I think this is a very good point. We don’t empirically know the particles exist, we know the observations exist. We infer the particles.
If you’ve ever had the air bag stolen from your car then you have no reason to doubt the reality of virtual particles.
One of the reasons the sensors in airbags are so expensive to make (and hence make good targets for thieves) is that they have to take into account the Casimir Effect. This is the pressure applied to very small parallel plates by virtual particles appearing out of nothing in the quantum vacuum.
That would though require that the Casmir effect as simply being artifact of the Feynman picture (?), which whilst, as I say, I don’t know an awful lot about QFT, I’m still confident enough to say is incorrect (for the simple reason that differrent ‘pictures’ in quantum physics don’t produce different empirical results).
Not all states in QFT are eigenstates of the particle number operator, that’s true; but not all states are eigenstates of the position operator, either – this doesn’t mean that QFT fails to reproduce the ‘real positions’ of things, it just means that things get a bit more subtle.
We know of the existence of real particles through the outcomes of experiments – which are in accordance with QFT, and in fact, confirm it. So to say that our knowledge of the existence of real particles is in tension with QFT amounts to the strange position that the theory’s confirmation somehow can be taken as evidence against it.
Things would be different if we had some independent confirmation of the existence of real particles as the fundamental constituents of our world; but of course, we don’t.
The question is rather whether the concept of particles provides a good ontology for QFT, and indeed, quite a lot of people would argue that it doesn’t, proposing a field ontology or other, quite wild things instead.
I don’t disagree with you as even with limited knowledge of QFT I was always under the impression it was in good agreement with observation from which we know that particles are in fact useful concepts.
My understanding is that particles are problematic as they’re not as easy a concept to pin down in QFT as perhaps we’d like them to be.
Even if the real particle stays in the same spot, doesn’t it still propagate to infinity?
It doesn’t freeze in time, and further from a photon’s reference frame the photon is still and the universe moves at the speed of light. Meaning all real particles are always moving toward infinity. It’s just a question of how much of the speed applies to time, and how much to space, even if they’re not moving relative to each other.
I hope this makes sense.
Meanwhile the virtual particle ceases to exist as soon as the interaction is over, unless a black hole turns the little pinocchio into a real boy. Then it propagates off into infinity.
There is a distinction to be made here. Take a photon propagating from one interaction to another produced from a known source. You never measure these photons to have a mass; they are always on mass shell. On the other hand, if you study the scattering between the two states that the photon propagated between, your analyses may involve looking at the invariant mass of the byproducts and you may conclude that a virtual photon propagated between the two states in order to mediate a force exchange. But this is just an artifact of perturbation theory attempting to describe force exchange in terms of things we are familiar with. Ultimately it is true that we can’t directly measure a photon: we have to do it indirectly, and so if it is our want we can always describe it as merely an internal leg in force mediation. But on the other hand the description of force mediation is perturbative, while the description of real free fields are exact. Free field photons are real, and in the perturbation calculations that describe interacting fields in terms of free fields, there is a very clear distinction between real and virtual states. But while there are no truly free fields in nature, the analogy still holds: there is still a clear distinction between the states that the perturbation series is built in terms of, and the intermediate states that are purely mathematical artifacts. In the real world we build our perturbation series in terms of free fields even though that is itself an approximation, but in any case the approximate fields that we use as external legs have a different ontological status from the the internal propagators: they are the ‘real’ entities we have grown up observing in the real world that are interacting with each other; the interactions themselves involve purely fictitious ‘virtual’ diagrams whose descriptions are completely dependent on how you define your perturbation expansion.
I would say that that’s a problem with our expectations, not with QFT.
And iamnotbatman, in situations where a particle propagates a long ways between interactions, if you treat it as an internal line rather than an external one, you’ll find that the process with that particular internal line has a much higher amplitude than any other contributing processes, but that the other amplitudes are still nonzero. “Virtual” particles become what we think of as “real” when the terms involving them come to dominate the total amplitude.