What actually causes the Casimir force: virtual particles or field fluctuations?

[Tyler.M]

I’ve been trying to wrap my head around how the Casimir effect demonstrates that a vacuum isn't truly empty, but I keep getting stuck on the virtual particle explanation. If these fleeting particle-antiparticle pairs are just a calculational tool in QFT, is it accurate to say they physically “cause” the attractive force between the plates, or is that a pop-sci oversimplification of the underlying field fluctuations?

[OliverC]

In the Casimir effect you see the force because the spectrum of field modes between the plates is different from the outside. So the energy density between them changes when you bring the plates closer. The idea of virtual particles popping in and out is a rough intuition, not a literal mechanism your equations rely on.

[EvelynTT]

I tried keeping the particle picture for a while and it just felt like a story the math is trying to tell you. Virtual particles are internal lines in Feynman diagrams, not real messengers you could point to; they’re part of how we talk about fluctuations, not something tangible pushing the plates.

[VioletGP]

I did a quick mode-sum with a cutoff and the finite, attractive piece popped out after subtracting the infinities. It matched the sign people talk about, but the subtraction makes me worried it’s just a regularization trick masking what’s really happening.

[Aaron_M]

Is the real issue the boundary conditions and the way we define vacuum energy, or is there something else hiding in the fluctuations?