Einstein's General Theory Of Relativity provides a rock-solid description of gravity, black holes and even the Big Bang, but fails to explain the very ‘singularities’ that signal towards infinity.

The extraordinary force of gravity can be researched with new-age engineering experiments but there is a risk of pushing too far and risking extreme damage by accidentally creating a black hole.

The infinite weirdness of black holes makes it difficult for a physicist to combine gravity with quantum mechanics (QM).

The fact that gravity works on everything makes it difficult to construct an apparatus to measure the QM properties of gravity. This is because the measuring apparatus can itself collapse into a black hole if it is heavy, and would not be able to measure accurately if it is too light.

Any proposed theory of quantum gravity would need to take care of the principles of locality, which state that any variable can change independently while measuring the properties of any point in space-time. These variables are called the degrees of freedom.

A worthy theory of quantum gravity has to minimize the degrees of freedom.

"The number of degrees of freedom in a spatial region is proportional to its surface area instead of its volume” - Daniel Harlow, Quantum gravity theorist at MIT.

Quantum gravity experiments and observations show some interesting phenomena in particles and even space-time. It is the cause of the creation of entire universes, not just particles.

The more the universe expands, the more vacuum is created, due to the expansion of space-time, leading to further perplexing questions.

• While performing quantum calculations, certain infinite terms appear, making these theories ‘non-renormalizable’.
• Theories that are describing the forces of nature other than gravity, can easily be renormalized(put to scale) and that makes gravity an anomaly.
• Gravity, for the most part, becomes a special force of nature that cannot be calculated.

The reason Einstein’s theory of general relativity works well in lower energy calculations and standard quantum techniques but not in higher energies and scales (like a black hole), could be because certain key features or ingredients of gravity are yet unknown to us, and are therefore missing from the famous theory.