Researchers might be on the brink of solving one of the most significant challenges in physics, potentially laying the groundwork for groundbreaking theories.

At present, two distinct theories—quantum mechanics and gravity—are employed to elucidate various facets of the universe. Numerous attempts have been made to fuse these theories into a cohesive framework, but a compelling unification remains elusive.

“Integrating gravity with quantum theory into a single framework is one of the primary objectives of contemporary theoretical physics,” states Dr. Mikko Partanen, the lead author of the recently published research in Report on Progress in Physics. He elaborates on this innovative approach in the context of BBC Science Focus, calling it “the holy grail of physics.”

The challenge of formulating a theory of “quantum gravity” arises from the fact that these two concepts operate on entirely different scales.

Quantum mechanics investigates the minutest scale of subatomic particles, leading to the development of standard models. These models link three fundamental forces: electromagnetic, strong (which binds protons and neutrons), and weak (responsible for radioactive decay).

The fourth fundamental force, gravity, is articulated by Albert Einstein’s general theory of relativity, which portrays gravity as a curvature of spacetime. Massive objects and high-energy entities distort spacetime, influencing surrounding objects and governing the domain of planets, stars, and galaxies. Yet, gravity seems resistant to aligning with quantum mechanics.

The Duality of Theories

A significant issue is that gravity is rooted in a “deterministic classical” framework, meaning the laws predict specific outcomes. For instance, if you drop a ball, gravity guarantees it will fall.

In contrast, quantum theory is inherently probabilistic, offering only the likelihood of an event rather than a definitive outcome.

“These are challenging to merge,” Partanen comments. “Attempts to apply quantum theory within gravitational contexts have yielded numerous nonsensical results.”

For example, when quantum physicists measure the electron’s mass, the equations spiral into infinity. Similarly, applying gravity in extreme conditions, like at the edge of a black hole, renders Einstein’s equations meaningless.

“While intriguing approaches like string theory [which substitutes particles with vibrating energy strings] exist, we currently lack unique, testable predictions to differentiate these theories from standard models or general relativity,” notes Partanen.

Instead of crafting an entirely new theory for unification, Partanen and his colleague, Professor Jukka Tulkki, approached gravity through the lens of quantum mechanics by reformulating the gravitational equations using fields.

Fields represent how quantum theory elucidates the variation of physical quantities over space and time. You may already be acquainted with electric and magnetic fields.

This novel perspective allowed them to replicate the principles of general relativity in a format that combines effortlessly with quantum mechanics.

Testing the Theories

A particularly promising aspect of this new theory is that it does not require the introduction of exotic new particles or altered physical laws, meaning physicists already possess the necessary tools for its verification.

According to him, this new theory generates equations that account for phenomena like the bending of light around massive galaxies and redshifts—the elongation of light’s wavelength as objects recede in the expanding universe.

While this validates the theory, it does not confirm its correctness.

To establish this, experiments must be conducted in extreme gravitational environments where general relativity falters.

If quantum gravity can make superior predictions in such scenarios, it would serve as a crucial step towards validating this new theory and suggesting that Einstein’s framework might be incomplete.

However, this is challenging due to the minimal differences between the two theories.

For instance, when observing how the sun’s mass bends light from a distant star, the predictive discrepancy is a mere 0.0001%. Current astronomical tools are insufficient for precise measurements.

Fortunately, larger celestial bodies can amplify these differences dramatically.

“For neutron stars with intense gravitational fields, relative differences can reach a few percent,” Partanen observes. While no observatory currently exists to make such observations, advancements in technology could soon enable this.

The theory remains in its nascent stages, with the team embarking on a mission to finalize mathematical proofs to ensure the theory avoids diverging into infinities or other complications.

If progress remains encouraging, they will then apply the theory to extreme situations, such as the singularity of a black hole.

“Our theory represents a novel endeavor to unify all four fundamental forces of nature within one coherent framework, and thorough investigation may unveil phenomena beyond our current understanding,” concludes Partanen.

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About Our Experts

Mikko Partanen is a postdoctoral researcher in the Department of Physics and Nanoengineering at Aalto University in Espoo, Finland. He specializes in studying light and its quantum properties, with his research appearing in journals such as Physics Chronicles, New Journal of Physics, and Scientific Reports.