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The debate about whether gravity is a force since it has just flared up yet again so I thought now would be a good time to give Substack a start, and wade in. The debate is partly semantics, and sometime a bit of physics-trolling but it’s weirdly heated, I think because it hinges on differing attitudes to geometry and field theory.

So, while all the arguments in this debate have been well rehearsed across the internet, I think it’s worth outlining here, since I think it gets to the heart of Einstein's theory of gravity, General Relativity, and what makes it different to the quantum forces of nature. It’s also a good starting point for this Substack, because in a subsequent post, I will explain that not only is gravity not a force, it is not a gauge theory (like electromagnetism). I'll argue that this makes it important to question whether we should be attempting to reconcile quantum theory and gravity by quantising spacetime. The interactions we know how to quantise are gauge theories, but gravity is not a gauge theory. This is what forms part of my motivation for considering an alternative to quantum gravity, the postquantum theory of classical gravity.

In this post, I’ll just consider the question of whether we should call gravity a force. Newton certainly did, and Newton's law of gravity superficially looks like Coulomb's force law for electrostatics, so it would be natural to call gravity a force. But we've learned a lot since Newton’s time, and we can now be more precise about what constitutes a force. To explain this, I’ll start with Einstein’s equivalence principle, but mostly to say that I think we talk too much about the equivalence principle! Instead, we can jump straight to the conclusion Einstein drew from the equivalence principle — that we can describe gravity purely in terms of spacetime geometry. If you accept that we live in a spacetime which can have curvature, then I argue you should conclude that no force is acting on a test particle moving freely in this curved spacetime. Hence gravity is not a force. To call gravity a force, is to privilege flat space as somehow being special.

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