# What's the importance of all four fundamental forces being "curvature"?

I've heard about how, in a gauge theory, the gauge covariant derivative of the field around a closed curve is generally not zero, and this is how you can quantify force or field strength. And that this is the same basic idea as curvature, with the gauge field being equivalent to the connection.

So since gravity is already known to be curvature, we can say that all the forces of nature are curvature in their own way. So what's the significance of that? Is there some deeper reason that we should expect that to be the case? And are the current unification programs based on that similarity?

• This seems to be a related question: @ physics.stackexchange.com/q/272902 Commented Jul 23, 2022 at 20:21
• Curvature is the simplest local invariant (as opposed to a global invariant like the number of holes in spacetime) you can cook up from the data that goes into a physical theory. Once we specify a theory, curvature is something we can measure -- it's a "force." Commented Jul 24, 2022 at 18:15
• @CharlesHudgins That's interesting, can you expand on that or provide a reference that explains that idea? Commented Jul 24, 2022 at 21:45
• I would turn it around and say that the significance is that gravity can be thought of as a gauge theory like the other forces (see, e.g.:physics.stackexchange.com/questions/71476/…). And the fact that all the fundamental forces (except, arguably, one associated with the Higgs field) share this broad structure does seem deep indeed, although I can't say more than that. Commented Jul 24, 2022 at 23:54
• @AdamHerbst Unfortunately I don't have a reference ready because this is just something I picked up from browsing. But here's a sketch: we want to differentiate something with symmetry in a way that respects the symmetry. That gives us a connection. The connection necessarily couples to the object we're trying to differentiate, and so should be physically observable. How do we observe the connection? Well a connection tells us how things should change as we move around space. The curvature tells us what happens as we move in a loop. So we measure the curvature by traversing a loop... Commented Jul 25, 2022 at 0:10

For example, electromagnetism's $$U(1)$$ symmetry (let's put electroweak unification aside for the moment) is the rotational invariance of $$|\phi|$$ for $$\phi\in\Bbb C$$ with $$|\phi|:=\sqrt{\phi^\ast\phi}$$, or equivalently for $$\phi\in\Bbb R^2$$ with $$|\phi|:=\sqrt{\phi\cdot\phi}$$. (I'm denoting the set of values $$\phi$$ can have at each point in spacetime, say $$\Bbb R^4$$, so as a function $$\phi\in X^{\Bbb R^4}$$ for $$X=\Bbb C$$ or, somewhat less helpfully in QFT, $$X=\Bbb R^2$$.) So if there is a space which is "curved" in this context by electromagnetism, it is not spacetime per se.
For ordinary Lie algebras, the gauge covariant derivative on the space symmetries... cannot be intertwined with the internal gauge symmetries... this is the content of the Coleman–Mandula theorem. However, a premise of this theorem is violated by the Lie superalgebras (which are not$${}^\dagger$$ Lie algebras!) thus offering hope that a single unified symmetry can describe both spatial and internal symmetries: this is the foundation of supersymmetry
$${}^\dagger$$ emphasis in the original.