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Gravity is a habit that is hard to shake off.
Terry Pratchett

Einstein’s general theory of relativity is our best notion of gravity. The presence of mass warps spacetime, creating the gravitational phenomena we observe. Currently we grasp many of its properties. We use it to study the expansion of the universe and precisely position GPS satellites. But sometimes the concepts break down.

Gravity is the weakest of the fundamental forces. This is a big problem when we try to decipher its effects on a microscopic scale. Contributions from strong, weak and electromagnetic interactions effectively drown out any chance of spotting gravity. The hypothesised messenger particle, the graviton - the gravitational equivalent of a photon - has never been detected experimentally.

For the most part, small scale gravity is of no concern: we can safely ignore its tiny impact on the behaviour of atoms. This avenue has been pursued with great success in modern particle physics. The framework of quantum field theory is an incredibly accurate description of matter, despite not incorporating gravity.

But sometimes we want to understand gravity in minuscule regions. The Big Bang theory predicts that the early universe was minute and dense. In such a situation all forces are on an equal footing. To model the development of the cosmos we must figure out how they work in conjunction. We need a quantum theory of gravity.

Black holes are also shrouded in mystery. These dead stars are so massive that they crush spacetime to an infinitesimal point. Here quantum effects come into play. Naively combining the calculations from general relativity and quantum mechanics leads to nonsensical results.

Our comprehension of gravity is incomplete. For small spacetime volumes or large gravitational forces Einstein has little to offer. We need a way to reconcile general relativity and quantum field theory. A successful approach must modify physics on a fundamental level, allowing us to solve for quantum gravity.

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