〈 Berger | Dillon 〉

What's an Effective field theory (EFT), and why we care about it? 𝐈𝐧𝐝𝐞𝐱 (yes, there's an index for this twitter thread): 1. Decoupling 2. Effective 3. Renormalization 4. References

1. Decoupling We sometimes find that physics at scale S₁ decouples from physics at S₂. When physics at two scales decouple, we mean that they’re autonomous. They minimally depend on each other. If physics changes at S₂, physics at S₁ is likely to remain unchanged.

1. Decoupling Rejoice: we therefore don’t need to know everything to do physics. We can study physics at S₁ while ignoring physics at S₂. Examples: In fluid mechanics, we ignore subatomic structure. Particle interactions are left out, while we study fluids as a continuum.

1. Decoupling In nuclear physics, things like protons are made of quarks and gluons. But it’s difficult to use quarks and gluons to study protons. Because physics at the level of protons (sometimes) decouples from physics at the level of quarks, we ignore quark interactions.