, the weak nuclear force and the strong nuclear force. The Standard Model of particle physics is a gauge theory which describes those three forces, and it has certain 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,' says Partanen, the study's lead author.
Without such a theory, physicists cannot reconcile our two most powerful theories, quantum field theory and general relativity. Quantum theory describes the world of the very small—tiny particles interacting in probabilistic ways—while general relativity describes the chunkier world of familiar objects and their gravitational interaction. They are descriptions of our universe from different perspectives, and both theories have been confirmed to extraordinary precision—yet they are incompatible with each other. Furthermore, because gravitational interactions are weak, more precision is needed to study true quantum gravity effects beyond general relativity, which is 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,' says Partanen. Those are the conditions around black holes and in the very early universe, just after the Big Bang—areas where existing theories in physics stop working.
Always fascinated with the very big questions of physics, he discovered a new symmetry-based approach to the theory of gravity and began to develop the idea further with Tulkki. The resulting work has great potential to unlock a whole new era of scientific understanding, in much the same way as understanding gravity paved the way to eventually creating GPS.
Open invite to the scientific community
Although the theory is promising, the duo point out that they have not yet completed its proof. The theory uses a technical procedure known as renormalization, a mathematical way of dealing with infinities that show up in the calculations. So far Partanen and Tulkki have shown that this works up to a certain point—for so-called 'first order' terms—but they need to make sure the infinities can be eliminated throughout the entire 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,' explains Tulkki. 'We still have to make a complete proof, but we believe it's very likely we'll succeed.'
Partanen concurs. There are still challenges ahead, he says, but with time and effort he expects they'll be overcome. 'I can't say when, but I can say we'll know much more about that in a few years.'
For now, they've published the theory as it stands, so that the rest of the scientific community can become familiar with it, check its results, help develop it further, and build on it.
'Like quantum mechanics and the theory of relativity before it, we hope our theory will open countless avenues for scientists to explore,' Partanen concludes.
