Here in our Universe, there are a series of fundamental forces that all appear to operate independently of one another. There’s gravity, which is an attractive force between all objects with mass and/or energy, governed by Einstein’s general relativity and the curvature of spacetime. There’s electromagnetism, where any particle with an electric charge or an electric or magnetic field is affected by other charges and electromagnetic fields in the environment.
There’s the weak nuclear force, which enables particles to change species and to decay from a heavier species into a lighter one. And there’s the strong nuclear force, which holds atomic nuclei together and binds quarks (and antiquarks) together into hadrons. We aren’t sure whether gravity is an inherently quantum force or not, but the other three forces — electromagnetism, the weak nuclear force, and the strong nuclear force — all have quantum field theories that describe them.
Although it took decades to ultimately make sense of these three forces, we now know that at least two of them are interrelated: the electromagnetic and weak nuclear force. These are not two completely independent forces, but rather are two “broken” manifestations of a more fundamental, unified force that exists under high-energy conditions: the electroweak force. It makes sense to wonder whether, at still higher energies, the strong nuclear force doesn’t unify with the electroweak force as well: the big idea of grand unification, or of grand unified theories as they’re sometimes known.
We don’t know the answer to whether nature ultimately unifies or not, and if so, under what conditions, at which energy scale, or under what framework. But it’s a fascinating idea that continues to generate theoretical interest, and continues to drive the search for experimental signatures. Here’s what we’re seeking.
The idea of unification holds that all three of the Standard Model forces, and perhaps even gravity at higher energies, are unified together in a single framework. This idea, although it remains popular and mathematically compelling, does not have any direct evidence in support of its relevance to reality. Only electroweak unification, among all the unified possibilities, has been established.
If you’ve ever seen the Standard Model of elementary particles, you could easily be forgiven for thinking that: This oversimplified picture is one that we often paint, and it’s mostly true. For most applications, it’s a close enough approximation to being true — a good enough model — that we simply run with it and don’t worry about the tiny caveats where we need to be a little more subtle. But in fundamental physics, those subtle details are often where the next vital breakthroughs are hiding.
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