What Exactly Are Virtual Particles?

For those unfamiliar, virtual particles are represented as internal lines in Feynman Diagrams. Feynman Diagrams themselves are tools that illustrate every potential interaction between fields in quantum field theory. Below is an example of such a diagram.

In this diagram, the squiggly line symbolizes a virtual particle, depicting two particles annihilating to create a photon (the virtual particle), resulting in their scattering in opposite directions. With this understanding, we can delve into the flaws in Siegel's assertions.

Analysis of Siegel's Arguments

Siegel's criteria for determining the "reality" of a phenomenon in physics includes the requirement that it must influence observable quantities in a measurable way. He asserts that validation of predictions stems solely from empirical measurements, a statement that seems somewhat redundant.

One significant concern arises when Siegel cites the Casimir effect as evidence for virtual particles. The Casimir effect occurs when two closely spaced metal plates in a vacuum attract each other—not due to gravitational forces. He claims, “The vacuum should be filled with energetic contributions from all the allowed states,” suggesting that the imbalance of energy outside the plates leads to their attraction, which he interprets as evidence of virtual particles.

While his explanation regarding vacuum energy's role in the Casimir effect is somewhat valid, his leap to concluding that it supports the existence of virtual particles is flawed. The Casimir effect doesn't involve virtual particles; Siegel's derivation relies solely on vacuum energy. This is akin to claiming that a historical fact about a president proves the existence of virtual particles.

This leads me to believe that Siegel may not fully grasp the definition of virtual particles. They are not synonymous with vacuum energy; virtual particles are merely lines in Feynman diagrams, and the Casimir effect does not utilize these diagrams.

Furthermore, Siegel states, “This doesn’t mean that virtual particles are physically real. It means that their use in calculations enables us to make quantitative predictions about matter and energy interactions.” However, it’s crucial to note that virtual particles were not factored into the calculations for the Casimir effect.

He concludes that while the effects of virtual particles are genuine, the particles themselves are not measurable, creating a contradiction. If something can influence a tangible entity, it must be considered real. Siegel seems to adopt a more experimental definition of reality, requiring observation for validation. I maintain that entities like quarks are indeed real, even if they have not been directly observed. By Siegel's standards, quarks, gluons, and the Higgs boson might not qualify as "real," despite their observable effects.

If I concur with Siegel that virtual particles lack reality, where is the disagreement? The crux of the issue lies in his incorrect assertion that virtual particles can instigate real-world phenomena. He attributes the cause of the Casimir effect to virtual particles, but mathematical constructs lack causal power. Additionally, Siegel discusses vacuum polarization’s observable impacts without linking them to virtual particles, reinforcing the disconnect.