Recent mathematical methods have paved the way for the theory of asymptotically safe gravity to gain the attention of physicists once more.
To date, our best understanding of everything, which includes the fundamental particles and four forces that govern them, has been summed up by the Standard Model of particle physics. With the passing of time and several experiments, the Standard Model has established itself to become a well-tested physics theory. However, an old theory of everything, the asymptotically safe gravity concept, has once again started gaining traction in the world of modern physics.
While four forces supposedly affect the elementary particles in our universe, the Standard Model only describes three of them: electromagnetic, weak, and strong forces. The theory excludes the gravitational force which, according to German theoretical physicists Sabine Hossenfelder, makes it not only aesthetically unpleasing but a mathematical dilemma as well.
Unlike the three other fundamental forces, gravity is not considered a quantum theory. In essence, particles have both quantum properties and gravitational fields. So, it’s safe to say that a gravitational field has quantum properties like the particles that cause it. However, that is not the case as a comprehensive or even provable theory of quantum gravity has not yet been produced.
Quantization of Gravity
In the quest for a theory of quantum gravity, American theoretical physicists Bryce DeWitt and Richard Feynman tried to quantize gravity by applying the same methods that led to the successful transformation of electromagnetism into quantum electrodynamics. However, when the technique was applied to gravity, it only resulted in a theory plagued by an infinite number of infinities.
Several other theories such as string theory, loop quantum gravity, and causal dynamical triangulation, to name a few, were used by physicists in hopes of finding an approach to quantize gravity. Unfortunately, none of the efforts succeeded. But just recently, despite the failures, a traditional theory of everything resurfaced once again.
The Theory of Asymptotically Safe Gravity
Known as the theory of asymptotically safe gravity, it is an old concept first proposed by theoretical physicists Steven Weinberg in late 1970s. Weinberg, who shares a Nobel Prize in Physics with Sheldon Lee Glashow and Abdus Salam for the unification of the weak force and electromagnetic interaction between elementary particles, supposedly realized that the problems encountered in quantization of gravity were not dead ends.
Meaning, even if it appears that the theory breaks down when extrapolated to high energies, the breakdown might not actually transpire. However, to test this concept, researchers have to wait for new mathematical methods to be available.
According to Hossenfelder, the theory of asymptotically safe gravity has been around for over four decades but was only able to catch up in the 1990s through the research of Christof Wetterich and Martin Reuter. The work of the two physicists allegedly provided the necessary mathematical formulas to calculate what really transpires with the quantum theory of gravity at higher energies.
The program starts with the theory at low energies, using the new mathematical methods to explore how gravity may reach asymptotic safety. To date, no one has really proven if gravity is asymptotically safe. However, several independent arguments have emerged to support the idea.
Of course, like all other theories, the concept has its problems, and one of it is that calculations are not possible in the full theory space. Researchers were only able to make feasible predictions by studying just a part of the theory space, which in turn only produces limited knowledge. This keeps the situation inconclusive despite the consistency of the calculations with asymptotic safety.
Also, another issue that is yet to be solved revolves around the possibility that even if gravity is asymptotically safe, it might be rendered physically meaningless at high energies because it could potentially break some fundamental elements of quantum theory.
Still, despite the challenges, modern physicists can now put the ideas behind asymptotic safety to the test. And, if gravity is found to be asymptotically safe, given that the theory will behave at high energies, it might potentially restrict the current number of elementary particles mentioned in the Standard Model of particle physics.
While many physicists still believe that nothing new will come out of asymptotic safety, Hossenfelder wrote that it provides the link between testable low energies and inaccessible high energies. Plus, it is also not necessarily in conflict with other techniques used to quantize gravity. She believes that somehow, “asymptotic safety might allow us to finally connect the known universe to the quantum behavior of space-time.”