Written by: Marina David

Editors: Lisa Pinatti, Christina Del Greco, and Sarah Kearns

Imagine you are waiting at a broken traffic light and both the green and red light are on. Should you keep waiting in hopes that it will fix itself eventually? Or should you pretend that you only see the green light and continue driving? You don’t want to cause a traffic jam, but you will be late for that 8 am meeting if you continue just sitting in the car. You would be confused about what to do, right? Quite surprisingly, this is similar to our current understanding of gravity.

We are all pretty familiar with gravity: it keeps us on the ground. If we throw a ball in the air, we know that it will come back down. These are the ideas that evoked one mathematician, Sir Isaac Newton, to establish the notion of gravity; now four hundred years later, we call these ideas *Newtonian Mechanics*. With Newtonian Mechanics, we can write mathematical equations describing projectiles, oscillatory motion, and even astronomical objects, such as planets and galaxies.

Newton’s perception was challenged three hundred years later by the German physicist, Albert Einstein, who took a unique approach by perceiving gravity as geometry. Einstein postulated that the universe is a geometrical structure and every object that has mass bends this geometry, giving us what we experience as* gravity.* The theory involved strange cosmological objects, such as black holes, and predicted that gravity can produce waves propagating throughout the universe, called gravitational waves [1]. His ideas were renowned but did not become widely accepted without a fight. After several astronomical tests of his theory [2], his viewpoint became accepted, and his ideas are now rightfully known as *Einstein’s Theory of General Relativity*. It is now understood that Newtonian Mechanics is a limiting case of General Relativity. As long as we are traveling slow enough, at velocities much smaller than the speed of light, both theories give similar and accurate results.

What about the smallest of all scales? At scales smaller than an atom, physicists developed *quantum mechanics* to understand the little building blocks inside atoms and to describe, for example, how electrons orbit an atom’s nucleus [3]. The results of numerous experimental and theoretical studies have now led us to a more appropriate viewpoint on how subatomic particles interact by focusing on three values: energy, position, and momentum. Particles have quantized energy, just like steps on a ladder. The position and momentum of a particle are described by probabilities and uncertainty. They can be here, or they can be there, until they are observed and measured by the observer.

Some theories pair well together to explain physical phenomena better than one theory on its own. For example, quantum electrodynamics, also known as QED, is a theory that combines quantum mechanics and electromagnetism to explain how light and matter interact [4]. Can quantum mechanics be combined with general relativity to describe a quantum gravity theory? Does general relativity still hold when quantum mechanics is taken into account? Most of the time, both theories can work synchronously. This is because either the gravitational effects or the quantum effects are small enough that they can be neglected [5]. However, these theories become problematic together at the center of black holes, a region in spacetime with such strong gravity that nothing in its vicinity can escape. At the center of the black hole, the curvature of spacetime becomes very large, and the equations from general relativity that characterize these black holes break down. At the same time, quantum fluctuations become increasingly important [5, 6] and cannot be neglected. This is the clue that our ideas about the universe are not as coherent as we initially expected them to be. We are still missing some concrete concepts.

This is where the two theories collide, just as you and another vehicle may if you decide to continue driving past that broken traffic light. The conflicting ideas of quantum mechanics and gravity are why our current understanding of merging the quantum realm with the gravitational realm is like a malfunctioning traffic light. Should we follow the green light onto the intersection telling us to use our notions of gravity, or should we stop because it is a red light and obey quantum mechanics? The answer: somewhere in between. There is a strong need to go beyond Einstein’s notion of gravity and into the quantum realm, giving us instead a “yellow” light to proceed on. This becomes important as we probe certain scales. The result of merging these two fundamental ideas is called quantum gravity and although we are confident in some aspects of this theory, quite a bit still remains unknown.

The unresolved complications regarding black holes make them captivating to study, and physicists are eager to crack their code. One success is understanding black hole entropy [7] from a quantum mechanical point of view [8]. Entropy from this viewpoint is a measurement of the number of different possible arrangements of a system’s energy configuration, or microstates. What makes this result remarkable is that the quantum theory, which gives us the microstate counting, matches with what we expect from the gravity interpretation, also known as the macroscopic viewpoint.

Despite many successes, there are still more puzzling results than answers. This includes the black hole information paradox [9] posed by the late Stephen Hawking, who suggested that physical information may permanently disappear inside a black hole, violating several key physical principles. Several mathematical tools and experimental setups [10] still need to be developed as physicists race towards a unifying theory of quantum gravity. Although physicists may be inching closer to the correct theory of quantum gravity, you may still be stuck in that traffic light. But, hey, in that spare time in the car, consider the intricate dynamics of quantum gravity.

**References**

[1] https://www.ligo.caltech.edu/page/what-are-gw

[2] https://asd.gsfc.nasa.gov/blueshift/index.php/2015/11/27/testing-general-relativity/

[3] https://plato.stanford.edu/entries/qm/

[4] https://www.britannica.com/science/quantum-electrodynamics-physics

[5]https://www.scientificamerican.com/article/the-illusion-of-gravity-2007-04/

[6] https://blogs.scientificamerican.com/observations/maybe-we-could-see-a-singularity-after-all/

[7] https://youtu.be/YM-uykVfq_E

[8] https://arxiv.org/abs/hep-th/9601029

[9] https://www.scientificamerican.com/article/escape-from-a-black-hole/

[10] https://www.scientificamerican.com/article/quantum-gravity-in-the-lab/

Marina David is a PhD candidate in Physics at the University of Michigan. Her research focuses on studying aspects of quantum gravity and is particularly interested in black hole physics. She is passionate about science communication and how to incorporate DEI efforts in STEM. Outside of physics, she enjoys playing the ukulele, reading and baking.