What Is Quantum Gravity And Why Is it Considered To Be The Hardest Problem In Physics?

Simran Buttar

Editor at ‘The Secrets Of The Universe’, I have completed my Master’s in Physics from Punjab, India and I am currently pursuing my doctoral studies on Radio Emissions of Exoplanets in Barcelona, Spain. I love to write about a plethora of topics concerned with planetary sciences, observational astrophysics, quantum mechanics and atomic physics, along with the advancements taking place in the space industry.

Whenever the word “gravity” strikes our minds, we think of planets revolving around their respective stars, the gravitational interaction between massive galaxies, the giant gravitational force possessed by neutron stars, black holes, etc. But, what about the tiny small particles existing in our universe? What about the microscopic quantum world? Do they know and experience gravity the way we do? Or are things different at their end? Well, in a bid to find answers to all these questions, the theory of “quantum gravity” has come into existence.

So, What is quantum gravity? Why is it needed? What if it’s really true? Let’s dig in to answer these questions!

In the simplest words, Quantum gravity is a theoretical framework that aims to describe how the force of gravity works for the universe’s smallest bits. Being a quantum theory of gravity, it seeks to describe gravity according to quantum mechanics principles, and in situations where quantum effects cannot be ignored, at the so-called Planck scale.

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Why do we need a quantum theory of gravity?

About a century ago, Albert Einstein came up with his General Theory of Relativity and scrapped off the long-existing notion of Newton’s gravity of simple attraction between objects with a description of matter or energy bending space and time around it. Undoubtedly, general relativity has successfully aced all the tests thrown at it time and again and completely explains the gravitational interaction at the macroscopic scale. But,  when physicists try to calculate the curvature of space around an electron or other such small entities, the math becomes impossible to handle. 

Moreover, at distances very close to the center of the black holes that are closer than the Planck length, quantum fluctuations of spacetime play an important role. So, when one tries to outline the gravitational field of a black hole in the general theory of relativity, the spacetime curvature diverges at the center, thus signaling the break down of the general theory of relativity and hinting towards the need for a theory that goes beyond general relativity and takes into account the quantum effects as well.

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• What are Feynman diagrams and why are they so important?
• An introduction to the standard model of physics
• What are monopoles and how would their discovery change physics?

Gravitons and the theory of everything :

Four fundamental forces govern our universe, the gravitational force, which governs planets’ motion, the electromagnetic force that studies the interaction between charges. The strong force explains how a nucleus is stable, and the weak force is concerned with radioactivity. Now, quantum mechanics suggests that everything is made of quanta, or packets of energy, that can behave like both a particle and a wave; for instance, a photon is a quantum of light and so on. So, each force must have a quantum or a force carrier associated with it if it is completely true in the quantum world.

Except that of gravity, the three fundamental forces are already known to follow these laws of quantum mechanics and have a force carrier associated with them. Hence, there is no issue regarding their credibility in the quantum world. Things are different for the force of gravity because general relativity is entirely based on the classical framework. Over time, physicists have associated a hypothetical force carrier with gravity as well, and this hypothetical quantum of the force of gravity is what we call a graviton.

If one day, graviton’s hypothetical status gets changed to a real one, it will prove that gravity also fits well into quantum mechanics, which would eventually bring the scientists a giant leap closer to a “theory of everything.”

Various theories of quantum gravity:

Over the years, several approaches have been put forward to explain quantum gravity, and hence, several candidate theories have been proposed. The most known approaches in this context are the string theory, canonical quantization theory, loop quantum theory, Euclidean quantum theory, and the recent one being a quantum gravity theory based on quantum computation. I’m not delving into these theories’ complexities in this one article as it would become a lot to gulp in one go.

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However, some of these theories tend to directly quantize gravity, while others indirectly prefer to do this task. Gravity is a theory based on geometry and distance, so usually, the normal approach to quantize gravity is to quantize the metric of spacetime. None of these mentioned theories are complete and consistent quantum theories of gravity yet. All are constantly evolving, with new ideas hitting every day, which makes quantum gravity one of the most active research areas these days with an ample number of opportunities.

Undoubtedly, quantum gravity theories have a lot of challenges to face as far as their experimental confirmation is concerned due to the limitation of resources. However, it is widely hoped that a theory of quantum gravity would one day allow us to understand problems of very high energy and minimal dimensions of space, such as the behavior of black holes, the origin of the universe, and a lot more. Till then, Keep exploring! You never know, one day you might be the one winning a Nobel for some breakthrough in the field of quantum gravity!

Before you go, make sure you also read:

• What is quantum tunneling and why is it the reason behind our existence?
• The concept of Schrodinger’s cat in quantum mechanics
• Understanding the Dirac equation and anti-matter