The biggest challenge in theoretical physics for more than a century has been to develop a unified theory that combines gravity and the other three fundamental forces of the Universe: the strong and weak nuclear forces as well as electromagnetism. Physicists have tried but failed to unite two apparently incompatible pillars of contemporary science: the theory of Albert Einstein’s general relativity and quantum field theory.

Now, a new breakthrough by researchers at Aalto University in Finland could mark a turning point in this quest. In a recently published article in the journal Reports on Progress in Physics, scientists Mikko Partanen and Jukka Tulkki present a new quantum theory of gravity that is formulated in a way that is compatible with the Standard Model of particle physics. This proposal opens the door to a more complete understanding of the origins of the universe and extreme phenomena such as black holes.

According to Partanen, the study’s lead author, it is possible that within a few years the foundations of this theory will make it possible to unravel critical questions that have so far challenged even the brightest minds in contemporary physics. If this theoretical framework becomes consolidated as a complete quantum field theory for gravity, it could offer answers to such complex enigmas as the nature of the singularities present in black holes or the conditions of the universe in the immediate moments following the Big Bang.

Although Partanen prefers to avoid the term, this type of theory that aims to coherently describe all the fundamental forces of nature is often referred to as a Theory of Everything.

A new way of understanding gravity

The Finnish researchers’ main breakthrough lies in having conceived a formulation of gravity as a gauge theory—a type of theory in which particles interact with each other through a field—similar to those used to describe the other three fundamental forces. As Jukka Tulkki explains, the most familiar gauge field is the electromagnetic field, which mediates interactions between particles with electric charge. In the same way, if we consider particles that possess energy, the interactions that arise due to that energy should take place through the gravitational field.

As opposed to using the symmetries of the spacetime found in general relativity, the new theory involves a symmetry closer to that of the Standard Model as it enables gravity to be included in a consistent quantum conceptual framework and bridges the gap between two incompatible but both correct visions of the universe: the vision of quantum physics in terms of a world of infinitesimal particles and probabilistic phenomena and the vision of general relativity that describes the motion of macro bodies in the presence of gravity.

Both have been verified in many experiments to a surprising accuracy but have stayed incompatible in their mathematical and conceptual structures.

Since gravitational interaction is considerably weaker than the other forces, detecting quantum effects in gravity requires extreme conditions of energy and precision. It is in these scenarios—such as those found around black holes or during the early moments of the universe—where a quantum theory of gravity becomes indispensable.

Partanen, long fascinated by the great questions of physics, developed along with Tulkki a new approach based on symmetries that could open a new era of scientific understanding, much like the theory of general relativity laid the groundwork for technological applications such as the Global Positioning System (GPS), now ubiquitous in our mobile phones.

SOURCES

Aalto University

Mikko Partanen and Jukka Tulkki, Gravity generated by four one-dimensional unitary gauge symmetries and the Standard Model. Rep. Prog. Phys. 88 057802. DOI 10.1088/1361-6633/adc82e

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