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Existence is a polysemic and difficult word to define. Almost certainly numbers (and other well-defined mathematical objects) exist in a different way than a real physical object (the chair I sit in, the computer I write on). I do not intend to discuss the concept of existence in all its possible meanings, because I want to concentrate on the notion of "physical existence".

I think it is reasonable to accept that an object "physically exists" if:

  1. there is a physical procedure (using other physical objects) capable of determining whether the object is present or not (such verification should be intersubjective and different "agents" could carry it out independently):
  2. if the object whose existence is conjectured "exists physically" has observable consequences, if it does not exist the result of the measurement or physical process would give observable consequences.

I am not quite sure if this condition of detectability by some physical procedure would encompass all interesting cases of "physical existence", but certainly the vast majority. The question is whether there can be "physically existing objects" that do not satisfy the above condition.

For example, some artifacts within theories are doubtful. Numbers and mathematical concepts used in theories do NOT physically exist, for example. In some computational schemes of Quantum Field Theory (QFT) "virtual particles" are used which are treated as existing, but cannot actually be detected by physical means. For example, a virtual photon could move at a different speed than light, for example!!!!

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    At least they exist as virtual particles. Virtual has also many meanings in physics. One is that they may be created and destroyed so fast that can only be conjectured to have existed. Other times they are used as mere bookkeeping mathematical devices
    – Nikos M.
    Jun 6 at 22:40
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    If you ask Carnap "Do electrons exist?” he would reply it's not a legitimate question implying philosophical import, but pseudo-questions asked from outside a language framework of science since we're not acquainted with it and can only observe some related attributes such as mass, charge, etc as its trace. Even more so for your virtual particles. But inside a certain scientific language (say the common textbook one), not only they exist but very useful for scientists. Here you can treat virtual particles as some relations demanded by theoretical content and thus exists in such a language. Jun 7 at 5:01

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  1. According to the current interpretation of QFT, virtual particles are not physical objects but mathematical artefacts. Virtual particles arise when applying Feynman‘s pertubation theory to solve interaction problems in QFT.

  2. E.g., perturbation theory of quantum electrodynamics (QED) approximates the final result of the interaction of two particles like electron and positron by an infinite series of interaction summands. Each summand describes the interaction of the two particles by only a finite number of interaction events. Eventually, the sum over all these events describes the physical process by an infinite series.

    Each summand considers a non-physical subprocess which can be computed more easily than the whole process. During each step the finitely many interaction events can be formalized as the exchange of non-physical particles, the virtual particles.

  3. In QFT the primary ontological entity is the field – not the particle. The physical particle is considered a ripple moving through its corresponding field. A virtual partical is considered a disturbance of the field due to two or more locally interacting ripples from physical particles. Hence virtual particles are a mathematical tool. They do not exist as physical particles. Solving interaction problems in a non-perturbative way even avoids the concept of virtual particles.

  4. Consider also the blog of Strasser and the subsequent discussion on StackExchange Physics.

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  • When I read Strasser (profmattstrassler.com/articles-and-posts/…) I get the impression they are physical. They relate in the same way to the field as particles, "Exactly the same equations that tell us about photons also tell us about how these disturbances work; in fact, the equations of quantum fields guarantee that if nature can have photons, it can have these disturbances too."
    – J Kusin
    Jun 7 at 20:26
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    "This disturbance is important, because the force that the two electrons exert on each other — the repulsive electric force between the two particles of the same electric charge — is generated by this disturbance."
    – J Kusin
    Jun 7 at 20:27
  • @J Kusin: Please note that Strasser emphasizes that virtual particles are disturbances of the fields. Of course these disturbances can be computed from the original field equations of the physical particles which couple the electron field and the electromagnetic field: One approximates the Green-function of the original differential equation. But Strasser emphasizes that it's best to skip the term 'particle' when calculating the next interaction term of the approximation. - One cannot isolate a virtual particle. It is the idealization of a finite number of interaction events.
    – Jo Wehler
    Jun 7 at 20:37
  • Forget how it’s approximated mathematically. The mathematical objects aren’t the physical objects. We all seem to agree on that. There is something physical which is not entirely captured by physical fields and particles which must play the physical role of repelling like particles (among other roles). Isn’t that ultimately what is meant by virtual particles, this 3rd physical component? In physics, something physical must be playing that role since it has physical consequences. We can’t isolate many physical things. Einstein believed in hidden variables for example.
    – J Kusin
    Jun 8 at 15:15
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Some other interesting examples are:

  • The wave-function of quantum particles, given we can only take measurements that indicate it, never measure it.
  • Dark-matter, which has no direct observations since it was posited in the modern sense in the 1970s.
  • The Many Worlds of the Measurement Problem interpreted in the most ‘unadorned’ way.
  • Places beyond cosmological event horizons (and regions of space in a Big Rip longterm cosmological picture), given there seems to be no possible worldline for us to interact with them.
  • Hawking Radiation, which though very widely accepted will be extremely, extremely, difficult to observe directly (Hawking points may be evidence).
  • Unruh radiation which may mean the existence of some photons depends on your rest-frame.

I would say this is an area where physicists do not think enough about philosophy. Sabine Hossenfelder for instance holds the view that science is only concerned with making predictions, existence being necessary parts of a model that makes correct predictions. But when you consider Poppers point that measurements decide between models, you can see if we haven’t imagined a model yet, we can’t know about a truly existing thing implied by our methods yet. We take for granted existence is a property out there, and forget that science is tentative, it’s results provisional, and it’s determinations generally within confidence intervals. And, fundamentally in a relationship with our models that we are testing between.

I thought Rupert Sheldrake made this point very clearly in a debate with Hossenfelder, where he talked about his paper Is The Sun Conscious? (PDF of the article from his website, it was published in The Journal Of Consciousness Studies), which she immediately dismissed. Sheldrake thinks the movement of stars by non-random solar flares could account for the missing ‘dark matter’. If we don’t open our minds to possibilities like this, we can’t look for evidence for or against them.

“the universe is not only queerer than we suppose, but queerer than we can suppose” JBS Haldane, in his essay Possible Worlds

We are well to remember this. What we can use observations to understand, is limited by our hypothesis generation.

Existence is a simple word, and it deceives us into thinking it is a simple property. It is better to think about it having different qualities in different contexts, and to look at what our statements about it in those contexts are used to indicate. That is, a kind of parallel to the deflationary theory of truth, in regard to existence.

Virtual particles have no mass, they are only relevant within the bounds of the uncertainty principle related to their mass. But this has real consequences, including Hawking Radiation. I would say we should describe them as part of the potential of empty space, and related to the energy of it. That is, not as having existence separate to particles, but not as only a book-keeping exercise.

QFT is considered to have provided a more convincing formulation of the vacuum as empty, than the Dirac Sea model of This leaves the problem of the high vacuum energy which seems to indicate potential, eg in the Casimir effect.

In a deeper sense until QFT and GR are unified we don’t know what spacetime is, so we just don’t know yet.

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    Great examples of conjectured physical existence
    – Nikos M.
    Jun 7 at 18:21

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